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

	Erstellt von Holly Bamford vor etwa 9 Jahre

Complete up to calculating energy changes

AQA GCSE C3 Notes