reverse the enthalpy sign
🌡️ACS GENERAL CHEMISTRY STUDY GUIDE
Heat, Enthalpy, and Thermodynamics for the ACS Exam
Practice calorimetry, Hess's law, entropy, free energy, state functions, and thermodynamic sign conventions.
8 key ideas8 flashcards5 practice questions
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Reach the target by adding known reaction steps.
multiply every coefficient and ΔH
sum the remaining reaction
Key ideas to know
Start with the relationships between ideas. Then close the notes and explain each one from memory.
- 01
Heat flows because of a temperature difference and is not stored as a substance.
- 02
At constant pressure, reaction heat equals the enthalpy shift for the process.
- 03
Calorimetry uses measured temperature movement and heat capacity to infer transferred heat.
- 04
Hess's law adds reaction equations and their enthalpies to obtain a target reaction.
- 05
Entropy counts energy dispersal and the number of accessible microscopic arrangements.
- 06
The second law requires the total entropy shift of system plus surroundings to be positive for a spontaneous process.
- 07
Gibbs free energy combines enthalpy and entropy through ΔG = ΔH − TΔS.
- 08
Standard free energy and the equilibrium constant are connected by ΔG° = −RT ln K.
PART OF THE ACS GENERAL CHEMISTRY GUIDE
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Open the full ACS General Chemistry guide →Two ideas worth correcting now
NOT QUITE
→Spontaneous means fast
USE THIS INSTEAD
Spontaneity concerns thermodynamic direction, while rate belongs to kinetics.
NOT QUITE
→An exothermic reaction is always spontaneous
USE THIS INSTEAD
Entropy and temperature also enter the free-energy test.
Flashcards
Answer before opening each card. The effort to retrieve is part of the learning.
1What does q = mcΔT calculate?Show answer ＋
Heat absorbed or released by a sample with mass m and specific heat c.
2What sign does an exothermic system have for q?Show answer ＋
Negative, because heat leaves the system.
3What is a state function?Show answer ＋
A property determined by state rather than the path taken.
4Why can Hess's law add reaction enthalpies?Show answer ＋
Enthalpy is a state function.
5What does positive ΔG indicate under stated conditions?Show answer ＋
The forward process is not spontaneous under those conditions.
6At equilibrium, what is ΔG?Show answer ＋
Zero.
7What does K greater than one imply about ΔG°?Show answer ＋
ΔG° is negative.
8Can a spontaneous process be slow?Show answer ＋
Yes. Thermodynamics does not specify reaction rate.
Explain it in your own words
Use the answer as a check after you have written or spoken your response.
01A hot metal warms cooler water in an insulated cup. How are their heats related?
Heat lost by metal equals the negative of heat gained by water.
02How does reversing a thermochemical equation affect ΔH?
Its sign reverses.
03When is a process spontaneous at every temperature?
When ΔH is negative and ΔS is positive.
04Why can ice melt spontaneously above its melting point despite absorbing heat?
The favorable entropy term exceeds the positive enthalpy term, making ΔG negative.
05How can K be found from standard free energy?
Rearrange ΔG° = −RT ln K, use Kelvin temperature, then exponentiate.
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