Activation Energy and Catalysts: Quiz

Activation Energy and Catalysts: Quiz

7 questions

1. Question 1EasyWhat does a catalyst do?

   AIncreases the energy of the reactantsBProvides a pathway with a lower activation energyCIncreases ΔHDShifts the equilibrium toward products
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   Answer: Provides a pathway with a lower activation energy

   A catalyst lowers Ea. It does not change the energies of reactants or products, ΔH, or the equilibrium position.

2. Question 2EasyWhy does raising the temperature increase the rate of a reaction?

   AThe activation energy decreasesBA larger fraction of collisions have energy ≥ EaCThe reaction becomes more exothermicDThe orientation of collisions improves
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   Answer: A larger fraction of collisions have energy ≥ Ea

   Ea stays the same, but at a higher temperature many more molecules have enough energy (the area beyond Ea grows), and they also collide more often.

3. Question 3MediumFor an exothermic reaction, Ea (forward) = 80 kJ/mol and ΔH = −30 kJ/mol. What is Ea for the reverse reaction?

   A50 kJ/molB80 kJ/molC110 kJ/molD30 kJ/mol
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   Answer: 110 kJ/mol

   The reverse reaction starts from the lower-energy products, so its hill is higher: 80 kJ/mol + 30 kJ/mol = 110 kJ/mol.

4. Question 4MediumIn the Arrhenius equation, which units must Ea and T have when R = 8.314 J/(mol·K)?

   AkJ/mol and °CBJ/mol and KCkJ/mol and KDJ/mol and °C
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   Answer: J/mol and K

   Ea must be in J/mol to match the joules in R, and T must be in kelvin, so that Ea/RT has no unit.

5. Question 5HardA reaction has Ea = 50.0 kJ/mol. By about what factor does k increase from 298 K to 308 K?

   A1.03B1.93C10.0D0.52
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   Answer: 1.93

   ln(k₂/k₁) = (5.00 × 10⁴ J/mol ÷ 8.314 J/(mol·K)) × (1/298 K − 1/308 K) = 0.655, so k₂/k₁ = e^0.655 = 1.93.

6. Question 6MediumWhich statement about catalysts is correct?

   AThey are used up during the reactionBThey increase the equilibrium yieldCThey speed up the forward and reverse reactions equallyDThey only work for endothermic reactions
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   Answer: They speed up the forward and reverse reactions equally

   Because both directions speed up equally, equilibrium is reached faster but its position and K do not change. The catalyst is regenerated.

7. Question 7EasyWhat does the peak of an energy profile represent?

   AThe productsBThe transition stateCΔHDThe catalyst
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   Answer: The transition state

   The top of the hill is the transition state (activated complex); its height above the reactants is Ea.