TOPIC 3- chemical changes Flashcards
Acid
Substance w a pH of less than 7. Forms H+ ions in water.
Higher concentration of H+ ions, more acidic
Base
A substance that reacts with an acid to produce a salt and water.
Alkali
Soluble base that forms hydroxide ions (OH-) in water.
Higher concentration of OH-, higher pH.
Base + Acid»_space;
Salt + H2O
Indicator
Dye that changes colour depending on whether it’s above/below a certain pH.
How is litmus paper affected by pH?
Turns red in acidic solutions, purple in neutral, blue in alkali
How’s is Methyll orange affected by pH?
Red in acidic conditions, yellow in neutral/alkaline conditions.
How’s phenolphthalein affected by pH?
Colourless in acidic/neutral solutions and pink in alkaline conditions.
Neutralisation
Reaction between an acid and a base, producing salt and water.
What are the ions in a neutral solutions?
Hydrogen ions are equal to hydroxide ions.
Dissociation of strong acids in water.
Ionise almost completely w low pH’s (0-2).
Dissociation of weak acids in water
Don’t fully ionise in solution, pH of 4-6.
Acid strength
What proportion of acid molecules ionise in water.
Concentration
How much acid is in one litre (dm cubed)
How does changing concentration affect pH?
Concentration of H+ ions increase by factor of 10 and pH decreases by 1. Opposite effect for decrease in volume.
(Eg decrease of 10p = decrease of 2 pH)
Neutralisation reaction
Reaction between an acid and a base
Hydrogen ion
H+
Ammonium ion
NH4+
Hydroxide ion
OH-
Nitrate ion
NO3-
Oxide ion
O2-
Sulfide ion
S2-
Sulfate ion
SO42-
Carbonate ion
CO32-
Metal+ Acid»_space;
Salt + Hydrogen
Test for hydrogen
“Squeaky pop” test, putting lighted splint in test tube causes pop.
Carbonate+ acid»_space;
Salt+ H2O + CO2
Test for carbon dioxide
Bubble through limewater n will turn cloudy.
Solubility of sodium, potassium and ammonium ions
Soluble
Solubility of nitrates
Soluble
Solubility of chloride
Soluble
Except silver, barium and lead
Solubility of common sulfate
Soluble (except lead/ barium/ calcium)
Solubility of carbonates/ hydroxides
Insoluble
Except for sodium, potassium and ammonium
Which reaction can be used to make a pure, dry sample of insoluble salt?
Precipitation
2 soluble salts
Precipitation reaction method to make insoluble salt
- ) dissolve salts each into separate testubes of deionised water.
- ) tip both into a small beaker and stir (precipitate should form)
- ) pour through a filtered funnel into conical flask
- ) dry remaining precipitate from filter in a dessicator/oven.
2 methods of making insoluble salts
(Insoluble) Base+ acid»_space; Salt + water
Neutralisation reaction - acid + alkali > salt
How are soluble salts made when using a base + acid?
- ) heat acid in a water bath (speeds up reaction)
- ) add insoluble base to the acid (creates soluble salt + water)
- ) filter excess solid
- ) heat solution gently w b burner and leave to cool and so crystallise
Why’s it important to have excess base?
So there’s no leftover acid in the product.
How are soluble salts made in neutralisation reactions?
- ) measure set amount of acid into flask w pipette and add indicator.
- ) add alkali slowly to acid w burette, mixing until changes colour.
- ) carry out reaction w same volumes of alkali and acid (no indicator so is pure).
- ) solution left is salt and water which should be evaporated and so left to crystallise. Filter off and dry.
Why are titrations used?
To find exactly how much acid is needed to neutralise a given quantity of alkali (reversible)
What’s concentration measured in?
Moles per dm cubed
Concentration =
Moles number/ solution volume
What kind of reaction is the ionisation of a weak acid?
Reversible
Acid strength
What proportion of acid molecules ionise in water.
Acid concentration
How much acid is in a litre of water.
Opposite of concentrated
Dilute
What happens if H+ ions increase by a factor of ten?
Ph decreases by 1
Acid + metal hydroxide >
Salt + water
Acid + metal oxide»
Salt + water
Electrolysis
The breaking down of a substance using electricity.
Electric current is passed through an electrolyte, causing it to decompose.
Electrolyte
Molten / dissolved ionic compound
Cations
Positive ions
Anions
Negative ions
Where do cations go?
Towards the cathode to be reduced.
Cathode
Negative electrode
Anode
Positive electrode
Where do anions go?
Toward anode to be oxidised.
What happens to each ion when they reach an electrode?
They are discharged
Electrochemical cell
A circuit made up of anode, cathode, electrolyte, power source and wires connecting electrodes.
How do you set up an electrochemical cell if the electrolyte is a solution?
Clean 2 inert electrodes w Emery paper (sandpaper) and place both electrodes in beaker of electrolyte, connected to power supply.
How do you set up an electrochemical cell if the electrolyte is a molten ionic substance?
Put substance in crucible and heat w Bunsen burner until molten (in fume cupboard).
Dip 2 clean, inert electrodes into electrolyte and connect to power supply.
What happens at the cathode in aqueous solutions?
Hydrogen and metal ions are present.
Hydrogen gas is produced if metal is more reactive than hydrogen. If metal is less reactive, solid layer of pure metal is produced.
What happens at the anode in aqueous solutions?
Hydroxide and halide ions are present, therefore molecules of halides are formed.
Oxygen is formed if no halides are present.
Solution of copper sulfate ions
Cu2+
SO42-
OH-
H+
What happens to copper sulphate at cathode w inert electrodes?
Ionic equation?
Is less reactive than hydrogen, so is reduced to produce copper metal.
Cu2+ + 2e-»_space; Cu
What happens to copper sulphate at anode w inert electrodes?
Ionic equation?
No halide ions are present so oxygen is produced and oxidised.
4OH-»_space; 2H2O + O2 + 4e-
What happens to the mass of the anode in the electrolysis of copper sulfate with non-inert electrodes?
Mass of anode will decrease as copper is transferred from anode to cathode.
Will take longer time (eg 30 mins) to see change in mass
What happens to the mass of the cathode in the electrolysis of copper sulfate with non-inert electrodes?
It increases as copper is transferred from anode to cathode.
How does the electrical supply act in the non-inert electrolysis of copper sulfate?
Electrons are pulled off of copper atoms at the anodes and are offered at cathodes to copper ions.
How can copper be extracted from its ore?
Reduction w carbon
Is impure tho and so electrolysis is used to purify it.
What occurs in purification of copper via electrolysis?
Impure copper anode is oxidised, forming copper ions in electrolyte.
Ions are then reduced at cathode and are added as layer of pure copper.
Impurities sink to bottom as sludge.
