Acids & Bases

Question 1

According to Arrhenius’ theory what is an acid?

Answer 1

A substance that contains hydrogen and releases H+ ions when it dissolves in water

Question 2

What is a strong acid?

Answer 2

One that fully dissociates/ionises in water

Question 3

What is a weak acid?

Answer 3

One that does not fully dissociate/ionise in water. It forms an equilibrium with the acid and its constituent ions

Question 4

An H+ ion cannot exist on its own in aqueous solution,…

What is the importance of H3O+?

Answer 4

1) … it forms a bond with water to form a hydronium ion (H3O+)
2) It explains, for example, why solutions of hydrogen chloride in solvents such as methylbenzene that cannot form a bond with H+ and therefore do not have any acidic properties

Question 5

According to Bronsted and Lowry, what is an acid and what is a base?

Answer 5

An acid is a proton donator - donates protons in a reaction
A base is a proton acceptor - accepts protons in a reaction
This applies to a wider range of acid-base reactions, not just those that happen in aqueous solution

Question 6

An acid and base react in aqueous solution to produce what? What type of reaction is this?

Answer 6

1) Salt and water

2) Neutralisation reaction

Question 7

What is a spectator ion?

Answer 7

An ion that appears on both sides of the equation, the do not take part in the reaction. They do not appear in the ionic equation

Question 8

What is the ionic equation for all reactions between a strong acid and a strong base?

Answer 8

H+ + OH- → H2O

Question 9

Moving up or down the pH scale by one unit means…

Answer 9

… the hydrogen ion concentration changes by a factor of 10

Question 10

The strength of a weak acid is determined by…

Answer 10

… the position of equilibrium, the further to the right the stronger (more dissociated it is)

Question 11

What is Ka? What does a low Ka mean?

Answer 11

1) The acid dissociation constant, used to describe the strength of different acids
2) A weaker acid, it dissociates less

Question 12

What is a monoprotic acid? Diprotic acid?

Answer 12

1) An acid with one ionisable hydrogen, e.g. HCl - strong

2) An acid that can release more than one H+ ion, e.g. H2SO4 - strong or H2SO3 (sulphurous acid) - weak

Question 13

Why does the pH of water change at different temperatures?

Answer 13

The ionisation of water is an endothermic process. When the temperature changes, the equilibrium shifts to oppose the change. This changes the concentrations of the ions which therefore affects Kw. Despite this the solution is still neutral because the concentration of hydroxide ions equals the concentrations of hydrogen ions

Question 14

What happens when a strong acid is added to a strong base?

Answer 14

• Initially, when a small volume of H+(aq) is added to the base, the proportion of OH-(aq) removed is quite small compared with the total amount of OH-(aq) present. The decrease in pH is very small
• As more H+(aq) Is added, the proportion of OH-(aq) being removed each time relative to the total amount increases, so there is a greater decrease in pH
• The change becomes greater when the end point is close
• The graph then shows a steep drop with the ending pH at 7 (pH of pure water)

Question 15

What happens when you add a strong base to a strong acid?

Answer 15

• Initially, when a small volume of OH-(aq) is added to the acid, the proportion of H+(aq) removed is quite small compared with the total amount of H+(aq) present. The decrease in pH is very small
• As more OH-(aq) Is added, the proportion of H+(aq) being removed each time relative to the total amount increases, so there is a greater decrease in pH
• The change becomes greater when the end point is close
• The graph then shows a steep drop with the ending pH at 7 (pH of pure water)

Question 16

In titration, why do all strong and weak acids have the same quantities in a calculation?

Answer 16

Because due to Le Chatelier’s principle, eventually all available hydrogen ions will be released to react with the stoichiometrically equivalent amount of bases

Question 17

Why is it not possible to detect the end point of a titration between a weak acid and a weak base from the graph?

Answer 17

The variation in pH with volume of titre is more gradual, pH meter should be used

Question 18

Why are different indicators needed for different reactions?

Answer 18

Different indicators change colour at different pH ranges, it is important to select one that will change colour during the steep part of the curve on the graph. Best way to do this is to select one where the pH range matches the end point of that particular titration

Question 19

What are indicators? How do they work?

Answer 19

They are weak acids where the dissociated form and the undissociated form have different colours. They form an equilibrium and if acid is added or neutralised by a base then [H+] changes. This affects the equilibrium. The most significant change in [H+] is at the end point, so the indicator should change colour when there is a large change in pH

Question 20

Why should the two different colours of the dissociated and the undissociated indicator be distinctly different?

Answer 20

So only two or three drops are needed to to give a clear visible change

Question 21

What is a buffer?

Answer 21

A solution that can resist a change in pH when small amounts of acid or base are added

Question 22

Generally buffers contain either:

Answer 22

• a weak acid and a large amount of strong-base salt of the acid (an acid buffer), e.g. ethanoic acid with sodium ethanoate
• a weak base and a large a,out of strong-acid salt of a the base (an alkaline buffer), e.g ammonia with ammonium chloride