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Chemistry 30

Chemistry 30 · Worksheets

Mixed review · Thermochemistry and organic

Ten questions of mixed difficulty, covering Organic chemistry, Thermochemistry. Print it, or work through it on screen — the answer key starts on its own page.

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Mixed review · Thermochemistry and organic

Chemistry 30 · maddyhelps.com

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  1. How much heat is absorbed when 100.0 g of water is warmed from 20.0 °C to 30.0 °C? (c = 4.19 J/(g·°C))

    1. a) 4.19 kJ
    2. b) 12.6 kJ
    3. c) 0.419 kJ
    4. d) 41.9 kJ

  2. In the complete combustion of propane, C₃H₈ + __O₂ → 3CO₂ + 4H₂O, the coefficient of O₂ is

    1. a) 3
    2. b) 5
    3. c) 7
    4. d) 4

  3. Which set of units is used for a molar enthalpy of reaction?

    1. a) kJ
    2. b) J/(g·°C)
    3. c) kJ/mol
    4. d) mol/L

  4. Two compounds with the same molecular formula but different structures are

    1. a) allotropes
    2. b) isomers
    3. c) polymers
    4. d) isotopes

  5. Polyethene is formed from ethene by

    1. a) combustion
    2. b) addition polymerization, with no other product
    3. c) substitution, releasing hydrogen
    4. d) condensation polymerization, releasing water

  6. If the reaction A → B has ΔH = −120 kJ, then the reaction B → A has ΔH =

    1. a) −120 kJ
    2. b) 0 kJ
    3. c) +60 kJ
    4. d) +120 kJ

  7. Adding a catalyst to a reaction changes the

    1. a) ΔH, but not the activation energy
    2. b) both the activation energy and ΔH
    3. c) activation energy, but not ΔH
    4. d) neither one

  8. A ketone differs from an aldehyde because its carbonyl group is

    1. a) always attached to a benzene ring
    2. b) a double bond between two carbons
    3. c) bonded to carbons on both sides, rather than at the end of the chain
    4. d) bonded to a hydroxyl group

  9. A hydrocarbon is described as unsaturated when it

    1. a) contains only single bonds
    2. b) contains at least one double or triple bond between carbons
    3. c) dissolves readily in water
    4. d) contains an oxygen atom

  10. In a calorimeter, 0.0250 mol of a substance reacts and warms 50.0 g of water by 8.00 °C. The molar enthalpy of the reaction is closest to

    1. a) +67.0 kJ/mol
    2. b) −67.0 kJ/mol
    3. c) −1.68 kJ/mol
    4. d) −42.0 kJ/mol

Answer key · Mixed review · Thermochemistry and organic

Chemistry 30 · maddyhelps.com

  1. a) 4.19 kJ — q = mcΔT = (100.0 g)(4.19 J/(g·°C))(10.0 °C) = 4190 J = 4.19 kJ. ΔT is the change in temperature, not the final temperature.
  2. b) 5 — The products hold 3×2 = 6 oxygen atoms in CO₂ and 4 in H₂O, so 10 oxygen atoms are needed — that is 5 O₂ molecules. Complete combustion of a hydrocarbon always gives carbon dioxide and water.
  3. c) kJ/mol — Molar enthalpy is energy per mole of a named substance, so kJ/mol. Plain kJ is the energy for the equation as written, and J/(g·°C) is a specific heat capacity.
  4. b) isomers — Isomers share a formula but arrange the atoms differently, which is why C₄H₁₀ can be butane or methylpropane, with different properties. Isotopes are atoms of one element differing in neutrons.
  5. b) addition polymerization, with no other product — The double bonds in many ethene molecules open and link together into one long chain, with nothing left over. Condensation polymerization, which makes nylon and polyesters, releases a small molecule such as water at each link.
  6. d) +120 kJ — Reversing an equation reverses the sign of its enthalpy change. The energy released going one way has to be supplied to go back.
  7. c) activation energy, but not ΔH — A catalyst provides a lower-energy pathway, so the hump gets shorter. The reactants and products are unchanged, so the difference between them — ΔH — stays exactly the same.
  8. c) bonded to carbons on both sides, rather than at the end of the chain — Both families contain C=O. In an aldehyde it sits at the end of the chain with a hydrogen attached; in a ketone it is between two carbon atoms.
  9. b) contains at least one double or triple bond between carbons — Unsaturated means the carbons are not holding their maximum number of hydrogens, because some bonding is used up in a double or triple bond. Those bonds are also what make addition reactions possible.
  10. b) −67.0 kJ/mol — q = (50.0)(4.19)(8.00) = 1676 J = 1.68 kJ absorbed by the water, so the reaction released it: ΔH = −1.68 kJ ÷ 0.0250 mol = −67.0 kJ/mol. The water warming up is what tells you the sign is negative.