3.1.7 Redox

What does redox mean? (1)

Reactions which involve both reduction and oxidation

What is oxidation? (1)

Process of electron loss

What is reduction? (1)

Process of electron gain

what is oxidation in terms of oxidation states

• oxidation number increases

what is reduction in terms of oxidation states

• oxidation number decreases

what is a redox reaction in terms of oxidation states

• a reaction where the oxidation state of at least one element changes

what is a disproportionation reaction

a reaction in which an element is both oxidised and reduced

What is an oxidising agent? (1)

Electron acceptor

what part is an oxidising agent exactly

• the reactant

• not the element

What is a reducing agent? (1)

Electron donor

what does an oxidation state (number) tell you

total number of electrons it has donated or accepted.

summary table of the oxidation number rules

what are the oxidation state rules ranked in order of 

priority

mneumonic for remembering the order of priority for the later oxidation states

Fairies

Hate

Orange

Clover

What is the oxidation state of uncombined elements? (1)

0

What is the oxidation state of elements bonded to identical elements? (1)

0

what is the oxidation state of compounds/polyatomic ions?

• sum of the oxidation states equals the overall oxidation state

• This overall oxidation state is equal to the overall charge on the ion.

What is the oxidation state of oxygen in compounds, except in peroxides? (1)

-2

What is the oxidation state of oxygen in peroxides? (1)

-1

whats the molecular formula of hydrogen peroxide

H2O2

What is the oxidation state of hydrogen in compounds, except in metal hydrides? (1)

+1

What is the oxidation state of hydrogen in metal hydrides (e.g., NaH)? (1)

-1

What is the oxidation state of Group I metals in compounds? (1)

+1

What is the oxidation state of Group II metals in compounds? (1)

+2

What is the usual oxidation state of Group 7 in compounds? (1)

-1

do oxidation states have a +/_ before or after the number

before

how do we assign oxidation states in covalent molecules

assign oxidation states based on charges each element would have if the compound was ionic

Finding the oxidation states from systematic names

• If an element can have multiple oxidation states (or isn’t in its ‘normal’ oxidation state) its oxidation state is sometimes shown using Roman numerals,

• e.g. (I) = +1, (II) = +2, (III) = +3 and so on. The Roman numerals are written after the name of the element they correspond to.

what are examples of molecules that have multiple oxidation states

• ions that end in -ate

• transition metals

examples of naming ions with oxidation states

what are the 2 methods for balancing redox equations

1. Constructing and combining half equations

2. Using oxidation states

what does a half equation show

half-equations show oxidation or reduction

The electrons are shown in a half-equation so that the charges balance.

EXAMPLE - give the half equation for the oxidation of iron

Fe → Fe3+ + 3e–

EXAMPLE - give the half equation for the reduction of chlorine

Cl2 + 2e– → 2Cl–

What is the most important thing to remember when combining half equations to form full balanced equations

Just make sure both half-equations have the same number of electrons in

when do we use the half equation balancing method

if we know the products of that redox reaction

what is the method for constructing and combining half equations to find the balanced equation from the half equation

Construct half-equations:

1. Construct separate equations for oxidation and reduction

2. 2) Balance the atoms ...

   1. .. other than hydrogen and oxygen

   2. oxygen by adding H2O

   3. hydrogen by adding H+

3. Balance the charges by adding e

To combine half-equations:

4. Multiply the half-equations so that the electrons cancel (as in the number of electrons in both half equation is the same)

5. Combine the half-equations

6. Double check its all balanced

summary/example of finding the balanced equation from half equations

why can we balance an equation using oxidation states

•  the sum of the oxidation states in the reactants must equal the sum of the oxidation states in the products.

• this can be used to work out the stoichiometric coeficients

what is the method of balancing equations using oxidation states

1. by assigning oxidation states figure out which element is oxidised and which element is reduced

2. write these in a table, noting their initial and final oxidation state and the change in oxidation states

3. overall changes in oxidation state must add to zero. THEREFORE, multiply so the changes are equal for both elements

4. this multiplier is the coefficient that goes in front of the elements

5. deduce the rest of the equation

summary/example of the method of balancing equations using oxidation states

How to test for SO2

dichromate

How to test for H2S

test using lead nitrate