Three of the following change the equilibrium composition of a gas phase reaction with a change in moles. Which is the odd one out?
Answer: A
The constant volume addition leaves every partial pressure untouched, so nothing shifts. The other three each change the partial pressures of the reacting species and therefore move the equilibrium. Whether the mole count changes decides if pressure matters.
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The standard Gibbs energy of reaction is calculated from
Answer: B
Tabulated formation values are combined with the stoichiometric coefficients, products positive and reactants negative. Heat capacities enter only when the value has to be corrected to a temperature other than the tabulated one.
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Assertion (A): Heat capacity data are needed to compute an equilibrium constant at a temperature other than the reference.
Reason (R): Both the standard enthalpy and the standard Gibbs energy of reaction vary with temperature.
Answer: C
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the necessity. The standard enthalpy of reaction varies with temperature according to the difference in heat capacities between products and reactants, and the standard Gibbs energy varies with it in turn. Both must therefore be corrected from the reference temperature to the reaction temperature before \( K \) can be evaluated, and heat capacity data are what make that correction possible.
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A gas phase reaction has an equilibrium constant of 4 at some temperature for the conversion of A to B. What fraction of A remains at equilibrium starting from pure A?
Answer: D
With the ratio of B to A equal to four, one part in five is A, that is twenty per cent. The constant gives a ratio rather than a fraction, and converting between them is where most errors arise. Writing the ratio out in full avoids the confusion.
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Consider the following statements about the equilibrium constant.
1. It is dimensionless when written with activities.
2. Its value depends on how the reaction equation is written.
3. It changes if a catalyst is added.
Answer: A
The first two are correct, and the second matters: doubling every coefficient squares the constant. The third is false, since a catalyst changes only the rate at which equilibrium is reached. Its position is fixed by thermodynamics alone.
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For an exothermic reversible reaction, the plot of equilibrium conversion against temperature
Answer: B
Correct answer: falls as temperature rises
For an exothermic reaction \( K \) falls with temperature, so the equilibrium conversion falls with it, monotonically. The maximum offered as an alternative belongs to the *rate* curve of a reversible exothermic reaction, where the rising rate constant and the falling equilibrium limit combine to give an optimum temperature - which is a different plot answering a different question.
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State whether the following is true or false.
Thermodynamics tells you whether a reaction is fast.
Answer: C
It tells you only how far a reaction can go, not how quickly it will get there. A reaction with a very favourable equilibrium constant may be immeasurably slow without a catalyst. Hydrogen and oxygen at room temperature are the stock example.
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