S5 Chemistry: Enthalpy Change — Tutor Lesson Plan (45 min, q = mc dT, four errors)

2026-11-02

Tutor lesson-prep guides · Secondary chemistry (DSE)

Students write the right equation and lose the sign, or divide by the mass of the fuel instead of the water. This lesson fixes the three decisions in the calculation before any numbers appear.

Objectives — the student can calculate the heat released with q = mc dT, convert it to a molar enthalpy change with the correct sign, explain why an experimental value is smaller in magnitude than the data-book value, and distinguish bond breaking from bond making in energy terms.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Specific heat capacity	What does 4.2 J per g per degree mean?	4.2 J raises 1 g of water by 1 degree
Moles	How many moles in 2.0 g of ethanol, Mr 46?	0.043 mol
Energy units	Convert 1365 J to kJ	1.365 kJ

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Hook	Burn a few drops of ethanol under a test tube of water	Predict whether the water gets hotter or colder
5-14 min	Scene 1	Exothermic and endothermic with the sign convention written on the board	Assign signs to three reactions
14-25 min	Scene 2	q = mc dT with 50 g water and a 6.5 degree rise	Get 1365 J by hand
25-34 min	Scene 3	From joules to kJ per mole	Get 1365 kJ per mol
34-40 min	Scene 4	Why the measured value is too small in magnitude	Name two routes for heat loss
40-45 min	Exit check	Three decisions in section 08	Marked at once

03 Teaching script (read it out)

Scene 1: Two words, two signs

Exothermic: energy leaves the system, the surroundings warm up, delta H is negative. Endothermic: energy enters, the surroundings cool, delta H is positive. Combustion is exothermic, thermal decomposition of calcium carbonate is endothermic. The sign belongs to the system, not to the thermometer.

Ask: “The water gets hotter. Is delta H positive or negative?” Students who answer positive are following the thermometer, not the system.

Scene 2: One equation, three decisions

q = m c delta T, with m in grams, c = 4.2 J per g per degree for water, delta T in degrees. For 50 g of water and a rise of 6.5 degrees: q = 50 x 4.2 x 6.5 = 1365 J, which is 1.365 kJ. The three decisions are: which mass (the water, not the fuel), which sign, and whether the answer is per gram or per mole.

Warn: using the mass of the fuel is the most common single error in this topic.

Scene 3: Joules to kilojoules per mole

If 0.00100 mol of ethanol produced 1.365 kJ, then the enthalpy of combustion is 1.365 / 0.00100 = 1365 kJ per mol, written as -1365 kJ per mol because the reaction is exothermic. The data-book value is about 1366 kJ per mol, so this example is close; a school experiment usually gives a smaller magnitude because heat escapes to the air and to the apparatus.

Ask: “Our value is 900 instead of 1366. Is the reaction different?” No, the apparatus lost heat.

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	Find q for 50 g of water warmed by 6.5 degrees.	q = mc delta T, then convert to kJ	1365 J, 1.365 kJ
Calculation	Find the molar enthalpy of combustion from q and moles.	Divide kJ by moles, then attach the sign	-1365 kJ per mol
Explanation	Why is the experimental value smaller in magnitude?	Heat lost to the surroundings, incomplete combustion, evaporation of the fuel	Two named routes of loss

Sanity check (teach this): A molar enthalpy of combustion below 200 or above 4000 kJ per mol is almost always a unit or mole error. Check the arithmetic in this order: mass of water, temperature rise, division by 1000, then division by moles. If the sign is positive for a combustion, the student has described the surroundings.

05 Independent practice (3 questions)

Question	Answer	What to watch
25.0 g of water rises by 10.0 degrees. Find q in kJ.	25.0 x 4.2 x 10.0 = 1050 J = 1.05 kJ	Mass of water, not of fuel
0.0100 mol of a fuel gives 5.00 kJ. Find the molar enthalpy of combustion with sign.	-500 kJ per mol	Sign negative for combustion
Give two reasons the measured value is lower in magnitude than the data book.	Heat loss to surroundings and incomplete combustion	Two distinct reasons, not one written twice

06 Common mistakes and what to say

Mistake	Student writes	Say this
Using the mass of the fuel in q = mc dT	Use the mass of the water being heated	Ask which substance changes temperature
Giving a positive sign for combustion	Exothermic means delta H is negative	Ask where the energy goes: into the water or out of the fuel
Forgetting to divide by 1000	q comes out in joules; molar enthalpy is in kJ per mol	Write the unit at every step
Dividing by the mass of fuel instead of the moles	Molar means per mole, so convert grams to moles first	Ask for n before any division

07 Homework (paper, section and question numbers)

HKDSE Chemistry Paper 1 multiple-choice practice paper — Questions on energetics; then redo section 05 questions 1 to 3 of this guide
Secondary chemistry chapter index — Read the energetics chapter and the bond energy section

08 Exit check (last 5 minutes)

Question	Answer	Decision
Water gets colder as a salt dissolves. What is the sign of delta H?	Positive, the reaction is endothermic	All correct: move to bond energies
50 g of water rises by 6.5 degrees. Give q in kJ.	1.365 kJ	One wrong: repeat scene 2
Why divide by moles and not by grams?	Molar enthalpy is defined per mole of reaction	Two wrong: repeat scene 3

09 Tutor FAQ

Must students memorise 4.2 J per g per degree?

Yes, it is given in some papers but not all. It costs nothing to know and is used in every calorimetry question.

Do we need a bomb calorimeter?

No. A copper can and a thermometer are enough for the calculation; the gap to the data book is the teaching point.

How do bond energies fit in?

Bond breaking absorbs energy and bond making releases it. The net sign is exothermic when more energy is released than absorbed.

Find a tutor — free Tutor Registration →

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.