DSE Chemistry | Energetics Teacher Prep Guide | q = mcDT, enthalpy signs and 3-step Hess law

2026-11-04

Tutor lesson-prep guides · Secondary Chemistry

Students compute q = mcDT correctly, then fall apart on whether the enthalpy change is positive or negative. This lesson puts the sign first: decide exothermic or endothermic, then talk numbers.

Objectives — Students can: (1) compute the energy change in a calorimetry experiment with q = mcDT and convert to kJ per mol; (2) assign the correct sign and direction to exothermic and endothermic changes; (3) combine known equations with Hess law to reach a target equation.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Specific heat capacity	The value and units for water	4.18 J per g per K
Moles	Mass, moles and molar mass	n = m / M
Adding equations	What happens to enthalpy when equations add	Values add, signs follow the direction

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-6 min	Conservation of energy	Exothermic means surroundings warm; system enthalpy falls	Give three exothermic examples
6-16 min	q = mcDT	50 g water up 6.4 degrees: compute q, then convert	Get 1338 J
16-30 min	Convert to kJ per mol	0.025 mol of reactant maps to -53.5 kJ per mol	Redo the conversion
30-44 min	Hess law	Add known equations, add their enthalpy, flip signs on reversal	Build one target equation
44-56 min	Exam walk-through	A question mixing combustion and formation data	Write all three lines
56-60 min	Exit check	Three questions collected and marked	Independent work

03 Teaching script (read it out)

Scene 1: Decide the direction before the numbers

Before any calculation, ask whether the temperature rises or falls. Rise means exothermic, so enthalpy change is negative.
In calorimetry, q is the heat taken up by the water, so the reaction sign is the opposite of q. Get this wrong and even perfect arithmetic scores zero.

Students explain: magnesium burns and the temperature rises, so is the enthalpy change positive or negative, and why.

Scene 2: Three slots in q = mcDT

Use the mass of solution for m (50 g of water is 50 g), 4.18 J per g per K for c, and the temperature difference for delta T. The result is in joules; divide by 1000 for kJ, then divide by moles for kJ per mol.
A wrong slot costs an order of magnitude, so carry units on every line.

Students do: write the q calculation as three lines, each ending with its unit.

Scene 3: Hess law: add only, never invent

Put the target equation on the left, known equations on the right, and add what you need; 3 steps is normally enough. Reversing an equation flips the sign of its enthalpy; multiplying coefficients multiplies the enthalpy.
The classic failure is adding equations but forgetting to add the enthalpy values.

Students do: build the target from two known equations, naming each move (reverse, double, add) and updating enthalpy as they go.

04 Question types and worked steps

Type	Example	Steps	Answer
Calorimetry	50 g water up 6.4 degrees	q = mcDT, divide 1000, divide moles	1337.6 J, 53.5 kJ per mol released
Enthalpy sign	Burning magnesium warms up	Exothermic, so enthalpy is negative	Enthalpy below zero
Hess law	Two known equations to a target	Build, flip signs, update enthalpy	Enthalpies add
Bond enthalpy	Breaking and forming	Broken minus formed	Negative when exothermic

Sanity check (teach this): The q in calorimetry is heat absorbed by the water, so the reaction sign is opposite. Check the magnitude too: a few thousand kJ per mol means the division by moles was skipped.

05 Independent practice (3 questions)

Question	Answer	What to watch
Calorimetry	q = mcDT, then kJ, then per mol	Units on every line
Enthalpy sign	Exothermic or endothermic first	Reaction sign is opposite to q
Hess law	Build equations, update enthalpy	Reverse flips sign, scaling multiplies
Mean bond enthalpy	Breaking absorbs, forming releases	Enthalpy change = broken minus formed

06 Common mistakes and what to say

Mistake	Student writes	Say this
Confusing enthalpy sign with q	Writes +1337.6 for an exothermic reaction	“The water gained heat, so the reaction released it. Released heat is negative enthalpy; your plus sign describes the water.”
Forgetting to divide by moles	Reports joules as kJ per mol	“Joules are energy; kJ per mol is energy per mole. You skipped the division.”
Reversing an equation but not the sign	Flips the equation, copies the enthalpy	“Reversing means walking the same path backwards, so the energy direction reverses too.”
Subtracting 273 from a temperature difference	Treats a difference in degrees as a single reading	“A temperature difference is the same number in degrees Celsius and kelvin. The 273 is your own invention.”

07 Homework (paper, section and question numbers)

Secondary Chemistry chapter index — All chapters and practice
DSE Chemistry practice paper — Energetics questions
Redo this chapter — Redo the 50 g example and one Hess law question

08 Exit check (last 5 minutes)

Question	Answer	Decision
Sign of an exothermic enthalpy change	Negative	Positive means act one again
Steps from q to kJ per mol	Divide by 1000, then by moles	Skipping one means act two again
Reversing an equation in Hess law	Flip the enthalpy sign	Saying unchanged means act three again

09 Tutor FAQ

Why is 4.18 acceptable for solutions?

Dilute aqueous solutions are mostly water, so the error is small; careful questions supply their own value.

Must enthalpy carry units?

Yes, kJ per mol. A bare number is an incomplete answer.

Can bond enthalpies and combustion data be mixed?

Only for gaseous species; different states need the phase energy added first.

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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.