3.1.11 Electrode potentials and electrochemical cells - Chemistry Alevel

What are the rules for redox equilibria equations?

The electrons should be on the left hand side and the arrow should be reversible.

Electrode potential - what effects it

The type of metal (wether it tends to loose or gain electrons) and the concentration of solution.

How does a metal half cell work?

The excess liberated electrons are delocalised across the X strip making it negatively charged (as the [metal] X atoms show a tendency to form ions and join the solution leaving electrons on the strip). The solution is positively charged as it has excess Xⁿ⁺ ions. (dynamic equilibrium lies to the left is equation is proper)

X(s) → Xⁿ⁺(aq) + ne⁻ (not correctly written redox equilibria equation)

The Y metal strip is positively charged as the Y ions have taken electrons from it to be discharged as copper atoms on the strip. The solution is negatively charged as there is a deficit of Yⁿ⁺ ions as the dynamic equilibrium is lying to the right.

Yⁿ⁺(aq) + ne⁻ → Y(s) (not correctly written redox equilibria equation)

Half cell - what energy conversion takes place?

Chemical energy is converted to electrical energy when they are connected.

What is the salt bridge made of often?

A piece of filter paper or agar gel soaked in potassium nitrate or KCl or Na₂SO₄.

Why is the salt bridge made of ____?

Because it does not react with either of the solutions in the half cells.

What is the electrode made of in a non-metal half-cell?

It is made of an unreactive metal usually platinum.

Standard hydrogen electrode - why is a platinum block used as an electrode?

As it is porous and has a larger surface area than a strip and so absorbs more gas. Pt is unreactive and conducts electricity as well.

Half cell conditions

At 298K, pressure of any gases of 100kPa and each aqueous ion is at a concentration of 1 moldm⁻³. (you need to list this for all solutions and gases in diagrams).

Standard hydrogen electrode - what side from the salt bridge?

It is always on the left.

What does E⁰ being negative mean?

The metal electrode is more negative compared to the standard hydrogen half cell. The equilibrium lies to the left and oxidation is occurring. Metals & reducing agents.

What does E⁰ being positive mean?

The metal electrode is more positive compared to the standard hydrogen half cell. The equilibrium lies to the right and reduction is occurring. Non-metals & oxidising agents.

What effects electrode potentials?

Cell concentration, cell temperature and cell pressure (if gas). Think Le Chatelier's principle (more electrons liberated means more negative). If the temperature increases then tendency to dissolve increases.

Standard electrode potential - H₂. value

0.00V

Cell emf - equation

E⁰(RHS) - E⁰(LHS)

Using electrochemical cell - benefits

Portable source of electrical energy.

Using electrochemical cell - disadvantages

Waste issues.

Non-rechargable cells - benefits

They are cheaper short term [BUT more expensive long term.]

[Less likely to contain toxic metals like lead & cadmium. Therefore less hazardous in landfill if the contents leak and pollute water sources.]

Rechargeable cells - benefits

[Looses charge faster BUT is rechargeable and so is reusable.]

[More expensive short term BUT] cheaper long term.

[Supplies more power.]

Less waste[d (can be recycled but often isn't).]

Lore environmental impact. [More likely to contain toxic metals like lead & cadmium. Therefore more hazardous in landfill if the contents leak and pollute water sources.]

Non-rechargable cells - disadvantages

[Works for longer BUT must be replaced when it runs out of charge.

Supplies less power.

More produced as they can only be use once so more is wasted (can be recycled but often isn't).]

Waste issues.

Rechargeable cells - disadvantages

Some waste issues (at end of useful life).

Acidic hydrogen-oxygen fuel cell - half equations

Negative electrode: H₂(g) → 2H⁺(aq) + 2e⁻

Positive electrode: O₂(g) + 4H⁺(aq) + 4e⁻ → 2H₂O(l)

Alkaline hydrogen-oxygen fuel cell - half equations

Negative electrode: H₂(g) + 2OH⁻(aq) → 2H₂O(l) + 2e⁻

Positive electrode: O₂(g) + 2H₂O(l) + 4e⁻ → 4OH⁻(aq)

Acidic hydrogen-oxygen fuel cell - cell diagram

Pt(s)|H₂(g)|H⁺(aq)||H⁺(aq)|O₂(g)|H₂O(l)|Pt(s)

Alkaline hydrogen-oxygen fuel cell - cell diagram

Pt(s)|OH⁻(aq)|H₂(g)|H₂O(l)||O₂(g)|H₂O(l)|OH⁻(aq)|Pt(s)

Hydrogen-oxygen fuel cell - overall equation

2H₂ + O₂ → 2H₂O

Hydrogen-oxygen fuel cell - how are the electrodes separated & why

The electrodes are separated by an ion-exchange membrane which allows the passage of ions but not electrons.

Hydrogen-oxygen fuel cell - benefits/advantages

1) They do not need recharging[, unlike other storage cells: as long as hydrogen and oxygen are supplied the cell will produce electricity.]

2) The only waste product is water[: there are no toxic chemicals to dispose of and no CO₂ or NOₓ emissions from the cell itself.]

3) Very efficient.

Hydrogen-oxygen fuel cell - risks/disadvantages

1) Hydrogen is highly flammable [so it needs to be handled carefully when stored or transported.]

2) Hydrogen is usually made using fossil fuels. [Hydrogen and oxygen can be produced from the electrolysis of water but this required electricity which is normally generate from burning fossil fuels which means the whole process isn't carbon neutral.]

3) [Energy is required to produce the hydrogen and oxygen.]

4) Needs constant supply of fuels.

5) High cost of fuel cells.

Predicting feasibility of a reaction with E values - what to do if they ask you to do this

If the cell emf is positive than the reaction is possible.

1) Compare the magnitudes of the two E° values, he more negative electrode will release electrons.

2) Describe what will happen in terms of redox for example; the Mg will reduce the Cu²⁺ ions to form...

Predicting feasibility of a reaction with E values - problems

1) The actual conditions may be different from standard conditions that are use to measure E°.

2) E° refers to aqueous solutions but reactions may take place in different states.

3) E° refers to the position of equilibrium not the reaction rate, the reaction may have a very high activation energy and so be very slow.

Non-rechargeable cell - what is the function of the porous pot? (sometimes)

It allows the SO₄²⁻ ions to pass through but prevents the solutions from mixing, maintaining the charge across the cell and allowing the circuit to be complete.

Non-rechargeable cell - Why is a cell with solutions not practical? (sometimes)

There is a risk of spilling hazardous liquids.

Non-rechargeable cell - Why must E be used instead of E°? (sometimes)

The conditions are not standard and/or the compounds aren't in an aqueous state.

Non-rechargeable cell - Problems due to it not being reversible (sometimes)

The cell reaction cannot be reversed and so when a solid is formed in the reaction the electrode can become n electrical insulator, the casing formed by a reactant could be used up making the walls thinner making the cell prone to leakage.