Acids & Bases Flashcards
According to Arrhenius’ theory what is an acid?
A substance that contains hydrogen and releases H+ ions when it dissolves in water
What is a strong acid?
One that fully dissociates/ionises in water
What is a weak acid?
One that does not fully dissociate/ionise in water. It forms an equilibrium with the acid and its constituent ions
An H+ ion cannot exist on its own in aqueous solution,…
What is the importance of H3O+?
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
According to Bronsted and Lowry, what is an acid and what is a base?
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
An acid and base react in aqueous solution to produce what? What type of reaction is this?
1) Salt and water
2) Neutralisation reaction
What is a spectator ion?
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
What is the ionic equation for all reactions between a strong acid and a strong base?
H+ + OH- → H2O
Moving up or down the pH scale by one unit means…
… the hydrogen ion concentration changes by a factor of 10
The strength of a weak acid is determined by…
… the position of equilibrium, the further to the right the stronger (more dissociated it is)
What is Ka? What does a low Ka mean?
1) The acid dissociation constant, used to describe the strength of different acids
2) A weaker acid, it dissociates less
What is a monoprotic acid? Diprotic acid?
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
Why does the pH of water change at different temperatures?
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
What happens when a strong acid is added to a strong base?
- 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)
What happens when you add a strong base to a strong acid?
- 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)
In titration, why do all strong and weak acids have the same quantities in a calculation?
Because due to Le Chatelier’s principle, eventually all available hydrogen ions will be released to react with the stoichiometrically equivalent amount of bases
Why is it not possible to detect the end point of a titration between a weak acid and a weak base from the graph?
The variation in pH with volume of titre is more gradual, pH meter should be used
Why are different indicators needed for different reactions?
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
What are indicators? How do they work?
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
Why should the two different colours of the dissociated and the undissociated indicator be distinctly different?
So only two or three drops are needed to to give a clear visible change
What is a buffer?
A solution that can resist a change in pH when small amounts of acid or base are added
Generally buffers contain either:
- 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
