Chemical reactivity: Class 9 Chemistry

Key points

Elements are broadly metals and non-metals. Metals are usually shiny, hard, malleable (can be beaten into sheets), ductile (can be drawn into wires), good conductors of heat and electricity, and have high melting points. Non-metals are dull, brittle, poor conductors and have lower melting points. Mercury is a liquid metal, and graphite is a non-metal that conducts electricity.

Chemical reactivity is the tendency of an element to take part in chemical reactions. For metals it is the tendency to lose electrons and form positive ions. The more easily a metal loses electrons, the more reactive it is. For non-metals it is the tendency to gain electrons.

The reactivity series lists metals in order of decreasing reactivity: potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead, (hydrogen), copper, silver, gold. Potassium is the most reactive in this list and gold the least. A useful memory sentence is: Please Stop Calling Me A Zebra, I Like Her Cuddly Sister Girl.

Reaction of metals with oxygen (air):

• Metals form oxides when heated in air or burnt. Magnesium burns with a bright white flame: 2Mg + O₂ → 2MgO.
• Sodium and potassium react quickly with oxygen at room temperature and are kept under oil.
• Iron burns in oxygen as a powder or wool: 3Fe + 2O₂ → Fe₃O₄.
• Copper turns black when heated: 2Cu + O₂ → 2CuO. Gold does not react.
• Metal oxides are basic. Soluble ones give alkalis in water.

Reaction of metals with water:

• Potassium and sodium react violently with cold water to give hydroxide and hydrogen: 2Na + 2H₂O → 2NaOH + H₂.
• Calcium reacts steadily with cold water: Ca + 2H₂O → Ca(OH)₂ + H₂.
• Magnesium reacts very slowly with cold water but quickly with steam: Mg + H₂O → MgO + H₂.
• Zinc and iron react only with steam, and only when heated.
• Copper, silver and gold do not react with water or steam.

Reaction of metals with dilute acids:

• Metals above hydrogen in the series give a salt and hydrogen gas: Zn + 2HCl → ZnCl₂ + H₂.
• Magnesium reacts fast: Mg + H₂SO₄ → MgSO₄ + H₂. Iron reacts slowly.
• Metals below hydrogen, such as copper, do not react with dilute hydrochloric or sulphuric acid.
• The test for hydrogen is that it burns with a squeaky pop in a lighted splint.

Displacement reactions: a more reactive metal displaces a less reactive metal from a solution of its salt. Zinc added to copper sulphate solution gives zinc sulphate and copper: Zn + CuSO₄ → ZnSO₄ + Cu. The blue colour fades and a brown deposit of copper forms. Copper does not displace zinc from zinc sulphate, because copper is less reactive.

A thermite reaction is a displacement from an oxide by a more reactive metal: Fe₂O₃ + 2Al → Al₂O₃ + 2Fe. So much heat is given out that iron is formed molten, and the reaction is used to weld railway tracks.

Non-metals also differ in reactivity. Among halogens, reactivity decreases down the group: fluorine, chlorine, bromine, iodine. A more reactive halogen displaces a less reactive one from its salt solution. Chlorine passed into potassium bromide solution gives bromine, which turns the solution orange: Cl₂ + 2KBr → 2KCl + Br₂. Iodine does not displace chlorine.

Extraction of metals depends on reactivity. Very reactive metals (sodium, potassium, calcium, magnesium, aluminium) are extracted by electrolysis of their molten compounds. Metals of medium reactivity (zinc, iron, lead) are extracted by reduction of their oxides with carbon or carbon monoxide: Fe₂O₃ + 3CO → 2Fe + 3CO₂ in the blast furnace. Unreactive metals such as gold and silver occur native (free) or are easily obtained by heating their compounds.

An alloy is a mixture of a metal with one or more other elements, usually metals, made to improve properties. Brass (copper and zinc) is harder and stronger than copper. Bronze (copper and tin) is used for statues and bells. Steel (iron with a little carbon) is stronger than iron, and stainless steel has chromium and nickel to resist rust. Solder (tin and lead) melts at a low temperature and joins wires.

Study tip: Learn the reactivity series by heart and use it to answer any 'will it react' question: write the reacting pair and the products or write 'no reaction' with the reason. Always balance equations first in calculations, and write the mole ratio from the equation as a separate step.

Definitions and flashcards

Say the meaning first, then open the card to check.

Short questions with model answers

These are the kind of questions asked for 2, 3 and 5 marks. Try each one before opening the answer.

Numericals with worked steps

Common mistakes

• Saying that copper reacts with dilute hydrochloric acid to give hydrogen.
• Writing the products of metal and acid reactions as metal oxide and water instead of salt and hydrogen.
• Not balancing the equations before doing mole calculations.
• Believing carbon can extract all metals. Very reactive metals need electrolysis.
• Reversing the displacement order and claiming a less reactive metal displaces a more reactive one.

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Practice questions and timed test Printable worksheet