Practical Q's

suggest why ethanoic anhydride is preferred to ethanoyl chloride for preparing aspirin

less corrosive

role of H3PO4 in producing aspirin

catalyst

suggest why reflux is not essential when flask is heated to 85^oC for 10 minutes

boiling points of reactants above 85^oC

describe two ways that testing melting point would show a product is impure (producing aspirin)

• melting point lower than data book value

• melts over a wide range of temperatures

draw a diagram to show how an impure solid is filtered under reduced pressure

describe the process of recrystallisation for purifying aspirin/6

• dissolve crude product in minimum volume

• of hot solvent

• filter whilst hot

• leave to cool

• filter under reduced pressure with Buchner funnel

• wash with water and dry with pieces of filter paper

NaOH reacts with ethyl benzoate to produce benzoic acid

suggest why an excess of NaOH is used in making benzoic acid

ensures all the ethyl benzoate is completely reacted

state why reflux is used in this hydrolysis/1

prevents reactant vapours from escaping, returned to reactant mixture

suggest why sodium benzoate is soluble in cold water but benzoic acid is insoluble in cold water/2

• sodium benzoate is ionic and therefore very soluble

• benzoic acid has a non polar benzene ring making it insoluble

suggest why the percentage yield of recrystallisation was more than 100% /1

products were not dried properly

give two practical steps needed before recording the initial burette reading/2

• make sure there are no air bubbles

• remove the funnel

why might NaCl/brine be added to the immiscible liquids in rp10c?

to separate the aqueous and organic layer

how does the EMF of a cell change when the reduced metal increases?

increases

how does increasing the concentration of oxidised metal change EMF?

decreases, fewer electrons produced

how does increasing pressure of an EMF cell change EMF?

E value decreases

what equipment would you used to prepare a solution for an electrochemical cell?

volumetric flask

why should large and small masses of hydrated solids not be heated in a crucible?

decomposition likely to be incomplete

percentage uncertainty too high

why is there a lid on a crucible, and why is it loose

prevents loss of solid, allows gas to escape

why must crucible be dry when heating?

prevents additional mass

potential errors in a gas syringe

• syringe sticks

• soluble gases like CO2 and SO2 are dissolved in water, so true mass of gas unknown

• gas escapes before bung inserted

what can be rinsed with distilled water in titration and why?

conical flask, does not affect moles

why must acid be added for manganate titrations?

to supply H+ ions - not acidic enough MnO2 produced not Mn2+

errors in rp2 enthalpy change

• heat loss

• reaction incomplete

• density of solution is taken to be same as water

values that must be measured for combustion calorimeter

• mass of spirit burner

• volume of water

• temp. change of water

errors in calorimetry

• incomplete combustion

• heat loss

• evaporation of fuel after weighing

• heat capacity of calorimeter not included

why use vacuum filtration not gravity?

• faster, more efficient

• continuous process

• air drawn through filter funnel and pulls liquid down

• wash solid with cold solvent

• compress crystals - ensures air passes through crystals rather than around - better drying

how to improve validity of rp8 emf practical

• wipe electrodes with propanone to remove grease

• rub electrodes with emery paper to remove oxide layer

improving accuracy of rp9 acid-base titration

maintain constant temp.

when nitrating methylbenzene, why is the temperature kept very low?

prevents multiple substitutions of nitro groups onto benzene

the acids are also exothermic

why are MgSO4 and CaCl2 suitable drying agents?

they absorb water well and are insoluble and inert in organic liquids

why might measuring boiling point not be the most accurate method of identifying a substance

multiple substances may have the same boiling point

why does MCO3 with 2+ ions, but not M2CO3 with 3+ ions?

greater polarising power of 3+ ions with higher charge density

in an esterification reaction to work out Kc that is reversible, why might the phenolphthalein pink fade?

adding NaOH may shift eqm towards reactants

in a disappearing cross experiments, suggest why the student records the times to the nearest second and not to the nearest 0.01s /1

difficult to judge end-point to 0.01s