Redox II

Question 1

Reduction

Answer 1

Reduction is the gain of electrons

The oxidation number will decrease

Question 2

Oxidation

Answer 2

Oxidation is the loss of electrons

The oxidation number will increase

Question 3

Standard Electrode Potential

Answer 3

A standard electrode potential is a measure of the emf of a half-cell relative to a standard hydrogen electrode under standard conditions

The potential difference can be recorded as a voltage - the bigger the difference between the positiveness and negativeness, the bigger the voltage

Conditions:

• 298 K temperature
• 100 kPa pressure of gases
• 1.00 mol dm^-3 concentration of ions

Question 4

Standard Hydrogen Electrode Set Up

Answer 4

A standard hydrogen electrode is a half cell in which a 1.00 mol dm^-3 solution of hydrogen ions is in equilibrium with hydrogen gas at 100kPa on the surface of a platinum electrode coated with platinum black at 298k

Typically, the standard electrode potential is the left-hand electrode in an electrochemical cell, and the cell emf is the electrode potential of the right hand electrode

2H⁺ + 2e^- ⇒ H₂

The equilibrium position can be changed by changing conditions, so the conditions must be standardised to make fair comparisons

Using the Standard Hydrogen Electrode

1. The electrode is attached to the electrode system you are investigating e.g. magnesium in a solution containing magnesium ions
2. Each of the two beakers and their contents are described as half cells
3. The salt bridge is included to complete the circuit and allow current to flow - the electrolyte used is chosen so that it doesn’t react with the contents of the beakers

Reference Electrode

In order to deduce where the electrodes lie relative to each other, e.g. further left, we just need to be able to compare the voltage with a standardised system

It is used to measure electrode potentials in place of the standard hydrogen electrode

Question 5

E_cell Feasability and Entropy

Answer 5

Cell potentials will only give information about whether a reaction is thermodynamically favourable:

Reactions that are predicted to occur may not because of:

• High activation energy
• E_cell value may be quoted for a different temperature
• Reaction is taking place under non-standard conditions

E_cell = E_right - E_left

• the more positive electrode should go on the right hand side

Entropy

Positive E_cell = feasible reaction

Negative E_cell = unfeasible reaction

E_cell is directly proportional to the total entropy change and to ln K for a reaction

ΔS_tot (total entropy change) = nFE_cell

ΔG (free energy change) = -TΔS_tot

ΔG = -RTlnK_c

E_cell = ∆S^Ø

E_cell = lnK

Question 6

Standard Conditions

Answer 6

• 298 K
• 100kPa
• 1.00 mol dm^-3 concentration

If the concentration is less, then the individual E_cell value will be more negative, meaning throuh calculation, it will become more positive, and the overall E_cell value will be more positive

Question 7

Disproportionation Reactions

Answer 7

Electrode potentials predict whether or not a disproportionation reaction will occur

“When an element in the same species is both oxidised and reduced simulatenously during the same reaction”

If a substance has a positive E_cell value for reactions at both more positive and more negative electrodes, then it will disproportionate

Question 8

Limitations of Making E_cell Predictions

Answer 8

• Reaction may be too slow for any change to be observed
• There may be something to inhibit the reaction kinetically
• Activation energy too high
• Changes in standard conditions - all electrode potentials become more positive if the concentration is increased and less positive if it is decreased

e. g. Reaction of Cu²⁺ ions to oxidise H₂ to H⁺
* nothing happens as when hydrogen is bubbled into copper (II) sulfate, the activation energy is so high that the rate of reaction is zero

Question 9

Storage Cells

Answer 9

A storage cell is an electrochemical cell that is based on reversible chemical changes so that it can be recharged by an external electricity supply

When a storage cell is recharged, an electric current passes through it in the opposite direction to the current that the cell produces

This is how recharging occurs

Question 10

Lead Acid Cells

Answer 10

• Used for motor vehicles such as cars and wheelchairs
• Provide back up power to computer systems, emergency lighting in case of power cuts

Negative Terminal = Lead

Electrolyte = Concentrated Sulfuric Acid

Positive Terminal = Lead Coated with Lead (IV) Oxide

1. Lead gives up electrons, forming lead (II) ions when the cell is discharging
2. In the presence of sulfate ions, the lead (II) ions precipitate to form lead (II) sulfate

Pb(s) + SO42- (aq) → PbSO4 (s) + 2e-

PbO2(s) + SO42- (aq) + 4H+ (aq) + 2e- → PbSO4 (s) + 2H2O (l)

• The positive terminal is coated with lead (IV) oxide
• During discharge the lead (IV) oxide reacts with H+ ions in the sulfuric acid electrolyte and takes electrons
• The lead (II) ions precipitate as lead (II) sulfate
• The production of insoluble lead (II) sulfate means that after long periods of time it will become coarser and thicker and the process cannot be reversed when the cells are recharged

Question 11

Le Chatelier’s Principle

Answer 11

The Ecell is a measure of how far from the equilibrium the cell reaction lies, so the more positive it is, the more likely it is to occur

If current can flow, the cell reaction occurs and the Ecell falls to zero as the reaction proceeds until the reactant concentration drops

If non standard conditions are used, the reaction may or may not go until completion

Concentration

Increasing the concentration increases the standard electrode potential due to Le Chateliers principle

Temperature

Most cells are exothermic in the forward direction so if there was a temperature rise, there would be a decrease in the standard electrode potential as the equilibrium shifts backwards in the endothermic direction

Question 12

Electrode potentials in Electrochemical series

Answer 12

Electrochemical cells produce an electric potential difference from a redox reaction

1. The two half reactions happen in separate half cells
2. Electrons flow from one cell to the other through a wire connecting the electrodes

Question 13

Lithium Cells

Answer 13

Modern phones and laptops use lithium batteries

• Lithium is low density so cells can be light
• It is very reactive so the electrode potential is relatively high and each cell has a large e.m.f.
• However, due to its reactivity, it readily reacts with oxygen, forming a layer of a non conducting oxide on the surface of the metal

Electrodes have been designed with lithium atoms and ions inserted into the crystal lattices of other materials

The electrolyte used is also a polymeric materials rather than in an aqueous solution

Positive electrode: Li+ + MnO2 + e- → LiMnO2

Negative electrode: 2Li → 2Li+ + 2e-

Question 14

Fuel Cells

Answer 14

Fuel cells are electrochemical cells in which the chemical energy is converted directly into electrical energy

Hydrogen fuel cells are a renewable energy source that hold a constant voltage

Fuel cells have a continuous supply of reactants from which to produce a steady electric current

• Hydrogen
• Hydrocarbons
• Alcohols

Inside a fuel cell, the energy from the redox reaction between a fuel and oxygen is used to create a potential difference

• They have approximately a 70% efficiency

Positive electrode: 0.5 O2 +2H+ + 2e- → H2O

Negative electrode: H2 → 2H+ + 2e-

Overall reaction: H2 + 0.5 O2 → H2O

• These types of cells are less polluting as they only release water, so have a greater efficiency than other electrochemical cells
• However, they are expensive as storing hydrogen is difficult and they have a limited lifetime