UNIT 3 – CHEMICAL WORLD


Method 1. Using a graduated cylinder, measure 10 cm3


sodium hydroxide solution and pour into a polystyrene cup.


2. Using a thermometer, measure the starting temperature of the sodium hydroxide solution and record it in your Experimental Investigations Log.


3. Measure 10 cm3 of the hydrochloric acid solution and carefully pour into the sodium hydroxide


solution. Place the lid on as quickly as possible and start recording the temperature change. Record the highest and lowest temperature reached during the reaction.


4. Follow steps 1–3 to test the following reactions: • Sodium hydrogen carbonate and citric acid • Calcium chloride and water • Calcium chloride and baking soda • Vinegar and sodium carbonate • Water and Epsom salts


j Go to page 65 of your Experimental Investigations Log to write up this investigation.


Energy profile diagrams


The energy transfer that takes place in a chemical reaction can be shown using an energy profile diagram. An energy profile diagram shows:


• the amount of energy the chemicals reacting together have (the reactants)


• the amount of energy needed to start the reaction (the activation energy)


• the amount of energy the substances formed during the reaction have (the products)


Energy profile diagram for an exothermic reaction


The energy profile diagram in Fig. 23.6 shows an exothermic reaction. Notice that the products of the reaction have less energy than the reactants. This is because energy has been given out during the reaction.


? TEST TUBE


2. a. What will happen to the temperature of the surroundings as a result of the reaction shown in Fig. 23.6?


b. How could you test if your answer is correct?


Activation energy


Thermometer


of the


SAFETY • Handle solutions with care.


Lid


Polystyrene cup Reactants


:Fig. 23.5 To test if a reaction is exothermic or endothermic


Reactants


Energy given out


Products Reaction progress


:Fig. 23.6 Energy profile diagram of an exothermic reaction


226 Elements of Science


Energy


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