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31/07/2014

31/07/2014. Exothermic and Endothermic Reactions. L.O. I understand that reactions can be exothermic or endothermic and can give an example of each. I can show experimentally if an energy change has taken place by using temperature changes. 4:26 PM.

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31/07/2014

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  1. 31/07/2014 Exothermic and Endothermic Reactions • L.O. • I understand that reactions can be exothermic or endothermic and can give an example of each. • I can show experimentally if an energy change has taken place by using temperature changes 4:26 PM

  2. When fuels are burned energy is released as heat. This is combustion. e.g. methane + oxygen  carbon dioxide + water + heat energy (natural gas) ethanol + oxygen  carbon dioxide + water + heat energy (methylated spirits) carbon + oxygen  carbon dioxide + heat energy (coal) butane + oxygen  carbon dioxide + water + heat energy (camping gas) octane + oxygen  carbon dioxide + water + heat energy (in petrol) 4:26 PM

  3. Magnesium reacting with acid Thermit reaction Exothermic Reactions Exothermic reactions increase in temperature. • Examples include: • Burning reactions including the combustion of fuels. • Detonation of explosives. • Reaction of acids with metals.

  4. 45o C 25o C magnesium Hydrochloric acid Exothermic Reactions • Magnesium + Hydrochloric acid Heat energy given out Gets hot

  5. Endothermic Reactions Endothermic reactions cause a decrease in temperature. • Endothermic chemical reactions are relatively rare. • A few reactions that give off gases are highly endothermic - get very cold. • Dissolving salts in water is another process that is often endothermic.

  6. Heat energy taken in as the mixture returns back to room temp. Ammonium nitrate Water Endothermic Reactions Endothermic reactions cause a decrease in temperature. Cools Starts 25°C Cools to 5°C

  7. transferred energy Whenever chemical reactions occur, energy is usually __________ to or from the surroundings. An exothermic reaction is one which gives out _________ to the surroundings, usually in the form of heat and usually shown by a ____ in temperature. In exothermic reactions heat exits the reaction. The best example of an exothermic reaction is __________ fuels, also known as ____________. Heat exits the reaction. rise combustion burning

  8. surroundings drop An endothermic reaction is one which takes in energy from the ___________, usually in the form of heat and usually shown by a ____ in temperature. In endothermic reactions, heat enters the reaction. Photosynthesis is endothermic because it takes in energy from the ___. Energy enters the reaction. Sun

  9. Remember: Exothermic Heat exits (is given out). The temperature outside goes up. Endothermic. Heat enters (is taken in). The temperature outside goes down.

  10. Your teacher will show you how to carry out the practical.

  11. Breaking chemical bonds • Most chemicals will decompose (break up) if we heat them strongly enough. • This indicates that breaking chemical bonds requires energy – is an endothermic process. Heat taken in Energy needed to overcome the bonding between the atoms

  12. Making chemical bonds • It is reasonable to assume that bond making will be the opposite of bond breaking • Energy will be given out in an exothermic process when bonds are formed. Heat given out Energy given out as bonds form between atoms

  13. Plenary : Use traffic light cards to show whetheror endo exo exo endo Are these endothermic or exothermic? 1. A red glow spread throughout the mixture and the temperature rose. 2. The mixture bubbled vigorously but the temperature dropped 150C. 3. Hydrazine and hydrogen peroxide react so explosively and powerfully that they are used to power rockets into space. 4. The decaying grass in the compost maker was considerably above the outside temperature. exo exo

  14. 5. Which of the following is not an exothermic reaction. • The burning of petrol • Respiration • Photosynthesis

  15. 6. What is an endothermic reaction? • One which requires heat to start it. • One which gives out heat. • One which takes in energy.

  16. 7. Two chemicals at room temperature (250C) were mixed together. The reaction was slightly endothermic. Which of the following could be the final temperature? • 270C • –270C • c) 220C

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