Electrochemistry

What is the purpose of an electrochemical cell?

They use redox reactions to create a flow of electrons around a “circuit”

What is produced by an electrochemical cell?

A potential difference

What are most electrochemical cells made up of?

2 solutions with metal electrodes and a salt bridge

What is a salt bridge?

A tube of unreactive ions that can move between solutions to carry the flow of ions and balance the charge

What solution is a salt bridge normally made of?

KCl or KNO₃

Give 2 essential properties of a salt bridge?

Unreactive

Allows movement of ions

What is a cell potential?

A value that indicates whether the half-cell will react by oxidation or reduction

State the 4 rules for drawing cell diagrams.

The half-cell with the most negative potential (oxidation) goes on the left

The most oxidised species goes next to the salt bridge

A salt bridge is shown using a double-line

Always include state symbols

What is the standard hydrogen electrode (SHE)?

The measuring standard for half-cell potentials

What is the cell potential of the SHE?

0.00V

What are the standard conditions to measure the SHE potential?

1 moldm^-3

298K

100kPa

What solution is used to measure the SHE?

HCl with platinum electrodes

Why are platinum electrodes used for electrochemical cells?

They are inert but conduct electricity

What does a negative cell potential mean?

The substance is more easily oxidised and will lose electrons

What does a positive cell potential mean?

The substance is more easily reduced and will gain electrons

Give the equation to calculate the EMF of a cell composed of 2 given half cells.

EMF = E_right - E_left

Which half cell is always on the left in a conventional cell?

The one where oxidation occurs

What is an easy way to remember the emf equation?

Positive potential - negative potential

What does a positive emf value mean about the reaction taking place?

It is spontaneous and favourable

How does a more positive emf effect the favourability of the reaction?

More positive = more favourable

Describe how to use the anticlockwise rule?

1. Write the most negative emf equation on the top

2. Draw anticlockwise arrows around the equations

3. Balance

4. Write out the cell reaction

What electrode potential signifies a better oxidising agent?

A more positive one

What electrode potential signifies a better reducing agent?

A more negative one

How will an increase in concentration of solutions in a cell effect the EMF?

It will be higher as fewer electrons are produced in the reaction

What effect does an increase in pressure have on the EMF of a cell?

EMF will be more negative as more electrons are produced

Give the 3 types of commercial cells.

Non-rechargeable

Rechargeable

Fuel cells

State briefly how a rechargeable cell works?

It uses a reversible chemical reaction to produce energy, which can be reversed when an external current is applied to recharge the cell

What electrodes are used in a Lithium-ion cell?

Lithium cobalt oxide and carbon

What electrolyte is used in a lithium ion cell?

A lithium salt electrolyte

Give the half-equation for the negative electrode of a lithium ion cell.

Li —> Li⁺ + e-

Give the half-equation for the positive electrode of a lithium ion cell.

Li⁺ + e- + CoO₂ —> Li⁺[CoO₂]^-

How is a rechargeable cell recharged?

A current is applied to it forcing the electrons back in the opposite direction

Give the 2 electrode equations for a zinc-carbon cell.

negative = Zn²⁺ + 2e- —> Zn

positive = 2NH₄⁺ + 2e- —> 2NH₃ + H₂

Why are E values of commercial cells approximate?

They are not measured under standard conditions

Give the overall equation for a zinc-carbon electrode.

Zn(s) + 2NH₄⁺(aq) —> Zn²⁺(aq) + 2NH₃(g) + H₂(g)

What are some disadvantages of zinc-carbon batteries?

Ammonia and hydrogen gases cause battery to expand

Zinc casing thins

How are gaseous ammonia and hydrogen removed from non-rechargeable batteries?

MnO₂ is used to oxidise H₂ to water which dissolves the ammonia

How does a hydrogen fuel cell work?

A constant supply of H₂ and O₂ is supplied to the cell, producing a constant current flow

What electrodes are used in a hydrogen fuel cell and why?

Pt as it is inert and conducts electricity

What are the 2 main advantages of hydrogen fuel cells?

Water is the only waste product

It is more energy efficient than a combustion engine

Give the equation for the negative electrode of a hydrogen fuel cell under alkaline conditions.

H₂(g) + 2OH^-(aq) —> 2H₂O(l) + 2e-

Give the equation for the positive electrode of a hydrogen fuel cell under alkaline conditions.

O_2(g) + 4e- + 2H₂O(l) —> 4OH^-(aq)

Give the overall equation for hydrogen fuel cells.

2H₂(g) + O₂(g) —> 2H₂O(l)

What are the disadvantages of hydrogen fuel cells?

Hydrogen is flammable

The cells are expensive to produce