Video: Applying Knowledge of the Energetics of Boiling and Water’s Specific Heat Capacity

For statements I and II, state for each if they are true or false. I) The boiling of water is an exothermic process. II) Water has a relatively high specific heat capacity. 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) The boiling of water is an exothermic process. II) Water has a relatively high specific heat capacity. If both are true, state if II is a correct explanation for I.

An exothermic process is a process that releases energy to the environment. This shows up as a negative change in enthalpy. If you’ve ever boiled water on the stove, you most likely intuitively expect the statement to be false. Boiling water requires energy, not releases it. But let’s dig into why boiling water requires energy. As we can see from this cartoon, when the substance is a liquid, the molecules are much closer together. When the substance is a gas, the molecules are free to move around their container. And they’re fully separated.

Separating the molecules so that the substance can go from a liquid to a gas is a process that requires energy. A process that requires energy is called endothermic, not exothermic. So statement I is false. The boiling of water is not an exothermic process. It’s an endothermic process because it requires energy. The second statement says that water has a relatively high specific heat capacity. The specific heat capacity is the energy that’s required to raise the temperature of a substance. And it’s expressed per one gram of the substance per one degree Celsius.

So if the substance has a relatively high specific heat capacity, that means that it will take a lot of energy to change the temperature of that substance. Again, intuitively, we might expect this to be true based on how long it takes water to boil. But let’s think about this in more detail. When the temperature of a substance is increased, what’s really going on is that the kinetic energy of the substance is increasing because temperature and kinetic energy are proportional. At the molecular level, this corresponds to more movement of the atoms or molecules. They will vibrate faster and rotate faster.

If we take a look at liquid water at the molecular level, we would see that molecules of liquid water are strongly attracted to each other due to the presence of an intermolecular force called hydrogen bonding. In hydrogen bonding, a partially positive hydrogen is attracted to the lone pair on an oxygen of another neighboring water molecule. Because this intermolecular attraction between water molecules is so strong, when we add energy to liquid water, most of the energy goes towards breaking this hydrogen bonding and not towards increasing the kinetic energy of the water molecules.

So the presence of hydrogen bonding in water causes it to have a relatively high specific heat capacity. So statement II is true. If we compare the specific heat capacities of water to other various common substances, like concrete and vegetable oil, we’ll see that water does in fact have an extremely high specific heat capacity compared to these other things that we’re familiar with. Water’s high specific heat capacity compared to other substances is why your pool can stay at a relatively stable and comfortable temperature during the summer. But the surrounding concrete will burn your feet.

Water’s high specific heat capacity also explains why lakes and oceans and other bodies of water are able to sustain life despite fluctuations in the temperature outside of them. Because water’s heat capacity is so high, it takes a lot of energy to raise the temperature. So the temperature of the water can stay fairly stable. Since statement I of this question was false, we don’t have to answer the last part of the question.

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