GCSE Chemistry Flashcards

24 sample cards from the Chemistry deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. Write the half equations for the electrolysis of molten aluminium oxide.

   Cathode: $\ce{Al^3+ + 3e- -> Al}$. Anode: $\ce{2O^2- -> O2 + 4e-}$. The carbon anodes burn away as they react with oxygen.

2. Define exothermic and endothermic reactions in terms of energy transfer.

   An exothermic reaction transfers energy to the surroundings (temperature rises), e.g. combustion. An endothermic reaction takes in energy from the surroundings (temperature falls), e.g. thermal decomposition.

3. On a reaction profile, how do you tell an exothermic from an endothermic reaction?

   In an exothermic reaction the products are at a lower energy than the reactants ($\Delta H$ negative). In an endothermic reaction the products are at a higher energy than the reactants ($\Delta H$ positive).

4. What is activation energy?

   The minimum energy that colliding reactant particles must have for a reaction to occur; on a reaction profile it is the height from the reactants up to the peak of the curve.

5. How is the overall energy change of a reaction calculated from bond energies?

   $$\Delta H = \sum E_{\text{bonds broken}} - \sum E_{\text{bonds made}}$$ Breaking bonds is endothermic; making bonds is exothermic.

6. What is a hydrocarbon, and what is the general formula of the alkanes?

   A hydrocarbon is a compound containing only hydrogen and carbon atoms. Alkanes are saturated hydrocarbons with the general formula $\ce{C_{n}H_{2n+2}}$, e.g. methane $\ce{CH4}$.

7. Describe how the properties of hydrocarbons change as the molecules get larger.

   As chain length increases: boiling point rises, viscosity increases, flammability decreases, and the fractions become less volatile.

8. How does fractional distillation separate crude oil?

   Crude oil is heated and vaporised, then enters a fractionating column with a temperature gradient (hot at bottom, cool at top). Hydrocarbons condense and are collected at the height where the temperature matches their boiling point.

9. Write the general equation for the complete combustion of a hydrocarbon.

   Hydrocarbon + oxygen $\rightarrow$ carbon dioxide + water. E.g. $$\ce{CH4 + 2O2 -> CO2 + 2H2O}$$ Both carbon and hydrogen are oxidised.

10. What is cracking and why is it carried out?

    Cracking breaks long-chain hydrocarbons into shorter, more useful ones using heat with a catalyst (catalytic) or steam. It produces smaller alkanes (fuels) and alkenes (for polymers) to match demand.

11. What is the general formula of alkenes and how are they tested for?

    Alkenes are unsaturated hydrocarbons with a $\ce{C=C}$ double bond, general formula $\ce{C_{n}H_{2n}}$. They decolourise orange bromine water (from orange to colourless).

12. Describe how addition polymers are formed from alkene monomers.

    Many small unsaturated monomers with $\ce{C=C}$ double bonds join together in an addition reaction, the double bonds opening to form a single long saturated polymer chain, e.g. ethene forms poly(ethene).

13. How does condensation polymerisation differ from addition polymerisation?

    Condensation polymerisation involves monomers with two functional groups reacting and joining together with the loss of a small molecule (usually water) for each bond formed. Addition polymerisation loses no small molecule and uses monomers with a $\ce{C=C}$ bond.

14. State the functional group of alcohols and give the first four members.

    The functional group is $\ce{-OH}$ (hydroxyl). The first four are methanol, ethanol, propanol and butanol; e.g. ethanol is $\ce{C2H5OH}$.

15. What are the products when ethanol is oxidised, and what functional group do carboxylic acids contain?

    Ethanol is oxidised to ethanoic acid (a carboxylic acid). Carboxylic acids contain the $\ce{-COOH}$ functional group; e.g. ethanoic acid is $\ce{CH3COOH}$.

16. Define a pure substance in chemistry and describe how purity is checked using melting point.

    A pure substance is a single element or compound not mixed with any other substance. A pure substance melts (and boils) at a specific sharp temperature; impurities lower the melting point and make it melt over a range.

17. What is a formulation? Give two examples.

    A formulation is a mixture made in specific, measured quantities so that each component gives the product the required properties. Examples include paints, fuels, medicines, cleaning products, alloys and fertilisers.

18. Explain how paper chromatography separates a mixture.

    Components dissolve in the mobile phase (solvent) and move up the paper (stationary phase). More soluble components that are less attracted to the paper travel further, so the mixture separates into spots.

19. How is an $R_f$ value calculated in chromatography?

    $$R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}}$$ The value is always between 0 and 1 and is characteristic of a substance in a given solvent.

20. How do you use chromatography to tell whether a substance is pure?

    A pure substance produces only one spot on the chromatogram in all solvents, whereas a mixture separates into two or more spots.

21. Describe the flame test colours for lithium, sodium, potassium, calcium and copper ions.

    Lithium: crimson/red. Sodium: yellow. Potassium: lilac. Calcium: orange-red. Copper: green (blue-green).

22. How do you test for and identify the gas carbon dioxide?

    Bubble the gas through limewise (calcium hydroxide solution); carbon dioxide turns the limewater cloudy/milky: $$\ce{Ca(OH)2 + CO2 -> CaCO3 + H2O}$$

23. Describe the tests for hydrogen, oxygen and chlorine gases.

    Hydrogen: a lighted splint gives a squeaky pop. Oxygen: relights a glowing splint. Chlorine: bleaches damp blue litmus paper (turns it white, sometimes red first).

24. How are carbonate, sulfate and halide ions identified in solution?

    Carbonate: add dilute acid, effervescence of $\ce{CO2}$ that turns limewater cloudy. Sulfate: add dilute HCl then barium chloride, a white precipitate forms. Halide: add dilute nitric acid then silver nitrate, giving white ($\ce{Cl-}$), cream ($\ce{Br-}$) or yellow ($\ce{I-}$) precipitate.

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