Chemistry Unit 3

when an acid dissolves in water, it forms H+ ions. this is a hydrogen atom which has lost an electron. it is a proton. these produced protons become surrounded by water molecules to keep them in solution- they are hydrated. hydrated hydrogen ions are shown with H+ (aq). an alkali is a base which dissolves in water, and produces OH- ions (hydroxide ions). 

because acids act as a source of protons, we call them proton donors. the hydroxide ions from an alkali which combine with protons to form water:

OH- (aq) + H+ (aq) ---> H2O (l)

and because alkalis behave like this, we call them proton acceptors.

the strength of an acid depends on the extent to which it ionises in water. a strong acid or a strong alkali is one which is 100% ionised in water.

hydrochloric acid, sulphuric acid + nitric acid are all strong acids.

sodium hydroxide + potassium hydroxide are both strong alkalis.

ethanoic acid, citric acid and carbonic acid are all weak acids

ammonia is a weak alkali.

a strong acid will be completely ionised, so the concentration of hydrogen ions is 1mol/dm3. however, a weak acid is only partly ionised, so the concentration of hydrogen ions will be much lower than 1mol/dm3.

adding an acidic solution to an alkaline solution will produce a neutralisation reaction. they react together + neutralise each other , producing a salt in the process. when a neutralisation reaction takes place, the quantities of each solution used must be correct, because if a very strong acid and a very strong alkali were mixed, if there was more acid solution, the whole alkali solution would be neutralised, but not all of the acid solution would be- so the mixture would become slightly acidic overall. precise volumes of acids and alkalis needed to react with each other can be measured using titrations.

in the neutralisation reaction, the point at which the acid and the alkali have completely reacted is called the end point. end points can be shown using a chemical indicator. indicators change colour over different pH ranges.

strong acid + strong alkali = any indicator

weak acid + strong alkali = phenolphthalein

strong acid + weak alkali = methyl orange

1. measure a known volume of the alkali solution into a conical flask using a pipette

2. add indicator to the solution

3. put the acidic solution into the burette. record the starting volume

4. open the tap to release the acid solution. the solution from the burette is released one drop at a time, alongside swirling the flask to ensure the solutions are mixed

5. repeat step 4 until the indicator changes colour to let you know the acid and alkali solutions have mixed

6. record the amount of acid you entered by reading the measurement on the burette

concentration is usually described as the amount of the solute (in terms of moles) dissolved in the solution (in one cubic decimetre), so the units are mol/dm3 so if we know the amount of a substance dissolved in a known amount of solution we can calculate the concentration.

e.g. imagine we were making a sodium hydroxide solution in water by dissolving exactly 40g of sodium hydroxide to make 1dm3 of solution

 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 

we know that the solution is 1dm3, so the concentration is 1 mol/dm3

an exothermic reaction releases energy. exothermic reactions are used in burning fuels as a source of energy. some reactions give off more energy than others so the amount of energy released in a given reaction can be calculated. the apparatus used to calculate this is called a calorimeter. at school, a simple calorimeter may be used- but a more accurate instrument is available called a bomb calorimeter. a bomb calorimeter works by measuring the temperature of the water inside it – because the energy produced in an exothermic reaction increases the temperature of its surroundings, in this case the water. The change in energy is calculated using the temperature change and amount of water  

breaking bonds is an endothermic process, because energy has to be taken in from the surroundings to break the bonds

making bonds is an exothermic process, because energy is released in the formation of new chemical bonds 

when chemicals react + give off/take in energy, calculations can be used to work out how much energy has changed. 

4.2 joules of energy raises 1g of water by 1°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.

First, calculate the temperature change:

26.1°C – 19.6°C = 6.5°C

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

So energy change = 100g x 6.5°C x 4.2J/g/°C = 2,730J = 2.73kJ

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

So the final energy change was -27.3kJ. the temperature increased so the reaction is exothermic where energy is released. the energy change is negative

the energy required to break apart a bond between two particular atoms is known as bond energy. bond energies are measured in kJ/mol and can be used to work out ΔH in energy calculations

to calculate energy change we need to know- a) the amount of energy needed to break the bonds between the atoms b) the amount of energy released in a formation of new chemical bonds

Making and breaking the same bond always involves the same amount of energy, just different + and – signs

energy level diagrams can be drawn to show energy changes in a reaction. they show the relative amounts of energy stored in the products and reactants of a reaction measured in kJ/mol

endothermic diagrams go up the stairs

exothermic diagrams go down the stairs

(More Complicated Energy Level Diagram Calculations)

Although bond making and breaking is always the same energy levels back and forward – different chemical reactions mix and match the type of bonds being made and broken. This is why energy levels can begin to look a bit more complicated. A good example of a chemical reaction where different bonds are involved is the Haber process. This is the making of ammonia from nitrogen and hydrogen.

the sun supplies rivers, lakes and the ocean with energy, allowing the water there to evaporate. the water vapour formed rises into the atmosphere, where it cools and condenses to form droplets which clouds are made of. eventually the water droplets fall as rain, replenishing the water sources they originally came from. this is the water cycle.