chemistry alevel aqa practicals

required practical 1

Making a Standard Solution & Titration

why do things like concentrations vary for things liek river samples

samples vary over time or not consistent accross river

measuring the volume of gas produced apparatus set up

why might you need to seal the containers straight away

prevent loss or gain of H2O/ gasses/ certain reasgent

how could all the gasses in a sealed container escape

an equilibrium will be set up and so depeninding on the conditions it could force the equilibrium to the side with no gas

why might apparatus explode if sealed

build up of gas produced will ead to high pressure

weighing compounds and making up volumetric solutions

weigh by difference

weigh sample bottle on balance, transfer solid to beaker then reweigh bottle and record mass by difference

add distilled water to beaker and swirl until solid dissolves- pour into volumetric flask

ensure risning out + add washings to volumetric flask

ensure bottom of the meniscus is on the mark and then invert the flask to mix

what would happen if you filled a volumetric flask significantly above the graduation mark

acid in flask would be more dillute so would appear more acid had reacted

Distillation

what is ditilation used for

used to separate substances with different boiling points

the one with lower boiling point boils first and can be condensed and collected

step 1 distillation

turn on water

in at low point + out at top

step 2 distillation

heat flask with bunsen burner to allow reagents to evaporate

step 3 distillation

vapours pass into fractioning column where they separate due to boiling points

step 4 distillation

substance with lower BP travels into condenser

higher BP condenses back into flask

step 5 distillation

use thermometer to ensure that the temperature is at, or just below, the BP of the substance you wish to collect

step 6 distillation

the substance you want will pass into condenser and be cooled back into a liquid

step 7 distillation

cool the reception vessel using ice to prevent loss of product by evaporation

heating under reflux set up

what is heating under reflux used for

convert alcohols into acids

when is reflux used

when need to heat for a long time but dont want to lose any product/ reactant through evaporation

anti-bumping granules

used t prevent uneven boiling by forming smaller bubbles

things to remember when drawing a reflux container

not sealed anywhere

flask with condenser vertically above it with no gaps

label

condenser must have outer tube for water with 2 openings

heating substances in a cruicible

why does cruicibkle ahve to be dry

to ensure water isnt lost on heating + gives a greater mass loss than actual

why use a lid

prevnt loss of the solid/ reactant

when to stop heating

when reached a constant mass on several checks

how to ensure reaction has gone to completion

there is a constant mass

main risks

handling a hot crucible

what isnt a good method to see if the reaction is at completion

colour change

heat loss can be improved through

adding a lid/ reducing distance bwetween flame and beaker or using a water bath

if only small amounts of H2O is used then there will be less heat loss than if a large amount is used

why is this

a smaller heat loss by evaporation from the surface

decomposition recations and heat loss

if a smaple is left over night then it can have an effect on the temperature change measured in the reaction as some will have already decomposed

there would be a smaller temperature change as there is less sample and what is left can react exothermically rather than endothermically

why might the mass of a solid obtained during decomposition be LESS than expected from the mass of original substance

some solid could have been lost during weighing

why might the mass of a solid obtained during decomposition be MORE than expected from the mass of original substance

sample is impure

reaction didn't go to completion

themometers often arent adequate for experiemnts even with uncertainties of +- 0.5

why

the temperature change is often bigger than the uncertainty or heat loss is a more significant issue

Calorimetry

calorimetry equation

q=mcΔT

when does -'ve sign have to be put in

for exothermic reactions only

what is temperature measured in

Degrees Celsius (°C)

Calorimetry method

weigh spirit burner before and after combustion.

put measured volume of water in calorimeter

burn alcohol to heat water and measure temperature rise of water

why do values often differ from data book

incomplete combustion

heat transfer

heating of calorimeter or container

evaporation of fuel

for mixing 2 solutions or solution and a solid and finding the accurate minimum temperature that isn't influenced by heat from the surroundings

step 1

start stop watch + take temperature of 1 solution every minuite 0-3 minuites

step 2

add 2nd substance at minuite 4 do not take temperature at this time

step 3

record temperature at every subsequent minuite for about 5 minuites

step 4

plot graph temp vs time

step 5

extrapolate both lines to time of mixing + determine the temperature change

% error

(error/your volume) x100

to reduce % error

increase volume by changing a concentration or larger volume of other reactant.

when must there be operator error

if % error is greater than the calculated error from the equipment then there must be operator error as well

small volumes in tire will give...

largest error so would be least accurate values

if toxic

wear gloves/avoid skin contact

if corrosive

wear gloves + goggles

if irritant

wear goggles

if toxic gas

carry out in fume cupboard to avoid toxic vapours

how can harmful effects be reduced

can be reduced by diluting with water if needs returned to environment

why do you have less yield than expected

some of it dissolves/ lost in filtration/ gets oxidised (if relevant)

why do reactions stop when some reagents left

one of the products formed could coat the reactants (if solid)

% atom economy

(mass required product/ mass of all products) x100

how can impurities be removed

wash and filter, re-crystallisation

why do we use an excess of a reagent

to ensure all of the reactants has reacted/ improve yield/ displace equilibrium to the right.

the error in weighing substance that are used in excess does not matter

why use one reagent rather than another excess

teh one using in excess will probably be a solid and is therefore easier to remove by filtration

why might another substance be used

might be less corrosive, more readily available

oxidising agents and flammable materials

keep away from flammable materials; they are o if only have a low concentration

if you need to heat flammable materials you would use a water bath or electrical heater this will eliminate fire risks

oxidising agents

keep away from flammable materials

ok in low concentration

timing errors

in experiments where you are timing a way to reduce errors is to repeat the experiment with lower concentrations

this will slow the experiment to make the % error less

repeating experiments

improves reliabilty

determining solubility of a saturated solution

take a know volume of the solution, evaporate the filtrate to dryness, weigh the residue

ionic equations

just show the ions that are reacting, leave out the spectator ions

why are small samples used?

substance might be corrosive/ toxic/ expensive (and vice versa, toxic substances might be used as only small amounts)

graphs

if smooth curve or straight line can be drawn which runs close to all points then you can use it with confidence to predict intermediate points

scaling up

some things that you might be able to do easily in the lab might not work on an industrial scale because the cost of heating etc. may be too high

stirring

ensures all the solution is at the same temperature

why do you filter to remove insoluble impurities before adding chemical to give a precipitate

prevents you getting an incorrect/ too large a value for the final mass

why do you wash precipitates with distilled water

to remove soluble impurities/ excess of the reactants

how do you obtain a pure dry sample of a soluble substance

add it to water then stir and heat it so it dissolves

filter the mixture to get rid of the insoluble impurities and then evaporate the filterer to leave behind the dry substance

properties of drying agents

must be insoluble and not react with the substance you are trying to dry

drying compounds

can use propanone/ acetone etc. as evaporates at a lower temperature and removes the water

to determine the mass of a solid needed to ensure the acid is in excess

workout the solid needed to reacts fully with the acid and then use a smaller amount

if given a large sample of solid to react

ensure that the beaker is big enough for it and the other reactant

problems with decanting when trying to keep a solid

you can lose some of the solid, a better method would be to filter and wash with distilled water

how could you measure a set mass of water without using a balance

density of water= 1gcm3 so you could measure the volume and it would equal the mass, e.g. using a pipette

during rate of reaction experiments what effect would increasing the size of the reaction vessel have on the time taken

the time taken would increase as the particles will be more spread out

issue with using drops from a Pasteur pipette rather than a normal pipette

drop sizes vary so could get inconsistencies in your results.

how to avoid these inconsistencies with pasteur pipette

use a larger measured volume of the liquid, dissolve it in suitable solvent, transfer to 250cm3 volumetric flask and make up with more of the sample. then titrate a 25cm3 portion of this solution

how would you obtain a pure dry sample of a substance formed in a precipitation reaction?

filter the solution off, wash the residue with water and dry by pressing between filter paper or in air

purification by recrystallization how to do

step 1

dissolve the product in the minimum volume of hot solvent- to obtain saturated solution, to increase yield an enable crystallisation

step 2

solvent should have a high solubility for the solid when hot and less solubility for the solid when cold so lots of crystals appear cooling

step 3

filter hot- to remove insoluble impurities that haven't dissolved

step 4

allow solution to be cooled in ice - to increase amount of crystals formed

step 5

wash with cold water- remove any soluble impurities

step 6

cool/ crystallise, filter the pure product using a buchner funnel + side arm flask

step 7

compress crystals in the funnel so air passes through the sample and not just round it

filter under reduced pressure / vaucuum filtration

ensure you include a side arm flask, flat bottomed filter funnel + clearly show filter paper

equilibrium mixtures

if flask isnt sealed properly then you would get an incorrect value for Kc because one of the substances would escape and so incorrect concentrations would be determined and therefore wrong values used to determine Kc

why calibrate pH meter

to improve accuracy by using known pH

when does callibration have to occur

before use as do not give accurate results after storage

how to callibrate pH

use buffer solution, rinse probe between measurements

measure pH of more than one buffer solution

plot a graph of pH of buffer against pH on probe