1.1.4 - Redox

Specification

Context and exemplification - Assessable learning outcomes

Oxidation number

* Candidates will not be expected to use oxidation numbers in peroxides or metal hydrides.ƒn
* Ionic equations will only be required in Group 7 chemistry. Candidates should be able to:
(a) apply rules for assigning oxidation number to atoms in elements, compounds and ions;

(b) describe the terms oxidation and reduction in terms of:

(i) electron transfer,

(ii) changes in oxidation number;

(c) use a Roman numeral to indicate the magnitude of the oxidation state of an element, when a name may be ambiguous, eg nitrate(III) and nitrate(V);

(d) write formulae using oxidation numbers;

Redox reactions

* For nitric acid, reactions of metals are not expected.
* Ionic equations not required.
(e) explain that:

(i) metals generally form ions by losing electrons with an increase in oxidation number to form positive ions,

(ii) non-metals generally react by gaining electrons with a decrease in oxidation number to form negative ions;

(f) describe the redox reactions of metals with dilute hydrochloric and dilute sulfuric acids;

(g) interpret and make predictions from redox equations in terms of oxidation numbers and electron loss/gain

Oxidation Number

Apply rules for assigning oxidation number to atoms in elements, compounds and ions

GENERAL RULES:

Describe the terms oxidation and reduction in terms of

(i) electron transfer:

(ii) changes in oxidation number;

Use a Roman numeral to indicate the magnitude of the oxidation state of an element, when a name may be ambiguous, eg nitrate(III) and nitrate(V);

The roman numerals state the oxidation state of an ambiguous element, usually an element from the transition metals.

Example – Sodium Chlorate(V), NaClO3

Sum of all oxidation numbers = zero

Therefore: (3 x O) + Na + Cl = 0

(3 x -II) + +I + Cl = 0

Cl = +V

Therefore the chlorine needs to be +5 in order to make the overall oxidation number 0.


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Redox Reactions

Explain that:

Describe the redox reactions of metals with dilute hydrochloric and dilute sulphuric acids;

In redox reactions of dilute hydrochloric acid and dilute sulphuric acid, a salt of the acid plus hydrogen are made:

Hydrochloric acid reaction:

Mg(s) + 2HCl(aq) ----> MgCl2(aq) + H2(g)

Sulphuric Acid reaction:

Mg(s) + H2SO4(aq) ---> MgSO4(aq) + H2(g)


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