Unit 6 · 7–9% of the exam
Thermodynamics
Unit 6 (thermochemistry) tracks the energy that flows as heat during physical and chemical changes. You will use calorimetry to measure heat, then find enthalpies of reaction from bond enthalpies, enthalpies of formation and Hess's law. The skills carry straight into Unit 9's look at entropy and free energy.
Calorimetry and energy diagrams
Run a coffee-cup calorimeter, then shape the reaction's energy profile: ΔH, Ea and the effect of a catalyst.
Topics in this unit
6.1
Endothermic and Exothermic Processes
Exothermic processes release energy to the surroundings (ΔH < 0); endothermic processes absorb energy from the surroundings (ΔH > 0).
6.2
Energy Diagrams
An energy diagram compares the energy of reactants and products; products below reactants means exothermic, above means endothermic.
6.3
Heat Transfer and Thermal Equilibrium
Heat flows from the hotter to the colder object until both reach the same temperature, and the energy lost by one equals the energy gained by the other.
6.4
Heat Capacity and Calorimetry
q = mcΔT lets you measure heat flow; in a calorimeter, q(reaction) = −q(solution).
6.5
Energy of Phase Changes
During a phase change, temperature stays constant while energy goes into overcoming (or forming) intermolecular attractions: q = n × ΔH(fus or vap).
6.6
Introduction to Enthalpy of Reaction
ΔH of reaction is the heat exchanged at constant pressure for the reaction as written; it scales with the amount reacting.
6.7
Bond Enthalpies
ΔH ≈ Σ(bonds broken) − Σ(bonds formed): breaking bonds absorbs energy and forming bonds releases it.
6.8
Enthalpy of Formation
ΔH°rxn = ΣΔH°f(products) − ΣΔH°f(reactants), where ΔH°f of an element in its standard state is zero.
6.9
Hess's Law
Enthalpy is a state function, so reactions can be added: reversing a reaction flips the sign of ΔH and multiplying a reaction multiplies ΔH.
Must know
- q = mcΔT (c of water = 4.18 J·g−1·°C−1)
- q(surroundings) = −q(system); in a calorimeter q(rxn) = −q(solution)
- ΔH = q(rxn) ÷ moles reacting (kJ/mol)
- Phase change heat: q = n × ΔH(fusion) or n × ΔH(vaporization)
- ΔH°rxn = ΣΔH°f(products) − ΣΔH°f(reactants)
- ΔH ≈ Σ bond enthalpies broken − Σ bond enthalpies formed
- Hess's law: reverse a reaction → change the sign of ΔH; multiply by n → multiply ΔH by n
- ΔH°f of an element in its standard state (e.g., O2(g), C(graphite)) = 0
Common mix-ups
Temperature and heat are the same thing.
Temperature measures average kinetic energy; heat is energy transferred because of a temperature difference. A bathtub at 40 °C holds far more thermal energy than a cup at 90 °C.
If a solution gets cold when a salt dissolves, the dissolving released cold.
The process is endothermic: it absorbs energy from the water, so the water's temperature drops.
Temperature keeps rising steadily while ice melts on a hot plate.
During melting, the added energy overcomes intermolecular forces and the temperature stays at 0 °C until all the ice has melted.
Practice questions
AP-style multiple choice. Every answer comes with the reasoning, including why the tempting wrong answer is wrong.
How much heat is absorbed when 50.0 g of water is warmed from 22.0 °C to 30.0 °C? (c = 4.18 J·g−1·°C−1)
Free-response practice
In a coffee-cup calorimeter, 50.0 mL of 1.00 M HCl and 50.0 mL of 1.00 M NaOH, both at 21.0 °C, are mixed. The highest temperature reached is 27.7 °C. Assume the solution has a density of 1.00 g/mL and c = 4.18 J·g−1·°C−1. (a) Calculate the heat absorbed by the solution. (b) Calculate ΔH of neutralization in kJ per mole of water formed. (c) If some heat escapes from the calorimeter, how would the magnitude of the calculated ΔH be affected? Explain.
Flashcards
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See it in real life
Everyday reactions that use Unit 6 ideas, animated atom by atom.
In the kitchen
What happens in a gas stove flame
Combustion of methane
In the kitchen
Why baking soda and vinegar fizz
Acid–carbonate neutralization
In the kitchen
What makes bread dough rise
Alcoholic fermentation of glucose
In your body
Where your body's energy comes from
Aerobic cellular respiration
In your body
Why hydrogen peroxide fizzes on a cut
Decomposition of hydrogen peroxide
In your body
How an instant cold pack gets cold
Dissolution of ammonium nitrate