If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

### 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?
(3 votes)
• 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.
(7 votes)
• more i am inspird
(3 votes)
• When would our whole universe reach thermal equilibrium?
(1 vote)
• You’re referring to the heat death of the universe. It essentially takes the second law of thermodynamics to its eventual conclusion. As the universe continues to expand, energy also wishes to spread out due to entropy. At a certain point energy is as dispersed as it can be (maximum entropy), leaving everything in the universe in thermal equilibrium. This also means nothing can really happen anymore and the universe is devoid of any activity. As for when that could happen, it’s a matter of debate which depends on how much dark energy the universe contains. I’ve seen estimates around 10^(100) years though. If true, it means the universe has a predictable end.

Hope that helps.
(3 votes)

## Video transcript

- [Instructor] Let's say we have two samples of helium gas. One sample of helium gas is at temperature T1 and the other sample of helium gas is at temperature T2. If T2 is greater than T1, that means on average, the particles of helium gas in the second box are moving faster than the particles of helium gas in the first box. We can tell the particles in the second box are moving faster because on average, the length of these arrows indicating the velocity vector in the box and the right are longer than the length of the arrows in the first box. The equation for kinetic energy is equal to 1/2 mv squared, where m is the mass of a particle and v is the velocity of a particle. Since the gas particles in the box on the right are on average traveling faster and have higher velocities, the average kinetic energy of the particles in the box on the right is higher than the average kinetic energy for the particles in the box on the left. And so the average kinetic energy is proportional to the temperature. The higher the temperature, the higher the average kinetic energy of the particles. Instead of gases, let's look at two metal blocks made of the same material. So one of the metal blocks is at temperature T1 and the other metal block is at temperature T2. Let's say the temperature T2 is greater than T1. What that means is the particles in the metal box on the right are on average moving faster than the particles in the metal box on the left. And that means that the average kinetic energy of the particles in the box on the right is greater than the average kinetic energy in the particles in the box on the left. Also notice, right now are two pieces of metal are not touching each other. So there's a little bit of space between them. Next, we bring the two objects and we put them in contact with each other. So now there are collisions between the particles that touch and this results in the transfer of energy from the hotter object to the cooler object. So heat flows from the object at the higher temperature to the object at the lower temperature. And so the hot atoms in the metal object on the right start to move a little slower, whereas the colder atoms in the metal on the left start to move a little bit faster. And the transfer of energy continues until both objects have the same final temperature. And we say that thermal equilibrium has been reached. So here we have our two objects, and the two objects are at the same final temperature, which means we have reached thermal equilibrium. And since we've reached thermal equilibrium, there's no more flow of heat between the two objects, and since both objects are at the same final temperature, that means the average kinetic energy of the particles in both objects is now the same.