M2 Acids and Redox

Question 1

Describe acids

Answer 1

All acids contain hydrogen. When dissolved in water an acid releases hydrogen ions as protons, into the solution.

Question 2

Define a strong acid

Answer 2

A strong acid releases all its hydrogen atoms into solution as H+ ions and completely dissociates in aqueous solution. eg. HCl, H2SO4, HNO3

HCl (aq) = H+ (aq) + Cl- (aq)

Question 3

Define a weak acid

Answer 3

A weak acid only releases a small proportion of its available hydrogen atoms into solution as H+ ions. A weak acid partially dissociates in aqueous solution. eg. CH3COOH

CH3COOH (aq) <> H+ (aq) + CH3COO- (aq)

*only the hydrogen atom on the COOH group is released as H+ ions as ethanoic acid is a weak acid
*it is in equilibrium so the reaction is reversible

Question 4

Describe bases

Answer 4

Metal oxides, metal hydroxides, metal carbonates and ammonia are classified as bases. A base neutralises an acid to form a salt.

Question 5

Describe an alkali

Answer 5

An alkali is a base that dissolves in water, releasing hydroxide ions into the solution.
NaOH + aq = Na+ (aq) + OH- (aq)

Question 6

Describe neutralisation

Answer 6

In neutralisation of an acid, H+ ions react with a base to form a salt and water. The H+ ions from the acid are replaced by metal or ammonium ions from the base.
An acid is neutralised by a metal oxide or metal hydroxide to form a salt and water only.
With alkalis, the reactants are in solution, forming a salt and water only:
acid + alkali = salt + water
The ionic equation for neutralisation:
H+ (aq) + OH- (aq) = H2O (l)
Metal carbonates neutralise acids to form a salt, water and carbon dioxide.

Question 7

Describe a titration

Answer 7

A titration is a technique used to accurately measure the volume of one solution that reacts exactly with another solution.
Titrations can be used for:
- finding the concentration of a solution
- identification of unknown chemicals
- finding the purity of a substance

Question 8

What is a standard solution?

Answer 8

A standard solution is a solution of known concentration.
A volumetric flask is used to make up a standard solution, they have typical tolerances of:
a 100cm3 volumetric flask: +/- 0.20 cm3
a 250cm3 volumetric flask: +/- 0.30 cm3

Question 9

Apparatus in an acid-base titration

Answer 9

The solution of an acid is titrated against a solution of a base using a pipette and a burette.
a 10 cm3 pipette: +/- 0.04 cm3
a 25 cm3 pipette: +/- 0.06 cm3
a 50 cm3 burette: +/- 0.10 cm3
A burette reading is recorded to the nearest half division, with the bottom of the meniscus on a mark or between two marks (nearest +/- 0.05 cm3)

Question 10

What values should you use for the mean titre?

Answer 10

• Repeat titres until two agree within 0.1 cm3, and use these to calculate the mean

Question 11

What do you know from a titration result?

Answer 11

both the concentration and reacting volume of one of the solutions
only the reacting volume of the other solution

Question 12

Define the oxidation number

Answer 12

Oxidation number is based on a set of rules that apply to atoms, the number of electrons involved in bonding to a different element.

Question 13

Rules for elements (oxidation number)

Answer 13

The oxidation number is always 0 for elements.

Question 14

Rules for compounds and ions (oxidation number)

Answer 14

Each atom in a compound has an oxidation number, which has a sign placed before the number.
Sun of oxidation numbers = total charge

Question 15

Describe how Roman numerals in naming

Answer 15

Roman numerals are used in the names of compounds of elements that form ions with different charges. The Roman numeral shows the oxidation state (number) of the element, without a sign.
Iron (III) representa Fe3+ with an oxidation number +3
NO2 - is nitrate (III), nitrogen has oxidation number of +3
NO3 - is nitrate (V) nitrogen has oxidation number of +5

Question 16

What is a redox reaction

Answer 16

A reaction that involves oxidation and reduction

Question 17

What is oxidation?

Answer 17

• The addition of oxygen
• The loss of electrons
• The increase in oxidation number
• The loss oh hydrogen

Question 18

What is reduction?

Answer 18

• The gain of electrons
• The loss of oxygen
• The decrease in oxidation number
• The gain of hydrogen

Question 19

Describe the redox reaction of metals and acids

Answer 19

metal + acid = salt + hydrogen
Zn + HCl = ZnCl2 + H2
Zinc is oxidised as it’s oxidation number increases by +2
Hydrogen is reduced as its oxidation number decreases by -2

Question 20

Which elements have fixed oxidation states?

Answer 20

Group 1 ions = always +1
Group 2 ions = always +2
Group 7 ions always -1
Oxygen = usually -2, unless peroxide where it is -1
Fluorine = always -1, can be +2 in oxygen difluride
Hydrogen = always +1, unless it is a metal hydride eg. NaH