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
list some general safety rules you should carry out during an experiment
wear eye protection at all times
take care when handling hot apparatus
ensure there are no naked flames in the lab when using flammable substances
dispose of chemicals as directed, take particular care of disposing organic chemicals
what does a good risk assessment include
a list of all the hazards and why they are hazardous
a list of all the potential risks relating to what you do in the experiment
suitable control measures you could take that will reduce or prevent the risk
what does this sign mean
dangerous to the environment
what does this sign mean
toxic
what does this sign mean
gas under pressure
what does this sign mean
corrosive
what does this sign mean
explosive
what does this sign mean
flammable
what does this sign mean
caution (used for less serious health hazards like skin irritation)
what does this sign mean
oxidising
what does this sign mean
Longer term health hazards such as carcinogenicity
what is sodium hydrogen carbonate used for in solvent extraction
to remove acidic impurities in the organic liquid
why is pressure released when you shake the separating funnel in solvent extraction
because of the build up of carbon dioxide in an acid + carbonate reaction
how do you know which layer is the organic layer in solvent extraction
- refer to the density (more dense layer at the bottom)
- add water and observe which layer increases (aqueous layer)
what are immiscible liquids
liquids that do not mix and form two layers
why is the percentage yield in the preparation of an organic liquid less than 100%
theoretical reasons
- side reactions occur so by-products may be produced instead of the expected products
- the reagents used may be impure
- the reaction is incomplete
practical reason
- some product is lost in the purification steps (e.g. in washing, separating in the separating funnel and in transfer between apparatus)
- some product is lost in distillation
what colour will acidified potassium or sodium dichromate change when warmed with a reducing agent such as ethanol
they are oxidising agents so they will change from orange to green
what would you observe when you shake bromine water in a test tube where C=C (alkenes) are present
orange solution changes to colourless solution
How can you differentiate between primary/secondary alcohol or aldehyde vs. a tertiary alcohol using a test tube experiment
warm the samples with acidified potassium dichromate (vi)
- if there is a primary/secondary alcohol or aldehyde the orange solution will change to green
- if it is a tertiary alcohol the solution remains orange (it could not be oxidised)
How can you use Tollens reagent (ammoniacal silver nitrate) to differentiate between an aldehyde and a ketone
warm with the substance:
with an aldehyde you will observe a silver mirror
with a ketone the solution remains colourless
How can you use Fehling’s solution to differentiate between an aldehyde and a ketone
warm with the substance:
- with an aldehyde you will observe a red precipitate
- with a ketone the solution remains blue
you warmed 3 substances with silver nitrate solution in ethanol, one produced a white ppt, one produced a cream ppt, the other produced a yellow ppt. what are the three substances?
white ppt: chloroalkane
cream ppt: bromoalkane
yellow ppt: iodoalkane
you added sodium carbonate to a substance and you observed effervescence, the gas produced turns colourless limewater to cloudy. name the substance and the gas produced
carboxylic acid
carbon dioxide
what would you expect to observe if you add sodium carbonate or sodium hydrogen carbonate to carboxylic acid
effervescence, the gas produced will turn colourless limewater cloudy
(note that phenol is weakly acidic and can react with bases and metals but it is not acidic enough to react with carbonates)
what would you expect if you add magnesium to carboxylic acid
what gas is produced and how would you test it
effervescence
hydrogen - a pop is heard when a lighted splint is applied to the gas produced
what can you deduce if you warm a substance with ethanol and a few drops of conc. sulphuric acid and a sweet smell is noticeable
an ester has been produced - the organic compound could be a carboxylic acid
