Chemical Changes
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Metal oxides and redox
Metals react with oxygen to form metal oxides. Metal oxides are bases.
In any reaction where one substance is oxidised, another is reduced.
Metal oxides and redox — Key Knowledge
- Oxidation gain of oxygen
- Reduction loss of oxygen
- Redox oxidation and reduction happening together
The reactivity series
Metals are ranked by how vigorously they react. A more reactive metal can displace a less reactive metal from its compound.
Carbon and hydrogen are included in the series as reference points for extraction and acid reactions.
The reactivity series — Key Knowledge
- Reactivity order potassium, sodium, lithium, calcium, magnesium, aluminium, carbon, zinc, iron, hydrogen, copper, silver, gold — most to least reactive
- Displacement more reactive metal replaces less reactive metal
Extraction of metals
The method used to extract a metal depends on its position in the reactivity series relative to carbon.
More reactive metals hold onto their oxygen more tightly, so carbon cannot remove it.
Extraction of metals — Key Knowledge
- Reduction with carbon metals less reactive than carbon, e.g. iron from iron oxide in a blast furnace
- Electrolysis metals more reactive than carbon, e.g. aluminium from molten aluminium oxide
Acids reacting with metals
Metals above hydrogen in the reactivity series react with dilute acids to produce a salt and hydrogen gas.
acid + metal → salt + hydrogen
The fizzing observed is hydrogen gas being released.
Acids reacting with metals — Key Knowledge
- Salt naming hydrochloric acid gives chlorides, sulfuric acid gives sulfates, nitric acid gives nitrates
- Unreactive metals copper, silver, gold do not react with dilute acids
Neutralisation reactions
Acids react with bases, alkalis and metal carbonates. Each reaction produces a salt.
acid + base → salt + water; acid + alkali → salt + water; acid + metal carbonate → salt + water + carbon dioxide
The H⁺ ions from the acid react with OH⁻ ions from the alkali to form water.
Neutralisation reactions — Key Knowledge
- Base a substance that neutralises an acid
- Alkali a soluble base that produces OH⁻ ions in water
Making soluble salts
To make a particular salt, choose the right acid and the right base, metal or carbonate. The metal in the base provides the first part of the salt name; the acid provides the second.
Excess insoluble base is added then filtered off to ensure all the acid has reacted.
Making soluble salts — Key Knowledge
- Naming rule metal/base gives first name, acid gives second — e.g. copper oxide + sulfuric acid → copper sulfate
The pH scale
The pH scale runs from 0 to 14 and measures how acidic or alkaline a solution is.
Each whole number step on the pH scale represents a tenfold change in H⁺ ion concentration.
The pH scale — Key Knowledge
- Acidic pH below 7, higher concentration of H⁺ ions
- Neutral pH 7, e.g. pure water
- Alkaline pH above 7, lower concentration of H⁺ ions
- Universal indicator changes colour across the pH range
- pH meter gives a numerical reading
Electrolysis — the process
Electrolysis uses direct current to decompose an ionic compound into its elements. The compound must be molten or dissolved so the ions are free to move.
In a solid ionic compound the ions are locked in a lattice and cannot carry charge.
Electrolysis — the process — Key Knowledge
- Electrolyte the ionic compound being decomposed
- Cathode negative electrode — attracts positive ions
- Anode positive electrode — attracts negative ions
- Direct current required — not alternating current
Electrolysis of molten compounds
Electrolysis of a molten ionic compound is straightforward — the metal is deposited at the cathode and the non-metal is released at the anode.
Aluminium is extracted industrially by electrolysis of molten aluminium oxide because it is too reactive for carbon reduction.
Electrolysis of molten compounds — Key Knowledge
- Metal at cathode positive metal ions gain electrons
- Non-metal at anode negative non-metal ions lose electrons
Electrolysis of aqueous solutions
Aqueous solutions are more complex because water molecules are also present, providing additional H⁺ and OH⁻ ions.
Electrolysis of brine (sodium chloride solution) produces hydrogen at the cathode and chlorine at the anode.
Electrolysis of aqueous solutions — Key Knowledge
- At the cathode hydrogen produced if the metal is more reactive than hydrogen; metal deposited if less reactive
- At the anode oxygen produced unless halide ions are present, in which case the halogen is produced