Endothermic and Exothermic Reactions: The Grade 11 Project
Some reactions get hot. Some get cold. The difference comes down to a single comparison: how much energy it takes to break the old bonds against how much is released making the new ones.
This page covers what endothermic and exothermic actually mean, why bond energies decide it, the four reactions in the Grade 11 formal assessment project, and the three chemicals on the textbook's own list that should not be in a school laboratory.
The definition, and the bit everyone gets backwards
| Energy | The surroundings | ΔH | |
|---|---|---|---|
| Exothermic | Released by the reaction | Get warmer | Negative |
| Endothermic | Absorbed by the reaction | Get colder | Positive |
"Exothermic" does not mean the reaction has more energy. It has less.
That is exactly why the surroundings have more. The products hold less chemical potential energy than the reactants did, and the difference has gone into warming everything nearby, which is what your thermometer measures.
It is the deepest misconception in this topic and it costs marks every year.
Why it happens: bond breaking and bond making
Every chemical reaction does two things.
- Breaking the bonds in the reactants takes energy IN
- Forming the bonds in the products gives energy OUT
Whichever is bigger decides what you feel.
| If | Then |
|---|---|
| More released forming bonds than absorbed breaking them | Exothermic. The surplus warms the surroundings |
| More absorbed breaking bonds than released forming them | Endothermic. The shortfall is taken from the surroundings |
Nothing is created or destroyed. Energy moves between chemical potential energy and thermal energy, and the total stays the same.
Enthalpy and the sign of ΔH
ΔH is the heat of reaction: the energy of the products minus the energy of the reactants.
- Exothermic: ΔH is negative. The products are lower in energy than the reactants, because energy left the system
- Endothermic: ΔH is positive. The products are higher in energy, because energy came in
The sign catches people out because it feels backwards. A reaction that makes things hot has a negative ΔH. Remember that ΔH describes what happened to the chemicals, not to the beaker.
Energy profile diagrams and activation energy
Draw the reactants on the left, the products on the right, energy up the page.
| Products sit | The curve | |
|---|---|---|
| Exothermic | Lower than the reactants | Ends below where it started |
| Endothermic | Higher than the reactants | Ends above where it started |
Both curves have a hump in the middle. That is the activation energy, the minimum energy needed to get the reaction started, and every reaction has one whether it is exothermic or not.
A reaction can release enormous energy and still need a push to begin. Petrol and air are wildly exothermic together and they sit in a tank all day. The spark supplies the activation energy.
Where this fits in the curriculum
| Subject | Physical Sciences |
|---|---|
| Grade | 11 |
| Topic | Energy and chemical change |
| Status | Formal assessment PROJECT |
| Marks | 40 |
This one is a project, not a practical investigation, and it changes how it is assessed. Learners are told to design their own methods, with the textbook supplying five suggestions as a guideline. So the marks are for planning, measuring, explaining and concluding, rather than for following instructions.
The four reactions
Part 1: Dissolving salts in water
Half-fill a beaker with water, take the temperature, stir in one spatula of a salt, feel the outside of the beaker, take the temperature again. Rinse everything and repeat with the next salt.
| Salt | Temperature | Type |
|---|---|---|
| Ammonium chloride | Falls several degrees | Endothermic |
| Potassium nitrate | Falls | Endothermic |
| Magnesium sulfate | Rises slightly | Exothermic |
The third one is in the method precisely because it goes the other way. Two salts cool the water and one warms it, which stops the class concluding that dissolving is always endothermic. It is not.
Touching the beaker matters. It is the only moment in the whole project where a learner feels the result instead of reading it off a scale.
Part 2: An acid and a base
Dissolve citric acid in water, then stir in sodium hydrogen carbonate a little at a time while tracking the temperature against the clock.
It fizzes hard and it gets noticeably colder. The temperature falls several degrees over a minute or two and then levels off.
Take a reading every 15 seconds, not every minute. Three points give a straight line and the shape of the curve is what the question is about.
Part 3: Rusting iron
Seal a thermometer through a rubber stopper into a dry test tube and read it after five minutes. Then soak steel wool in vinegar, squeeze it out, wrap it round the bulb, seal it back in and read again after five minutes.
The temperature rises a few degrees. Rusting is exothermic.
The vinegar is not incidental. It strips the protective layer off the steel wool so oxidation can start fast enough to see in five minutes. Without it, nothing happens.
Start this one first and walk away. It is ten minutes of waiting and there is no reason to stand over it while Parts 1 and 2 are undone.
Part 4: A metal and a metal salt
Add small pieces of magnesium to concentrated copper(II) sulfate solution until the blue colour disappears.
Mg(s) + CuSO4(aq) → MgSO4(aq) + Cu(s)
The blue fades, red-brown copper appears and the temperature climbs sharply. This is the most exothermic thing in the project, often ten degrees or more.
Open beaker only. Never in a closed container, which is the textbook's own warning.
Three chemicals on the textbook list that should not be in a school
The chapter lists fifteen chemicals. Only nine are used in its own five suggested methods. Three of the fifteen are a problem, and one of them is not needed at all.
Barium chloride: leave it out entirely
Acutely toxic. Fatal if swallowed. Soluble barium salts affect the heart at low doses.
It appears in the apparatus list and in none of the five suggested methods. Nothing in this project needs it. Do not buy it and do not let it near a school bench.
Ammonium chloride instead of ammonium nitrate
Ammonium nitrate is used in Part 1 and it works beautifully. It is also a Class 5.1 oxidiser and a restricted explosives precursor.
Ammonium chloride dissolves just as endothermically, gives just as clear a temperature drop, and is neither of those things.
If your school already has ammonium nitrate, use it. If you are buying, buy ammonium chloride.
Magnesium ribbon instead of magnesium powder
Part 4 calls for magnesium powder, and the textbook's own margin note reads "This reaction is violent."
Powder has an enormous surface area, so the reaction runs away and spatters. Ribbon cut into 5 mm pieces gives the same colour change and the same temperature rise at a rate you can actually watch.
And one thing in the list that does not exist
The apparatus list includes "ammonium hydroxide crystals". There is no such substance. Ammonium hydroxide is ammonia dissolved in water, and it is a solution.
Ammonium chloride is almost certainly what was meant, which resolves itself if you make the substitution above.
Where you have already met these reactions
Two of the four are sold in shops.
| Product | What is happening | Which part |
|---|---|---|
| Instant cold pack for a sports injury | A salt dissolving endothermically. Squeeze it, the barrier breaks, the salt meets the water and it goes cold | Part 1 |
| Hand warmer | Iron powder rusting in a permeable sachet. Exothermic | Part 3 |
| Self-heating tin of coffee | Calcium oxide and water. Exothermic | |
| Thermite welding of railway lines | Aluminium and iron oxide. Violently exothermic | |
| Respiration | Exothermic, and it is why you are warm | |
| Photosynthesis | Endothermic, driven by sunlight |
Buy a cold pack from a pharmacy for about R30 and read the ingredients. It is Part 1, in a bag.
Safety
- Goggles for the whole project, and Part 4 in particular
- Part 4 in an open beaker, never a closed container. The textbook says so and it is right
- No barium chloride. See above
- Potassium nitrate is an oxidiser. Keep it away from anything organic and give it its own spatula
- Copper(II) sulfate is harmful if swallowed and stains everything it touches
- Citric acid and sodium hydrogen carbonate fizz hard. Use a beaker with room to spare
- Do not taste anything, including the effervescent tablets the textbook mentions
If it does not work
| What you see | What caused it |
|---|---|
| No temperature change in Part 1 | Not enough solid, or it has not dissolved. Keep stirring |
| The change is tiny | Too much water. Half-fill a 100 cm³ beaker, not a 250 |
| Part 3 shows nothing after five minutes | The steel wool was not soaked in vinegar, or it was left dripping. Squeeze it out |
| Part 3 shows a fall instead of a rise | The wet steel wool was still cold. Squeeze harder and let it stand a moment |
| The blue never disappears in Part 4 | Not enough magnesium. Keep adding |
| Part 4 spatters | Magnesium powder was used instead of ribbon |
| The Part 2 graph is a straight line | Readings taken too far apart. Every 15 seconds |
| Every group gets different numbers | Normal. The size of the change depends on how much water and how much solid |
The failure worth keeping
A group that gets a temperature RISE with magnesium sulfate in Part 1 will usually assume they did it wrong.
They did not. Dissolving is not always endothermic, and magnesium sulfate is in the method for exactly that reason.
A learner who trusts a result that contradicts the pattern they had started to assume has done the most valuable thing in the whole project.
How the 40 marks are made up
| Section | Marks |
|---|---|
| Planning, questions and variables | 8 |
| Part 1, dissolution | 6 |
| Part 2, acid and base, with the graph | 8 |
| Part 3, rusting | 4 |
| Part 4, metal and metal salt | 6 |
| Part 5, research applications | 4 |
| Conclusion | 4 |
The textbook prints no mark allocation. It says the teacher assesses comprehending, synthesising, generalising, communicating results and drawing conclusions. The split above is ours.
Eight marks for planning is deliberate. This is a project and learners are told to design their own methods, so the planning is the thing being assessed.
The mark most often dropped is writing that an exothermic reaction "has more energy". It has less. Second most often: answering "it is endothermic because it absorbs heat", which is a definition rather than a justification. The mark is for pointing at your own falling temperature.
Free worksheet and marking memo
Both free, no sign up, straight to the PDF.
- Learner worksheet, 40 marks, with the planning section, all four results tables, the graph question and the applications research set out
- Marking memorandum, with worked sample results for all four parts, the balanced equation, model answers for the applications and a note on the seven places learners most often drop marks
Related practicals
- Rate of reaction, Grade 12. Same chemistry bench, and activation energy connects the two
- Intermolecular forces, Grade 11. Prescribed, and the other page where we tell teachers which chemical to leave out
- Boyle's Law, Grade 11
Buy this experiment
We are putting together an Energy and Chemical Change Kit for Grade 11 with the beakers, thermometers, test tubes, stopwatch and goggles in one box, plus the seven chemicals and a printed teacher guide. Coming shortly.
You supply the vinegar and the steel wool. Both are supermarket items, both are used only in Part 3, and both are cheaper and fresher bought locally on the morning.
And we will not be supplying barium chloride, for the reason above.