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    Unit 7 Chemical Changes: Summary Questions

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    Lei Yin

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    Unit 7 Chemical Changes: Summary Questions

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    Lei Yin
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    Ess-Chem-Unit-7-Answers

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    45 views2 pages

    Unit 7 Chemical Changes: Summary Questions

    Uploaded by

    Lei Yin

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    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
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    E s s e n t i a l C h e m i s t r y f o r C a m b r i d g e I G C S E ® 2 nd E d i t i o n

    Unit 7 Chemical changes


    SUMMARY QUESTIONS:
    1 (a) Exothermic
    (b) Endothermic
    (c) Endothermic
    (d) Exothermic
    2 Endothermic – a reaction which absorbs heat energy
    Exothermic – a reaction which gives out heat energy
    Isotopes – atoms of the same element having different numbers of neutrons
    Radioactive – an atom giving out energy as waves or particles
    Combustion – burning in oxygen
    3 Uranium; carbon; pollutant; oxygen; water; decay; energy; turbine; electricity
    4 Bond breaking – endothermic
    Bond making – exothermic
    Coal – fossil fuel
    Electrochemical cell – portable source of energy
    Uranium–235 – a radioactive isotope

    PRACTICE QUESTIONS:
    1 B The reaction absorbs heat energy
    2 D Hydrogen
    3 (a) Exothermic
    (b) Take temperature of the acid and alkali separately; add them together in a beaker;
    record temperature after mixing.
    (c) HCl and KOH on left dotted line; KCl and H2O on right hand dotted line; 20 kJ by red arrow.
    (d) 40 kJ
    4 (a) (i) Sodium hydroxide OR calcium chloride
    (ii) Potassium nitrate
    (b) Endothermic
    (c) Sodium hydroxide
    (d) (i) 10 °C
    (ii) 40 °C

    © OUP 2015: this may be reproduced for class use solely for the purchaser’s institute
    E s s e n t i a l C h e m i s t r y f o r C a m b r i d g e I G C S E ® 2 nd E d i t i o n

    5 (a) Releases heat energy (on reaction).


    (b) Bond breaking is endothermic; bond making is exothermic; more energy released on
    forming bonds than in breaking bonds.
    (c) As in Figure 7.1.2a with CH4 + 2O2 in place of ‘reactants’, CO2 + 2H2O in place of products
    and arrow pointing down; reactants on left and products on right; product level below
    reactant level; arrow downwards showing exothermic reaction.
    (d) (i) 4 × 435 = 1740
    2 × 498 = 996
    total = 2736 kJ
    (ii) 2 × 805 = 1610
    4 × 464 = 1856
    total = 3466 kJ
    (iii) −3466 + 2736 = −730 kJ (ignore wrong sign)
    6 (a) Any three of:
    Hydrogen burns to give water; water is non-polluting; not a greenhouse gas; more energy
    per gram with hydrogen; fossil fuels form carbon dioxide when burned; carbon dioxide is a
    greenhouse gas.
    (b) 2 × 436 = 872 kJ to break bonds of hydrogen, 498 kJ to break the bonds in oxygen.
    Total = 1370 kJ for breaking bonds in reactants; 4 × 464 = 1856 kJ released when
    water forms; −1856 + 1370 = −486 kJ released (ignore sign).
    (c) (i) O2 + 2H2O +4e− → 4OH−
    (ii) 2H2 + 4OH− → 4H2O + +4e−
    (iii) At cathode / negative electrode / negative pole electrons are released; at anode /
    positive electrode / positive pole electrons are taken up / positive pole less good at
    releasing electrons; electrons move around circuit from where there are more electrons
    (or greater electron density of electrons) to where there are less electrons (or lower
    electron density) / electrons move in the direction to equalise the electron density.
    7 (a) Aqueous potassium nitrate / (dilute) sulfuric acid
    (b) Zinc higher in the electrochemical / reactivity series; zinc better at releasing electrons
    (or reverse argument).
    (c) Electron density greater on zinc than on copper; electrons move in direction to equalise
    electron density / move from high to low electron density.
    (d) (i) Voltage decreases
    (ii) Voltage increases
    (iii) No difference in electron density (or electrode potential) on electrodes / each electrode
    has same ability to release electrons.

    © OUP 2015: this may be reproduced for class use solely for the purchaser’s institute

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