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Chapter 4 Thermochemistry

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Chapter 4 Thermochemistry

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Chapter 4 Thermochemistry

  1. 1. Chapter 4 Thermochemistry
  2. 2. A. Energy Changes in Chemical Reactions Thermochemistry The study of changes in heat energy which take place during chemical reactions Classify into: • Exothermic reaction • Endothermic reaction TIPS: EX mean to go out/exit EN mean to come in/enter
  3. 3. EXOTHERMIC REACTION
  4. 4. ENDOTHERMIC REACTION
  5. 5. Energy level diagram The energy of the products is lower than the total energy of the reactants The energy of the products is higher than the energy of the reactants Energy Reactants Products ∆H = negative Energy Reactants Products ∆H = positive Exothermic reaction Endothermic reaction Definition A chemical reaction that gives out heat to the surroundings A chemical reaction that absorbs heat from the surroundings What happen?  During exothermic reaction, temperature of the surrounding increases.  This is because heat given out from the reaction is transferred to the surroundings.  During endothermic reaction, temperature of the surrounding decreases.  This is because the reactants absorb heat energy from the surroundings. Heat of reaction, ∆H The change in the amount of heat in a chemical reaction. ∆H negative: heat is given out ∆H positive: heat is absorbed
  6. 6. Step to construct energy level diagrams Step 6 Label ∆H as positive or negative Step 1 Identify whether the reaction is exothermic or endothermic Step 2 Draw and label the energy axis Step 3 Draw the energy level for reactants and products Step 4 Draw an arrow from reactants level to the products level Step 5 Write the reactants and products based on the balanced chemical equations
  7. 7. Construct energy level diagrams for the following thermochemical equations • Zn + 2HCl → ZnCl2 + H2 ∆H = -152 kJ • N2 + 2O2 → 2NO2 ∆H = +220 kJ • KOH + HNO3 → KNO3 + H2O ∆H = -57 kJ • C + 2S → CS2 ∆H = +220 kJ • Ca(NO3)2 + K2CO3 → CaCO3 + 2KNO3 ∆H = +12 kJ
  8. 8. Energy change during formation and breaking of bonds Usually a chemical reaction involves bond breaking and bond formation. • Bond breaking : always requires energy • Bond formation : always releases energy
  9. 9. Endothermic Energy absorb for bond breaking is more than energy released from bond formation ∆H positive Type of Reaction Energy Change Sign of ∆H Exothermic Energy absorb for bond breaking is less than energy released from bond formation ∆H negative
  10. 10. Application of exothermic and endothermic reaction Cold packs Contain chemicals (water & solid ammonium nitrate, NH4NO3) that react to absorb heat from surroundings. • Help to reduce high temperature • Help to reduced swelling
  11. 11. Application of exothermic and endothermic reaction Hot packs Contain chemicals (calcium chloride, CaCl2 or magnesium sulphate, MgSO4 and water)that react to release heat. • Help to warm up something • Help to lessen the pain of aching muscles
  12. 12. Application of exothermic and endothermic reaction Reusable heat pack Contain sodium acetate crystallization and re-solution system Lye (drain cleaner) Contain sodium hydroxide, NaOH
  13. 13. Heat of Reaction • The change in the amount of heat in a chemical reaction. • Symbol: ∆H Different types of reactions Combustion Heat of Combustion Types of Reaction Heat of Reaction Precipitation Heat of Precipitation Displacement Heat of Displacement Neutralization Heat of Neutralization
  14. 14. Heat of Combustion The heat change when one mole of a substance is completely burnt in oxygen under standard conditions Heat of reaction Definition Heat of Precipitation The heat change when one mole of a precipitate is formed from their ions in aqueous solution Heat of Displacement The heat change when one mole of a metal is displaced from its salt solution by a more electropositive metal Heat of Neutralization The heat change when one mole of water is formed from reaction between an acid and an alkali
  15. 15. The Ways to Calculate Heat of Reaction
  16. 16. Heat of Precipitation Experiment to investigate the heat of precipitation between silver nitrate solution and sodium chloride solution
  17. 17. Heat of Precipitation Procedure: 1. Measure 25 cm3 of 0.5 mol dm-3 sodium chloride, NaCl solution using a measuring cylinder and pour the solution into a polystyrene cup. 2. Measure and record the initial temperature of sodium chloride solution. 3. Measure 25 cm3 of 0.5 mol dm-3 silver nitrate, AgNO3 solution using a measuring cylinder and pour the solution into another polystyrene cup. 4. Measure and record the initial temperature of silver nitrate solution. 5. Pour silver nitrate, AgNO3 solution into the polystyrene containing sodium chloride, NaCl solution. 6. Stir the mixture and record the highest temperature, θ3
  18. 18. Tabulation of data: Initial temperature silver nitrate solution : Initial temperature sodium chloride solution : Highest temperature :
  19. 19. Heat of Displacement Experiment to investigate the heat of displacement of copper by zinc Copper(II) nitrate 0.5 g zinc powder Polystyrene cup
  20. 20. Heat of Displacement Procedure: 1. Measure 25 cm3 of 0.2 mol dm-3 copper(II) nitrate solution using a measuring cylinder. 2. Pour the solution into a polystyrene cup. 3. Measure and record the initial temperature, θ1 of copper(II) nitrate solution. 4. Measure 0.5 g of zinc powder and added into the polystyrene quickly. 5. Stir the mixture and the highest temperature is recorded, θ2
  21. 21. Tabulation of data: Initial temperature copper(II) nitrate solution : Highest temperature :
  22. 22. Heat of Neutralization Describe an experiment to determine the heat of neutralization between dilute hydrochloric acid and sodium hydroxide solution. 100cm3 of 2 mol dm-3 dilute hydrochloric acid 100 cm3 of 2 mol dm-3 aqueous sodium hydroxidePlastic cup
  23. 23. Procedure: 1. 50 cm3 of 2.0 mol dm-3 sodium hydroxide solution is measured using a measuring cylinder and poured into a plastic cup. 2. The initial temperature of sodium hydroxide solution is measured after a few minutes. 3. 50 cm3 of 2.0 mol dm-3 hydrochloric acid is measured using another measuring cylinder and poured into a plastic cup. 4. The initial temperature of hydrochloric acid solution is measured after a few minutes. 5. The hydrochloric acid is then poured quickly and carefully into the sodium hydroxide solution. 6. The mixture is stirred using thermometer and the highest temperature reached is recorded.
  24. 24. Heat of Neutralization All neutralization process can be presented by the following ionic equation H+ + OH- → H2O ∆H = -57 kJ mol-1 • Heat of neutralization for strong acid and strong alkali is same, that is -57 kJ mol-1 • Heat of neutralization for weak acid and strong alkali is less than -57 kJ mol-1 • Heat of neutralization for weak acid and weak alkali is much lesser
  25. 25. Strong acid & strong alkali Explanation: Strong acid ionise completely in water to produce high concentration of hydrogen ions HCl → H+ + Cl- 1 mol of hydrogen ions reacts with 1 mol of hydroxide ions to form I mol of water to release 57 kJ of heat energy KOH/NaOH with HCl/HNO3
  26. 26. Weak acid & strong alkali Explanation: Weak acid ionise partially in water to produce low concentration of hydrogen ions CH3COOH + CH3COO- + H+ Most of the ethanoic acid still remains in the form of molecule Thus the heat release is always less than 57 kJ
  27. 27. Weak acid & weak alkali Explanation: More heat energy is needed to dissociate both the weak acid and weak alkali completely to produce hydrogen ions.
  28. 28. Monoprotic acid & Diprotic acid H2SO4 + 2NaOH → Na2SO4 + 2H2O ∆H = - 114.6 kJ HCl + NaOH → NaCl + H2O ∆H = - 57.3 kJ Explain the differences of heat of reaction. 1. H2SO4 is a diprotic acid while HCl is monoprotic acid. 2. H2SO4 will produce 2 mole of H+ ions and HCl produce 1 mole of H+ ion 3. Neutralization of a diprotic acid will produce twice heat energy than monoprotic acid
  29. 29. Heat of Combustion Describe an experiment to determine the heat of combustion of butanol in the laboratory. In your description include a labelled diagram, procedure and tabulation of data
  30. 30. Heat of Combustion Procedure: 1. 100 cm3 of water is measured using a measuring cylinder. 2. Poured into a copper tin. 3. The initial temperature of water is measured and recorded, θ1 . 4. The spirit lamp is filled with butanol and weighed, x g 5. A spirit lamp is light and put under the copper tin.
  31. 31. Heat of Combustion Procedure: 6. The water is stirred continuously with a thermometer 7. When the temperature of water increased by 30 °C, the flame is put off 8. The spirit lamp is weighed again, y g 9. The highest temperature is recorded, θ2
  32. 32. Heat of Combustion Results: Mass of weight of spirit lamp + butanol / g x Final mass of spirit lamp + butanol / g y Mass of butanol used / g x-y = z Highest temperature of water / °C θ2 Initial temperature of water / °C θ1 Increased in temperature / °C θ2 - θ1 = θ3
  33. 33. Heat of Combustion When 2.7 g of glucose (C6H12O6) is burnt completely in excess oxygen, the heat released increases the temperature of 600 g of water by 12.5 °C. Calculate the heat of combustion of glucose. [Specific heat capacity of water = 4.2 J g-1 °C-1, density of water = 1.0 g cm-3, RAM: H = 1, C = 12, O = 16]
  34. 34. When 1 mole of butanol is burnt in excess of oxygen, 2600 kJ of heat is produced. Calculate the mass of butanol needed to burn completely in oxygen in order to raise the temperature of 250 cm3 of water by 30 °C. [Specific heat capacity of water = 4.2 J g-1 °C-1, density of water = 1.0 g cm-3, RAM: H = 1, C = 12, O = 16]
  35. 35. Comparing & contrasting heats of combustion Draw a graph of the number of carbon atoms in alcohol againts the magnitude of the heat of combustion. Alcohol Molecular formula Heat of combustion (kJ/mol) Methanol -728 Ethanol -1376 Propanol -2016 Butanol -2678 Pentanol - 3332 Hexanol
  36. 36. From the graph: 1. State the relationship between the number of carbon atoms in an alcohol and the heat of combustion 2. Predict the heat of combustion of hexanol
  37. 37. Compare the heat of combustion between ethanol and butanol. Explain why there is a difference in the heat of combustion between ethanol and butanol. 1. The heat of combustion of butanol is higher than ethanol. 2. Butanol has higher number of carbons per molecules than ethanol 3. Butanol will produced more carbon dioxide and water than ethanol 4. The combustion of butanol produced more heat than ethanol.

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