Chemical Changes Flashcards
what is the pH scale?
- measure of how acidic/alkaline solution is
the lower the pH scale…
the more acidic it is
the higher the pH scale…
the more alkaline it is
what has a pH of 1?
stomach acid
what has a pH of 3?
lemon juice
what has a pH of 8?
washing up liquid
what has a pH of 12?
bleach
what 2 things can you use to measure pH?
- indicator
- pH probe
how can you use an indicator to measure pH?
- changes colour depending on whether its above/below certain pH
- wide range indicators: contains mixture of dyes gradually change colour over pH
- useful for estimation
how can you use a pH probe to measure pH?
- measure electronically
- attached to pH probe in solution
- displays numerical value
- more accurate than indicator
what is an acid?
- forms aqueous solution of pH less than 7
- forms H+ ions in water
what is an alkali?
- base that dissolves in water to form solution
- with pH greater than 7
- forms OH- ions in water
what happens when acid mixes with base?
- neutralisation
acid + base
salt + water
what is the ionic equation of neutralisation?
H+(aq) +OH-(aq) –> H2O(l)
what pH is the product of a neutralisation reaction?
- pH 7
what is titration used for?
what can this then help you work out?
- find volume of acid needed ro neutralise a quantity of alkali
- concentration of acid/alkali
how do you carry out titration to find out the concentration of an alkali?
- using pipette and pipette filler, add set volume of alkali to conical flask
- add 2/3 drops of indicator
- use funnel to fill burette with some acid of known concentration
- record initial volume of acid in burette
- using burette, add acid to alkali gradually - regularly swirl flask
- go slow when you think end-point is near
- indicator changes colour when all alkali neutralised
- record final volume of acid left in burette to calculate volume of acid used to neutralise alkali
what is a safety precaution when carrying out titration?
- fill burettte with acid below eye level
- acid may spill over
- wear safety glasses
how do you increase the accuracy of titration?
- several consistent readings to spot anomalies
- first is rough to get approximate idea of end-point
- repeat to get same number within 0.1cm3
- calculate mean of results ignoring anomalies
what is universal indicator?
- mixture of different indicators
- estimate pH of solutions cuz can turn variety of colours
- each colour is narrow range of pH values
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what happens when UI is used in an acid or alkali?
goes from red to violet
why is indicator used in titration not UI?
want to see sudden colour change between alkali and acid
what does litmus change to in acids?
blue in alkali
red in acid
what does phenolphthalein change to in acids?
pink in alkali
colourless in acid
what does methyl orange change to in acids?
yellow in alkalis
red in acids
what happens when strong acids dissolve in water?
- ionise completely
- all particles dissociate to release H+ ions
what happens when weak acids dissolve in water?
- partially ionise in water
- small portion of particles dissociate to release H+ ions
what does it mean when ionisation of weak acids are reversable?
- equilibrium between dissociated and undissociated acid
- only few H+ so position of equilibrium to left (more reactants)
what is the difference between strong and weak acids of the same concentration?
- strong acids more reactive
- concentration of H+ higher so faster rate of reaction
what is acid strength?
-tells you what proportion of acid molecules ionise in water
what is concentration of acid?
what does it affect?
- measures how much acid there is in certain volume of water (how dilute)
- larger amount means more concentrated acid
- pH decreases
what does the pH tell you about the concentration of H+ in solution?
factor H+ ion changes by = 10-x
-pH of strong acid will always be less than weak acid of same concentration
what are examples of strong acids?
- sulfuric
- hydrochloric
- nitric
what are examples of weak acids?
- ethanoic
- citric
- carbonic
acid + metal oxide
salt + water
acid + metal hydroxide
salt + water
acid + metal carbonate
salt + water + CO2
how would you make copper chloride with hydrochloric acid and copper oxide?
- warm dilute acid with bunsen burner and turn off
- add insoluble base (CuO) to HCl bit at a time until no more reacts and base in excess
- filter out excess to get salt solution
- crystallisation
state the reactivity series
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- potassium
- sodium
- lithium
- calcium
- magnesium
- carbon
- zinc
- iron
- hydrogen
- copper
the higher the metal is in the reactivity series…
- more easily lose electrons to form positive ions
- more easily it reacts with water or acid
acid + metal
salt + hydrogen
how can you investigate the reactivity of metals?
- speed of reaction by rate at which hydrogen bubble are given off
- more reactive means faster reaction
- more reactive means greater temp change: use same mass and SA
metal + water
metal hydroxide + hydrogen
which metals react with water and which don’t?
-sodium, lithium, calcium do
zinc, iron, copper don’t
formation of metal ores involves…
oxidation
extraction of metal involves…
-reduction
how are metals which are more reactive than carbon extracted?
- electrolysis
- expensive
how are metals that are less reactive than carbon extracted?
- carbon
- e.g iron oxide reduced in blast furnace to make iron
- carbon can take away oxygen from less reactive metals
oilrig
- oxidation is loss
- reduction is gain
what is the rule of displacement reactions?
- a more reactive metal will displace a less reactive metal from its compound
- metal ion gains electrons so reduced to atom
- metal atom loses electrons so oxidised to ion
what happens during electrolysis?
- electric current passed through electrolyte (dissolved ionic compound)
- ions move towards electrodes to react and compound decomposes
where do cations in electrolysis move to?
- negative cathode
- gain electrons and reduced to metal atom
where do anions in electrolysis move to?
- positive anode
- lose electrons and oxidised to element
how can you extract aluminium from its bauxite ore using electrolysis?
- contains Al2O3 with high mp so mix with cryolite to lower
- molten mixture contains free electrons so it will conduct electricity
- positive Al3+ attracted to negative cathode where they each gain 3 electrons and reduce to Al atoms which sink to bottom
- negative O2- attracted to positive anode where they each lose 2 electrons and oxidise which combine to form O2
what happens at the negative cathode in aqueous solutions in electrolysis?
- if H+ and metal ions more reactive than H+ = hydrogen gas produced
- if less reactive = pure metal produced
what happens at positive anode in aqueous solution in electrolysis?
- if OH- and halide ions present = halide molecules formed
- no halide present = OH- and oxygen produced
