Acids and bases

What’s the Bronsted-Lowry definition of an acid?

Proton donors

What’s the Bronsted-Lowry definition of a base?

Proton acceptors

What ions causes a solution to be acidic?

The H+ ion

But in aqueous solutions, H+ ions bond with H2O to form hydroxonium ions H3O+ which cause acidity

Give 2 equations to show the ionisation of water

2H2O (l) → H3O+ (aq) + OH- (aq)

H2O (l) → H+ (aq) + OH- (aq)

What ion causes a solution to be basic?

OH-

Give examples of weak acids

Carboxylic acids

Give examples of strong acids

HCl

H2SO4

HNO3

Give an example of a weak base

Ammonia (NH3)

Give examples of strong bases

NaOH

KOH

What’s the Arrhenius definition of an acid?

Ionise in water to produce H+ ions

What’s the Arrhenius definition of a base?

Ionise in water to produce OH-

What’s the Lewis definition of an acid?

Electron pair acceptor

What’s the Lewis definition of a base?

Electron pair donor

What’s a strong acid/base?

Fully ionises in aqueous solution

What’s a weak acid/base?

Only partially ionises in water

What’s the equation for an acid and a base?

Acid + base → salt + water

What’s the equation for an acid and a metal oxide?

Acid +metal oxide → salt + water

What’s the equation for an acid and a metal carbonate?

Acid + metal carbonate → carbon dioxide + salt + water

What’s the equation for an acid and a metal?

Acid + metal → salt + hydrogen

What happens when an acid and a base react?

Protons are exchanged

What is a conjugate base?

The substance that forms once an acid has donated a proton

What is a conjugate acid?

The substance that forms once a base has accepted a proton

What happens to water when acid is added to it?

It acts as a base by accepting a proton to form H3O+

Where does equilibrium lie for strong acids?

To the right

Forwards reaction favoured strongly so lots of H+ produced

Where does equilibrium lie for weak acids?

To the left

Backwards reaction favoured so not as many H+ produced

Where does equilibrium lie for weak bases?

To the left

Backwards reaction favoured so not as many OH- ions produced

Where does equilibrium lie for strong bases?

To the right

Forward reaction favoured strongly so lots of OH- ions produced

Why does the concentration of water have a constant value?

Only a tiny amount of OH- and H+ ions are formed compared to H2O molecules, so they do not change the concentration of water noticeably

What is the expression and unit for Kw?

Kw = [H+][OH-]

Mol²dm-6

Whats the value of Kw?

1.00 × 10-14 mol2dm-6 (at 25C/298K)

What happens to the value of Kw if you increase temperature?

The dissociation of water is endothermic

Increasing temperature shifts equilibrium to the right (to decrease temperature) so more OH- and H+ are produced

Since Kw = [H+] [OH-], Kw increases

Define pH

pH is the logarithmic scale that measures the concentration of H+ ions in solution

Whats the equation for pH?

pH = -log10[H+]

What equation do you use to work out [H+] if you know pH?

[H+] = 10 ^-pH

What is a monoprotic acid? Give 3 examples

An acid that only donates 1 H+ ion per acid molecule when it dissociates in water

Eg - HCl, HNO3, CH3COOH

What is a diprotic acid? Give an example

An acid that can donate 2 H+ ions per acid molecule when it dissociates in water

Eg - H2SO4

What is Ka and why is it used?

Ka is the acid dissociation constant

It is used to find the pH of weak acids which only partially dissociate in aqueous solution

What is the expression and unit for Ka?

Ka = [H+][A-] / [HA]

Units - moldm-3

Give the simplified Ka expression and explain what assumption can be made in order to simplify it

Ka = [H+]² / [HA]

• The disassociation of acids is greater than the disassociation of water present in the solution. We can assume that all the H+ ions come from the acid. Every H+ comes with an A- so [H+]=[A-]

Give an equation to show how weak acids exist in equilibrium

HA → H+ + A-

How can you work out the pH of a weak acid?

• Use the Ka expression to sub in values for Ka and [HA] to calculate [H+]

• Use pH = -log10[H+] by substituting in [H+]

What is pKa used for?

To measure the strength of an acid

The lower the value of pKa, the … the acid

Stronger

Give the expression for pKa?

pKa = -log10Ka

Rearrange the pKa expression to find Ka

Ka = 10^-pKa

What are titrations used for?

To work out the concentration of an acid or base

What is the end point of a titration?

When the volume of acid or alkali added causes the indicator to change colour

What is the equivalence point of a titration?

The point at which the exact volume of base or acid has been added to neutralise the solution

What generally happens to the pH of the solution around the equivalence point?

There is a large and rapid change

What is the half neutralisation point in a titration?

When volume is half the volume that has been added at the equivalence point

What are 3 properties of a good indicator?

1. Sharp colour change - no more than one drop of acid/alkali needed for change

2. End point same as equivalence point

3. Distinct colour change

What colour is methyl orange in acid and in alkali?

• Red in acid

• Yellow in alkali

What colour is phenolphthalein in acid and in alkali?

• Colourless in acid

• Pink in alkali

Which indicator would you use for a strong acid-strong base titration?

Methyl orange

Which indicator would you use for a strong acid-weak base titration?

Methyl orange

Which indicator would you use for a strong base-weak acid titration?

Phenolphthalein

Which indicator would you use for a weak acid-weak base titration?

Neither methyl orange or phenolphthalein as neither give a sharp change at the end point

We have to use a pH meter

How would you calculate the concentration of a reactant if you know the volume and concentration of the other reaction in a titration?

1. Calculate moles of one reactant

2. Use balanced equation to work out moles of the other

3. Use concentration=moles/volume

What is a buffer?

A chemical that resists the change in pH when small amounts of acid or base are added

What are the 2 types of buffer?

• Acidic

• Basic

What are acidic buffers made from?

A weak acid and it’s salt

How does an acidic buffer resist added acid?

• The H+ ions from the acid react with the conjugate base from the buffer

• There’s a high concentration of the conjugate base from the salt as it dissociates fully

• This removes extra H+, so pH barely changes

How does an acidic buffer resist added base?

• The OH- ions react with the H+ ions from the buffer

• This neutralises the OH-, so pH stays constant

What are basic buffer made from?

A weak base and its salt (which dissociates fully to form its conjugate acid)

How does a basic buffer resist added base?

• The OH- ions from the base react with the conjugate base produced by the salt

How does a basic buffer resist added acid?

• The H+ ions from the acid react with the OH- ions from the solution that were produced by the base

• There is a low concentration of these as the base weakly dissociates but they can be reproduced by adding a high concentration of these base and water

Give 3 uses of buffers

• Shampooc

• Washing powder

• Blood