RP 8 - Measuring EMF of an Electrochemical Cell

Question 1

What’s an electrochemical cell?

Answer 1

• 2 different half-cells connected by a salt bridge, with their electrodes connected to a voltmeter (measures EMF). This allows the flow of electrons
• The electrical energy generated is from chemical redox reactions

Question 2

What does an electrochemical cell look like as a diagram?

Answer 2

There are 2 beakers next to each other with solutions in both, both labelled ‘ion inside THAT solution* solution’. There’s also an electrode in both beakers, both labelled ‘element that THAT electrode is made of electrode’. The left-hand side electrode has a wire which is connected to a ‘voltmeter’ which is connected to the other electrode by wire. There’s also a ‘salt bridge’ connected the solutions of both beakers*

Question 3

What does a half-cell consist of?

Answer 3

• Half-cell are usually metal/metal ion (metal electrode in the metal ion solution) or ion/ion (ions that are the same element but with different oxidation states, in solution)
• Each half-cell will contain the chemical species from one half of a redox reaction (redox half-equation)

Question 4

What’s a salt bridge?

Answer 4

• A salt bridge allows the transfer of ions
• It’s typically a concentrated solution of an electrolyte i.e. KNO3 (aq) that doesn’t react with either half-cell solution

Question 5

Why may a graphite or platinum electrode be used?

Answer 5

They’re very unreactive i.e. won’t react with half-cell solutions and won’t affect the voltmeter readings. Usually used in ion/ion half cells

Question 6

How do you measure comparative electrode potentials of different metals?

Answer 6

1. File a piece of copper using emery paper and connect it to the positive voltmeter terminal
2. Cut a piece of filter paper, saturate with KNO3 solution and place on top of the copper
3. Connect the voltmeter to another piece of metal
4. Hold the metal against the filter paper and record the voltmeter value
5. Repeat with different metals and record the results in a table

Question 7

Why do you need to file/sand away the outer layer of the metal when measuring comparative electrode potentials of different metals?

Answer 7

It removes the oxide layer on the outside of the metal