AQA GCSE C3 Notes

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Acids and

bases

Proton Donors & Acceptors

When an acid is forms H­­+

ions- a hydrogen atom, that has lost an electron or just a proton

The protons become surrounded by water molecules to keep

them in solution-hydrated

Hydrated hydrogen ions are written as H+ ­­(ag)

An alkali is a base which dissolves in water and produces OH- ions – Hydroxide ions

Acids act as a source of protons – proton donors

Hydroxide ions combine with protons to form water-OH- (aq) + H+ (aq) → H2O (l)

Alkalis are called proton acceptors

Strength of Acids and Alkalis

Strength of acid depends on extent to which it ionises in water-strong acid/alkali=100% ionised in water

A weak acid/alkali are only partly ionised in water

Can find the strength of acids by testing for pH

A strong acid

will be completely ionised so

concentration hydrogen ions =1 mol/dm³

Weak acids are

only partly ionised so concentration

of hydrogen ions is less than 1

mol/dm³

Titrations

Adding an acidic solution to an alkaline solution will

produce a neutralisation reaction

Acid + alkali = base + salt

We can measure exact volumes of acids and alkalis

needed to react with each other using titrations

The point at which the acid

and the alkali have completely reacted is called the end point

End point can be shown using an indicator-phenolphthalein/methyl

orange

How to carry out titration:

1 Measure known

volume of the alkali solution into conical flask using pipette

2 Add indicator

solution to the alkali

3 Put the acidic

solution into a burette-record the reading on the burette (i.e. starting

volume)

4 Open tap to

release the acid solution. The solution from the burette is released one drop

at a time, alongside swirling of the flask to ensure the solutions are mixed

5 Repeat Step 4

until the indicator changes colour to let you know the acid and the alkali have

completely mixed

6 Record the

amount of acid you entered by reading the measurement on the burette

Be sure to repeat the entire process two or three times at

least to ensure accuracy.

Titration calculations

Units of concentration are mol/dm3

·

For example, imagine we were making a sodium

hydroxide solution in water by dissolving exactly 40g of sodium hydroxide to

make 1dm³ of solution:

·

We know that the mass of one mole of NaOH is the

sum of the atomic masses of sodium, oxygen and hydrogen: 23 + 16 + 1 = 40g

·

Because 40g is in the solution, we know that

there is exactly one mole of NaOH in the solution

·

And we know that the solution is

1dm³, so the concentration is 1 mol/dm³

Energy Calculations

Energy and reactions

Breaking bonds is an endothermic

process

Making bonds is exothermic

process-releases energy

To measure the amount of energy released

in a reaction a calorimeter is used

Energy Level Diagrams

Exothermic reaction, we say that

the change in energy is negative.

Temperature of the surroundings

decreases because the change in energy is positive.

Endothermic reaction, more energy is

needed to break to bonds

The products are at a lower energy level than the reactants-energy

has been released

The amount of energy needed to

start a reaction is called the activation energy

Adding a catalyst will reduce the activation

energy

Calculating Energy Changes

4.2 joules

of energy raises 1g of water by 1°C

The units involved are kJ/g/°C (kilojoules per gram per degree)

A + B → C

Question: 60cm³ of a solution

containing 0.1 moles of A is mixed with 40cm³ of a solution containing 0.1

moles of B. Prior to mixing, their temperature was 19,6°C. After mixing, the

maximum temperature reached was 26.1°C

1.

First, calculate the temperature change: 26.1°C

– 19.6°C = 6.5°C

2.

Since 60cm³ of A added to 40cm³ of B makes

100cm³ overall, we are looking at 100g (assuming the density of the solution is

the same as water density). And we know that 4.2J raises 1g by 1°C

3.

So energy change = 100g x 6.5°C x 4.2J/g/°C =

2,730J = 2.73kJ

4.

BUT – don’t forget the solutions are only 0.1

molar – so we have to multiply our value by 10 to find out a 1.0M solution

2.73kJ x 10 = 27.3kJ

5.

So the final energy change was -27.3kJ

Bond Energies

The energy required to break a bond between two atoms is

called bond energy

To calculate energy change we need to know:

a) The amount of energy needed to

break the bonds between atoms

b) The amount of energy released in the formation of new

chemical bonds.

Water and

Solubility

The water cycle

The sun supplies lakes, rivers and oceans

with energy-allows water to evaporate.

The water vapour forms clouds

as the water vapour condenses

The water droplets fall as rain

This is the water

cycle

Solubility

Amount of solute which can be dissolved in an amount of

solvent-called the solubility of the

substance

Measured in grams (of solute) per 100g (of solvent)

Solubility of a solid

solute increases as temp. Increases

Saturated solution-solution with maximum amount of solute

dissolved