Video: Applying Knowledge of the Energetics of State Changes

For statements I and II, state for each if they are true or false. I) When a sample of ice melts, the process is endothermic. II) Water is in a higher potential energy state than ice. If both are true, state if II is a correct explanation for I.

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Video Transcript

For statements I and II, state for each if they are true or false. I) When a sample of ice melts, the process is endothermic. II) Water is in a higher potential energy state than ice. If both are true, state if II is a correct explanation for I.

In this question, we’re considering water or H₂O going from a solid to a liquid. Statement I is asking us if this process is endothermic. If a process is endothermic, that means that the system absorbs energy from the surroundings in the form of heat. An endothermic process has a positive change in entropy, or ΔH. So statement I is asking us if ice needs to absorb energy in order to melt and turn into liquid water. The process of ice melting does require energy. So when ice melts, it absorbs heat from the surroundings, which makes the process of ice melting endothermic. So statement I is true.

Statement II is asking us if water has a higher potential energy than ice. This question is actually getting at a pretty complicated point. So before we answer it, let’s really try to understand what’s going on when water goes from a solid to a liquid. To answer this part of the question, we really need to understand what’s going on during a phase change such as ice melting. So let’s try to dig into this. Solid water melts at a temperature of zero degrees Celsius. As long as the water is still melting, the temperature is constant. Temperature is proportional to the average kinetic energy of the system. So when we say that the temperature of the water is constant while it’s melting, what we’re really saying is that the average kinetic energy for the water is constant.

We’ve already discussed how when ice melts, it absorbs energy from the surroundings. So energy is being added to the water as it melts. But since the process of ice melting is occurring at a constant temperature, this change in energy is not due to a change in kinetic energy. Since the kinetic energy is not changing but we’re gaining energy, this increase in energy must be due to an increase in the average potential energy of the water. So since the average potential energy increases when we go from solid water to liquid water, it’s true that water must have a higher potential energy than ice.

Another way to think about the change in potential energy between a liquid and a solid is to consider how potential energy behaves in general. Potential energy is often related to an object’s position relative to other objects. If you throw a ball up into the air, when it’s farthest away from the earth, it will have a higher potential energy than when it finishes falling and lands on the ground. So now, let’s think of a solid versus a liquid. In a solid, the particles are all packed together and they don’t have much freedom to move around. But in a liquid, the particles can be a little bit further from each other.

Since the particles in a liquid can be further away from each other than the particles in a solid, the liquid will have a higher potential energy than the solid will. So once again, it’s true that liquid water will have a higher potential energy than ice does. Since both statement I and statement II are true, we need to state if II is a correct explanation for I. So we need to determine if water having a higher potential energy than ice is the reason that when a sample of ice melts, the processes is endothermic. We’ve already discussed how liquid has a higher potential energy than a solid. So when ice melts, it has to gain potential energy. And it does this by absorbing energy from the surroundings, which makes the process of ice melting endothermic. So it’s true that statement II is a correct explanation for statement I.

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