Energy Changes

1. Energy Changes You will learn about: • Exothermic & Endothermic Reactions • Energy level diagrams • Bond Making and Breaking • Calculating enthalpy change • Activation energy • Energy profile diagrams • Photography • Photosynthesis

2. Exothermic & Endothermic Reactions • Exo:out, Thermic: Heat energy • Exothermic reaction: one in which heat energy is released and there is an increase in temperature of the mixture. • Endo:in • Endothermic reaction: one in which heat energy is absorbedand there is a decrease in the temperature of the mixture.

3. Endothermic reaction • Dissolution of potassium nitrate in water - eg. of endothermic reaction • - heat energy is absorbed • => decrease in the temperature of the mixture

4. Exothermic reaction • Dissolution of calcium oxide in water eg. of exothermic reaction • - heat energy is released • => increase in temperature of the mixture

5. Enthalpy Change • Heat energy given out or taken in, during a reaction is measured in units called Joules (J). • Large amounts are measured in kiloJoules (kJ). • The symbol for the heat of reaction (heat energy change or enthalpy change) is H. • Numerically,H = Change in heat energy content = Total energy content of products – total energy content of reactants

6. Enthalpy Change • If Total energy content of products > Total energy content of reactantsH > 0Reaction is endothermic • IfTotal energy content of products < Total energy content of reactantsH < 0Reaction is exothermic • The sign of H will tell whether the reaction is exothermic or endothermic!

7. Examples • Example 1: When 1 mole of hydrochloric acid reacts with one mole of sodium hydroxide, 57.3 kJ of heat is produced. Draw an energy level diagram to represent this reaction.

8. Examples • Example 2: When 1 mole of hydrogen reacts with 1 mole of iodine to give 2 moles of hydrogen iodide, 52 kJ of heat is absorbed. Draw an energy level diagram to represent this reaction.

9. Bond Making and Breaking • When 2 atoms joined together by chemical bond, heat energy is given out. Bond making is a exothermic change. (H = -ve) • Energy is needed to break chemical bond. Bond breaking is a endothermic change. (H=+ve). • Energy for making and breaking a covalent bond is the same. This is the bond energy.

10. Enthalpy Change in a Reaction • Old bonds are broken in a reaction (endothermic) while new bonds are formed (exothermic). • The Enthalpy Change, H, (overall heat change) of the reaction = [heat absorbed to break old bonds in reactants] - [heat given out to form new bonds in products] • The sign of H will tell whether reaction is exothermic or endothermic.

11. Enthalpy Change • If the energy absorbed for bond breaking is less than that released during bond forming, the reaction is exothermic.   • If the energy absorbed for bond breaking is more than that released during bond forming, the reaction is endothermic.

12. QUIZZES 1. Classify each of the following changes as either endothermic, exothermic or no heat exchange. Exothermic No heat change Exothermic Endothermic Endothermic

13. 2) Fe + 2HCl  FeCl2 + H2; H=-156kJ (i) How do you know from the equation above that the reaction is exothermic? (ii) How much heat is produced in the reaction of 1 mole of HCl? The negative sign in front of H. Amount of heat produced by 1 mole HCl = 156/2=78kJ

14. Worked example Method 1: • E.g. 2H2 + O2 2H2O • Heat released when new bonds are formed = bond energies of 4 O-H bonds = 4x 463kJ = 1852kJ • Heat absorbed when old bonds are broken = bond energies of 2 H-H bonds + 1 O=O bond = (2x436) + (1x496) = 1368kJ • Enthalpy Change, H of reaction = 1368 -1852 = -484kJ (the -ve =>exothermic)

15. Method 2: E.g. 2H2 + O2 2H2O • Heat involved in bond making of 4 O-H bonds = 4x (-463)kJ = -1852kJ • Heat involved in bond breaking of 2 H-H bonds + 1 O=O bond = 2x(+436) + 1x(+496) = +1368kJ • Enthalpy Change, H of reaction = +1368 -1852 = -484kJ (-ve =>exothermic)

16. Activation Energy • For a chemical reaction to occur, the reactant particles must collide • Most particles do not have enough energy when they collide to start a reaction. They just bounce apart and do not react.

17. Activation Energy • Some particles collide with enough energy to start a reaction.

18. Activation Energy • The minimum energy that molecules must possess in order for a chemical reaction to occur is called the activation energy, Ea. • A reaction that takes place easily at room temperature has a fairly low activation energy. • A reaction that requires a higher temperature to occur has a higher activation energy.

19. Activation Energy • Activation energy is the energy barrier that the colliding molecules must overcome in order for a reaction to occur

20. Activation Energy • Consider the reaction of hydrogen with oxygen: 2H2(g) + O2(g) → 2H2O (l) • The reaction takes place in two steps: 1. A burning match provides the necessary activation energy. This energy gives the reactant molecules enough energy to break their covalent bonds when they collide, forming hydrogen and oxygen atoms. 2. Once the reaction starts, the hydrogen and oxygen atoms combine to form water. As new bonds are formed, energy is given out.

21. Activation energy – exothermic rxn • If a reaction is exothermic, enough energy is given out in the reaction of a few particles to provide the activation energy for the remaining particles. So the reaction keeps going. • This is the case for the reaction between hydrogen and oxygen; when the first hydrogen molecule reacts, the heat give out enables other molecules to react.

22. Activation energy- endothermic rxn • If the reaction is endothermic, insufficient energy is given out when bonds are made to provide the activation energy needed for the reaction to continue. • Thus, heat energy must be continually added to provide sufficient activation energy.

23. Energy profile diagrams • An energy profile diagram is a way of representing the energy changes that occur during a chemical change. The energy difference between the products and reactants represents the heat of reaction. • It includes the energy barrier, the activation energy.

24. Energy Profile for exothermic rxn • Exothermic reaction

25. Energy profile for endothermic rxn • Endothermic reactions

26. Examples of exothermic changes • Changes of state (condensation and freezing) • Dissolving of some salts in water eg. Na2CO3(s) → Na2CO3(aq) + heat • Combustion reactions All combustion reactions are exothermic. eg. 2H2(g) + O2(g) → 2H2O(l) + heat • Neutralisation reactions H+(aq) + OH-(aq) → H2O(l) + heat • Metal displacement reactions Zn(s) + Cu2+ (aq) → Zn2+ (aq) + Cu(s) + heat

27. Examples of endothermic changes • Changes of state (melting & boiling) • Dissolving of some salts in water eg. NH4Cl(s) + heat → NH4Cl(aq) • Decomposition of compounds eg. CuCO3(s) + heat → CuO(s) + CO2(g) • Reactions between acids and hydrogencarbonates eg. when some headache tablets (mixture of solid acids such as citric acid and sodium hydrogencarbonate) are added to water, heat energy is absorbed. • Formation of nitrogen oxide in a car engine N2(g) + O2(g) → 2NO(g)

28. Combustion of fuels • What are fuels? • Fuels are substances that can burn easily in air to give out energy. • The most commonly used fuels are fossil fuels such as coal, petroleum and natural gas. They are formed from decayed plant and animals that lived millions of years ago.

29. Combustion of fuels • What happens when fuels burn? • Coal is mainly carbon. C(s) + O2(g) → CO2(g) + heat energy • Most fuels contain carbon and hydrogen. When these fuels burn, carbon dioxide, water and heat energy are produced. The equation for the combustion of natural gas, which contains mainly methane is CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + heat energy • All combustion reactions are exothermic processes.

30. Combustion of fuels • We can calculate the heat of combustion of fuels given the bond energy values. Table: Bond energy values

31. Combustion of fuels • Calculate the enthalpy change for the complete combustion of butene with the following structure.

32. Calculation of heat of combustion C4H8 + 6O2 →4CO2 + 4H2O Heat involved in breaking 8C-H, 2C-C, 1C=C & 6O=O bonds = 8(+412) + 2(+348) +612 + 6(+496) = +7580 kJ

33. Calculation of heat of combustion • Heat involved in making 8C=O & 8O-H bonds = 8(-743) + 8(-463) = -9648 kJ • Enthalpy change = +7580 – 9648 = -2068 kJ

34. Group work – Alternative Fuels Research on the following, highlighting one of the fuels and do a powerpoint presentation, including the chemistry behind the fuel, the social-economic issues involved, and show examples where this fuel is being used. Do a comparative study on the alternative energy that is being employed in Singapore and another country. • 1. Hydrogen fuel & other types of fuel cells • 2. Biofuels • 3. Nuclear energy & solar energy

35. Light and Chemical Reactions • Reactions in which light energy is absorbed are endothermic. • Photography • Photosynthesis

36. Photography • Photographic film: coated with crystals of silver bromide or silver chloride. • Light hit film electrons transferred from the chloride or bromide ions to the silver ions. • Silver metals formed the dark areas on film. • This is an example of redox reaction. • 2AgBr 2Ag + Br2H = +199kJ

37. Photosynthesis • Chlorophyll absorbs light energy used to make sugars from carbon dioxide and water. • 6CO2 + 6H2O  C6H12O6 + 6O2H = +2816kJ

38. Group work – Alternative Fuels Research on the following, highlighting one of the fuels and do a powerpoint presentation, including the chemistry behind the fuel, the social- economic, environmental issues involved, and show examples where this fuel is being used. The pros and cons as well. Do a comparative study on the alternative energy that is being employed in Singapore and China. • 1. Hydrogen fuel & other types of fuel cells • 2. Solar energy • 3. Biofuels • 4. Nuclear energy