redox and electrochemistry

How do you write half equations in acidic conditions eg: oxidation of manganese to manganate ions

FEWH = Formula, Electrons, Water, Hydrogen

1. First balance any Atoms that are not H or O

2. Add e- to balance the charge (work out oxidation number of element at start and element at end)

3. Add H2O to balance O

4. Add H+ to balance H+ ions

What does oxidation mean

Increase in oxidation number

Gaining of oxygen

Loss of electrons

What is reduction

Gain of electrons

Decrease in oxidation number

Loss of oxygen

When determining oxidation states

What is oxidation state of a molecule?

Oxidation state of a compound?

Oxidation state of an ion

Molecule = 0

Compound depends on charge eg: NaCl Na= +1 Cl= -1

For an ion eg: SO4 2- Oxygen = -8 S= +6 = -2

Oxidation state of oxygen

Oxidation state of hydrogen

Oxidation state of fluorine

Oxygen = -2 except in H2O2 (-1) F2O (+2)

Hydrogen = +1 except NaH (-1)

F = -1

How do you write half questions for alkaline conditions

1. Balance atoms that are not H or O first

2. Add twice as much OH needed to balance O atoms

3. Add H2O to balance H atoms

4. Add e- to balance charge ( to work it out, work out oxidation number of element at start and end)

How do you balance redox equations

Write down the change of each oxidation number

Determine the ratio between oxidation number increase and decrease

Determine the ratio required for the elements

How to combine half equations

Write out two half equations

Multiply equations so number of electrons are same in both

Add 2 equations left side on left side and right side on right side and cancel electrons

when doing half equations are big numbers involved

yes

What is a half - cell/electrode

A metal dipping into a solution of its ions

1st of type of half cell

Metal in contact with solutions of its ions

Eg: Zn(s) electrode and Zn2+ (aq)

With the metal as the electrode

What is the second type of half cell

Solutions of ions in 2 different oxidation states

Eg: Fe3+ (aq) and Fe2+ (aq)

Electrode = platinum

What is the third type of half cell

Gases in contact with solutions of their ions

Eg: 2H+ (aq) —> H2(g)

Electrode = platinum

How do you draw an electrochemical cell

What are the conditions

298k

100kpa (1atm)

1moldm^-3

Half cells that have a gaseous element have what over the top of platinum electrode

Gas filled chamber

What is standard electrode potential under standard conditions

0.00v

If a half cell is oxidised (loses electrons) when connected to standard hydrogen half cell what happens to electrode potential

Becomes negative

If a half cell is reduced (gains electrons) when connected to standard hydrogen half cell what happens to electrode potential

Becomes positive

In electrode potential the half equation with the more negative electrode potential will undergo what reaction

In electrode potential the half equation with the more positive electrode potential will undergo what reaction

Oxidation

Reduction

How do you calculate E cell

Value of reduction - value of oxidation

the value of E cell is always what

Positive

The reaction with the more negative value will go where

Right to left

The reaction with the more positive value will go where

Left to right

Is the cathode the positive or negative terminal and what reaction occurs there

positive, reduction

Is the anode the positive or negative terminal and what reaction occurs there

negative, oxidation

How do you write redox half equations

Eg: 2 reactions

Zn2+ + 2e- —> Zn

Cu 2+ +2e- —> Cu

Equation = Cu2+ + Zn —> Cu + Zn2+

Cu2+ goes to Cu so is reduced as it gains electrons

Zn goes to Zn2+ so is oxidised as it loses electrons

When writing half equations in electrochemical series what does the equation with the higher E value do the the equation with the lower E value

It reverses its reaction

How to write the overall equation for a cell reaction

Write the half cell with the more positive E value as reduction

Write the half cell with the more negative E value as oxidation

Combine the half equations to get an overall equation

In redox half equations how do you know if a reaction is feasible

If the oxidising agent (thing being reduced) is more positive than the reducing agent (thing being oxidised)

A feasible reaction will have the E cell greater than E > 0

The E cell value that is more positive reverses reaction of less positive one

If it doesn’t match the equation not feasible

Feasibility question example

Since Br equation is more positive the Fe equation reverses so the reaction is not feasible

What are the observations of oxidation with half cells

Metal dissolves

Made into ions

Changes colour( in some cases)

What are the observations of reduction in half cells

Metal is deposited on electrode

Decrease in metal ions

Colour fades or disappears( in some cases)