Everything Science Grade 11

7.3 CHAPTER 7. ENERGYCHANGES IN CHEMICAL REACTIONS

C + H 2 O→ CO + H 2 ΔH = +131 kJ

Some of the informationrelating to exothermic and endothermic reactionsis summarised in Table 7.1.

DEFINITION: Enthalpy

Enthalpy is the heat content of a chemical systemfor a given pressure, andis given the

symbol ’H’.

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 lessthan

energy of product

Sign of ΔH Negative Positive

Table 7.1: A comparisonof exothermic and endothermic reactions

Writing equations using ΔH
There are two ways towrite the heat of the reaction in an equation. For the exothermic reaction
C(s) + O 2 (g)→ CO 2 (g), we can write:
C(s) + O 2 (g)→ CO 2 (g) ΔH =− 393 kJ· mol−^1 or
C(s) + O 2 (g)→ CO 2 (g) + 393 kJ· mol−^1
For the endothermic reaction, C(s) + H 2 O(g)→ H 2 (g) + CO(g), we can write:
C(s) + H 2 O(g)→ H 2 (g) + CO(g) ΔH = +131 kJ· mol−^1 or
C(s) + H 2 O(g) + 131kJ· mol−^1 → CO + H 2
The units for ΔH are kJ· mol−^1. 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 alsobe written as kJ, whichthen
gives the total amount of energy that is releasedor absorbed when the product forms.

Activity: Endothermic and exothermic reactions

Apparatus and materials:

Approximately 2 g eachof calcium chloride (CaCl 2 ), sodium hydroxide (NaOH), potassium

nitrate (KNO 3 ) and barium chloride (BaCl 2 ); concentrated sulphuric acid (H 2 SO 4 ) (Be Careful,

this can cause serious burns) ; 5 test tubes; thermometer.

Method:

1. Dissolve about 1 g ofeach of the following substances in 5-10 cm^3 of water in a test tube:
   CaCl 2 , NaOH, KNO 3 and BaCl 2.


2. Observe whether thereaction is endothermicor exothermic, either byfeeling whether
   the side of the test tubegets hot or cold, or using a thermometer.


3. Dilute 3 cm^3 of concentrated H 2 SO 4 in 10 cm^3 of water in the fifth test tube and observe
   whether the temperaturechanges.


4. Wait a few minutesand then carefully addNaOH to the H 2 SO 4. Observe any energy
   changes.


5. Record which of theabove reactions are endothermic and which areexothermic.

Results: