Acids and bases

What is an arrhenius acid

A substance which dissolves in water and dissociates to give H₃O⁺

What is a Bronsted-Lowry acid

• A proton donor

What is a Bronsted-Lowry base

A proton acceptor

Conjugate Acid base pairing

• A strong acid has a weak conjugate base pair

• A weak base has a strong conjugate acid pair

Amphiprotic molecules

• Can either accept or donate a proton, thus acting as an acid or a base

• E.g water and amino acids

pH equation

• pH runs from 0 to 14

• pH = -log [H+]

pH curve

• Has a logarithmic curve

• To change the pH by a factor of 1, the [H+] must change by a factor of 10

How temperature affects pH of water

• The dissociation of water is endothermic

• An increase in temperature causes equilibria to shift to the right, increasing the concentration of hydrogen ions

• This causes the pH to decrease

Ionic product and concentration of water

• K = [H+][OH-]

• Concentration of water is 55.5

Strong vs Weak acids

• Strong acids have a greater Kc

• They fully dissociate to form H+

• Strong acids are often mineral acids

• Weak acids do not fully dissociate in water and are often organic acids

Carboxylic acids vs Alcohols

• Carboxylic acids tend to be stronger

• This is due to their functional groups, which enable carboxylic acids to lose their H+ more easily than alcohols

• This is because the C=O in the -COOH pulls electrons away from the carbon, which pulls electrons away from the oxygen and thus the H+

Using equilibrium constant to calculate pH

• If we know Kc, we can put it equal to the Kc equation to calculate H+, which can be used to calculate pH

• E.g, if Kc = [CH₃CO₂^-][H+] and we know Kc = 1.8×10^-5 , pH can be calculated

• It can be assumed that [CH₃CO₂^-] = [H₊]

• This means x² = Kc = 1.8×10^-5

• ^{From here, [H+] can be found, and so can the pH}

Conjugate acid base pairing of H₂O

• When water behaves as a weak base, it does not gain H+ easily. It has a strong acid conjugate base

• When water behaves as a weak acid, it loses H+ easily. It has a strong base conjugate pair

Why a titration cannot be used to work out whether an acid is strong or weak

• The base will continue to react with the H+ produced in the reaction

• This causes the equilibrium to shift to the left as the concentration of products increases

• The reaction will continue until all the reactants are used up

• The same volume of base would be required to neutralise a strong or weak acid

Methods of measuring acid strength

• pH meter / indicator

• Measure the conductivity

• Measure the rate of reaction

Shape of a pH curve

• Makes a sigmoidal (S) shape

• The end point is the region where the graph goes vertically up

• The equivalence point is the point where the reaction is complete and all reactants have been converted into products

Suitable indicator for reaction with strong acid and strong base

Phenolphthalein

Suitable indicator for reaction with strong acid and weak base

Methyl red

Suitable indicator for reaction with weak acid and strong base

Phenolphthalein

why is there no suitable indicator for a reaction between a weak acid and weak base

The indicator will not rapidly change colour as there is no sudden increase in pH

Reactions between metal and acid

• Metals will displace hydrogen from acids in a redox reaction if they are more reactive than hydrogen

Relationship between acid strength and pKa

Stronger acids have a higher Ka and thus lower pKa.

This means equilibrium lies to the right

Ka, Kb and Kw equation

Kw = Ka x Kb

pKa + pKb = 14

Salt combinations

• Salts can be made by reacting weak or strong acids and bases

• The pH of the salts equivalence depends on the strength of the acids and bases the salt was derived from

• The pH of the salt solution comes as a result of the reaction of the conjugate acids and bases with water - salt hydrolysis

How indicators work + phenolphthalein

• Weak acids / bases which have different colours depending on the pH of the solution they are in

• Occurs because they react when placed in solution

• The pH in the centre of the range of a indicator is very close to the pKa

• Phenolphthalein - colourless in acid and pink in alkaline solutions

What is a buffer

A solution whose pH does not change significantly when small amounts of acid or alkali are added

Weak acids and bases as buffers

• Weak acids and bases are slightly resistant to neutralisation

• E.G ethanoic acid. When small amounts of alkali are added, the OH- reacts with the H+ produced by the acid. The H+ is used up, meaning the position of equilibrium will move to the right to replace the H+, which remains relatively constant.

• However, the addition of acid will immediately result in a change of pH. The concentration of CH₃COO- ions is very low, and there is little available to react with the added H+. The position of equilibrium cannot shift further to the left to remove the H+

Acid buffer

A mixtur of a weak acid and the salt formed between the weak acid and a strong base

Basic buffer

A mixture of a weak base with the salt formed between the weak base and a strong acid

How a basic buffer works

• When H+ is added to the buffer solution, the concentration of OH- drops, causing the position of equilibrium to move to the right to counter this change. The concentration of the weak base is high (because it is weak) and thus a significant amount of acid can be added before the pH changes

• When OH- is added, its concentration rises. The position of equilibrium moves to the left to counter this. The concentration of the weak alkali in the salt dissociates, meaning its concentration is high and thus lots of alkali can be added before the pH changes

How to make the perfect buffer

• The amount of alkali required to neutralise an acid is called the equivalence point

• To make a perfect buffer, we titrate to the half equivalence point

• This ensures there will be equal amounts of CH₃COOH to dissociate if OH- is added and CH₃COO- to use up if H+ is added

Expression to predict pH when a weak acid is half titrated

pH = -logKa

The buffer zone

• The point (at the start of the pH curve) during a neutralisation where the the acid acts as an alkali, limiting its change in pH

Lewis acids and bases

• A lewis acid is a lone pair acceptor. It is an electrophile (lover of electrons)

• A lewis base is a lone pair donor. It is an electrophile (lover of protons)