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Energy changes
The document explains energy changes in chemical reactions, distinguishing between exothermic (heat released) and endothermic (heat absorbed) processes. It details how enthalpy changes (△h) indicate the type of reaction and discusses bond breaking and forming in relation to energy changes. Additionally, it covers activation energy, combustion of fuels, and the advantages and concerns of using hydrogen as a fuel.
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Energy changes
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- 2. ENERGY CHANGES Energycannot be created or destroyed, it can only be changed from one form to another. All chemical reactions involve energy changes. Energy change has taken place when there is a rise or drop in temperature to the surroundings. Two types of energy change Exothermic Endothermic
- 3. EXOTHERMIC CHANGES Anexothermic change is a change in which heat is given out to the surroundings. When an exothermic change occurs, Heat is released from the reactants and transferred to the surroundings The temperature of the reaction mixtures rises The container feels warmer
- 4. EXOTHERMIC CHANGES Physical processChemical reaction Condensation Combustion of fuels Freezing Respiration Dissolving of anhydrous salts (sodium carbonate) in water Neutralisation (reaction between acid and alkali) Dissolving of acids (e.g. concentrated sulfuric acid) in water Corrosion of metal (e.g. rusting of iron)
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- 6. EXOTHERMIC CHANGES Duringan exothermic change, the temperature of the reaction mixtures rises until it reaches the highest temperature. When the reaction is completed, no more heat is given off and the temperature of the reaction mixtures falls back to room temperature.
- 7. ENDOTHERMIC CHANGES Anendothermic change is a change in which heat is absorbed from the surroundings. When an endothermic change occurs, Heat is absorbed and transferred from the surroundings to the reactants The temperature of the reaction mixture falls The container feels cooler
- 8. ENDOTHERMIC CHANGES Physical processChemical reaction Evaporation Photosynthesis Melting Action of light on silver bromide in photographic film Dissolving of some ionic compounds (e.g. ammonium chloride, potassium chloride and hydrated copper (II) sulfate) in water Thermal decomposition (e.g. heating of calcium carbonate to produce calcium oxide and carbon dioxide)
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- 10. ENDOTHERMIC CHANGES Duringan endothermic change, the temperature of the reaction mixtures falls until it reaches the lowest temperature. When the reaction is completed, no more heat is absorbed and the temperature of the reaction mixtures rises back to room temperature.
- 11. ENTHALPY CHANGE INA REACTION The enthalpy change refers to the amount of energy involved in the reaction. Measured in kilojoules (kJ) and represented by the symbol △H. △H simply means “change in energy content”.
- 12. ENTHALPY CHANGE INA REACTION The enthalpy change, △H, represents the difference in energy content of the reactants and products. △H = total energy of products – total energy of reactants. When △H is negative, it is an exothermic reaction. When △H is positive, it is an endothermic reaction.
- 13. ENERGY LEVEL DIAGRAMS Energylevel Reactants Products heat of reaction (△H = negative) Exothermic reaction: △H < 0 Exothermic reaction
- 14. ENERGY LEVEL DIAGRAMS Energylevel Reactants Products heat of reaction (△H = positive) Endothermic reaction: △H > 0 Endothermic reaction
- 15. BOND BREAKING ANDBOND MAKING All reactions involve the breaking of bonds or the forming of new bonds. Energy changes in reactions are caused by the making and breaking of chemical bonds. Heat energy is absorbed when breaking bonds while heat energy is released when forming bonds. Bond breaking is an endothermic process. Bond forming is an exothermic process.
- 16. EXOTHERMIC OR ENDOTHERMIC CHANGE? The overall enthalpy change will indicate whether a reaction is exothermic or endothermic. Overall enthalpy change (△H) = total energy absorbed for bonding breaking – total energy released for bond making
- 17. EXOTHERMIC OR ENDOTHERMIC CHANGE? Exothermic: total energy absorbed for bond breaking < total energy released for bond making Endothermic total energy absorbed for bond breaking > total energy released for bond making
- 18. EXOTHERMIC OR ENDOTHERMIC CHANGE? Let’s consider the reaction between hydrogen and oxygen to produce water: hydrogen + oxygen water 2H2(g) + O2(g) 2H2O(l) Is this reaction exothermic or endothermic? We will need to do some calculations to find out.
- 19. EXOTHERMIC OR ENDOTHERMIC CHANGE? Given data: Energy required to break 1 mol of O=O bonds = 496 kJ Energy required to break 1 mol of H=H bonds = 436 kJ Energy required to break 2 mol of H=H bonds = 2 X 436 kJ = 872 kJ Total energy absorbed for bond breaking = 872 + 496 = 1368 kJ
- 20. EXOTHERMIC OR ENDOTHERMIC CHANGE? Given data Energy released on forming 1 mol of O-H bonds = 463 kJ Energy released on forming 2 mol of O-H bonds in 1 mol of H2O molecules = 2 X 463 kJ = 926 kJ Energy released on forming 2 mol of H2O molecules = 2 X 926 kJ = 1852 kJ
- 21. EXOTHERMIC OR ENDOTHERMIC CHANGE? Overall heat change: △H = total energy of products – total energy of reactants = 1368 – 1852 = -484 kJ The negative △H indicates that the reaction is exothermic.
- 22. ACTIVATION ENERGY Activationenergy, Ea, is the minimum amount of energy that reactant particles must possess in order for a chemical reaction to occur. For a reaction to occur, reactant particles must collide to form products. They must possess energy equal to or more than the activation energy in order for bond breaking, followed by bond forming.
- 23. ENERGY PROFILE DIAGRAMS Energy profile diagrams show the enthalpy change and activation energy of a reaction.
- 24. FUELS Fuels area source of energy and we burn these substances to give off energy, an exothermic process. Examples include wood, coal, petrol and natural gas.
- 25. COMBUSTION OF FUELS Coal is made up of carbon and it burns in excess air to give carbon dioxide and produce heat energy. carbon + oxygen carbon dioxide + heat energy C(s) + O2(g) CO2(g) + heat energy
- 26. COMBUSTION OF FUELS Most fuels are hydrocarbons (compounds containing hydrogen and carbon only) and when burnt, produces carbon dioxide, water and heat. We have an example of the combustion of natural gas in excess air. methane + oxygen carbon dioxide + water vapour + heat energy CH4(g) + 2O2(g) CO2(g) + 2H2O(g) + heat energy
- 27. INCOMPLETE COMBUSTION Incompletecombustion occurs when there is limited supply of oxygen. Carbon particles (soot) and a poisonous gas, carbon monoxide are produced. We have an example of the incomplete combustion of natural gas in limited air. methane + oxygen carbon monoxide + water vapour + carbon + heat energy 4CH4(g) + 5O2(g) 2CO(g) + 8H2O(g) + 2C(s) + heat energy
- 28. FUEL CELLS Afuel cell uses a fuel and oxygen to produce electricity directly. Example of a fuel cell is the hydrogen-oxygen fuel cell.
- 29. FUEL CELLS Ahydrogen-oxygen fuel cell is a chemical cell which uses hydrogen as the fuel. The combustion process produces water and heat energy. hydrogen + oxygen water 2H2(g) + O2(g) 2H2O(g) △H = -484 kJ (exothermic reaction)
- 30. ADVANTAGES OF USINGHYDROGEN AS A FUEL Hydrogen does not cause pollution because only water is produced during combustion. Hydrogen is a renewable source of energy. Hydrogen is an efficient source of energy. It gives off more than twice as much energy compared to other fuels.
- 31. ADVANTAGES OF USINGHYDROGEN AS A FUEL There are no cheap sources of hydrogen. Hydrogen is obtained by electrolysis, which is an expensive process. Hydrogen is currently via cracking of alkanes. Hydrogen is highly flammable and explosive.
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