Section 7 - Oxidation, Reduction and Redox reactions

What is oxidation in terms of electrons?

The loss of electrons by an atom, ion, or molecule.

What is reduction in terms of electrons?

The gain of electrons by an atom, ion, or molecule.

What is a redox reaction?

It’s a reaction in which both oxidation and reduction occur simultaneously.

What is an oxidising agent?

An oxidising agent causes another substance to be oxidised and is itself reduced.

What is a reducing agent?

A reducing agent causes another substance to be reduced and is itself oxidised.

What is the oxidation number of an element in its standard state?

0

What is the oxidation number of a simple ion?

Equal to the charge on the ion.

What is the oxidation number of hydrogen in compounds?

• +1

• Except in metal hydrides, where it is -1.

What is the oxidation number of oxygen in compounds?

• -2

• Except in:

  • peroxides, where it is -1

  • OF2, where it is +2

What is the oxidation state of fluorine in all compounds?

-1

What is the usual oxidation state of other halogens (Cl, R, I)?

• -1

• Unless combined with oxygen or a more electronegative halogen.

What should the sum of oxidation states be in a neutral compound?

0

What should the sum of oxidation states be in a polyatomic ion?

Equal to the charge of the ion.

How do you determine if a reaction is a redox reaction using oxidation numbers?

If there is a change in oxidation numbers of elements.

How do you identify which substance is oxidised and which is reduced?

• Oxidised: oxidation number increases (loses electrons)

• Reduced: oxidation number decreases (gains electrons)

How can you identify the oxidising and reducing agents in a reaction?

• Oxidising agent: substance reduced (gains electrons)

• Reducing agent: substance oxidised (loses electrons)

What is a half-equation?

A chemical equation showing either oxidation or reduction, including electrons.

What is the purpose of combining half-equations?

To give the overall redox reaction, ensuring electrons lost = electrons gained.

How does a metal react with an acid in terms of redox?

• Metal is oxidised to a metal ion.

• H+ ions are reduced to H2 gas.

How does a metal react with water in terms of redox?

• Metal is oxidised.

• Water is reduced to hydrogen and hydroxide ions.

How are displacement reactions redox reactions?

A more reactive metal is oxidised, and it displaces a less reactive metal ion, which is reduced.

What is disproportionation?

A reaction in which the same element is both oxidised and reduced.

What occurs at the anode in electrolysis?

Oxidation does

What occurs at the cathode in electrolysis?

Reduction does

How do you determine which species is discharged at the electrodes?

The most easily oxidised/reduced species is discharged, considering ion concentration and reactivity.

How is redox important in corrosion?

• Iron oxidises to Fe2+/Fe3+, leading to rust.

• Oxygen and water act as oxidising agents.

How is redox important in biology?

Cellular respiration and photosynthesis involve electron transfer (redox reactions).

When balancing redox reactions in acidic solution, what is the order?

1) Write half equations

2) Balance all the elements except H and O

3) Balance O using H2O

4) Balance H using H+

5) Balance charge with electrons

6) Combine half-equations

How do you balance redox reactions in alkaline solution?

Balance as acidic solution first, then addOH- to both sides to neutralise H+, forming H2O where needed.