c4 Flashcards
Describe a method to make pure, dry crystals of copper sulfate from a metal oxide and a dilute acid.
- use copper oxide and sulfuric acid
- add acid to beaker and warm
- add copper oxide with a spatula and stir
- keep adding until it is in excess
- filter mixture using filter paper and funnel to remove excess
- heat solution in evaporating basin until crystals begin to form
- leave solution to crystalise and pat dry with filter paper
Plan an investigation to find the order of reactivity of three metals.
You should use the temperature change when each metal reacts with hydrochloric acid. (6)
- measure out dilute hydrochloric acid into polystyrene cup
- using a thermometer measure initial temperature
- add metal, stir and measure highest temperature
- find difference in temperature and repeat with other metals
- metal with highest reactivity will have largest temperature change
- to ensure valid results use same volume and concentration of acid and same mass of metal
what is the reactivity series
potassium, sodium, lithium, calcium, magnesium, carbon, zinc, iron, hydrogen, copper
Describe what is happening during the electrolysis of molten PbBr2 (3 marks)
- lead ions are positively charged
- lead ions attracted to negative electrode (cathode)
- lead is formed
- bromide ions are negatively charged
- bromide ions are attracted to positive electrode (anode)
- bromine gas is formed
in molten electrolysis, if a halide is not present, what is formed?
oxygen
in molten electrolysis, if a halide is present, what is formed?
halogen
in molten electrolysis, if the metal is more reactive than hydrogen, what is formed?
hydrogen gas
in molten electrolysis, if the metal is less reactive than hydrogen, what is formed?
metal
what is the test for chlorine gas
hold damp blue litmus paper over gas. it will turn white
why does aluminium oxide need to be melted before electrolysis?
so ions are free to move to electrodes
what is aluminium oxide mixed with before electrolysis and why?
cryolite it has a high melting point so cryolite lowers it and saves energy
why do anodes need to be replaced regularly in the electrolysis of aluminium oxide?
Anodes must be regularly replaced because they are made from carbon which will react with the oxygen go at the anode so anode will lose carbon (wear away)
why does the blue colour of copper nitrate solution fade during the electrolysis (1)
copper ions are discharged from solution
when forming crystals, why is excess solid used instead of excess acid? (1)
because solid can be filtered off- separation of solid is easier
how would a student obtain crystals from solution? (2)
heat solution until crystallisation point is reached (until crystals start to form)
leave the solution to crystallise
explain why the pH of an avid depends on:
- the strength of the acid
- the concentration of the acid (4)
- pH depends on H+ ion concentration
- the higher the concentration of H+ ions, the lower the pH
- the stronger an acid, the greater the ionisation in aqueous solution so the lower the pH
- the higher the conc of acid, the more acid in the same volume of solution so the lower the pH
