Question

As the intermolecular attractive forces between molecules increase in magnitude, do you expect each of the following to increase or decrease in magnitude? (a) Vapor pressure, (b) heat of vaporization, (c) boiling point, (d) freezing point, (e) viscosity, (f) surface tension, (g) critical temperature.

Short answer

As intermolecular attractive forces increase, the following changes in magnitude occur: (a) Vapor pressure decreases, (b) Heat of vaporization increases, (c) Boiling point increases, (d) Freezing point increases, (e) Viscosity increases, (f) Surface tension increases, and (g) Critical temperature increases.

Step-by-step solution

a) Vapor Pressure

Vapor pressure is the pressure exerted by the vapor in equilibrium with its liquid or solid phase. When the intermolecular attractive forces increase, it becomes harder for the molecules to escape the liquid or solid phase and enter the vapor phase. As a result, the equilibrium will shift towards the condensed phase, and hence, the vapor pressure will decrease in magnitude.

b) Heat of Vaporization

Heat of vaporization is the amount of heat required to vaporize a certain amount of a substance at its boiling point. If the intermolecular attractive forces increase, it will require more energy to overcome these forces and turn the substance into the vapor phase. Thus, as intermolecular attractive forces increase, the heat of vaporization will also increase in magnitude.

c) Boiling Point

Boiling point is the temperature at which the vapor pressure of a substance is equal to the atmospheric pressure, and the substance turns from a liquid to a gas. When the intermolecular forces increase, the vapor pressure decreases, as discussed in part (a). To achieve the same atmospheric pressure, the temperature needs to be increased, and hence, the boiling point will increase.

d) Freezing Point

Freezing point is the temperature at which a liquid turns into a solid. When the intermolecular attractive forces increase, it means the molecules are held together more tightly. Therefore, it takes less cooling of the substance to make it solidify. As a result, the freezing point will increase.

e) Viscosity

Viscosity is a measure of the resistance of a fluid to flow. When the intermolecular forces increase, the molecules in a liquid are more strongly attracted to each other. This makes it harder for them to flow past one another, which increases the resistance to flow. Therefore, as intermolecular attractive forces increase, the viscosity will also increase in magnitude.

f) Surface Tension

Surface tension is a measure of the energy required to increase the surface area of a liquid. It is caused by the unbalanced intermolecular forces at the surface of the liquid. When intermolecular attractive forces increase, the surface molecules are more strongly attracted to the bulk molecules and experience a greater inward pull. This makes it more difficult to increase the surface area, hence increasing the surface tension.

g) Critical Temperature

Critical temperature is the temperature above which a gas cannot be liquefied, no matter how much pressure is applied. When intermolecular attractive forces increase, the gas molecules are more strongly attracted to each other. This means that at higher temperatures (with higher intermolecular forces), it is more likely for the gas to condense into a liquid under increased pressure. Therefore, as intermolecular attractive forces increase, the critical temperature will also increase.

Key concepts

Vapor Pressure

Vapor pressure plays a pivotal role in understanding how substances change state. It's the pressure created by the vapor of a substance when it's in a closed system, in equilibrium with its liquid self. Imagine a sealed container half filled with water; the space above the water is filled with water vapor. Now, if the intermolecular forces (the attractive forces between molecules) get stronger, imagine each water molecule as being a bit 'stickier'. They'll cling to one another more tightly. This means fewer molecules will have the energy to break free and become vapor. As a result, the vapor pressure decreases. A lower vapor pressure indicates that a liquid is less willing to turn into a gas, which is an intuitive way to imagine the tightening grip molecules have over one another with stronger intermolecular forces.

Explaining this concept visually is powerful. Picture a group of people (molecules) holding hands (intermolecular forces) in a room. If they grip tighter, it's harder for someone to leave the room (change to vapor), just as stronger intermolecular forces lead to decreased vapor pressure.

Heat of Vaporization

The heat of vaporization is the energy needed for a liquid to become gas. When you boil water, the energy you're adding doesn't just warm up the water; it also helps water molecules break away from their liquid-state hugs and fly off as steam. If the bonds between the molecules become stickier due to stronger intermolecular forces, you'll need more energy (heat) to separate them. Thus, the heat of vaporization increases.

In everyday terms, think about blowing up a very tough balloon. If the rubber is extra stretchy (weaker bonds), it takes less effort. But if the rubber is very firm (stronger bonds), you'll huff and puff more to inflate it. The heat of vaporization behaves similarly; the 'tougher' the molecular 'rubber', the more 'breath' (heat) you need.

Boiling Point

Picture the boiling point as a mountain's peak where water finally jumps into the air as steam. This 'peak' is reached when the vapor pressure of the water equals the air pressure pushing down on it. If water molecules prefer each other's company due to stronger intermolecular forces, it takes higher temperatures to reach the point where they can become vapor—thus hiking up the boiling point. It's much like raising the mountain further into the sky; climbers (molecules) need more energy (temperature) to reach the top under these new, tougher circumstances.

Freezing Point

The freezing point is where liquids turn into solids, like water into ice. Stronger intermolecular forces tie the molecules down earlier (at a higher temperature) to settle them into a solid-state. It's a bit like a group deciding to sit down earlier on their hike because the pathway became more challenging. With 'stickier' molecules, less cooling is needed to snap into a rigid, solid structure, and so the freezing point rises.

Asking students to visualize a team sitting down to rest while hiking an icy trail can effectively convey why increased intermolecular forces would mean an earlier rest stop - or a higher freezing point.

Viscosity

Viscosity measures how thick and resistant to flow a liquid is—think honey versus water. With stronger intermolecular forces, the liquid behaves more like honey than water because its molecules grip each other more firmly. This increased thickness makes it harder for the substance to flow smoothly and quickly, causing an uptick in viscosity. Informing students to think of viscosity as the 'stickiness' between liquid molecules can be a helpful comparison to daily experiences, like stirring various thicknesses of syrups.

Surface Tension

Surface tension is that invisible 'skin' on water that allows insects to skate across ponds. Molecules on the surface of liquids don't have buddies above them, so they stick more tightly to their neighbors on the surface and below. If the molecules have stronger attractions thanks to increased intermolecular forces, this 'skin' gets even stronger. The 'stickier' the molecules, the tougher it is to break or stretch the surface, leading to higher surface tension. An illustrative way to convey this is by imagining a tightrope made of different materials. A rope with more 'cling' between its fibers will be harder to twist or stretch - analogous to a liquid with high surface tension.

Critical Temperature

Critical temperature is the highest temperature at which a gas can be forced back into a liquid state, no matter the pressure. Think of it as the ultimate limit of gas molecules sticking together before they become perpetual wanderers. When the attraction between the molecules intensifies, they prefer being closer to each other, even at higher temperatures. As a result, the critical temperature climbs higher. It's like people preferring to stay in a warm, crowded room (liquid state) even as it gets hotter outside, rather than heading out to the cool, open air (gas state).

Aiding students to grasp this concept might involve inviting them to imagine a situation where they'd prefer the cosiness of a warm room to the harsh conditions outside, regardless of the indoor temperature's increase, akin to molecules at high critical temperatures.