ARCH-C-10: Calorimetry (q = mcΔT, ΔH = -q/n), Hess's Law Cycles & Mean Bond Enthalpies (ΣE_broken - ΣE_formed)

Full 15-Q Timed Drill

Step 1 of 6 • The Core Idea

ARCH-C-10: Why the 150s Classroom Method Fails Under the 90s Clock

Standard Classroom Method (Too Slow)~150s (Blows 90s Clock)

First calculate the heat absorbed by the

100\text{ g}

of combined solution:

q = mc\Delta T = 100 \times 4.2 \times 6.0 = 2520\text{ J} = 2.52\text{ kJ}

. Next, find the moles of water formed:

n = 0.050\text{ dm}^3 \times 1.0\text{ mol dm}^{-3} = 0.050\text{ mol}

. Since the temperature rose, the reaction is exothermic (

\Delta H < 0

\Delta H = -\frac{2.52}{0.050} = -50.4\text{ kJ mol}^{-1}

(~115s).

The Fast ShortcutTarget: 40s (Saves 110s)

**Line 1 (Total Mass

100\text{ g}

):**

q = 100 \times 4.2 \times 6.0 = 2520\text{ J} = 2.52\text{ kJ}

.
Line 2 (Exothermic Sign & Moles):

\Delta H = -\frac{2.52}{0.050} = -50.4\text{ kJ mol}^{-1}

(25s).

Where Students Lose Time: In 19 past UAT-UK questions (8.44% of Chemistry), candidates lose 110s by reversing bendomex (breaking = endothermic +, making = exothermic -) or forgetting δh < 0 for a temperature rise. Instead, apply C11 with Maths 1 skill M1.2.

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