Unit 6 practice test: Thermochemistry

Step 1 breaks H–H (436 kJ) and Cl–Cl (242 kJ): ΔH₁ = +678 kJ, endothermic.

• −678 kJ, because breaking bonds releases energy: Bonded atoms attract each other; separating them takes energy, so the sign is positive.
• Correct: +678 kJ, because breaking bonds absorbs energy: Right: 436 + 242 = 678 kJ must be supplied to pull the atoms apart.
• +862 kJ, because two bonds are broken: The two bonds broken are H–H and Cl–Cl, not two H–Cl bonds.
• 0 kJ, because no new substance is made: Separating atoms changes the energy even though no product has formed yet.

Forming bonds releases energy: ΔH₂ = −2 × 431 = −862 kJ.

• +862 kJ, because forming bonds requires energy: Atoms that attract each other release energy as they bond.
• −431 kJ, because one H–Cl bond forms: Two HCl molecules form, so two H–Cl bonds: 2 × 431.
• Correct: −862 kJ, because forming two H–Cl bonds releases energy: Right: 2 × 431 = 862 kJ is released as the atoms bond.
• −184 kJ, because that is the overall change: −184 kJ is the overall ΔH, the sum of Steps 1 and 2, not Step 2 alone.

Total bonds formed (2 × 431 = 862 kJ) outweigh total bonds broken (436 + 242 = 678 kJ), so 184 kJ is released per mole of reaction.

• Correct: The H–Cl bonds formed are stronger in total than the H–H and Cl–Cl bonds broken.: Right: forming 862 kJ worth of bonds releases more than the 678 kJ needed to break the old ones.
• Breaking the H–H and Cl–Cl bonds releases more energy than forming H–Cl absorbs.: This reverses both signs: breaking absorbs and forming releases.
• Each H–Cl bond is stronger than each H–H bond, so less energy is needed overall.: An H–Cl bond (431) is actually weaker than an H–H bond (436); the totals decide.
• The reaction makes more molecules than it uses, which releases energy.: Two molecules react and two form; molecule count is not the reason.

ΔT = T(lowest) − T(start) = 19.04 − 22.40 = −3.36 °C. The lowest point is where the dissolving stops absorbing energy.

• −3.13 °C, from 22.40 °C to the last reading, 19.27 °C: The slow rise after 150 s is energy leaking in from the room, not part of the dissolving.
• Correct: −3.36 °C, from 22.40 °C to the lowest point, 19.04 °C: Right: the lowest temperature marks the end of the dissolving; after that, the room warms the cup.
• 19.04 °C, the lowest temperature reached: That is a temperature, not a change. ΔT = final − initial.
• −1.10 °C, the drop in the first 15 s after adding the solid: The temperature keeps falling until about 150 s, when the dissolving is complete.

q(dissolving) = −q(solution). The solution's temperature fell, so q(solution) < 0 and q(dissolving) > 0: endothermic.

• Correct: q for the solution is negative and q for the dissolving is positive.: Right: the solution lost energy (negative) and the dissolving gained the same energy (positive).
• q for the solution is positive and q for the dissolving is negative.: This would fit a temperature rise. Here the solution cooled.
• Both are negative, because the temperature went down.: Energy lost by one is gained by the other, so the signs are opposite.
• Both are positive, because energy was transferred.: Transfer means one side loses (negative) and one side gains (positive).

A foam cup slows energy exchange but does not stop it. A solution colder than the room slowly gains energy, which is why the lowest point, not the last reading, gives ΔT.

• The dissolved NH₄Cl starts releasing the energy it absorbed earlier.: Dissolving does not reverse by itself while the salt stays in solution.
• Correct: Energy from the warmer room leaks into the cold solution through the cup.: Right: once the dissolving is done, the solution is colder than the room, so energy flows in.
• The solid begins to dissolve faster, which warms the solution.: Dissolving NH₄Cl absorbs energy, so faster dissolving would cool the solution.
• The thermometer warms the solution because of its own heat capacity.: The thermometer is at the solution's temperature; the rise comes from outside the cup.