Heat Transfer and Thermal Equilibrium
Explain thermal contact and a heat-capacity-weighted final temperature through energy conservation.
AP Chemistry Topic 6.3 · CED printed p. 114 / PDF p. 121. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- System and surroundings as a chosen boundary
- Moles, balanced equations and limiting reactants
- Intermolecular versus intramolecular interactions
- Activation energy versus overall energy change
Particles in a warmer sample have greater average thermal kinetic energy in the AP particle model. The distribution is broad, so this does not mean every warm-sample particle has more energy than every cool-sample particle. Different-mass particles at the same temperature need not have the same speed. Temperature concerns the thermal energy scale, not equal velocities or equal total energy in differently sized samples.
When bodies make thermal contact, particle interactions and collisions exchange energy. On average there is net transfer from higher temperature to lower temperature. The warmer body cools while the cooler body warms, provided no other process or external energy source dominates. Heat describes this transfer across a boundary due to a temperature difference; it is not a substance sitting inside one body. A large cold object and a small hot object illustrate why total stored energy alone does not set heat-flow direction.
At thermal equilibrium their temperatures are equal, and there is no net heat flow between them. Microscopic exchanges can continue in both directions while balancing on average. The final temperature is not automatically the arithmetic average: energy conservation weights each temperature change by heat capacity. For an isolated pair with no phase or chemical change, m1c1(Tf−T1) +m2c2(Tf−T2) =0. Here m₁ and m₂ are the two masses, c₁ and c₂ are their specific heat capacities, T₁ and T₂ are their initial temperatures, and Tf is the common final temperature. Equal masses only give the midpoint if their specific heat capacities are equal too.
Let's work through it
Supplied data: 40.0 g copper at 80.0 °C and 40.0 g water at 20.0 °C exchange heat in an ideal insulated container. cCu =0.385 and cwater =4.184 J g−1 °C−1.
- Copper loses what water gains: 40.0(0.385)(Tf−80.0) +40.0(4.184)(Tf−20.0) =0.
- Solve Tf =[0.385(80.0)+4.184(20.0)]/(0.385+4.184) =25.1 °C.
- The result is close to water’s initial temperature because the water has much greater heat capacity. A midpoint of 50 °C would require equal heat capacities, not merely equal masses.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
At thermal equilibrium do collisions stop?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
At thermal equilibrium do collisions stop?
Worked answer and reason
No. Exchanges continue, but there is no average net transfer between equal-temperature bodies.
Equilibrium is balanced microscopic exchange.
Retrieval 2
Why is the final temperature not always the average of the two starting temperatures?
Worked answer and reason
The heat capacities weight energy conservation; only equal total heat capacities give the arithmetic midpoint in the ideal two-body model.
Equal energy loss/gain does not imply equal temperature changes.
Catch the tempting mistake
- Thermal equilibrium means equal total energies.
- It means equal temperatures; mass and heat capacity can make total energies different.
- Heat always flows from the object with greater mass.
- Net heat transfer is governed by temperature difference.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 6 problem 3 · ap application · multiple
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For the teacher
- Ask students to predict which initial temperature the final value lies closer to before calculating.
- Require a collision/interaction explanation plus the equal-temperature equilibrium condition.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 6.3.A
- Explain the relationship between the transfer of thermal energy and molecular collisions.
- 6.3.A.1
- The particles in a warmer body have a greater average kinetic energy than those in a cooler body.
- 6.3.A.2
- Collisions between particles in thermal contact can result in the transfer of energy. This process is called “heat transfer,” “heat exchange,” or “transfer of energy as heat.”
- 6.3.A.3
- Eventually, thermal equilibrium is reached as the particles continue to collide. At thermal equilibrium, the average kinetic energy of both bodies is the same, and hence, their temperatures are the same.
Science references for this teaching
- College Board AP Chemistry Course and Exam Description, Fall 2024 — Exact unit learning requirements and exclusions; supplied PDF is pinned by the integration catalog.
- OpenStax Chemistry 2e: Energy Basics — Scientific check: system boundaries, energy conservation, thermal transfer and heat capacities.
- OpenStax Chemistry 2e: Calorimetry — Scientific check: heat-balance equations, calorimeter heat capacity, reaction/solution signs; newly authored numerical data.
Glossary and deeper reading · 5 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Mole
A mole is an amount of substance containing exactly 6.02214076 × 10²³ specified entities. The entities can be atoms, molecules, ions, or formula units, so the counted entity must be named. For example, one mole of oxygen molecules contains two moles of oxygen atoms.
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Limiting reactant
The limiting reactant is consumed first under the specified complete-reaction model and limits the product amount. Identifying it requires comparing available amounts with the balanced equation's ratios. For example, abundant hydrogen cannot produce more water after the available oxygen has been consumed.
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Activation energy
Activation energy is the energy barrier associated with a reaction pathway in an introductory energy-profile model. It concerns the pathway and is distinct from the reactant-to-product energy change. For example, a thermodynamically favored reaction can remain slow when its activation barrier is large.
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Thermal equilibrium
Thermal equilibrium means that bodies in thermal contact have the same temperature and no net heat transfer between them. It does not mean that they contain equal amounts of energy or have the same mass. For example, a small metal sample and a large water bath can reach the same temperature.
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Heat capacity
Heat capacity is the heat required to raise a specified object's temperature by one temperature unit under stated conditions. It refers to the entire object and differs from specific heat capacity, which is expressed per unit mass. For example, doubling the amount of the same material approximately doubles its heat capacity.