Topic 3 Redox 1

Question 1

What is a redox reaction?

Answer 1

A reaction that involves both reduction and oxidation.

Question 2

What is an oxoanion?

Answer 2

An ion with the general formula XₓOᵧᶻ⁻ (X is any element and O is an oxygen atom). Metal and non-metals can become oxoanions. The most common include NO₂⁻, NO₃⁻, SO₃²⁻ etc.

Question 3

What is a half equation?

Answer 3

An ionic equation used to describe either the gain or the loss of electrons during a redox reaction.

E.g. Cu²⁺(aq) + 2e⁻ –> Cu(s)

Question 4

What is a displacement reaction?

Answer 4

A type of redox reaction in which the relative strengths of metals as reducing agents and non-metals as oxidising agents can be compared.

E.g. A more reactive metal displaces a less reactive metal from one of it’s salts.

Question 5

What are spectator ions?

Answer 5

Ions which are present in solution but take no part in the reaction.

Question 6

What are ionic equations?

Answer 6

Equations that describe chemical changes by showing only the reacting ions and any other reacting atoms or molecules leaving out any spectator ions.

E.g. Cu²⁺(aq) + Zn(s) –> Cu(s) + Zn²⁺(aq)

Question 7

What is the definition of oxidation?

Answer 7

A change that occurs to molecules, atoms and ions which makes the oxidation number more positive / less negative.

Zn(s) –> Zn²⁺(aq) + 2e⁻ is an example of oxidation.

Question 8

What is the definition of reduction?

Answer 8

A change that occurs to molecules, atoms and ions which makes the oxidation number more negative / less positive.

Cu²⁺(aq) + 2e⁻ –> Cu(s) is an example of reduction.

Question 9

What is the definition of oxidation states?

Answer 9

Oxidation states show how much an atom is oxidised or reduced in a compound.

E.g. Li has an oxidation state of +1 and Cl has an oxidation state of -1.

Therefore the overall oxidation number in LiCl = +1 + -1 = 0

Question 10

What is a disproportionation reaction?

Answer 10

A reaction in which one element is simultaneously being oxidised and reduced.

Question 11

What are the rules for oxidation numbers?

Answer 11

• An uncombined element has an oxidation number of 0.
• A simple ion has an oxidation number equal to its charge.
• Hydrogen has an Oxidation number of +1 with non-metals and -1 with metals.
• Oxygen has an oxidation number of -2, except when in peroxides where it has -1 or when bonded to fluorine where it is +2.
• The sum of the oxidation numbers in a neutral molecule is 0.
• The sum of the oxidation numbers in a multi-atom ion equals the charge on that ion.

Question 12

What does OIL RIG stand for?

Answer 12

Oxidation
Is
Loss of electrons

Reduction
Is
Gain of electrons

Question 13

What are cations and anions?

Answer 13

Cation - An ion with a positive charge

Anion - An ion with a negative charge

Question 14

What are the most common cations?

Answer 14

1+ = Na⁺, K⁺, Ag⁺, Cu⁺, H⁺ & NH₄⁺

2+ = Mg²⁺, Ca²⁺, Zn²⁺, Cu²⁺ & Fe²⁺

3+ = Al³⁺ & Fe³⁺

Question 15

What are the most common anions?

Answer 15

1- = Cl⁻, Br⁻, I⁻, OH⁻, NO₂⁻, NO₃⁻

2- = O²⁻, S²⁻, SO₃²⁻, SO₄²⁻ & CO₃²⁻

3- = N³⁻ & PO₄³⁻

Question 16

What rules can you follow to name inorganic compounds?

Answer 16

• If there are only two elements, the name ends with ‘ide’ and the more electronegative element comes second. E.g. CO₂ - Carbon Dioxide
• The roman numbers in the name indicate the oxidation number of the element before. E.g. Iron (II) Sulphate is FeSO₄ where irons oxidation number is +2
• Oxoacids (acids containing oxygen) always end ‘ic’ or ‘ous’. The ‘ic’ ending is used when the central atom has the highest oxidation number (of the two types of acid). E.g. HNO₃/Nitric acid(N = 5+) compared to HNO₂/Nitrous acid (3+).
• The corresponding endings for the salts of oxoacids are ‘ite’ for ‘ous ‘and ‘ate’ for ‘ic’.

Question 17

How do you combine half equations to make an ionic equation?

Answer 17

• If both half equations show reduction (X + e⁻ ⇌ X⁻) then one equation needs to be flipped to show oxidation. (Electrons must be on different sides)
• Balance the equations in terms of electrons (e.g. if one has 2e⁻ and the other e⁻, then the e⁻ half equation needs to be doubled/x2)
• Cancel out any atoms/ions/electrons that are the same in both half equations. (All electrons should cancel out if balanced correctly)
• Finally, add the two half equations together by putting all the atoms/ions/molecules on the left side together and the same on the right. The charges must also make 0 here (if not there is a mistake in balancing).

E.g.
Cl₂ + 2e⁻ –> 2Cl⁻, Fe³⁺ + 3e⁻ –> Fe
Cl₂ + 2e⁻ –> 2Cl⁻, Fe –> Fe³⁺ + 3e⁻ (e⁻ on diff sides)
3Cl₂ + 6e⁻ –> 6Cl⁻, 2Fe –> 2Fe³⁺ + 6e⁻ (balanced)
3Cl₂ –> 6Cl⁻, 2Fe –> 2Fe³⁺ (e⁻ cancelled out)

3Cl₂ + 2Fe –> 2Fe³⁺ + 6Cl⁻ (charges make 0)