Chemical changes flashcards
15 cards on Chemical changes: the key points, worked examples and the slips to watch. Swipe through them, or print them.
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Chemical changes
The reactivity series
What order do the metals come in the reactivity series, and what does the order tell you?
Answer it in your head, then turn the card over.
The reactivity series
Key points
- Most to least reactive: potassium, sodium, lithium, calcium, magnesium, aluminium, zinc, iron, copper, silver, gold.
- Carbon fits between aluminium and zinc, and hydrogen between iron and copper.
- Higher metals lose their outer electrons to form positive ions more easily.
- Potassium, sodium, lithium and calcium react with cold water. Metals above hydrogen fizz in dilute acid; copper does not.
Watch out
Carbon and hydrogen are not metals: they sit in the series to compare with, for extraction and for reactions with acid.
Chemical changes
Metal displacement reactions
What is a displacement reaction, and how do you predict whether one will happen?
Answer it in your head, then turn the card over.
Metal displacement reactions
Key points
- A more reactive metal takes the place of a less reactive metal in its compound.
- If the metal added is less reactive than the one in the compound, nothing happens.
- Which metals displace which is a way to work out the order of the reactivity series.
An iron nail in copper sulfate solution
iron + copper sulfate → iron sulfate + copper
A brown coat of copper grows on the nail, and the blue solution fades to pale green.
Watch out
Copper in magnesium sulfate solution does nothing: a metal can only displace one below it.
Chemical changes
Extracting metals with carbon
How are metals such as iron extracted from their ores, and why not aluminium?
Answer it in your head, then turn the card over.
Extracting metals with carbon
Key points
- Most metals are found as compounds, such as oxides, in ores. Gold is so unreactive it is found as the metal itself.
- A metal below carbon in the reactivity series is extracted by heating its oxide with carbon.
- The carbon takes the oxygen: the metal oxide is reduced and the carbon is oxidised.
- A metal above carbon, such as aluminium, holds its oxygen too tightly, so it needs electrolysis, which costs far more.
Iron from iron oxide
iron oxide + carbon → iron + carbon dioxide
2Fe₂O₃ + 3C → 4Fe + 3CO₂
Watch out
Carbon can only take oxygen from a metal below it: it cannot reduce aluminium oxide.
Chemical changes
Oxidation and reduction
What are oxidation and reduction in terms of oxygen?
Answer it in your head, then turn the card over.
Oxidation and reduction
Key points
- Oxidation is the gain of oxygen. Burning and rusting are both oxidation.
- Reduction is the loss of oxygen.
- They always happen together: one substance gains the oxygen that another loses.
- In extraction, the metal oxide is reduced to the metal.
Watch out
When carbon removes oxygen from iron oxide, the iron oxide is reduced, not oxidised: the carbon is oxidised.
Chemical changes
HigherOxidation and reduction with electrons
What are oxidation and reduction in terms of electrons?
Answer it in your head, then turn the card over.
Oxidation and reduction with electrons
Key points
- OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain of electrons.
- A metal atom turning into a positive ion loses electrons, so it is oxidised.
- A metal ion turning back into the metal gains electrons, so it is reduced.
- In an ionic equation, ions that do not change (spectator ions) are left out.
Magnesium and copper sulfate
Mg + Cu²⁺ → Mg²⁺ + Cu
Magnesium loses 2 electrons: oxidised. Cu²⁺ gains 2 electrons: reduced.
Watch out
The sulfate ions do not change, so they are not in the ionic equation.
Chemical changes
How acids react
What does an acid make with a metal, a base and a carbonate, and how do you test the gases?
Answer it in your head, then turn the card over.
How acids react
Key points
- Acid + metal → salt + hydrogen. Hydrogen burns with a squeaky pop when a lit splint is held near it.
- Acid + base (a metal oxide or hydroxide) → salt + water.
- Acid + carbonate → salt + water + carbon dioxide. Carbon dioxide turns limewater milky.
- Only metals above hydrogen in the reactivity series react with dilute acids.
Example
zinc + sulfuric acid → zinc sulfate + hydrogen
calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
Watch out
Only a carbonate gives carbon dioxide: a metal gives hydrogen, and a base gives no gas at all.
Chemical changes
Naming salts
How do you name the salt an acid makes?
Answer it in your head, then turn the card over.
Naming salts
Key points
- A salt forms when the hydrogen in an acid is replaced by a metal.
- The metal gives the first part of the name, the acid the second.
- Hydrochloric acid makes chlorides, sulfuric acid makes sulfates and nitric acid makes nitrates.
Example
magnesium + hydrochloric acid → magnesium chloride + hydrogen
zinc oxide + sulfuric acid → zinc sulfate + water
copper oxide + nitric acid → copper nitrate + water
Watch out
Sulfuric acid makes sulfates, not sulfides: a sulfide has no oxygen in it.
Chemical changes
The pH scale
What does the pH scale show, and how do you measure pH?
Answer it in your head, then turn the card over.
The pH scale
Key points
- The pH scale runs from 0 to 14: below 7 is acidic, 7 is neutral, above 7 is alkaline.
- Acids release hydrogen ions, H⁺, in water. The more H⁺, the lower the pH.
- Alkalis release hydroxide ions, OH⁻, in water.
- Measure pH with universal indicator (red for strong acid, green for neutral, purple for strong alkali) or a pH probe.
Watch out
A lower pH means more acidic: pH 2 is more acidic than pH 5.
Chemical changes
Neutralisation
What happens in neutralisation, and what is the reaction between the ions?
Answer it in your head, then turn the card over.
Neutralisation
Key points
- Acid + base → salt + water. An alkali is a base that dissolves in water.
- The H⁺ ions from the acid react with the OH⁻ ions from the alkali to form water.
- The pH moves towards 7 as the acid and alkali use each other up.
- Indigestion tablets and lime spread on acidic soil both work by neutralisation.
Formula
H⁺(aq) + OH⁻(aq) → H₂O(l)
Example
hydrochloric acid + sodium hydroxide → sodium chloride + water
Watch out
The Na⁺ and Cl⁻ ions do not react: they stay in solution and make up the salt.
Chemical changes
HigherStrong and weak acids
What is the difference between a strong and a weak acid, and how is pH linked to the hydrogen ions?
Answer it in your head, then turn the card over.
Strong and weak acids
Key points
- A strong acid is fully ionised in water: hydrochloric, nitric and sulfuric acids.
- A weak acid is only partly ionised: ethanoic, citric and carbonic acids.
- Concentrated and dilute say how much acid is in a given volume, not how ionised it is.
- Each fall of 1 on the pH scale means 10 times as many hydrogen ions.
From pH 4 to pH 2
A fall of 2 on the pH scale
Hydrogen ion concentration × 10 × 10 = 100 times higher
Watch out
Strong does not mean concentrated: a dilute solution of a strong acid is perfectly possible.
Chemical changes
Making a soluble salt
How do you make pure, dry crystals of a soluble salt from an acid and an insoluble base?
Answer it in your head, then turn the card over.
Making a soluble salt
Key points
- Warm the acid, then add the base a little at a time, stirring, until no more reacts: some solid is left over.
- The excess base makes sure all the acid is used up.
- Filter off the leftover base.
- Heat the solution gently to evaporate some water, then leave it to cool so crystals form. Pat them dry.
Copper sulfate crystals
copper oxide + sulfuric acid → copper sulfate + water
CuO + H₂SO₄ → CuSO₄ + H₂O
Watch out
Do not boil the solution dry: slow cooling gives bigger, purer crystals, and boiling dry can spoil some salts.
Chemical changes
What electrolysis is
What is electrolysis, and what forms when a molten ionic compound is electrolysed?
Answer it in your head, then turn the card over.
What electrolysis is
Key points
- Electrolysis uses an electric current to break down an ionic compound that is molten or dissolved.
- Melting or dissolving lets the ions move. The liquid is the electrolyte.
- Positive ions go to the negative cathode; negative ions go to the positive anode (PANIC: Positive Anode, Negative Is Cathode).
- A molten compound gives the metal at the cathode and the non-metal at the anode.
Molten lead bromide
Cathode: Pb²⁺ ions gain electrons and form lead.
Anode: Br⁻ ions lose electrons and form bromine, a brown vapour.
Watch out
A solid ionic compound cannot be electrolysed: its ions are held in place and cannot move.
Chemical changes
Electrolysis of solutions
How do you predict the products when a solution is electrolysed?
Answer it in your head, then turn the card over.
Electrolysis of solutions
Key points
- A solution also holds H⁺ and OH⁻ ions from the water.
- Cathode: hydrogen forms if the metal is more reactive than hydrogen; otherwise the metal forms, such as copper.
- Anode: oxygen forms, unless there are halide ions (chloride, bromide, iodide): then the halogen forms.
- Tests: hydrogen pops with a lit splint, oxygen relights a glowing splint, chlorine bleaches damp litmus paper.
Sodium chloride solution
Cathode: hydrogen, as sodium is more reactive than hydrogen.
Anode: chlorine, as chloride ions are present.
Watch out
Sodium never forms from sodium chloride solution: it is too reactive, so hydrogen forms instead.
Chemical changes
HigherHalf equations at the electrodes
How do you write the half equations for what happens at each electrode?
Answer it in your head, then turn the card over.
Half equations at the electrodes
Key points
- At the cathode, positive ions gain electrons (reduction): the electrons go on the left.
- At the anode, negative ions lose electrons (oxidation): the electrons go on the right.
- Balance the charges: a 2+ ion needs 2 electrons.
Example
Cathode: Cu²⁺ + 2e⁻ → Cu
Cathode: 2H⁺ + 2e⁻ → H₂
Anode: 2Cl⁻ → Cl₂ + 2e⁻
Anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻
Watch out
Cu²⁺ → Cu + 2e⁻ is back to front: a positive ion gains electrons, so they go on the left.
Chemical changes
Extracting aluminium
How is aluminium extracted by electrolysis, and why is it so expensive?
Answer it in your head, then turn the card over.
Extracting aluminium
Key points
- Aluminium oxide is mixed with cryolite, which lowers its melting point and saves energy.
- Aluminium forms at the cathode and oxygen at the anode.
- The oxygen reacts with the carbon anodes to make carbon dioxide, so the anodes burn away and must be replaced.
- Melting the ore and running the current take huge amounts of electricity.
Watch out
The anodes are replaced because they react with oxygen, not because the electricity wears them out.
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