Flashcards · 9202 · Topic 9

Energy changes flashcards

17 cards on Topic 9, subtopics 3.9.1 to 3.9.3. Pick a subtopic or study them all.

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Exothermic and endothermic reactions: Exothermic: transfers energy to the surroundings, which get hotter; ΔH negative. Endothermic: takes in energy from the surroundings, which get colder; ΔH positive.
Three types of exothermic reaction, and two everyday uses: Combustion, many oxidation reactions (e.g. rusting) and neutralisation. Uses: self-heating cans and hand warmers.
A type of endothermic reaction, and an everyday use: Thermal decomposition (e.g. calcium carbonate → calcium oxide + carbon dioxide). Use: sports injury packs that go cold (e.g. ammonium nitrate dissolving in water).
Hydrated copper sulfate ⇌ anhydrous copper sulfate + water: colours and energy changes: Heating: blue → white, endothermic. Adding water: white → blue and it gets hot, exothermic. The same amount of energy is transferred each way.

Q = mcΔT: what each symbol means, with units: Q: energy transferred (J). m: mass of water or solution heated (g) — not the fuel. c: specific heat capacity (4.2 J/g °C for water). ΔT: temperature change (°C).
0.50 g of a fuel heats 100 g of water by 15.0 °C. Energy released per gram? (c = 4.2 J/g °C): Q = 100 × 4.2 × 15.0 = 6300 J = 6.30 kJ. 6.30 ÷ 0.50 = 12.6 kJ/g
Why are calorimetry results usually lower than the true value?: Energy heats the container, thermometer and air; fuels burn incompletely; fuel evaporates from the wick; slow reactions lose more heat before the highest temperature is reached.
Reaction profile: how are activation energy and ΔH read, and what does a catalyst change?: Activation energy = peak − reactant level. ΔH = product level − reactant level. A catalyst gives a lower peak with the same start, finish and ΔH.
Bond breaking and bond making: energy and the overall sign: Breaking bonds takes energy in; making bonds gives energy out. Energy change = bonds broken − bonds formed: negative (exothermic) when more is released making bonds.
CO + H₂O → CO₂ + H₂. Bond energies: C≡O 1077, O–H 464, C=O 805, H–H 436 kJ/mol. Energy change?: Broken: 1077 + 2 × 464 = 2005. Formed: 2 × 805 + 436 = 2046. 2005 − 2046 = −41 kJ/mol (exothermic).

Chemical cell, and why it stops producing a potential difference: It contains reactants that react to transfer energy electrically. It produces a pd until a reactant is used up — not because it "runs out of electricity".
Fuel cell, and how it differs from a chemical cell: It produces electricity from the reaction of a fuel with oxygen, both supplied from outside, so it works as long as they keep being supplied. It is refuelled, not replaced.
Hydrogen–oxygen fuel cell: overall reaction, waste product and uses: 2H₂ + O₂ → 2H₂O, without burning. Water is the only waste product. Uses: cars and spacecraft.
Why do fuel cells cause less pollution only "when in use"?: Only water is released at the vehicle, but making the hydrogen may use fossil fuels or electricity from them, so pollution can be released elsewhere.
Two disadvantages of storing hydrogen as a fuel: It is a very low-density gas, so a useful mass has a huge volume and must be kept at high pressure in strong, heavy tanks (or cooled to a liquid); it is highly flammable and leaks can explode.
Why might a hydrogen engine release a pollutant that a fuel cell does not?: The engine burns hydrogen in air at high temperature, so oxides of nitrogen can form. The fuel cell does not burn it.
Mass of water made from 3.0 kg of hydrogen in a fuel cell (H = 1, O = 16): 3000 ÷ 2 = 1500 mol H₂ → 1500 mol H₂O. 1500 × 18 = 27 000 g = 27 kg (always 9 × the mass of hydrogen).