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Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Addition of NaOH to water Sodium hydroxide pellets are added to water. The.

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Presentation on theme: "Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Addition of NaOH to water Sodium hydroxide pellets are added to water. The."— Presentation transcript:

1 Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Addition of NaOH to water Sodium hydroxide pellets are added to water. The mixture is carefully stirred to dissolve the pellets. The temperature of the water is recorded before and after adding sodium hydroxide.

2 Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Addition of NH 4 Cl to water Ammonium chloride crystals are added to water. The mixture is carefully stirred to dissolve the pellets. The temperature of the water is recorded before and after adding ammonium chloride.

3 Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Results of Both Experiments ExperimentSolidInitial temperature (°C) Final temperature (°C) INaOH2834 IINH 4 Cl2822

4 Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. What conclusions about energy changes can be made from these 2 experiments? In experiment I, heat energy was given out when the solid dissolved in water. Thus, the temperature of the solution rose. We say this change is exothermic.

5 Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. What conclusions about energy changes can be made from these 2 experiments? In experiment II, heat energy was taken in when the solid dissolved in water. Thus, the temperature of the solution dropped. We say this change is endothermic.

6 Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. We have just seen that heat energy is given out during a physical process, such as when sodium hydroxide dissolves in water. Heat energy may also be given out during a chemical reaction, such as when sodium hydroxide reacts with hydrochloric acid.

7 Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Exothermic Reactions Reactions that give out heat energy to the surroundings are called exothermic reactions.

8 Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. What are the characteristics of exothermic reactions? When an exothermic reaction occurs, a) heat is liberated and is transferred from the chemicals to the surroundings, and b) the temperature of the reaction mixture rises. The container feels warm.

9 Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. How does temperature change in an exothermic reaction? Initially, the temperature of the reaction mixture rises until the highest temperature is reached. When the reaction is completed, the temperature of the reaction mixture falls until it reaches room temperature.

10 Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Examples of exothermic reactions Combustion of fuels Rusting of iron Corrosion of metals Reaction between acid and alkali (neutralisation), Respiration

11 Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Endothermic Reactions Reactions that absorb heat from the surroundings are called endothermic reactions.

12 Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. What are the characteristics of endothermic reactions? When an endothermic reaction occurs, a) heat is absorbed and is transferred from the surroundings to the reactants, and b) the temperature of the reaction mixture falls. It feels cold.

13 Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Initially, the temperature of the reaction mixture falls until the lowest temperature is reached. When the reaction is completed, the temperature of the reaction mixture rises until it reaches room temperature. How does temperature change in an endothermic reaction?

14 Copyright © Marshall Cavendish International (Singapore) Pte. Ltd. Photosynthesis Action of light on silver bromide in photographic film Thermal decomposition for example Examples of endothermic reactions


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