Energy changes in reactions · Investigation
Measuring a temperature change
Four groups run the same reaction and get four different answers. One of them is closest to right — and it is not the one with the neatest handwriting.
Start here
Same acid, same alkali, same volumes, same room. Four groups report +5, +7, +2 and +7 °C.
Nobody made anything up. All four wrote down what their thermometer said. The reaction cannot have released a different amount of energy in four beakers on the same bench.
What is the most likely reason for the spread?
How much heat each beaker lost to the room before the thermometer was read. A glass beaker on a cold bench with an open top is leaking energy the whole time the reaction is running, and the slower the group, the more escapes before they look. The reaction is not the variable. The measurement is.
Energy change is measured by measuring temperature — and a temperature reading is only as good as the arrangement around it. Heat leaks out of an open beaker, into the bench, and into the thermometer itself.
This lesson is about the decisions that decide whether your number means anything: what to measure, what to keep the same, and what to insulate against.
Before you run it · judge someone else's plan
A student has written a method. Five decisions. Rule on each.
0 of 5 ruled on
Two of the five are sound, two are flawed, and one would wreck the result on its own.
Step 1
Measure 25 cm³ of acid into a glass beaker.
Flawed — sound as far as it goes, but the glass beaker is the weak point.
Glass conducts heat away into the bench and the air, and it absorbs a share of the energy warming itself up. A polystyrene cup does neither, and costs less.
Step 2
Take the temperature of the acid before adding anything.
Sound — essential, in fact, and easy to skip.
Without a starting value there is no change to calculate — only a final number that means nothing on its own. It should be recorded, not remembered.
Step 3
Add the alkali, then go and write up the method while it reacts.
Flawed, and this is the one that ruins the experiment.
The peak temperature arrives within seconds and then falls as heat escapes to the room. Come back two minutes later and you record a number that is mostly a measure of how long you were away.
Step 4
Stir with the thermometer.
Flawed — stirring is right, stirring with the thermometer is not.
Without stirring the thermometer reads one warm pocket rather than the mixture. But it is a fragile instrument, and a broken one in a beaker of acid is a genuine incident. Use a stirring rod.
Step 5
Record the highest temperature reached, and subtract the starting temperature.
Sound, and the right quantity to record.
The peak is the closest the apparatus gets to the true value, because from that moment on heat loss is winning. Subtracting gives the temperature change, which is what the energy transfer is judged by.
Your turn · build the apparatus
Three choices, then run it and see what your setup was worth.
The true temperature rise for this reaction is 7.0 °C. Your reading will be lower than that by however much heat your apparatus lets escape. Both that value and the eight readings are illustrative figures chosen to show what heat loss does, not measurements from one afternoon.
The container
The top
When you read it
Glass beaker · open to the room · read after two minutes
Glass beaker · open to the room · read at the peak
Glass beaker · lid fitted · read after two minutes
Glass beaker · lid fitted · read at the peak
Polystyrene cup · open to the room · read after two minutes
Polystyrene cup · open to the room · read at the peak
Polystyrene cup · lid fitted · read after two minutes
Polystyrene cup · lid fitted · read at the peak
Your reading
+2.1 °C
True value
+7.0 °C
A glass beaker, open to the room, read two minutes late. Almost two-thirds of the rise has leaked away before anyone looked at the thermometer. This is the group that reported +2 °C.
Your reading
+4.9 °C
True value
+7.0 °C
Reading promptly saved a great deal — but an open glass beaker still loses heat upwards and into the bench the whole time.
Your reading
+3.4 °C
True value
+7.0 °C
The lid helps, and then two minutes of waiting throws the benefit away. Good apparatus does not rescue slow measurement.
Your reading
+5.6 °C
True value
+7.0 °C
Better. What is still missing is the insulation: glass conducts, and some of the energy is warming the beaker itself rather than the mixture.
Your reading
+4.2 °C
True value
+7.0 °C
The cup insulates well, but an open top loses heat by evaporation and convection — and two minutes is long enough for that to matter.
Your reading
+6.3 °C
True value
+7.0 °C
Close. The polystyrene is doing its job; what is escaping now is going straight up out of the open top.
Your reading
+5.1 °C
True value
+7.0 °C
Good apparatus, poor timing. The rig would have given you 6.8 if you had read it when the reaction finished.
Your reading
+6.8 °C
True value
+7.0 °C
The best a school bench will do: insulated, covered, read at the peak. The missing 0.2 °C went into warming the cup, the lid and the thermometer, and no arrangement of this apparatus recovers it.
You found the best rig — and it still reads low.
A polystyrene cup with a lid and a fast reading gets you to 6.8 of the 7.0 degrees. The last two tenths went into warming the cup, the thermometer and the lid, and no school apparatus recovers them.
That is worth saying plainly: every reading here is an underestimate, and the error runs one way. An error that always points the same direction is not random scatter — it is a systematic error, and repeating the experiment will not remove it.
Key fact
Measure the start temperature, the highest or lowest reading, and take the difference. Insulate, use a lid, and read quickly — every escape of heat makes the measured change too small.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“Repeating an experiment and averaging makes the result accurate.”
Repeating and averaging is genuinely good practice. Commit before you read on.
Averaging fixes random error — the scatter from reading a scale slightly differently each time, or stirring a bit harder. Those errors fall on both sides of the truth, so they cancel out.
Heat loss is not like that. It makes every single reading too low, so the average of three low readings is a low average. An error that always points the same way survives any number of repeats. The only cure is to change the apparatus — which is why the lid matters more than the third run.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What two readings do you need to find the temperature change of a reaction?
Rung 2 · The one that catches people
A group insulates the cup, fits a lid, and repeats the experiment five times. Their answers are all close together but all lower than the true value. What kind of error is this?
Rung 3 · Explain
Write a method for measuring the temperature change when an acid is neutralised by an alkali. Include what you would keep the same, and explain the reason behind two of your choices.
Rung 4 · Take it somewhere new
A group wants to compare which of three fuels releases the most energy, by heating a beaker of water with each in turn. Identify the main sources of error in that plan and say what they should keep the same.
Key note
To measure an energy change, record the starting temperature and the highest or lowest temperature reached, and subtract. Insulating the container, fitting a lid and reading promptly all reduce heat loss to the surroundings. Heat loss makes every reading too small, and because the error always runs the same way, repeating the experiment will not correct it.
Going further
Professional versions of this experiment are done in a bomb calorimeter: a sealed steel vessel sitting in a measured mass of water inside an insulated jacket, with the whole assembly weighed and its own heat capacity measured beforehand. The reaction is ignited electrically and the water temperature is tracked to a hundredth of a degree. The calorie figures on the back of a food packet come from a machine like that, burning a sample of the food and measuring the water.
The polystyrene cup you chose is a genuine piece of scientific apparatus, not a school compromise. It works because polystyrene foam is mostly trapped air, and air is a poor conductor; the same principle is in a wetsuit, a duvet and the walls of a house. What makes it good enough for a classroom is that its own heat capacity is small — a glass beaker absorbs a noticeable share of the energy just warming itself up, and a plastic cup barely does.
Before this lesson
Next in this unit
At GCSE this becomes
- Calorimetry with real calculations, energy per gram of fuel, and evaluating systematic error in a required practical.
Where to next
- Next: Metals and non-metals
The periodic table
- Previous: Endothermic reactions
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