C4 - Chemical Changes Flashcards
What colour are acids on the pH scale?
Red / orange
What colour are alkalis on the pH scale?
Blue / purple
What does pH mean?
Power of hydrogen- a measure of the concentration of H+ ions in the solution.
What are 4 measuring instruments for pH?
- Litmus paper
- Phenolphthalein
- Universal indicator
- pH probe (goes in the solution) attached to a pH meter.
What colour does blue litmus paper turn in acids?
Red
What colour does blue litmus paper turn in alkalis?
Stays blue
What colour does red litmus paper turn in acids?
Stays red
What colour does red litmus paper turn in alkalis?
Blue
What is useful about universal indicator?
It matches a shade on the pH scale.
What is the benefit of measuring pH digitally?
It is more accurate and gives numerical values rather than a colour, which is harder to interpret.
What is the equation for neutralisation?
Acid + base = salt + water
How can neutralisation be shown in terms of hydrogen and hydroxide ions?
H+(aq) + OH-(aq) ⇒ H20(l)
What is the formula of sulfuric acid?
H2SO4
What is the formula of nitric acid?
HNO3
What is the formula of hydrochloric acid?
HCl
What is the formula of copper hydroxide?
Cu(OH)2
What is the formula of sodium hydroxide?
NaOH
What is the formula of magnesium oxide?
MgO
What makes a substance acidic?
H+ ions
What salt does sulfuric acid produce in neutralisation reactions?
Sulphate
What salt does hydrochloric acid produce in neutralisation reactions?
Chloride
What salt does nitric acid produce in neutralisation reactions?
Nitrate
What do acids donate in neutralisation reactions?
H+ ions
Why are strong acids more reactive than weak acids of the same concentration? What does this mean?
They dissociate more H+ ions so the rate of reaction is quicker
What makes an acid strong?
Strong acids ionise- dissociate hydrogen ions- completely in water. The percentage of dissociated H+ ions is higher per litre
What does a value of 1 on the pH scale mean in terms of hydrogen ions?
What does an increase in 1 mean?
There is one mole of dissociated hydrogen ions per litre of solution.
The concentration of dissociated H+ ions has decreased by a factor of 10.
Is the pH of a stronger acid lower or higher than of a weaker acid (for the same concentration in a solution)?
Lower
Name 3 strong acids
- Sulfuric
- Nitric
- Hydrochloric acid
Name 4 weak acids
- Ethanoic
- Citric
- Tartaric
- Carbonic acid
What is the difference between bases and alkalis?
Alkalis are soluble metal hydroxides. Bases are metal oxides or hydroxides and can neutralise acids. All alkalis are bases but not all bases are alkalis.
Potassium hydroxide + hydrochloric acid -> ______ + ______
Potassium chloride + water
What reacts to form copper chloride and water?
Hydrochloric acid and copper oxide.
What is the balanced symbol equation for the reaction between potassium hydroxide and sulfuric acid?
2KOH + H2SO4 → K2SO4 + 2H2O
What is the reactivity series?
It lists metals in order of their reactivity towards other substances
What is the order of the elements in the reactivity series, from most to least?
Potassium, sodium, lithium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper
What determines metals’ reactivity?
How easily they lose electrons (oxidise).
Acid + metal -> ______ + ______
Salt + hydrogen
______ + ______ -> salt + hydrogen
Acid + metal
Acid + metal → salt + hydrogen. What indicates the speed of this reaction?
The rate at which bubbles of hydrogen are given off, or the temperature change.
Metal + water -> ______ + ______
Metal hydroxide + hydrogen
______ + ______ -> metal hydroxide + hydrogen
Metal + water
Which metals will react with water?
The more reactive ones: potassium, sodium, lithium and calcium.
What is tarnishing?
Rusting - what happens to the surface of metals when they react with water.
What is oxidation?
1) A loss of electrons
2) When a metal reacts to bond with oxygen, forming an oxide.
What is reduction?
1) A gain in electrons
2) When a metal breaks a bond with oxygen.
How can metals be tested to see how reactive they are in comparison to others?
They can be tested in dilute acids
What does the reactivity of a metal depend upon?
Its tendency to give away outer electrons and form positive ions- oxidise
Why do metals have to be extracted?
Metals react with oxygen to form oxides (oxidation), forming ores from which they have to be extracted
What is the name given to a reaction that separates a metal from its oxide?
A reduction reaction
Acid + metal oxide -> ______ + ______
salt + water
______ + ______ -> salt + water
Acid + metal oxide/hydroxide
Acid + metal hydroxide -> ______ + ______
salt and water
Acid + metal carbonate -> ______ + ______ + ______
salt + water + carbon dioxide
______ + ______ -> salt + water + carbon dioxide
Acid + metal carbonate
Hydrochloric acid + sodium carbonate -> ______ + ______ + ______
sodium chloride + water + carbon dioxide
Give the equation for the oxidation of magnesium.
2Mg + O2 -> 2MgO
Which metals can be extracted from their ores chemically by reduction with carbon?
Metals below carbon in the reactivity series- zinc, iron, copper.
How do reduction reactions work?
Carbon reacts with a metal oxide to produce carbon dioxide and the pure metal. This only works if carbon is more reactive than the metal.
Describe the practical: making soluble salts using an insoluble base.
1) Fill a beaker with boiling water (heat gives energy which will speed up the reaction).
2) Measure 10ml of sulfuric acid into a test tube. Put the test tube into the beaker of water.
3) Allow the acid to heat up before adding copper oxide, which is an insoluble base.
4) The sulfuric acid and copper oxide react, producing a blue salt solution. Excess copper oxide sinks to the bottom.
5) Filter the salt solution from the excess copper oxide- this makes a filtrate.
6) Bring the solution almost to the boil over a bunsen burner, which makes the solution more concentrated (some water boils off) and gives it a higher temperature gradient so that more crystals form.
7) Place the solution into a petri dish; after time, it will become crystalline.
How can the reactivity of metals be tested?
1) Reacting them with acids: the violence indicates the metal’s reactivity. Increasing heat and concentration of acid can make the reaction faster. The rate of reaction is indicated by the rate at which hydrogen bubbles are released.
2) Reacting them with acid/water and measuring the temperature change. Temperature change will be greater with more reactive metals. Controls should be the time period, the mass and the surface area of the metal.
Give the balanced symbol equation for the extraction of iron by reduction with carbon.
2Fe2O3 + 3C → 4Fe + 3CO2
Which metals can be extracted by reduction with carbon?
Those less reactive than carbon, because carbon needs to bond with the oxygen. These are zinc, iron and copper.
Why has gold been mined for longer throughout history than other metals?
It is found in its elemental form, due to its low reactivity; it does not oxidise to form ores.
A gain of electrons =
Reduction
A loss of electrons =
Oxidation
What are redox reactions and why do they have this name?
A more reactive metal is put into the solution of a less reactive metal, which is displaced from its compound. re=reduction and ox= oxidation; these happen at the same time, hence redox.
Redox reactions are the same as…
Displacement reactions.
In displacement reactions, it’s always the metal ___ that is reduced (gains electrons) and the metal ___ that is oxidised (loses electrons).
1) Ion
2) Atom
What are spectator ions?
Ions which don’t change in a reaction
Write the ionic equation for this displacement reaction:
Iron sulfate + zinc →zinc sulfate + iron
1) Write symbol equation, including ions:
FeSO4(s) + Zn(aq) → ZnSO4(aq) + Fe(s)
Fe2+SO42- + Zn → Zn2+SO42- + Fe
2) Take out and ignore the spectator ion: SO42-
3) Write the ionic equation, including only the atoms/ions which are reduced or oxidised:
Fe2+ + Zn → Zn2+ + Fe
4) The iron ions are reduced by the zinc atoms.
5) The zinc atoms are oxidised by the iron ions.
What does electrolysis mean?
Splitting up with electricity
What is the negative electrode?
Cathode
What is the positive electrode?
Anode
What is an electrolyte?
A molten or dissolved ionic compound.
What type of power supply is used for electrolysis?
D.C. (direct current)
Why is electrolysis necessary for extracting some metals from their ores?
Some are too reactive for displacement reactions (more reactive than carbon).
What are ores?
Compounds in which most metals exist, combined with other elements, typically oxygen or sulphur
What happens in electrolysis?
An electric current is passed through an electrolyte. Each ion present is attracted to the electrode (charged metal plate) with an opposite charge. Ions are reduced/oxidised to become pure, uncharged elements
Positive ions will move towards the ___ and are ___
1) Cathode
2) Reduced
Negative ions will move towards the ___ and are ___.
1) Anode
2) Oxidised
Why is cryolite added to aluminium oxide during electrolysis?
It lowers the melting point of aluminium, reducing the energy cost
Give the half equation for chloride ions at the anode
2Cl- → Cl2 + 2e-
Is aluminium oxidised or reduced during electrolysis?
All metals are reduced
Is chlorine oxidised or reduced during electrolysis?
All non-metals are oxidised
What is the downfall of electrolysis?
It is expensive because it requires lots of energy to both produce the current and heat the ionic compound
What is different about electrolysis in aqueous solutions?
There are ions in the water too, which may react with the electrodes, depending on the reactivity of ions in the electrolyte
What is the rule for the reaction at the cathode in electrolysis of aqueous solutions?
Metal and H+ ions are present.
If hydrogen is less reactive than the elemental form of the metal, hydrogen gas is produced.
If hydrogen is more reactive, a solid layer of the pure metal forms on the cathode.
Whatever is less reactive reacts with the cathode
What is the rule for the reaction at the anode in electrolysis of aqueous solutions?
OH- and non-metal ions are present at the anode.
If halide (Cl-, Br-, I-) ions are present, halogens will be produced.
If not, oxygen gas and water will form
What are titrations?
Experiments which allow you to find the exact volumes of two solutions needed for them to react completely.
Can be used to find the unknown concentration of an acid/alkali, by finding the volume of this needed to neutralise a volume of alkali/acid of known concentration.
Describe how you would carry out a titration to find the concentration of an alkali needed to neutralise a volume of an acid of known concentration
1) Add a set volume of the alkali to a conical flask using a pipette and pipette filler, then add 3 drops of single indicator (phenolphthalein/litmus/methyl orange).
2) Wearing safety goggles, use a funnel to fill a burette with the acid of known concentration. Record the volume in the burette.
3) Use the burette to add the acid to the alkali a bit at a time, regularly swirling the flask. When all the alkali has been neutralised, the indicator changes colour. When this point is nearing, add only a drop at a time.
4) Once the colour change has occurred, record the final volume of acid in the burette and subtract it from the initial volume to find the volume of acid needed to neutralise the alkali.
5) Repeat steps 1-5 three times to increase the accuracy; your results should be within 0.1cm3 of each other. Omit any anomalous results and calculate the mean volume.
6) Calculate the concentration of the alkali (moles of acid = conc. x volume. Use ratios from balanced symbol equation to find moles of alkali. Conc. of alkali = moles/volume)
Why are single (rather than universal) indicators used for titrations?
You want to see a sudden and definitive colour change at the point when the acid/alkali has been neutralised
What colour does phenolphthalein turn in:
1) Acids?
2) Alkalis?
1) Acids: colourless
2) Alkalis: pink
What colour does litmus turn in:
1) Acids?
2) Alkalis?
1) Acids: red
2) Alkalis: blue
What colour does methyl orange turn in:
1) Acids?
2) Alkalis?
1) Acids: red
2) Alkalis: yellow
