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Electrochemistry, energetics and rates
8 MDCAT Chemistry questions with answers and explanations.
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What is the oxidation state of manganese in KMnO4?
- 1 +2
- 2 +4
- 3 +6
- 4 +7
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Correct answer: +7
K is +1 and four O atoms are -8 in total. So Mn = +8 - 1 = +7.
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What is the oxidation state of sulphur in H2SO4?
- 1 +2
- 2 +4
- 3 +6
- 4 -2
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Correct answer: +6
Two H atoms give +2 and four O atoms give -8, so S must be +6 to make the molecule neutral.
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In an electrolytic cell, oxidation takes place at the:
- 1 Cathode
- 2 Anode
- 3 Salt bridge
- 4 Electrolyte only
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Correct answer: Anode
Oxidation always happens at the anode and reduction at the cathode.
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The standard electrode potential of the standard hydrogen electrode is:
- 1 +1.00 V
- 2 -1.00 V
- 3 0.00 V
- 4 +0.34 V
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Correct answer: 0.00 V
The standard hydrogen electrode is the reference, and its potential is defined as exactly 0.00 V.
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An exothermic reaction has an enthalpy change (delta H) that is:
- 1 Positive
- 2 Negative
- 3 Zero
- 4 Equal to the activation energy
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Correct answer: Negative
Heat is released to the surroundings, so the system loses enthalpy and delta H is negative.
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Zinc has a standard electrode potential of -0.76 V and copper +0.34 V. What is the standard EMF of the Zn-Cu cell?
- 1 0.42 V
- 2 1.10 V
- 3 -1.10 V
- 4 0.76 V
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Correct answer: 1.10 V
EMF = E(cathode) - E(anode) = 0.34 - (-0.76) = 1.10 V.
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Raising the temperature increases the rate of a reaction mainly because:
- 1 The activation energy falls
- 2 More molecules have energy equal to or above the activation energy
- 3 The concentration of the reactants rises
- 4 The products become more stable
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Correct answer: More molecules have energy equal to or above the activation energy
A larger fraction of molecules has enough energy to react when they collide. The activation energy itself does not change.
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A catalyst increases the rate of a reaction by:
- 1 Raising the temperature
- 2 Providing a path with lower activation energy
- 3 Increasing the equilibrium yield
- 4 Being consumed in the reaction
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Correct answer: Providing a path with lower activation energy
A catalyst offers an alternative route with lower activation energy. It is not consumed and does not change the position of equilibrium.