AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/6/6-1)
Unit 6 · Topic 6.1
6.1 Endothermic and Exothermic Processes
Every chemical or physical change either pulls energy in from its surroundings (endothermic) or pushes energy out to them (exothermic). You can tell which by watching the temperature of the surroundings, and you can explain it by comparing the attractions that are broken with the ones that are formed.
Key terms
- system
- surroundings
- exothermic
- endothermic
- heat (q)
System and surroundings
To talk about energy flow, you first split the world in two. The system is the part you are studying, usually the chemicals that are reacting, dissolving or changing phase. The surroundings are everything else: the water the chemicals are dissolved in, the beaker, the air, your hand holding the cup.
Energy is never created or destroyed. Whatever energy the system loses, the surroundings gain, and the other way around. Heat, written q, is energy that moves from one place to another because of a temperature difference.
Signs are always written from the system's point of view. If the system absorbs heat, q is positive. If the system releases heat, q is negative. Energy can also move as work (for example, a gas pushing back the air as it forms), but in this course you will almost always track heat.
Exothermic versus endothermic
In an exothermic process the system releases energy to the surroundings, so the surroundings warm up. In an endothermic process the system absorbs energy from the surroundings, so the surroundings cool down. At constant pressure the heat is the enthalpy change, ΔH, so exothermic means ΔH < 0 and endothermic means ΔH > 0.
| Feature | Exothermic | Endothermic |
|---|---|---|
| Direction of energy flow | System → surroundings | Surroundings → system |
| What a thermometer in the surroundings shows | Temperature rises | Temperature falls |
| Sign of q and ΔH for the system | Negative | Positive |
| Physical examples | Freezing, condensing, deposition | Melting, boiling, sublimation |
| Chemical examples | Burning fuel, neutralizing HCl with NaOH | Decomposing CaCO₃, photosynthesis |
| Dissolving examples | CaCl₂ (hot packs) or NaOH in water | NH₄NO₃ in water (cold packs) |
Reading a temperature change correctly
A temperature change is your evidence that energy moved. The trick is knowing where the thermometer is. When you dissolve a salt in water or run a reaction in solution, the thermometer sits in the water, which is part of the surroundings.
So if the water gets warmer, the process released energy into it: exothermic. If the water gets colder, the process took energy out of it: endothermic. Students often flip this because they think about the chemicals 'getting cold'. Ask instead: where did the energy go?
Here is the particle-level picture. In an exothermic reaction, the products have less chemical potential energy than the reactants. That energy shows up first as faster-moving particles (more kinetic energy), which is a higher temperature, and then flows out to the cooler surroundings.
Why dissolving can go either way
Dissolving an ionic solid involves three things happening at once. The ions in the solid have to be pulled apart, which takes energy. Some water molecules have to be pushed apart to make room, which also takes energy. Then water molecules cluster around the ions and attract them (ion–dipole attractions), which releases energy.
If the new solute–water attractions release more energy than it took to break the old attractions, dissolving is exothermic and the solution warms up. If they release less, dissolving is endothermic and the solution cools down. You can't tell which way it goes just from the formula; you need an experiment or data.
Note that an endothermic process can still happen on its own. Ammonium nitrate dissolves readily even though it absorbs energy. The reason involves entropy, which you'll meet in Unit 9.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Classifying from a temperature change
A student dissolves 5.0 g of solid CaCl₂ in 100 mL of water in a foam cup. The temperature of the water rises from 21.0 °C to 29.4 °C. Is dissolving CaCl₂ exothermic or endothermic? Justify your answer in terms of energy transfer and in terms of attractions.
Show the solutionHide the solution
- Step 1: Identify the system and surroundings. The system is the CaCl₂ as it dissolves. The water is the surroundings, and the thermometer is in the water.
- Step 2: The water's temperature went up, so the water gained energy. That energy came from the dissolving process.
- Step 3: Energy flowed from the system to the surroundings, so the process is exothermic and ΔH for dissolving is negative.
- Step 4: Attractions: the energy released when water molecules surround the Ca²⁺ and Cl⁻ ions (ion–dipole attractions) is greater than the energy needed to separate the ions in the solid and to separate water molecules from each other.
Answer: Exothermic (ΔH < 0): energy moved from the dissolving CaCl₂ to the water, because the ion–dipole attractions formed release more energy than it takes to break the attractions in the solid and in the water.
- Example 2
Trap: the cold reaction
When solid Ba(OH)₂·8H₂O and solid NH₄Cl are mixed in a flask, they react and the flask gets so cold that water on its outside freezes. A student writes: 'The reaction is exothermic because it releases cold.' Correct the student's claim.
Show the solutionHide the solution
- Step 1: Cold is not a substance or a form of energy, so nothing can 'release cold'. Only energy moves, and it always moves from warmer to cooler.
- Step 2: The flask and the water on it are the surroundings. They lost energy (their temperature fell, and some water froze).
- Step 3: That energy went into the reacting chemicals, the system. The system absorbed energy, so q for the system is positive.
- Step 4: A process that absorbs energy from its surroundings is endothermic, and ΔH > 0.
Answer: The reaction is endothermic: it absorbs energy from the flask and surroundings, which is why they cool down. ΔH is positive.
Common mistakes
- Saying a process 'releases cold' or that 'cold flows'. Only energy flows. A cold pack feels cold because it is taking energy from your hand.
- Using the thermometer reading as the system's energy change with the same sign. If the water warms up, the water's q is positive but the reaction's q is negative.
- Claiming that breaking bonds releases energy. Breaking any bond or attraction always takes energy; energy is released only when new bonds or attractions form.
- Assuming all dissolving is endothermic (or all exothermic). It depends on how the attractions broken compare with the attractions formed.
On the exam
- Expect to be given a temperature change and asked to classify a process. Earn the point by naming the direction of energy transfer between system and surroundings, not just by writing 'exothermic'.
- For dissolving questions, a full justification compares the strength of the interactions broken (in the solute and solvent) with the interactions formed (solute–solvent).
Connected topics
Videos
Check yourself
4 questions on 6.1 Endothermic and Exothermic Processes. Pick an answer to see if you got it, and why.
When the inner pouch of an instant cold pack breaks, solid NH₄NO₃ dissolves in water and the pack becomes cold. Which of the following best describes this process?
A hand warmer contains iron powder that reacts with oxygen from the air: 4Fe(s) + 3O₂(g) → 2Fe₂O₃(s). The packet feels warm in your hand. Taking the reacting chemicals as the system, which of the following is correct?
Solid Ba(OH)₂·8H₂O and solid NH₄Cl are mixed in a flask sitting on a wet wooden board. The mixture turns slushy, and soon the water under the flask freezes, sticking the flask to the board. Which of the following correctly describes the reaction?
Which of the following processes is exothermic?
0 of 4 answered