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Chemistry · Unit 8

Thermochemistry & Energetics: MDCAT Notes and MCQs

Basic terms

  • Thermodynamics: study of energy changes accompanying physical and chemical processes; thermochemistry deals with heat changes in chemical reactions.
  • System: the part under study; surroundings: everything else; boundary: separates them.
  • State function: depends only on initial and final states, not path — P, V, T, internal energy (E/U), enthalpy (H), entropy. Heat and work are NOT state functions.
  • Heat (q): energy transferred due to temperature difference. Heat capacity: heat needed to raise temperature of a body by 1 K.
  • Internal energy (E or U): sum of all kinetic and potential energies of particles of the system. Absolute value can't be measured; only ΔE. Units: joule (J), kJ (also calorie; 1 cal = 4.184 J).
  • Work: pressure–volume work w = −PΔV (expansion work done by system).

Exothermic vs endothermic

ExothermicEndothermic
Heat released; ΔH negativeHeat absorbed; ΔH positive
Combustion, neutralization, respiration, formation of H2OPhotosynthesis, thermal decomposition of CaCO3, N2 + O2 → 2NO, dissolving NH4Cl

First law of thermodynamics

Energy can neither be created nor destroyed. ΔE = q + w (sign convention: heat absorbed and work done on the system positive). At constant volume ΔE = qv.

Enthalpy

  • H = E + PV; at constant pressure ΔH = qp = ΔE + PΔV.
  • Standard enthalpy changes (298 K, 1 atm): of formation (1 mol compound from elements; ΔHf of elements = 0), combustion (complete burning of 1 mol), atomization, neutralization (strong acid + strong base = −57.4 kJ/mol H2O), solution.
  • Measured with a bomb calorimeter (constant volume → ΔE) or glass calorimeter.

Hess's law of constant heat summation

The total enthalpy change of a reaction is the same whether it occurs in one step or several steps. E.g. C + O2 → CO2 (ΔH = −393.5) = C + ½O2 → CO (−110.5) + CO + ½O2 → CO2 (−283.0). Used with Born–Haber cycles to find lattice energies. ΔHreaction = ΣΔHf(products) − ΣΔHf(reactants).

PMDC learning outcomes for this unit
  • Thermochemistry: 8.1 Define thermodynamics
  • Thermochemistry: 8.2 Classify reactions as exothermic and endothermic
  • Thermochemistry: 8.3 Define the terms system, surrounding, boundary, state function, heat, heat capacity, internal energy, work done and enthalpy of a substance.
  • Thermochemistry: 8.4 Name and define the units of internal energy.
  • Thermochemistry: 8.5 Explain the first law of thermodynamics of energy conservation.
  • Thermochemistry: 8.6 Apply Hess's law to construct simple energy cycles.
  • Thermochemistry: 8.7 Describe enthalpy of the reaction

Thermochemistry & Energetics MCQs with answers

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Q1
Which is NOT a state function?
AInternal energy
BPressure
CWork
DEnthalpy
Show answer
Answer: C (Work). Work and heat depend on the path; they are not state functions.
Q2
For an exothermic reaction, ΔH is:
APositive
BZero
CNegative
DInfinite
Show answer
Answer: C (Negative). Heat is released, so enthalpy of products is lower than reactants.
Q3
The SI unit of internal energy is:
AKelvin
BJoule
CWatt
DCalorie
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Answer: B (Joule). Energy is measured in joules (J).
Q4
Mathematically, the first law of thermodynamics is:
AΔE = q + w
BΔG = ΔH − TΔS
CΔH = ΔE − PΔV
DPV = nRT
Show answer
Answer: A (ΔE = q + w). ΔE = q + w expresses conservation of energy.
Q5
At constant pressure, the heat change equals:
AΔE
Bw
CΔH
DΔS
Show answer
Answer: C (ΔH). qp = ΔH.
Q6
Hess's law is used to:
AMeasure pH
BCalculate enthalpy changes that are hard to measure directly
CFind equilibrium constant
DMeasure rate of reaction
Show answer
Answer: B (Calculate enthalpy changes that are hard to measure directly). Energy cycles allow indirect calculation of ΔH (e.g. lattice energy via Born–Haber cycle).
Q7
Standard enthalpy of formation of an element in its standard state is:
APositive
BNegative
CZero
DEqual to its atomization energy
Show answer
Answer: C (Zero). By convention ΔHf° of elements in standard state = 0.
Q8
Enthalpy of neutralization of a strong acid with a strong base is about:
A−57.4 kJ/mol
B−285.8 kJ/mol
C+57.4 kJ/mol
D−393.5 kJ/mol
Show answer
Answer: A (−57.4 kJ/mol). Essentially H⁺ + OH⁻ → H2O, ≈ −57.4 kJ per mole of water.
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