What Kind(S) Of Intermolecular Forces Must Be Overcome During The Following Phase Changes?

All around us we come across matter in different phases. The air we breathe is a gas, while the water you drinkable is a liquid and the chair yous are sitting on is a solid. In this affiliate we are going to look at one of the reasons that matter exists every bit solids and liquids.

In the previous affiliate, nosotros discussed the different forces that exist between atoms (interatomic forces). When atoms are joined to one another they form molecules, and these molecules in plow have forces that demark them together. These forces are known as intermolecular forces.

Intermolecular forces allow usa to decide which substances are likely to deliquesce in which other substances and what the melting and boiling points of substances are. Without intermolecular forces holding molecules together we would not exist.

Note that nosotros will apply the term molecule throughout this affiliate as the compounds we are looking at are all covalently bonded and do not exist as giant networks (recall from course \(\text{10}\) that there are iii types of bonding: metallic, ionic and covalent). Sometimes you volition see the term elementary molecule. This is a covalent molecular structure.

Interatomic (between atoms) forces are likewise known as intramolecular (inside molecules) forces. You can remember this by thinking of international which ways between nations.

four.1 Intermolecular and interatomic forces (ESBMM)

Intermolecular forces

Intermolecular forces are forces that act betwixt molecules.

You will also recall from the previous affiliate, that nosotros can describe molecules as being either polar or not-polar. A polar molecule is one in which there is a divergence in electronegativity between the atoms in the molecule, such that the shared electron pair spends more time shut to the atom that attracts it more than strongly. The result is that ane end of the molecule will have a slightly positive charge (\(δ^{+}\)), and the other cease will accept a slightly negative charge (\(δ^{-}\)). The molecule is said to exist a dipole.

A dipole molecule is a molecule that has 2 (di) poles. Ane terminate of the molecule is slightly positive and the other is slightly negative. We can depict this very simply as an oval with one positive side and i negative. In reality however, the molecules practice non expect like this, they look more than like the images in Effigy 4.1.

Effigy four.one: A different representation of dipole molecules. The red region is slightly negative, and the bluish region is slightly positive.

It is important to remember that just because the bonds inside a molecule are polar, the molecule itself may non necessarily be polar. The shape of the molecule may also affect its polarity. A few examples are shown in Table 4.1 to refresh your memory. Note that we have shown tetrahedral molecules with all the terminal atoms at \(\text{90}\)\(\text{°}\) to each other (i.east. flat or ii-dimensional), but the shape is actually 3-dimensional.

Molecule	Chemical formula	Bond between atoms	Shape of molecule	Polarity of molecule
Hydrogen	\(\text{H}_{two}\)	Non-polar covalent		Not-polar
Hydrogen chloride	\(\text{HCl}\)	Polar covalent		Polar
Carbon tetrafluoride	\(\text{CF}_{4}\)	Polar covalent		Non-polar
Trifluoro-methane	\(\text{CHF}_{iii}\)	Polar covalent		Polar

Table iv.one: Polarity in molecules with dissimilar atomic bonds and molecular shapes.

Types of intermolecular forces (ESBMN)

It is important to be able to recognise whether the molecules in a substance are polar or non-polar because this volition determine what type of intermolecular forces there are. This is important in explaining the properties of the substance.

1. Ion-dipole forces

   As the name suggests, this type of intermolecular force exists between an ion and a dipole (polar) molecule. You will recollect that an ion is a charged atom, and this will be attracted to i of the charged ends of the polar molecule. A positive ion volition be attracted to the negative pole of the polar molecule, while a negative ion will be attracted to the positive pole of the polar molecule. This can be seen when sodium chloride (\(\text{NaCl}\)) dissolves in water. The positive sodium ion (\(\text{Na}^{+}\)) will be attracted to the slightly negative oxygen atoms in the water molecule, while the negative chloride ion (\(\text{Cl}^{-}\)) is attracted to the slightly positive hydrogen atoms. These intermolecular forces weaken the ionic bonds between the sodium and chloride ions so that the sodium chloride dissolves in the water (Figure 4.2).

   

   Effigy four.2: Ion-dipole forces in a sodium chloride solution.

   This is a simplified diagram to highlight the regions of positive and negative charge. When sodium chloride dissolves in h2o it tin can more accurately be shown every bit:

   

2. Ion-induced-dipole forces

   Similar to ion-dipole forces these forces exist between ions and non-polar molecules. The ion induces a dipole in the not-polar molecule leading to a weak, short lived force which holds the compounds together.

   These forces are found in haemoglobin (the molecule that carries oxygen around your body). Haemoglobin has \(\text{Fe}^{2+}\) ions. Oxygen (\(\text{O}_{two}\)) is attracted to these ions by ion-induced dipole forces.

3. Dipole-dipole forces

   When 1 dipole molecule comes into contact with another dipole molecule, the positive pole of the 1 molecule will be attracted to the negative pole of the other, and the molecules volition be held together in this fashion (Figure 4.3). Examples of materials/substances that are held together by dipole-dipole forces are \(\text{HCl}\), \(\text{SO}_{2}\) and \(\text{CH}_{three}\text{Cl}\).

   

   Figure 4.three: Two dipole molecules are held together by the bonny force between their oppositely charged poles.

   One special instance of this is hydrogen bonding.

4. Induced dipole forces

   These intermolecular forces are also sometimes chosen "London forces" or "momentary dipole" forces or "dispersion" forces.

   We know that while carbon dioxide is a non-polar molecule, we can still freeze it (and we can as well freeze all other not-polar substances). This tells us that there must be some kind of attractive force in these kinds of molecules (molecules can but be solids or liquids if at that place are attractive forces pulling them together). This force is known as an induced dipole force.

   In not-polar molecules the electronic accuse is usually evenly distributed but it is possible that at a particular moment in time, the electrons might not be evenly distributed (remember that the electrons are always moving in their orbitals). The molecule volition accept a temporary dipole. In other words, each finish of the molecules has a slight charge, either positive or negative. When this happens, molecules that are side by side to each other attract each other very weakly. These forces are found in the halogens (e.1000. \(\text{F}_{2}\) and \(\text{I}_{2}\)) and in other non-polar molecules such as carbon dioxide and carbon tetrachloride.

   All covalent molecules have induced dipole forces. For not-polar covalent molecules these forces are the simply intermolecular forces. For polar covalent molecules, dipole-dipole forces are plant in addition to the induced dipole forces.

   When the noble gases condense, the intermolecular forces that hold the liquid together are induced dipole forces.

5. Dipole-induced-dipole forces

   This type of force occurs when a molecule with a dipole induces a dipole in a not-polar molecule. It is similar to an ion-induced dipole force. An example of this type of strength is chloroform (\(\text{CHCl}_{3}\)) in carbon tetrachloride (\(\text{CCl}_{4}\)).

The following image shows the types of intermolecular forces and the kinds of compounds that lead to those forces.

Figure 4.4: The types of intermolecular forces. The boxes represent the type of compound while the lines stand for the type of force.

The last three forces (dipole-dipole forces, dipole-induced dipole forces and induced dipole forces) are sometimes collectively known as van der Waals' forces. We volition now look at a special case of dipole-dipole forces in more detail.

Hydrogen bonds

As the name implies, this type of intermolecular bond involves a hydrogen atom. When a molecule contains a hydrogen atom covalently bonded to a highly electronegative atom (\(\text{O}\), \(\text{N}\) or \(\text{F}\)) this type of intermolecular force can occur. The highly electronegative atom on one molecule attracts the hydrogen atom on a nearby molecule.

Water molecules for example, are held together past hydrogen bonds between the hydrogen cantlet of ane molecule and the oxygen atom of another (Figure 4.5). Hydrogen bonds are a relatively potent intermolecular force and are stronger than other dipole-dipole forces. It is important to annotation even so, that hydrogen bonds are weaker than the covalent and ionic bonds that be between atoms.

Practise not misfile hydrogen bonds with actual chemical bonds. Hydrogen bonding is an case of a example where a scientist named something believing information technology to exist one matter when in fact it was some other. In this case the forcefulness of the hydrogen bonds misled scientists into thinking this was really a chemical bail, when it is really just an intermolecular strength.

Figure 4.5: 2 representations showing the hydrogen bonds between water molecules: infinite-filling model and structural formula.

The difference between intermolecular and interatomic forces (ESBMP)

It is important to realise that in that location is a divergence between the types of interactions that occur in molecules and the types that occur betwixt molecules. In the previous chapter we focused on the interactions between atoms. These are known as interatomic forces or chemical bonds. We as well studied covalent molecules in more detail.

Recall that a covalent bond has an electronegativity difference of less than \(\text{2,ane}\). Covalent molecules have covalent bonds betwixt their atoms. Van der Waals' forces only occur in covalent molecules. We tin can show the interatomic and intermolecular forces between covalent compounds diagrammatically or in words. Intermolecular forces occur between molecules and do not involve individual atoms. Interatomic forces are the forces that hold the the atoms in molecules together. Figure 4.5 shows this.

	Interatomic forces	Intermolecular forces
Atoms or molecules	Forces between atoms	Forces between molecules
Force of forces	Strong forces	Relatively weak forces
Distance betwixt atoms or molecules	Very short distances	Larger distances than bonds

Table 4.2: The differences between interatomic and intermolecular forces.

The worked examples on intermolecular forces condense a lot of data into the first step. You may need to remind learners how to determine molecular polarity. To practise this, yous can use the worked examples in diminutive combinations as a quick refresher of the topic. In tests and exams learners demand to be able to speedily identify a polar or non-polar molecule and then demand to be very comfortable with this skill.

Worked instance 1: Intermolecular forces

Which intermolecular forces are found in carbon tetrachloride (\(\text{CCl}_{4}\))?

Call back virtually what you know about the molecule.

Carbon has an electronegativity of \(\text{ii,5}\). Chlorine has an electronegativity of \(\text{3,0}\). The electronegativity divergence between carbon and chlorine is \(\text{1,0}\) (remember the section on electronegativity in the previous affiliate). We also know that the bond betwixt carbon and chlorine is polar.

Also from the previous chapter nosotros know that carbon tetrachloride is a tetrahedral molecule (retrieve molecular shape). Carbon tetrachloride is symmetrical so is non-polar overall.

Now decide which case it is

Carbon tetrachloride is non-polar and so the only kind of force that can exist is induced dipole.

Worked example 2: Intermolecular forces

Which intermolecular forces are found in the following solution: sodium chloride in water?

Think about what you know about the molecules

Sodium chloride is ionic. (the electronegativity deviation is \(\text{ii,1}\)). Water has polar bonds (the electronegativity difference is \(\text{one,4}\)). Water is a polar molecule (its molecular shape is aptitude or athwart).

Now decide which case it is

This is an ionic substance interacting with a polar substance. This interaction is an ion-dipole strength.

Textbook Practise 4.1

hydrogen fluoride (\(\text{HF}\))

Hydrogen fluoride is a polar covalent molecule. (Information technology is linear and not symmetrical.) So the blazon of intermolecular forcefulness is dipole-dipole forces.

methyl hydride (\(\text{CH}_{4}\))

Methane is a non-polar covalent molecule. (It is tetrahedral and symmetrical.) So the blazon of intermolecular force is induced dipole forces.

potassium chloride in ammonia (\(\text{KCl}\) in \(\text{NH}_{iii}\))

Potassium chloride is an ionic compound. Ammonia is a polar covalent molecule. (It is trigonal pyramidal and not symmetrical.) Then the type of intermolecular force is ion-dipole forces.

krypton (\(\text{Kr}\))

Krypton is a element of group 0. Then the blazon of intermolecular strength is induced dipole forces.

Agreement intermolecular forces (ESBMQ)

The types of intermolecular forces that occur in a substance will affect its properties, such as its phase, melting point and boiling point. You should recollect from the kinetic theory of matter (meet grade \(\text{x}\)), that the phase of a substance is determined by how strong the forces are betwixt its particles. The weaker the forces, the more probable the substance is to be as a gas. This is because the particles are able to move far autonomously since they are not held together very strongly. If the forces are very strong, the particles are held closely together in a solid construction. Remember also that the temperature of a material affects the free energy of its particles. The more energy the particles take, the more likely they are to be able to overcome the forces that are holding them together. This can cause a alter in phase.

Shown below are the three phases of water. Note that we are showing two dimensional figures when in reality these are iii dimensional.

Figure 4.half-dozen: The three phases of water.

The effects of intermolecular forces

The post-obit five experiments investigate the effect of various physical properties (evaporation, surface tension, solubility, boiling point and capillarity) of substances and determine how these properties relate to intermolecular forces. Each experiment will wait at a different property.

A formal experiment on the effects of intermolecular forces is included in this chapter. In this experiment learners will investigate how intermolecular forces impact evaporation, surface tension, solubility, boiling points and capillarity. Some of the substances that are used (smash polish remover (mainly acetone if you utilize the non acetone costless variety), methylated spirits (a mixture of methanol and ethanol), oil (a mostly non-polar hydrocarbon), glycerin (a fairly circuitous organic molecule)) are quite complex substances and learners may non have the skills needed to make up one's mind the types of intermolecular forces at piece of work here. You should guide learners in this and tell them the intermolecular forces for these substances.

You can help learners piece of work out the strength of the intermolecular forces by telling them that larger molecules have stronger intermolecular forces than smaller molecules. This is oft a big gene in determining which substance has the strongest intermolecular forces.

This experiment is split into five experiments. Each experiment focuses on a different property and sees how that property relates to intermolecular forces. It volition often not be easy for learners to meet the pocket-size differences between some of the molecules called and so they need to use a combination of experimental results and noesis about the strength of the intermolecular force to attempt and predict what may happen. Each experiment ends with a conclusion about what should be constitute to guide learners.

It is very important to work in a well ventilated room (one with lots of air menstruation) particularly when working with methanol and ethanol. Many of the substances used (particularly boom polish remover, ethanol and methylated spirits) are highly flammable and so care must be taken when heating these substances. It is recommended that learners utilize a hot plate rather than a Bunsen burner to heat these substances as this reduces the take a chance of burn. When doing chemical science experiments it is also extra of import to ensure that your learners exercise not run around, practice not try to drinkable the chemicals, do not eat and drink in the lab, practice non throw chemicals on the other learners and in general practice human activity in a responsible and safe way. The guidelines for safe experimental piece of work can exist found in the science skills chapter from grade \(\text{10}\).

The effects of intermolecular forces: Part \(\text{i}\)

Aim

To investigate evaporation and to determine the relation between evaporation and intermolecular forces.

Apparatus

You lot will need the following items for this experiment:

• ethanol, water, nail polish remover (acetone), methylated spirits

• evaporating dishes (or shallow basins)

Method

1. Place \(\text{20}\) \(\text{ml}\) of each substance given in split up evaporating dishes.
2. Carefully move each dish to a warm (sunny) spot.
3. Marker the level of liquid in each dish using a permanent marker. Make several marks at different positions around the dish. If the permanent marking is leaving a smudge rather than a noticeable mark, carefully wipe the side of the dish and try over again.
4. Discover each dish every minute and note which liquid evaporates fastest.

Results

Record your results in the table below. You lot do non need to measure the level of the liquid, but rather just write how much the level had dropped (e.yard. for water you might write did not notice whatever decrease in the level or for ethanol y'all might write well-nigh all the liquid had evaporated).

Substance	Liquid level after \(\text{1}\) \(\text{min}\)	\(\text{2}\) \(\text{min}\)	\(\text{3}\) \(\text{min}\)	\(\text{4}\) \(\text{min}\)	\(\text{5}\) \(\text{min}\)
Ethanol
Water
Nail shine remover
Methylated spirits

Give-and-take and conclusion

You should observe that water takes the longest fourth dimension to evaporate. H2o has potent intermolecular forces (hydrogen bonds). Ethanol (\(\text{CH}_{3}\text{CH}_{2}\text{OH}\)) and methylated spirits (mainly ethanol (\(\text{CH}_{3}\text{CH}_{ii}\text{OH}\)) with some methanol (\(\text{CH}_{3}\text{OH}\))) both take hydrogen bonds but these are slightly weaker than the hydrogen bonds in water. Boom polish remover (acetone (\(\text{CH}_{3}\text{COCH}_{3}\))) has dipole-dipole forces only and so evaporates quickly.

Substances with weaker intermolecular forces evaporate faster than substances with stronger intermolecular forces.

The furnishings of intermolecular forces: Office \(\text{2}\)

Aim

To investigate surface tension and to determine the relation between surface tension and intermolecular forces.

Appliance

You volition need the following items for this experiment:

• h2o, cooking oil (sunflower oil), glycerin, nail polish remover (acetone), methylated spirits

• small glass beakers or glass measuring cylinders

• small slice of drinking glass or clear plastic (about \(\text{5}\) \(\text{cm}\) past \(\text{v}\) \(\text{cm}\).)

Method

1. Identify about \(\text{l}\) \(\text{ml}\) of each substance given in separate small beakers or measuring cylinders.
2. Observe the shape of the meniscus. (This is the level of the liquid). Note what happens at the edges where the liquid touches the glass. (You can place a few drops of food colouring in each substance to help y'all see the meniscus.)
3. Now place a drib of the substance on a small piece of glass. Observe the shape of the drop.

Results

Tape your results in the table below. Yous but need to give a qualitative result (in other words what you come across in the experiment).

Substance	Shape of meniscus	Shape of droplet
Water
Oil
Glycerine
Blast polish remover
Methylated spirits

Discussion and decision

The meniscus for all these substances should be concave (i.e. higher at the edges than in the middle). This is because the forces holding the molecules in the substance together are weaker than the attraction between the substance and the glass of the tube.

You should likewise have noticed that water, oil and Glycerine tend to class a driblet, while boom smooth remover and methylated spirits do not. Strong intermolecular forces assistance hold the substance together, while weaker ones do not concur the molecules in the substance together equally much.

Water has the strongest intermolecular forces (hydrogen bonds) of all the substances used. Glycerine and methylated spirits also have hydrogen bonds, simply these intermolecular forces are slightly weaker than in water. Sunflower oil is mostly non-polar but has very long molecules which aid account for the higher surface tension.

Substances with strong intermolecular forces will generally have a greater surface tension than substances with weaker intermolecular forces.

The furnishings of intermolecular forces: Role \(\text{three}\)

Aim

To investigate solubility and to determine the relation between solubility and intermolecular forces.

Apparatus

You volition need the following items for this experiment:

• Solids: sodium chloride (table salt), iodine, potassium permanganate

• Solvents: water, ethanol, chloroform

• \(\text{9}\) beakers or examination-tubes

• \(\text{3}\) A4 sheets of paper

Method

1. Place nearly \(\text{20}\) \(\text{ml}\) of each solvent given in divide beakers. Identify this set on a piece of paper labelled "sodium chloride".
2. Echo this stride twice. The 2nd ready is for potassium permanganate (so your piece of paper will say "potassium permanganate") and the 3rd set is for iodine (so your piece of paper will say "iodine"). Yous should now accept ix beakers in total.
3. Into the first set, add together about \(\text{2}\) \(\text{g}\) of sodium chloride.
4. Into the 2nd set, add about \(\text{2}\) \(\text{g}\) of potassium permanganate.
5. Into the third fix, add together about \(\text{2}\) \(\text{g}\) of iodine.
6. Discover how much of each substance dissolves in the solvent.

Results

Record your results in the tabular array below. If you observe simply a modest corporeality of the solid dissolving and so write that very little solid dissolved. If all the solid dissolves then write that all the solid dissolved.

Substance	H2o	Chloroform	Ethanol
Sodium chloride
Potassium permanganate
Iodine

Give-and-take and decision

You should notice that the sodium chloride and potassium permanganate dissolved (at least a flake) in all the substances. The iodine did non dissolve in any of the substances. The three solvents (h2o, chloroform and ethanol) are all polar and have dipole-dipole forces. Sodium chloride and potassium permanganate are both ionic substances, while iodine is not-polar.

Substances will deliquesce in solvents that have similar intermolecular forces or in solvents where the ionic bonds tin be disrupted by the formation of ion-dipole forces.

The furnishings of intermolecular forces: Part \(\text{4}\)

Aim

To investigate boiling point and to determine the relation between boiling betoken and intermolecular forces.

Apparatus

You will need the following items for this experiment:

• h2o, cooking oil (sunflower oil), Glycerine, blast polish remover, methylated spirits

• examination-tubes and a beaker

• hot plate

Method

Methylated spirits and smash polish remover are highly flammable. They will hands catch fire if left near an open flame. For this reason they must be heated in a water bathroom. This experiment MUST be performed in a well ventilated room.

1. Identify nearly \(\text{20}\) \(\text{ml}\) of each substance given in separate test-tubes.
2. Half-make full the beaker with water and place on the hot plate.
3. Place the test-tubes in the beaker.
4. Detect how long each substance takes to eddy. Equally soon as a substance boils, remove it from the h2o bath.

Results

Write down the order in which the substances boiled, starting with the substance that boiled first and ending with the substance that boiled last.

Give-and-take and conclusion

You should have plant that the nail polish remover and the methylated spirits boil before the h2o, oil and Glycerine.

Glycerine, water and methylated spirits have hydrogen bonds between the molecules. However, in water and Glycerine these intermolecular forces are very strong while in the methylated spirits they are slightly weaker. This leads to the higher boiling betoken for water and Glycerine. Nail polish remover has weaker dipole-dipole forces.

Although cooking oil is non-polar and has induced dipole forces the molecules are very large and then these increase the strength of the intermolecular forces.

Substances with strong intermolecular forces volition have a college boiling point than substances with weaker intermolecular forces.

The effects of intermolecular forces: Part \(\text{v}\)

Aim

To investigate capillarity (how far up a tube a liquid rises or how far down a liquid falls) and to determine the relation between capillarity and intermolecular forces.

Apparatus

You volition need the following items for this experiment:

• water, cooking oil (sunflower oil), nail polish remover, methylated spirits

• large shallow dish, narrow glass tube (with one end closed)

Method

1. Place almost \(\text{twenty}\) \(\text{ml}\) of h2o in the shallow dish.
2. Hold the narrow tube but above the level of the water in the dish.
3. Observe how far up the tube the water travels.
4. Repeat for the other three substances, remembering to wash and dry out the dish and tube well between each one.

Results

Record your results in the table below. You do not need to mensurate how far up the tube the substance travels only rather say if it only travelled a curt distance or a long distance.

Substance	Distance travelled up tube
Water
Oil
Blast polish remover
Methylated spirits

Discussion and determination

Water travels the greatest distance up the tube. Blast shine remover travels the to the lowest degree distance.

Capillarity is related to surface tension. If the attractive strength between the drinking glass walls of the tube and the substance are stronger than the intermolecular forces in the substance, than the edges of the liquid volition be pulled in a higher place the surface of the liquid. This in turn helps pull the liquid up the tube.

Substances with strong intermolecular forces will travel farther up a narrow tube (accept a greater capillarity) than substances with weaker intermolecular forces.

From these experiments we can see how intermolecular forces (a microscopic property) bear on the macroscopic behaviour of substances. If a substance has weak intermolecular forces then it will evaporate easily. Substances with weak intermolecular forces also have low surface tension and do not ascension as far upwardly in narrow tubes as substances with strong intermolecular forces. Boiling points are lower for substances with weak intermolecular forces. Substances are more than likely to exist soluble in liquids with similar intermolecular forces.

We will now look at some more properties (molecular size, viscosity, density, melting and boiling points, thermal expansion, thermal conductivity) in item.

Molecular size

The alkanes are a group of organic compounds that incorporate carbon and hydrogen bonded together. The carbon atoms link together to grade chains of varying lengths.

The boiling indicate and melting point of these molecules is determined past their molecular construction, and their surface surface area. The more carbon atoms there are in an alkane, the greater the surface expanse and therefore the higher the boiling bespeak. The melting point also increases as the number of carbon atoms in the molecule increases. This can be seen in the table below.

Formula	\(\text{CH}_{4}\)	\(\text{C}_{2}\text{H}_{half-dozen}\)	\(\text{C}_{5}\text{H}_{12}\)	\(\text{C}_{6}\text{H}_{14}\)	\(\text{C}_{20}\text{H}_{42}\)
Name	methane	ethane	pentane	hexane	icosane
Molecular mass (\(\text{g·mol$^{-1}$}\))	\(\text{16}\)	\(\text{thirty}\)	\(\text{72}\)	\(\text{86}\)	\(\text{282}\)
Melting indicate (℃)	\(-\text{183}\)	\(-\text{183}\)	\(-\text{130}\)	\(-\text{95}\)	\(\text{37}\)
Boiling point (℃)	\(-\text{164}\)	\(-\text{89}\)	\(\text{36}\)	\(\text{69}\)	\(\text{343}\)
Stage at room temperature	gas	gas	liquid	liquid	solid

You will also observe that, when the molecular mass of the alkanes is low (i.east. there are few carbon atoms), the organic compounds are gases because the intermolecular forces are weak. Every bit the number of carbon atoms and the molecular mass increases, the compounds are more than likely to be liquids or solids because the intermolecular forces are stronger.

You should come across that the larger a molecule is the stronger the intermolecular forces are between its molecules. This is one of the reasons why methane (\(\text{CH}_{4}\)) is a gas at room temperature while pentane (\(\text{C}_{five}\text{H}_{12}\)) is a liquid and icosane (\(\text{C}_{twenty}\text{H}_{42}\)) is a solid.

It is partly the stronger intermolecular forces that explain why petrol (mainly octane (\(\text{C}_{8}\text{H}_{xviii}\))) is a liquid, while candle wax (\(\text{C}_{23}\text{H}_{48}\)) is a solid. If these intermolecular forces did not increment with increasing molecular size we would not be able to put liquid fuel into our cars or use solid candles.

Viscosity

Viscosity is the resistance to flow of a liquid. Compare how easy it is to cascade water and syrup or dearest. The water flows much faster than the syrup or honey.

Yous can see this if you take a cylinder filled with water and a cylinder filled with glycerin. Drop a small metallic ball into each cylinder and note how easy it is for the brawl to autumn to the bottom. In the glycerin the ball falls slowly, while in the water it falls faster.

Substances with stronger intermolecular forces are more viscous than substances with weaker intermolecular forces.

Automobile and motor oils

You lot are given the post-obit information about engine oils.

Oil	Use	Other info
SAE xxx monograde	Engines	Low viscosity
SAE 50 monograde	Engines	High viscosity
SAE 15W-40 multigrade	Engines	Medium viscosity
SAE 0W-40 multigrade	Engines	Medium viscosity

(Data from shell.com)

Multigrade oils can be used even in cold weather since they remain fluid (The first number is the rating for wintertime weather and the W shows that this is the rating in wintertime. The 2d number is the viscosity rating in summer). Monograde oils are given their viscosity rating at \(\text{100}\)\(\text{°C}\). The viscosity is an indication of how well the oil flows. The more than gluey an oil the larger the molecules that are in the oil.

• Which oil has the longest molecules?

• Which oil has the shortest molecules?

• Which oil has the strongest overall intermolecular forces?

• Which oil has the weakest overall intermolecular forces?

• What can you conclude about the link between the magnitude of the intermolecular force and viscosity?

Density

Density

Density is a measure of the mass in a unit of measurement volume.

The solid phase is ofttimes the most dense stage (water is one noteworthy exception to this). This tin can be explained by the strong intermolecular forces establish in a solid. These forces pull the molecules together which results in more molecules in one unit volume than in the liquid or gas phases. The more molecules in a unit volume the denser that substance will be.

Melting and boiling points

Intermolecular forces affect the humid and melting points of substances. Substances with weak intermolecular forces volition have low melting and humid points while those with strong intermolecular forces volition have loftier melting and boiling points. In the experiment on intermolecular forces you investigated the boiling points of several substances, and should accept seen that molecules with weaker intermolecular forces have a lower boiling point than molecules with stronger intermolecular forces.

One farther indicate to note is that covalent network structures (remember from class \(\text{x}\) that these are covalent compounds that class big networks and an example is diamond) will have loftier melting and boiling points due to the fact that some bonds (i.east. the potent forces between atoms) accept to interruption before the substance can melt. Covalent molecular substances (e.g. water, sugar) oft have lower melting and boiling points, considering of the presence of the weaker intermolecular forces holding these molecules together.

Thermal expansion

As substances are heated their molecules start moving more than vigorously (their kinetic energy increases). This causes the liquid to expand on heating. Yous can observe this in a thermometer. As the booze (or mercury) is heated it expands and rises up the tube.

This is why when you tile a floor yous have to leave gaps between the tiles to allow for expansion. Information technology is besides why ability lines sag slightly and bridges accept slight gaps for expansion.

Thermal conductivity

Different materials comport oestrus differently. The following activeness volition highlight this.

Thermal electrical conductivity

Take a long sparse slice of graphite and a long thin piece of copper (or other metallic). Adhere a scrap of wax to the ane end of each rod (you lot volition need to melt the wax a fleck first to make information technology stick). While the wax is nevertheless soft, press a toothpick into the blob of wax.

Now append the graphite and copper rods from a desk or chair using a piece of string and heat the other end. Find which toothpick falls off beginning. Try to explain why.

Heat is transferred through a substance from the betoken being heated to the other cease. This is why the bottom of a pot gets hot first (assuming yous are heating the pot on a stove plate). In metals at that place are some free, delocalised electrons which can help transfer the heat energy through the metallic. In covalent molecular compounds there are no free, delocalised electrons and the rut does not travel every bit easily through the material.

Worked example 3: Understanding intermolecular forces

Explain why the melting bespeak of oxygen (\(\text{O}_{ii}\)) is much lower than the melting bespeak of hydrogen chloride \(\text{HCl}\).

Write down what yous know about melting points and forces

The stronger the intermolecular force, the higher the melting bespeak. So if a substance has strong intermolecular forces, then that substance will take a high melting point.

Write down which forces occur in the two given compounds

Oxygen is not-polar and has induced dipole forces. Hydrogen chloride is polar and has dipole-dipole forces.

Combine all the facts to get the respond

We know that stronger intermolecular forces pb to higher melting points. We besides know that oxygen has weaker intermolecular forces than hydrogen chloride (induced dipole versus dipole-dipole forces). Therefore oxygen will take a lower melting betoken than hydrogen chloride since oxygen has weaker intermolecular forces.

Types of intermolecular forces

Textbook Do 4.2

Given the following diagram:

1. Name the molecule and circle information technology on the diagram
2. Characterization the interatomic forces (covalent bonds)
3. Label the intermolecular forces

The molecule is hydrogen chloride.

Given the post-obit molecules and solutions:

\(\text{HCl}\), \(\text{CO}_{2}\), \(\text{I}_{ii}\), \(\text{H}_{two}\text{O}\), \(\text{KI} (\text{aq})\), \(\text{NH}_{iii}\), \(\text{NaCl}(\text{aq})\), \(\text{HF}\), \(\text{MgCl}_{2}\) in \(\text{CCl}_{iv}\), \(\text{NO}\), \(\text{Ar}\), \(\text{SiO}_{2}\)

Complete the table below past placing each molecule next to the right blazon of intermolecular force.

Ion-dipole
Ion-induced-dipole
Dipole-dipole (no hydrogen bonding)
Dipole-dipole (hydrogen bonding)
Induced dipole
Dipole-induced-dipole

In which one of the substances listed above are the intermolecular forces:

1. strongest

2. weakest

Ion-dipole	\(\text{KI} (\text{aq})\), \(\text{NaCl}(\text{aq})\), \(\text{HF}(\text{aq})\)
Ion-induced-dipole	\(\text{MgCl}_{2}\) in \(\text{CCl}_{4}\)
Dipole-dipole (no hydrogen bonding)	\(\text{HCl}\), \(\text{NO}\)
Dipole-dipole (hydrogen bonding)	\(\text{H}_{ii}\text{O}\), \(\text{NH}_{3}\)
Induced dipole	\(\text{CO}_{2}\), \(\text{I}_{2}\), \(\text{Ar}\)
Dipole-induced-dipole	\(\text{SiO}_{two}\) in water

H2o or ammonia are likely to have the strongest forces, while argon, iodine and carbon dioxide are likely to have the weakest forces. Induced dipole forces are the weakest intermolecular forces and hydrogen bonding is the strongest.

The boiling point of \(\text{F}_{ii}\) is much lower than the boiling indicate of \(\text{NH}_{3}\)

\(\text{NH}_{3}\) has hydrogen bonds which are much stronger than the induced dipole forces in \(\text{F}_{2}\). In order for a liquid to boil the intermolecular forces must exist broken and if the intermolecular forces are very stiff then it will take a lot of energy to overcome these forces and so the boiling point will be college.

H2o evaporates slower than carbon tetrachloride (\(\text{CCl}_{4}\)).

H2o has stiff intermolecular forces (hydrogen bonds) while carbon tetrachloride simply has weaker induced dipole forces. (Carbon tetrachloride is non-polar). Substances with stronger intermolecular forces take longer to evaporate than substances with weaker intermolecular forces.

Sodium chloride is likely to dissolve in methanol (\(\text{CH}_{three}\text{OH}\)) .

Sodium chloride is ionic. Methanol is polar. The type of intermolecular force that can exist when sodium chloride dissolves in methanol is ion-dipole forces. The formation of these forces helps to disrupt the ionic bonds in sodium chloride then sodium chloride can dissolve in methanol.

Tumi and Jason are helping their dad tile the bathroom floor. Their dad tells them to go out small gaps between the tiles. Why practice they demand to leave these pocket-size gaps?

Materials (such as tiles) expand on heating then small gaps demand to be left between the tiles to allow for this expansion. If Tumi and Jason did not leave these gaps betwixt the tiles, the tiles would soon lift up.

Source: https://intl.siyavula.com/read/science/grade-11/intermolecular-forces/04-intermolecular-forces-01

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