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22.
Classic chemistry experiments
Exothermic or
endothermic?
Topic
Energy transfer.
Timing
30 min.
Description
Students look at four reactions to see if they are exothermic or endothermic.
Apparatus and equipment (per group)
▼ Beakers
▼ Thermometer.
Chemicals (per group)
3
▼ 10 cm Sodium hydroxide solution
3
▼ 10 cm Dilute hydrochloric acid
3
▼ 10 cm Sodium hydrogen carbonate solution
–3
0.4 mol dm
–3
0.4 mol dm
(Irritant)
(Irritant)
–3
0.4 mol dm (Irritant)
▼ Four spatula measures citric acid
3
▼ 10 cm Copper(II) sulfate solution
–3
0.4 mol dm
(Irritant)
▼ Four spatula measures magnesium powder (Highly flammable)
▼ 3 cm Magnesium ribbon (Highly flammable)
3
▼ 10 cm Dilute sulfuric acid
–3
0.4 mol dm
(Irritant).
Teaching tips
Students get confused about endothermic reactions. An endothermic reaction drops in
temperature as it takes in (or absorbs) heat. A temperature sensor attached to a
computer can be used in place of a thermometer in this experiment. The software can
plot the temperature change on a graph and for a demonstration, show it on a screensize digital display.
Background theory
Bond making and bond breaking.
Safety
Wear eye protection.
Answers
1. Neutralisation.
2. Carbon dioxide.
3. Displacement reaction – magnesium is higher in the reactivity series so it displaces
copper from its sulfate.
4. Sodium hydrogen carbonate solution + citric acid is endothermic, the other
reactions are exothermic.
5. Reactions involve making and breaking bonds. Energy is required to break bonds.
Energy is released when bonds are made. If more energy is required to break the
bonds in a reaction than that released by making the bonds then the reaction is
endothermic (heat is absorbed). If less energy is required to break the bonds than that
released by making the bonds then the reaction is exothermic (heat is produced).
Classic chemistry experiments
55
Exothermic or
endothermic?
Introduction
Some reactions give out heat and others take in heat. In exothermic reactions the
temperature goes up, in endothermic reactions the temperature goes down. In this
experiment, various reactions are examined. Temperatures are measured to decide
whether a particular reaction is exothermic or endothermic.
Thermometer
Plastic cup
Reaction mixture
What to record
Complete the table
Temperature
before mixing/°C
Reaction
Temperature.
after mixing/°C
Exothermic or
endothermic
Sodium hydroxide solution
+ dilute hydrochloric acid
Sodium hydrogen carbonate
solution + citric acid
copper(II) sulfate solution +
magnesium powder
Dilute sulfuric acid +
magnesium ribbon
What to do
1. Use the apparatus as shown.
3
2. Put 10 cm of sodium hydroxide solution in the beaker, record the temperature
3
then add 10 cm of dilute hydrochloric acid, stirring with the thermometer. Record
the maximum or minimum temperature.
3. Repeat the procedure for the following reactions: (a) sodium hydrogen carbonate
solution and citric acid; (b) copper(II) sulfate solution and magnesium powder; and
(c) dilute sulfuric acid and magnesium ribbon.
Safety
Wear eye protection. Some of the solutions are irritant.
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Classic chemistry experiments
Questions
1. The first reaction is between an acid and an alkali, what do we call this type of
reaction?
2. Which gas is given off when sodium hydrogen carbonate reacts with citric acid?
3. Which type of reaction takes place between copper(II) sulfate and magnesium?
4. Which reactions are exothermic and which are endothermic?
5. Describe in terms of bond breaking and bond making, why some reactions are
exothermic and some are endothermic.