OCR GCSE Chemistry B (Twenty First Century)
OCR GCSE Chemistry B C3 Chemicals of the Natural Environment Exam Questions
Free OCR GCSE Chemistry B revision for C3 Chemicals of the Natural Environment: what you need to know, common mistakes and exam-style questions with answers. Covers metallic bonding, the reactivity series, metal extraction, electrolysis, crude oil, alkanes, cracking, alkenes, alcohols and carboxylic acids.
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About OCR GCSE Chemistry B C3 Chemicals of the Natural Environment
Chapter C3 looks at the raw materials chemists take from the Earth and how they are turned into useful products. In OCR Chemistry B you explain the properties of metals with metallic bonding, use the reactivity series to decide how a metal is extracted, and study electrolysis of molten and aqueous ionic compounds with half equations. The second half covers crude oil: fractional distillation, covalent bonding in hydrocarbons, alkanes, cracking, alkenes and simple organic families. It builds on bonding from C2 and leads into materials in C4. Examiners regularly ask for half equations, extraction reasoning and explanations of boiling point trends.
What you need to know
- C3.1 Metallic bonding Metals are a lattice of positive ions in a sea of delocalised electrons. This explains high melting points, electrical conductivity and why layers can slide, making metals malleable and ductile.
- C3.2 Reactivity and extraction The reactivity series is worked out from reactions with water, dilute acid and displacement. Metals less reactive than carbon, such as iron and zinc, are extracted by heating their oxides with carbon; more reactive metals such as aluminium are extracted by electrolysis. Bioleaching and phytoextraction can extract metals from low-grade ores.
- C3.3 Electrolysis An electrolyte is a molten or dissolved ionic compound. Positive ions gain electrons at the cathode (reduction) and negative ions lose electrons at the anode (oxidation). In aqueous solutions hydrogen may form at the cathode and oxygen or a halogen at the anode.
- C3.3 Aluminium Aluminium oxide is dissolved in molten cryolite to lower the operating temperature. Aluminium forms at the cathode and oxygen at the anode, which reacts with the carbon anodes so they must be replaced.
- C3.4 Crude oil Crude oil is a finite mixture of hydrocarbons separated by fractional distillation. Larger molecules have stronger intermolecular forces and higher boiling points. Alkanes have the general formula CnH2n+2.
- C3.4 Cracking and alkenes Cracking breaks long alkanes into shorter alkanes and alkenes using heat and a catalyst. Alkenes contain C=C, decolourise bromine water and can form addition polymers. Alcohols contain -OH and carboxylic acids contain -COOH and are weak acids.
Common mistakes in C3 Chemicals of the Natural Environment
- Saying the covalent bonds break when hydrocarbons boil. Boiling overcomes the weak intermolecular forces between molecules, not the covalent bonds.
- Mixing up the electrodes. Cations go to the cathode and gain electrons; anions go to the anode and lose electrons (OIL RIG).
- Writing half equations with electrons on the wrong side, such as Al3+ to Al + 3e-. Reduction at the cathode is Al3+ + 3e- to Al.
- Stating that aluminium is extracted with carbon. Aluminium is more reactive than carbon, so electrolysis is needed.
- Saying alkenes turn bromine water colourless from clear. Bromine water changes from orange to colourless with alkenes.
Try three C3 Chemicals of the Natural Environment exam-style questions
1. Explain why aluminium is extracted by electrolysis instead of by heating its oxide with carbon. (2 marks)
Show the mark scheme answer
Aluminium is more reactive than carbon; so carbon cannot reduce aluminium oxide (remove the oxygen).
2. Molten lead bromide is electrolysed. Write the half equation for the reaction at the cathode and state whether it is oxidation or reduction. (2 marks)
Show the mark scheme answer
Pb2+ + 2e- to Pb; reduction, because electrons are gained.
3. Decane, C10H22, is cracked to form octane, C8H18, and one other product. Write the balanced equation and name the type of compound formed. (2 marks)
Show the mark scheme answer
C10H22 to C8H18 + C2H4; ethene, an alkene.
C3 Chemicals of the Natural Environment FAQs
What does OCR Chemistry B C3 Chemicals of the Natural Environment cover?
It covers metallic bonding, the reactivity series and metal extraction, electrolysis with half equations, and crude oil chemistry including fractional distillation, alkanes, cracking, alkenes, alcohols and carboxylic acids.
Which paper is it examined in?
OCR Chemistry B has two papers at each tier, Breadth in chemistry and Depth in chemistry, and both can assess content from every chapter, so C3 can appear on either paper.
Which half equations should I learn for C3?
Practise cathode reactions for metal ions and hydrogen ions (2H+ + 2e- to H2) and anode reactions for halide ions, such as 2Cl- to Cl2 + 2e-. Always check charges balance on both sides.
