Topic 4: Chemical changes Flashcards
What is the pH scale?
A measure of how acidic or alkaline a solution is
What can be used to measure of pH of a solution? [2]
- Universal indicator
- pH probe
What do acids produce in aqueous solutions?
H⁺ ions
What do alkalis produce in water?
OH⁻ ions
What is an alkali?
A base that dissolves in water
What happens in a neutralisation reaction between an acid and an alkali?
Hydrogen ions react with hyroxide ions to produce water
What is a strong acid?
Completely ionised in aqueous solution (all acid particles dissosiate to release H⁺ ions)
What is a weak acid?
Only partially ionised in aqueous solution (small proportion of acid particles dissosiate and release H⁺ ions)
What is pH a measure of?
The concentration of H⁺ ions in a solution
For every decrease of 1 on the pH scale, how much does the concentration of H⁺ ions increase by?
A factor of 10
3 examples of strong acids
- Sulfuric acid
- Hydrochloric acid
- Nitric acid
3 examples of weak acids
- Ethanoic acid
- Citruc acid
- Carbonic acid
Why is a pH probe better to use than universal indictor?
Removes human judgement, making results more reliable
What is the neutralisation word equation?
Acid + base —> salt + water
What are the 3 common acids?
- Sulphuric acid
- Hydrochloric acid
- Nitric acid
What are the 2 common bases?
- Sodium hydroxide
- Calcium carbonate
What type of reaction do weak acids form?
Reversible reactions
Why does equilibrium lie to the left in a weak acid reaction?
Only a few particles release H⁺ ions
What is the equation for an acid + a metal oxide?
Acid + metal oxide —> salt + water
What is the equation for an acid + metal hydroxide?
Acid + metal hydroxide —> salt + water
What is the equation for an acid + metal carbonate?
Acid + metal carbonate —> salt + water + carbon dioxide
What is the colour change of litmus? acid —> alkali
Red —> blue
What is the colour change of methyl orange? acid —> alkali
Red —> yellow
What is the colour change of phenolphthalein? acid —> alkaline
Colourless —> pink
What is the equation for an acid + metal?
Metal + acid —> salt + hydrogen
What is the equation for a metal + water?
Metal + water —> metal hydroxide + hydrogen
How can you investigate the reactivity of metals? [2]
- Rate at which hydrogen bubbles are produced
- Temperature change
What is oxidation?
Gain of oxygen
What is reduction?
Loss of oxygen
What happens in a reduction reaction?
A metal is separated from its oxide
Which metals can be reduced using carbon?
Metals below carbon on the reactivity series
Why can some metals be reduced using carbon?
Carbon can take the oxygen from metals which are less reactive than carbon itself, leaving behind a pure metal
Why are metals such as gold only ever found as the metal itself?
They are too unreactive to react with things like oxygen
What do metals above carbon on the reactivity series have to be reduced by?
Electrolysis
What is the problem with electrolysis?
Very expensive
What gas is produced if a metal is reduced?
CO₂
What is oxidation in terms of electrons?
Loss of electrons
What is reduction in terms of electrons?
Gain of electrons
What is electrolysis?
Splitting up with electricity
What is an electrolyte?
A molten or dissolved ionic compound
Is the cathode positive or negative?
Negative
What kind of ions move to the cathode?
Positively charged ions
Is the anode negative or positive?
Positive
What kind of ions move to the anode?
Negatively charged ions
THINK: electrode? reduced/oxidised? released as?
Describe what happens when lead bromide (a simple ionic compound) is electrolysed in a molten state [2]
- Lead is attracted to the cathode, reduced and falls to the bottom as pure lead
- Bromine is attracted to the anode, oxidised and is released as bromine gas
In electrolysis, which way do electrons travel?
Anode —> cathode
Why are the electrodes in electrolysis usually made from inert carbon?
Inert carbon is unreactive, so will not interfere with the reaction
In electrolysis, why does the compound you’re trying to separate need to be molten or dissolved?
So the ions are free to move and can conduct electricity
Draw and check on back
Draw an electrolysis set-up? [5]
- Beaker
- Electrolyte
- Anode and cathode
- Wire
- Battery (small line towards cathode)
When would a metal be extracted from its ore by electrolysis? [2]
- If the metal is too reactive to be reduced with carbon
- If the metal reacts with carbon itself
What ore is alumunium extracted from?
Bauxite
What steps have to be taken before aluminium oxide can be electrolysed? [3]
- Purify aluminium oxide from bauxite
- Mix aluminium oxide with cryolite - to lower the melting temperature
- Melt it to make it molten
Why is cryolite mixed with aluminium oxide before electrolysis?
To lower the melting temperature
THINK: electrode? reduced/oxidised? released as?
Describe what happens when aluminium oxide (a metal ore) is electrolysed in a molten state [2]
- Aluminium is attracted to the cathode, reduced and falls to the bottom as pure metal
- Oxygen is attracted to the anode, oxidised and is released as oxygen gas
Write and check
Write the half equations for the electrolysis of aluminium oxide [2]
- Al³⁺ + 3e⁻ —> Al
- 2O²⁻ —> O₂ + 4e⁻
Why must the positive electrode (anode) be continually replaced?
It is made from carbon so it will start to react with oxygen to produce carbon dioxide
Why is electrolysis expensive?
It requires a large amount of energy
In aqueous solutions, what else is present as well as the ions from the compound? [2]
- Hydrogen ions
- Hydroxide ions
What is the general rule at the cathode in aqueous solutions?
Hydrogen gas will be produced if the metal ion is more reactive than hydrogen
What is the general rule at the anode in aqueous solutions?
If no halide ions are present, then the hydroxide ions will be discharged as oxygen and water
Describe what happens when a solution of copper sulfate is electrolysed [2]
- Copper and hyrogen ions are attracted to the cathode. Copper is less reactive than hydrogen so it is discharged and coats the electrode
- Sulfate and hydroxide ions are attracted to the anode. No halide is present so hydroxide ions are discharged as oxygen and water
Describe what happens when a solution of sodium chlorine is electrolysed [2]
- Sodium and hydrogen ions are attracted to the cathode. Sodium is more reactive than hydrogen so hydrogen gas is produced
- Chloride and hydroxide ions are attracted to the anode. A halide is present so chlorine gas is produced
