The heat of reaction (ESBQQ)

The heat of the reaction is represented by the symbol ΔH, where:

ΔH=EprodEreact

  • In an exothermic reaction, ΔH is less than zero because the energy of the reactants is greater than the energy of the products. Energy is released in the reaction. For example:

    H2(g)+Cl2(g)2HCl (g)ΔH<0
  • In an endothermic reaction, ΔH is greater than zero because the energy of the reactants is less than the energy of the products. Energy is absorbed in the reaction. For example:

    C (s)+H2O (l)CO (g)+H2(g)ΔH>0

Some of the information relating to exothermic and endothermic reactions is summarised in Table 12.1.

Type of reaction

Exothermic

Endothermic

Energy absorbed or released

Released

Absorbed

Relative energy of reactants and products

Energy of reactants greater than energy of product

Energy of reactants less than energy of product

Sign of ΔH

Negative (i.e. <0)

Positive (i.e. >0)

Table 12.1: A comparison of exothermic and endothermic reactions.

Writing equations using ΔH

ΔH has been calculated for many different reactions and so instead of saying that ΔH is positive or negative, we can look up the value of ΔH for the reaction and use that value instead.

There are two ways to write the heat of the reaction in an equation. For the exothermic reaction C(s)+O2(g)CO2(g), we can write:

C(s)+O2(g)CO2(g)ΔH=393 kJ·mol1

or

C(s)+O2(g)CO2(g)+393 kJ·mol1

For the endothermic reaction, C(s)+H2O(g)H2(g)+CO(g), we can write:

C(s)+H2O(g)H2(g)+CO(g)ΔH=+131 kJ·mol1

or

C(s)+H2O(g)++131 kJ·mol1H2(g)+CO(g)

The units for ΔH are kJ·mol1. In other words, the ΔH value gives the amount of energy that is absorbed or released per mole of product that is formed. Units can also be written as kJ, which then gives the total amount of energy that is released or absorbed when the product forms.

The energy changes during exothermic and endothermic reactions can be plotted on a graph:

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Figure 12.2: The energy changes that take place during an exothermic reaction.
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Figure 12.3: The energy changes that take place during an endothermic reaction.

We will explain shortly why we draw these graphs with a curve rather than simply drawing a straight line from the reactants energy to the products energy.

In the investigation on exothermic and endothermic reactions learners work with concentrated sulfuric acid. They must work in a well ventilated room or, if possible, in a fume cupboard. This is a highly corrosive substance and learners must handle it with care. If they spill any on themselves they must immediately wash the affected area with plenty of running water. Either the learner or their friend should inform you as soon as possible so you can ensure that the learner is ok. If necessary the learner may need to go to the bathroom to remove and rinse clothing that is affected. Either you or another learner should accompany them.

Most of the salts that the learners will work with are hygroscopic and will quickly absorb water from the air. These salts can cause chemical burns and should be handled with care. If possible learners should wear gloves to protect their hands.

Endothermic and exothermic reactions

Aim

To investigate exothermic and endothermic reactions.

Apparatus and materials

  • Approximately 2 g of calcium chloride (CaCl2)
  • Approximately 2 g of sodium hydroxide (NaOH)
  • Approximately 2 g of potassium nitrate (KNO3)
  • Approximately 2 g of barium chloride (BaCl2)
  • concentrated sulphuric acid (H2SO4) (Be careful, this can cause serious burns)
  • 5 test tubes
  • thermometer

When working with concentrated sulfuric acid always wear gloves and safety glasses. Always work in a well ventilated room or in a fume cupboard.

Method

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  1. Dissolve about 1 g of each of the following substances in 5-10 cm3 of water in a test tube: CaCl2, NaOH, KNO3 and BaCl2.

  2. Observe whether the reaction is endothermic or exothermic, either by feeling whether the side of the test tube gets hot or cold, or using a thermometer.

  3. Dilute 3 cm3 of concentrated H2SO4 in 10 cm3 of water in the fifth test tube and observe whether the temperature changes.

    Remember to always add the acid to the water.

  4. Wait a few minutes and then carefully add NaOH to the diluted H2SO4. Observe any temperature (energy) changes.

Results

Record which of the above reactions are endothermic and which are exothermic.

Exothermic reactionsEndothermic reactions
  • When BaCl2 and KNO3 dissolve in water, they take in heat from the surroundings. The dissolution of these salts is endothermic.

  • When CaCl2 and NaOH dissolve in water, heat is released. The process is exothermic.

  • The reaction of H2SO4 and NaOH is also exothermic.