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Thermochemistry and spontaneity
7 MDCAT Chemistry questions with answers and explanations.
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The standard enthalpy of formation of an element in its standard state is:
- 1 Positive
- 2 Negative
- 3 Zero
- 4 Always 100 kJ/mol
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Correct answer: Zero
By definition, forming an element from itself involves no change, so its standard enthalpy of formation is zero.
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For an endothermic reaction, the sign of delta H is:
- 1 Positive
- 2 Negative
- 3 Zero
- 4 It cannot be known
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Correct answer: Positive
An endothermic reaction absorbs heat, so the system gains enthalpy and delta H is positive.
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Which change leads to an increase in entropy?
- 1 Water freezing
- 2 Ice melting
- 3 Steam condensing
- 4 A gas dissolving in water
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Correct answer: Ice melting
Melting turns an ordered solid into a more disordered liquid, so entropy increases.
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A reaction is spontaneous at constant temperature and pressure when:
- 1 Delta G is positive
- 2 Delta G is negative
- 3 Delta H is positive
- 4 Delta S is zero
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Correct answer: Delta G is negative
A negative change in Gibbs free energy means the reaction can proceed spontaneously.
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The heat released by a reaction, 5000 J, is all absorbed by 100 g of water. What is the rise in temperature? (Specific heat of water 4.18 J/g per degree C)
- 1 5 degrees C
- 2 12 degrees C
- 3 50 degrees C
- 4 1.2 degrees C
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Correct answer: 12 degrees C
Temperature rise = Q / (m x c) = 5000 / (100 x 4.18), which is about 12 degrees C.
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For a reaction, delta H = +40 kJ and T x delta S = 50 kJ. What is delta G?
- 1 +10 kJ
- 2 -10 kJ
- 3 +90 kJ
- 4 -90 kJ
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Correct answer: -10 kJ
Delta G = delta H - T delta S = 40 - 50 = -10 kJ, so the reaction is spontaneous.
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The heat change of a reaction measured at constant pressure is called the change in:
- 1 Entropy
- 2 Enthalpy
- 3 Free energy
- 4 Internal energy only
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Correct answer: Enthalpy
Enthalpy change is the heat exchanged at constant pressure.