Practical Chemistry Flashcards
when preparing a standard solution, what features should the chemical being used have
it should:
- not absorb moisture from or lose moisture to the environment
- have an accurately known relative formula mass so the number of moles dissolved can be determined (e.g. not a hydrated salt)
- be very pure
- have a relatively high relative formula mass so that weighing errors are minimised
what is a standard solution
a standard solution is a solution for which the concentration is accurately known
how do you calculate percentage error
(uncertainty/quantity measured) x 100
what is the percentage error of 1g if the measurement is to 1 decimal place
10%
what is the percentage error of 1g if the measurement is to 2 decimal places
1%
what is the uncertainty if the balance is to:
a) one decimal place
b) two decimal places
a) +/-0.1g
b+/-0.01g
what is the enthalpy change of combustion
the enthalpy change when 1 mole of a substance is burned completely in excess oxygen with all reactants and products in their standard states under standard conditions
how much does 100cm3 of water weigh
100g
what is the enthalpy change of neutralisation
the enthalpy change when an acid and alkali react to produce 1 mole of water
describe one way to investigate the rate of a precipitation reaction
place a piece of paper with a cross underneah the flask where the reaction is taking place and record the time it takes for the precipitate to obscure the cross
what is reflux
the continual boiling and condensing of a reaction mixture to ensure the reaction takes place without the contents of the flask boiling dry
why is it often necessary to heat the reactants under reflux for some time when preparing an organic liquid
Because the organic reactants contain strong covalent bonds
what are anti-bumping granules
small rough pieces of silica or unglazed pottery that are added to the reaction mixture before reflux
what is bumping
large bubbles forming at the bottom when a liquid boils and then moving abruptly upwards causing hazardous splashing
why should anti-bumping granules be used in distillation
Anti bumping granules must be used for smooth boiling in distillation because if bumping occurs the liquid can splash into the condenser causing an impure product or it could even blow the distillation apparatus apart
what is a risk assessment
a judgement of how likely it is that someone might come to harm if a planned action is carried out