Redox reactions

Question 1

Redox reaction

Answer 1

When reduction and oxidation reactions take place at the same time.

Question 2

Oxidation

Answer 2

• LOSS of electrons
• GAIN of oxygen
• REMOVAL of hydrogen

Question 3

Reduction

Answer 3

• GAIN of electrons
• LOSS of oxygen
• ADDITION of hydrogen

Question 4

Reducing agent

Answer 4

This is a substance which gives electrons to another substance.

It reduces the other substance and is itself OXIDISED.

Question 5

Oxidising agent

Answer 5

This is a substance which ACCEPTS electrons from another substance.

It oxidises the other substance and is itself REDUCED.

Question 6

Oxidation number

Answer 6

This is the charge on an atom of an element in a compound assuming all atoms are monatomic ( X+ and Y-).

It is the number of electrons that an atom uses to bond with atoms of anther element.

Also known as OXIDATION STATE.

Question 7

Rules for oxidation numbers

Answer 7

Pure element = 0

Oxygen in compounds = -2 ( -1 in H2O2 and +2 with fluorine)

Hydrogen in compounds = +1 ( -1 in hydride compounds such as LiH.

Group 7 = -1
Group 1 = +1
Group 2 = +2

Aluminium = +3

Transition metals = variable

Question 8

Oxidation numbers of a neutral compound and charged ions.

Answer 8

All of the oxidation numbers of the atoms in a compound must add up to the total charge of the compound.

Neutral add up to zero
-1 charge add up to -1

Example:
H3PO4
P has a charge of +5 as the hydrogens add up to +3 and oxygen adds up to -8.

Question 9

Half equations & Overall equations

Answer 9

Oxidation half equation:
X ——– X+ + e-

Reduction half equation:
X + e- ——— X-

Overall equation:

We have to multiply the half equations so that the electrons balance.

• All of the components on the left of each equation are moved to the left.
• All of the components on the right of each equation is moved to the right.
• Cancel out the electrons
• Left with the overall equation.

Question 10

Oxidation numbers in chemical names

Answer 10

• A ROMAN NUMERAL is included in the name of the compound or ion when an element can exist in DIFFERENT oxidation states.

Examples:

Cu2O ~ Copper has an oxidation state of +1
Systematically called Copper(I) oxide.

KMnO4 ~manganese has an oxidation state of +7.
Systematically called Potassium manganate (VII)

ClO3 ~ chlorine has an oxidation state of +5
Systematically called Chlorate (V)

Question 11

Oxidation numbers in Chemical reactions

Answer 11

• Elements by themselves always have an oxidation state of 0.
• The big number placed in front of an element or compound to balance the equation DOES NOT have an effect on the oxidation state as we only consider a single atom.

SPECTATOR ION ~ The element that has not changed oxidation state.

OXIDISED ~ The element that has increased its oxidation state.

REDUCED ~ The element that has decreased its oxidation state