AP Chemistry : Thermodynamics

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

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Example Question #1 : Electrochemistry

Toward which pole do the electrons travel in an electrolytic cell?

Possible Answers:

The electrons do not travel; only the protons travel

Towards the positive pole

More information is needed

Towards the cathode

Towards the anode

Correct answer:

Towards the cathode

Explanation:

Reduction always occurs at the cathode, and oxidation always occurs at the anode. Since reduction is the addition of electrons, electrons must travel toward the site of reduction.

In an electrolytic cell the negative charge is on the cathode, while the positive charge is on the anode. Since an electrolytic cell requires energy to perpetuate the reaction, we are pushing the electrons against their potential gradient. The electrons, which are negatively charged, are traveling towards the cathode, which is also negatively charged.

Example Question #1 : Help With Galvanic Cells

Which of the following differences between galvanic cells and electrolytic cells is false?

Possible Answers:

Electrolytic cells have oxidation take place at the cathode

Electrolytic cells have a positive Gibb's free energy

Electrolytic cells are non-spontaneous

Electrolytic cells have negative voltages

Correct answer:

Electrolytic cells have oxidation take place at the cathode

Explanation:

Electrolytic cells use non-spontaneous reactions that require an external power source in order to proceed. The values between galvanic and electrolytic cells are opposite of one another. Galvanic cells have positive voltage potentials, while electrolytic voltage potentials are negative. Both types of cell, however, have oxidation occur at the cathode and reduction occur at the anode.

Example Question #2 : Galvanic (Voltai) And Elecrolytic Cells

What is true about the current flow in a galvanic cell?

Possible Answers:

Galvanic cells do not have a current

It travels from cathode to anode

It travels from anode to cathode

It flows in the direction of electron flow

Correct answer:

It travels from cathode to anode

Explanation:

In a galvanic cell, the electrons will flow from the anode to the cathode; however, current travels in the opposite direction of the electrons by convention. In chemical cells, current is defined by the direction in which protons would flow. As a result, current flows from the cathode to the anode.

Keep in mind that reduction occurs at the cathode and oxidation occurs at the anode. Electrons are thus constantly moving from the anode to the cathode. Since the current will be opposite to the flow of electrons, we can conclude that it must flow from cathode to anode.

Example Question #2 : Electrochemistry

The following galvanic cell is created:

      

Which of the following takes place at the cathode?

Possible Answers:

Copper gains electrons

Gold ions receive electrons

Copper loses electrons

Gold loses electrons

Correct answer:

Gold ions receive electrons

Explanation:

Reduction always takes place at the cathode. This means that a substance is receiving electrons.

In the reaction, the gold ions are receiving electrons in order to create gold atoms. This takes place at the cathode.

Copper ions and gold are products, and will not be oxidized or reduced. Copper is reduced at the anode during this reaction.

Example Question #1 : Cell Potential And Free Energy

1.  

2.  

3. 

What is ? Is reaction 3 spontaneous?

Possible Answers:

, nonspontaneous

, spontaneous

, nonspontaneous

, spontaneous

Correct answer:

, nonspontaneous

Explanation:

There are two concepts to consider in this problem.  First, the question asks for the .  Reaction 3 has  being reduced so the potential for the half reaction becomes negative.  half reaction appears the same in reaction 3 so the potential is the same. Second, negative voltages indicate non-spontaneity and positive voltages are spontaneous.  

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