Electrochemical Cells

Question 1

How does a half cell work?

Answer 1

A rod of metal is dipped into a solution of its own ions and an equilibrium is set up where metal atoms are oxidised and metal ions are reduced. If the metal has a greater tendency to be oxidised than another metal in a different half cell, then it will have a higher (more -ve) potential in comparison.

Question 2

What makes up an electrochemical cell?

Answer 2

Two half cells connected by a salt bridge.

Question 3

Why is a salt bridge used instead of a wire?

Answer 3

To avoid further metal/ion potentials in the circuit.

Question 4

What are the standard conditions for a cell?

Answer 4

298K
101kPa
1M

Question 5

Describe the standard hydrogen electrode

Answer 5

Hydrogen gas is bubbled into a solution of H+ ions. Hydrogen doesn’t conduct so electrical contact is made by a piece of unreactive platinum metal. The metal is coated in finely divided platinum to increase surface area and allow any reaction to proceed rapidly. The electrode is used under standard conditions. It’s potential is defined as 0.

Question 6

Which electrode goes on the left of an electrochemical cell?

Answer 6

The more negative one. Unless you are using the SHE which always goes on the left.

Question 7

How are cells represented on paper?

Answer 7

All species involved in the negative electrode go on the left and those involved in the positive electrode on the right. They are separated by two vertical lines representing a salt bridge. The species with a metal of the greatest oxidation state are closest to the centre. A single vertical line represents a phase boundary between two different states. Species of the same state are separated by commas. Remember to include state symbols and Pt in electrodes with no solid.

Question 8

How to find the overall equation for a cell

Answer 8

The metal loses electrons at the negative electrode and gains them at the positive electrode. Write out the two half equations for each electrode. Multiply any equations you need to so the number of electrons involved is the same. Add the two half equations together and cancel out the electrons. Not a reversible reaction.

Question 9

How to calculate the emf of a cell

Answer 9

The emf of a cell is the emf of the right electrode minus the emf of the left electrode.

Question 10

What happens to the emf of an electrode when the concentration of ions decreases?

Answer 10

The equilibrium would move left (if written with the forwards reaction being a reduction) so the potential is more negative.

Question 11

Name 3 types of cell

Answer 11

Non-rechargeable, rechargeable, fuel cell

Question 12

What is a battery?

Answer 12

A number of cells connected together

Question 13

Advantage of using electrochemical cell as a source of electricity

Answer 13

Is a source of portable electricity.

Question 14

Non-rechargeable cells

Answer 14

The chemicals are used up over time and emf drops. Once one or more of the chemicals have been completely used up, the cell is flat and the emf is 0V. Have to be disposed of after their single use. Cheap.

Question 15

Rechargeable cells

Answer 15

The reactions are reversible by applying an external voltage (greater than the voltage of the cell) to drive the electrons in the opposite direction and regenerate the chemicals. Less waste. Cheaper in long run. Lower environmental impact.

Question 16

Advantages and uses of lithium ion cells

Answer 16

Used in mobile gadgets. Light as lithium is least dense metal. Electrolyte is solid polymer so doesn’t leak. Charge can be topped up at any time. Rechargeable. Cell can be bent or folded without leaking.

Question 17

How do lithium ion cells work?

Answer 17

Positive electrode is LiCoO2 and negative electrode is carbon. Each electrode arranged in layers with a sandwich of solid electrolyte in between. On charging, electrons forced through external circuit from positive to negative electrode and at the same time Li ions move through electrolyte towards positive electrode to maintain balance of charge. The reactions a teach electrode are reversed on discharging.

Question 18

Reactions at each electrode in a lithium ion cell

Answer 18

Charging:
-ve electrode: Li+ + e- -> Li
+ve electrode: Li+ + CoO2 + e- -> Li+(CoO2)-

Reactions are reversed for discharging.

Question 19

Describe a hydrogen-oxygen fuel cell

Answer 19

Has two electrodes of a porous Pt based material. Separated by semi-permeable membrane. Electrolyte is sodium hydroxide solution. Hydrogen enters at the negative electrode. Oxygen enters at the positive electrode.

Question 20

Hydrogen-oxygen fuel cell reactions

Answer 20

-ve electrode: 2H2 + 4OH- -> 4H2O + 4e-
+ve electrode: O2 + 2H2O + 4e- -> 4OH-
The same electrons released at the negative electrode are used in the reaction at the positive electrode.
Overall equation: O2 + 4H2 -> 2H2O

Question 21

Advantages and disadvantages of hydrogen-oxygen fuel cells

Answer 21

Advantages: only waste product is water, don’t need recharging, very efficient.
Disadvantages: need constant supply of fuels, H2 is flammable and explosive, most H2 made using crude oil, H powered vehicles need H filling stations to be built, H hard to store as is gas.

Question 22

How is the equipment set up for measuring the emf of an electrochemical cell?

Answer 22

Two beakers containing a strip of the metal and known concentrations of the solutions of ions of the metal. The metals are cleaned with sandpaper and degreased with cottonwool and propanone. Crocodile clips on metals to connect them via wires to voltmeter (more negative potential connected to negative terminal. Salt bridge connecting solutions in beakers. Salt bridge usually piece of filter paper soaked in solution of saturated potassium nitrate.