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## AP®︎/College Chemistry

### Course: AP®︎/College Chemistry>Unit 6

Lesson 2: Heat transfer and thermal equilibrium

# Heat transfer and thermal equilibrium

The particles in a warmer object have a greater average kinetic energy than the particles in a cooler object. When two objects of different temperatures come into contact with one another, the particles at the surface of each object collide, resulting in the transfer of energy as heat. This process continues until thermal equilibrium is reached, at which point the average kinetic energies of the particles—and therefore the temperatures of the objects—is the same. Created by Jay.

## Want to join the conversation?

• How does the speed of particles in a substance relate to the different heat capacities of different substances? For example, would it take more energy to make particles of water move at a certain speed that it would to make particles of iron to move at that same speed? And if so, theoretically speaking, if a block of iron at normal temp were dropped in hot water, could the block of iron become even hotter than the water, since less energy is needed to heat it up and the particles of water have lots of energy to transfer? • So the speed of particles depends on their kinetic energy (which we can use temperature to represent) and their mass (which we can use molar mass). The temperature aspect includes heat capacity since heat capacity describes how much energy is required to raise the temperature of something. So if we have a group of particles with lots of kinetic energy (which we experience as high temperatures) and have low masses, then they will have high speeds.

If we had water molecules and iron particles and we wanted to have them move at the same speed, again we have to consider both their kinetic energy and their mass. If we wanted them to have the same kinetic energy, we need them to be at the same temperature. This can be accomplished by heating them to the same temperature and giving them both additional energy. However the amount of energy needed to do so is different for the two materials since they have different heat capacities. Water has a higher heat capacity than iron and so will require more energy to achieve the same higher temperature.

So that covers temperature, but we also have to consider mass. An iron particle is more massive than a water molecule so iron requires more energy to achieve a high speed compared to water. So it's more difficult to answer this question if we're concerned with having the two particles move at the same speed. Water needs more energy to achieve the same temperature because of its higher heat capacity, but iron needs more energy due to its mass. So we're not going to supply the same amount of energy to both, and the exact amount we have to supply to get them to move at the same speed will have to incorporate both the kinetic energy and mass aspects.

Whenever we combine two objects with different temperatures, the object with a high temperature will transfer energy to the object with a lower temperature. This is because energy naturally moves from high to low (hot object heat up cold objects, not the other way around). So if we have hot water and room temperature iron and combine them then the water will transfer energy to the iron and they will reach thermal equilibrium, or the same temperature. This temperature will be some value in between their starting temperatures.

Hope that helps.