reaction or sample
🌡️AP CHEMISTRY STUDY GUIDE
Unit 6: Thermochemistry
Review energy transfer, calorimetry, enthalpy, phase transitions, bond energies, and Hess's law for AP Chemistry Unit 6.
8 key ideas8 flashcards5 practice questions
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Track heat from the system to the surroundings.
water, cup, or air
for an insulated pair
Key ideas to know
Start with the relationships between ideas. Then close the notes and explain each one from memory.
- 01
The system is the matter under study, while the surroundings include everything else involved in energy transfer.
- 02
Heat flows because of a temperature difference.
- 03
Endothermic processes absorb heat into the system, while exothermic processes release heat.
- 04
Calorimetry connects heat transfer to mass, specific heat capacity, and temperature difference.
- 05
At constant pressure, reaction heat equals the enthalpy difference.
- 06
Phase transitions transfer energy without shifting temperature during the transition at constant pressure.
- 07
Bond breaking requires energy, while bond formation releases energy.
- 08
Hess's law adds reaction equations and their enthalpy differences to obtain a target reaction.
PART OF THE AP CHEMISTRY GUIDE
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Open the full AP Chemistry guide →Two ideas worth correcting now
NOT QUITE
→Temperature and heat are the same quantity
USE THIS INSTEAD
Temperature tracks average kinetic energy, while heat is energy transferred because of a temperature difference.
NOT QUITE
→Breaking chemical bonds releases energy
USE THIS INSTEAD
Energy is required to break bonds. Energy is released when new bonds form.
Flashcards
Answer before opening each card. The effort to retrieve is part of the learning.
1What equation connects heat, mass, specific heat, and temperature difference?Show answer ＋
q = mcΔT.
2What sign does heat have for an endothermic system?Show answer ＋
Positive because the system absorbs heat.
3What sign does enthalpy have for an exothermic reaction?Show answer ＋
Negative because the system releases heat.
4What is specific heat capacity?Show answer ＋
The heat required to raise one gram of a substance by one degree Celsius.
5Why does temperature remain constant during melting at constant pressure?Show answer ＋
Added energy disrupts intermolecular attractions rather than increasing average kinetic energy.
6What does Hess's law permit?Show answer ＋
Adding known reaction equations and enthalpy differences to obtain a target enthalpy difference.
7Does bond breaking release energy?Show answer ＋
No. Bond breaking requires energy.
8What is standard enthalpy of formation?Show answer ＋
The enthalpy difference when one mole of a compound forms from its elements in their standard states.
Explain it in your own words
Use the answer as a check after you have written or spoken your response.
01A hot metal sample warms cooler water in an insulated cup. How are their heat transfers related?
The heat lost by the metal equals the heat gained by the water, apart from cup absorption or other losses.
02Why is an exothermic reaction assigned a negative enthalpy difference?
The system ends with less enthalpy after transferring heat to the surroundings at constant pressure.
03How can bond energies estimate reaction enthalpy?
Add energy required to break reactant bonds, then subtract energy released when product bonds form.
04Why can a calorimetry result differ from a reference result?
Heat loss, incomplete reaction, evaporation, and instrument heat capacity can affect the measurement.
05How do you reverse a reaction in a Hess's law calculation?
Reverse its equation and reverse the sign of its enthalpy difference.
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