DSE Chemistry | Thermochemistry, Calorimetry and Hess Law: a tutor guide with three worked steps

2026-11-05

Students ask whether delta H is positive or negative before they ask where the energy went. The fix is an order of three moves: decide the direction of heat flow, find the heat absorbed by the water, then divide by moles. Once the order is fixed, the sign stops being a guess.

Objectives — Students can: (1) decide from a temperature change whether a reaction is exothermic or endothermic and give the sign of delta H; (2) complete a calorimetry calculation with m x c x delta T and divide by moles to get kJ per mole; (3) combine two known equations on an energy cycle to find an unknown delta H.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Moles	How many moles in 0.30 g of ethanol (Mr = 46)?	0.00652 mol
Specific heat capacity	Value and unit for water	4.2 J per g per degree C
Conservation of energy	Where does the released heat go?	Into the water and the container

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Two thermometers	Show one warming and one cooling experiment, ask which delta H is negative	Write a sign and a reason for each
5-15 min	Five-step calorimetry	Model mass, temperature rise, Q, moles, per mole	Repeat the same data themselves
15-23 min	Units and signs	Convert J to kJ and per gram to per mole	Correct three wrong units
23-35 min	Hess energy cycle	Draw the cycle, then add the equations	Draw before calculating
35-41 min	One exam question	Full run with every intermediate line	Work alone and self-mark
41-45 min	Exit check	Collect and mark three questions	Work alone

03 Teaching script (read it out)

Scene 1: Ask where the heat went before quoting a formula

Students memorise the sign of delta H as a fact, so the same reaction gets opposite signs in a multiple-choice item and a long question. Open with two thermometers: one warms, one cools. Ask whether heat travelled from the reaction to the water or the other way. The answer fixes the sign: heat leaving the reaction means exothermic and delta H is negative.
Write that sentence at the top of the board and make the class say it before every calculation.

Students do: label each experiment “heat goes from ___ to ___” before writing any sign.

Scene 2: Calorimetry: five steps, none skipped

Errors are rarely arithmetic. They are order errors: using the fuel mass as the water mass, or forgetting to divide by moles. Fix five steps: mass of water, temperature rise, Q = m c delta T, moles of fuel, delta H = minus Q over n.
Model it: 100 g of water rises from 25.0 to 35.0 degrees, so Q = 8400 J. Burning 0.3 g of ethanol (0.00652 mol) gives delta H = 1288.0 kJ per mole. Every student should press the calculator themselves, because a keying error is more common than a conceptual one.

Students do: write the five steps down the left margin and tick each one as it is completed.

Scene 3: Hess law: draw the cycle before adding numbers

Students treat Hess law as “add the enthalpies” and then lose the signs. Draw first: reactants left, products right, an alternative route drawn underneath, arrows pointing down meaning negative. Once the drawing is right, the arithmetic is just walking the arrows.
Use C + O2 to CO2 (-393) and CO + half O2 to CO2 (-283) to find C + half O2 to CO: reverse the second, flip its sign, add, and get -110 kJ per mole. Say the reversal out loud every time, or students will copy numbers without flipping.

Students do: draw all three cycles and cross out the arrow that was reversed.

04 Question types and worked steps

Type	Example	Steps	Answer
Sign of enthalpy	Temperature rises; state the sign of delta H	Ask where heat flows, then assign the sign	Exothermic, delta H negative
Calorimetry	100 g water rises 20 degrees, 0.30 g ethanol burnt	Five steps, Q before moles	1288.0 kJ per mole
Hess law	Find a third enthalpy from two equations	Draw, reverse, add	-110 kJ per mole
Error analysis	The value is smaller than the data book value; why?	Split heat loss from incomplete combustion	Calculated -1288 against -1367, about 5.8 percent low

Sanity check (teach this): Sanity check: the fuel mass is never the water mass; the unit is kJ per mole, not kJ; the divisor is moles of fuel, not moles of water. The calculated 1288 sits about 5.8 percent below the data book value of -1367 and in the correct direction, since heat loss always makes calorimetry low. A student who reports a more negative value has swapped a mass or a mole count.

05 Independent practice (3 questions)

Question	Answer	What to watch
Sign decision	Three experiments, give the sign and the reason	Check that the reason mentions heat flow
Calorimetry	100 g water, 20 degree rise, 0.3 g ethanol	Check division by moles of fuel
Hess law	Find C + half O2 to CO from two equations	Check the reversed equation had its sign flipped

06 Common mistakes and what to say

Mistake	Student writes	Say this
Fuel mass used as water mass	Q = 0.30 x 4.2 x 20	“4.2 belongs to water. Which substance are you heating right now?”
No division by moles	Answer written as 8.4 kJ per mole	“8.4 kJ is for 0.3 g. The question asks per mole. Where is your mole count?”
Hess law without reversing	-393 plus -283	“Did you reverse the second equation? Which way does its arrow point now?”
Unit written as kJ	delta H = -1287.9 kJ	“Where did per mole go? Without mol to the minus one you lose the mark.”

07 Homework (paper, section and question numbers)

Redo this chapter (Paper 1B Q4-Q6 style) — Two calorimetry questions, all five steps, question numbers written
Secondary chemistry index — Other chapters and their paper question types
All prep guides — The same ten-section structure in other subjects

08 Exit check (last 5 minutes)

Question	Answer	Decision
Sign of delta H	Negative when exothermic, positive when endothermic, with the heat-flow reason	Sign only means repeat act one
Five calorimetry steps	m, delta T, Q, n, delta H	Missing division means repeat act two
First Hess step	Draw the cycle, then reverse	Adding numbers straight away means repeat act three

09 Tutor FAQ

Should calculators be banned when arithmetic keeps failing?

No. Require each intermediate value to be written before it is keyed in, so errors show as copying rather than keying.

Do students need the data book values?

Not by heart, but they must know calorimetry always reads low and why.

Can Hess law be done by formula only?

It can, but students who draw first flip the wrong sign about half as often.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.