Chemistry_Grade_10-12 (1).pdf

The chemical reaction that takes place when fuels

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environment. The chemical reaction that takes place when fuels burn therefore has both positive and negative consequences. Respiration Respiration is the chemical reaction that happens in our bodies to produce energy for our cells. The equation below describes what happens during this reaction: C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O + energy In the reaction above, glucose (a type of carbohydrate in the food we eat) reacts with oxygen from the air that we breathe in, to form carbon dioxide (which we breathe out), water and energy. The energy that is produced allows the cell to carry out its functions efficiently. Can you see now why you are always told that you must eat food to get energy? It is not the food itself that provides you with energy, but the exothermic reaction that takes place when compounds within the food react with the oxygen you have breathed in! 259
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14.5 CHAPTER 14. ENERGY CHANGES IN CHEMICAL REACTIONS - GRADE 11 Interesting Fact erest Fact Lightsticks or glowsticks are used by divers, campers, and for decoration and fun. A lightstick is a plastic tube with a glass vial inside it. To activate a lightstick, you bend the plastic stick, which breaks the glass vial. This allows the chemicals that are inside the glass to mix with the chemicals in the plastic tube. These two chemicals react and release energy. Another part of a lightstick is a fluorescent dye which changes this energy into light, causing the lightstick to glow! Exercise: Endothermic and exothermic reactions 1. In each of the following reactions, say whether the reaction is endothermic or exothermic, and give a reason for your answer. (a) H 2 + I 2 2 HI + 21 kJ (b) CH 4 + 2 O 2 CO 2 + 2 H 2 O Δ H = -802 kJ (c) The following reaction takes place in a flask: Ba ( OH ) 2 . 8 H 2 O + 2 NH 4 NO 3 Ba ( NO 3 ) 2 + 2 NH 3 + 10 H 2 O Within a few minutes, the temperature of the flask drops by approxi- mately 20C. (d) Na + Cl 2 2 NaCl ΔH = -411 kJ (e) C + O 2 CO 2 2. For each of the following descriptions, say whether the process is endothermic or exothermic and give a reason for your answer. (a) evaporation (b) the combustion reaction in a car engine (c) bomb explosions (d) melting ice (e) digestion of food (f) condensation 14.5 Spontaneous and non-spontaneous reactions Activity :: Demonstration : Spontaneous and non-spontaneous reactions Apparatus and materials: A length of magnesium ribbon, thick copper wire and a bunsen burner 260
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CHAPTER 14. ENERGY CHANGES IN CHEMICAL REACTIONS - GRADE 11 14.6 magnesium ribbon Method: 1. Scrape the length of magnesium ribbon and copper wire clean. 2. Heat each piece of metal over the bunsen burner, in a non-luminous flame. Observe whether any chemical reaction takes place. 3. Remove the metals from the flame and observe whether the reaction stops. If the reaction stops, return the metal to the bunsen flame and continue to heat it.
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