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iGCSE chemistry Section 4 lesson 2

iGCSE chemistry Section 4 lesson 2. Content. The iGCSE Chemistry course. Section 1 Principles of Chemistry Section 2 Chemistry of the Elements Section 3 Organic Chemistry Section 4 Physical Chemistry Section 5 Chemistry in Society. Content. Section 4 Physical Chemistry.

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iGCSE chemistry Section 4 lesson 2

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  1. iGCSE chemistrySection 4 lesson 2

  2. Content The iGCSE Chemistry course Section 1 Principles of Chemistry Section 2 Chemistry of the Elements Section 3 Organic Chemistry Section 4 Physical Chemistry Section 5 Chemistry in Society

  3. Content Section 4 Physical Chemistry • Acids, alkalis and salts • Energetics • Rates of reaction • Equilibria

  4. Lesson 2 b) Energetics b) Energetics 4.10 understand that chemical reactions in which heat energy is given out are described as exothermic and those in which heat energy is taken in are endothermic 4.11 describe simple calorimetry experiments for reactions such as combustion, displacement, dissolving and neutralisation in which heat energy changes can be calculated from measured temperature changes 4.12 calculate molar enthalpy change from heat energy change 4.13 understand the use of ΔH to represent enthalpy change for exothermic and endothermic reactions 4.14 represent exothermic and endothermic reactions on a simple energy level diagram 4.15 understand that the breaking of bonds is endothermic and that the making of bonds is exothermic 4.16 use average bond energies to calculate the enthalpy change during a simple chemical reaction.

  5. Exothermic and endothermic reactions

  6. Exothermic and endothermic reactions In a chemical reaction there is usually a temperature change.

  7. Exothermic and endothermic reactions In a chemical reaction there is usually a temperature change. In an EXOTHERMIC reaction, the temperature goes UP. Heat energy is given OUT.

  8. Exothermic and endothermic reactions In a chemical reaction there is usually a temperature change. In an EXOTHERMIC reaction, the temperature goes UP. Heat energy is given OUT. In an ENDOTHERMIC reaction, the temperature goes DOWN. Heat energy is taken IN.

  9. Exothermic and endothermic reactions Examples of EXOTHERMIC reactions: Combustion is a common example of an exothermic reaction. Methane + oxygen  Carbon dioxide + water + Heat energy Carbon + oxygen  Carbon dioxide + heat energy (coal)

  10. Exothermic and endothermic reactions Examples of EXOTHERMIC reactions: Neutralisation is another example of an exothermic reaction. Acid + Alkali  Salt + Water + Heat energy

  11. Exothermic and endothermic reactions Examples of EXOTHERMIC reactions: Neutralisation is another example of an exothermic reaction. Acid + Alkali  Salt + Water + Heat energy The symbol to show a change in the amount of heat energy is ∆H (pronounced delta H)

  12. Exothermic and endothermic reactions reactants Increasing energy Loss of energy For an EXOTHERMIC reaction, ∆H is negative. This means that heat has been lost from the reaction. products Time

  13. Exothermic and endothermic reactions Examples of ENDOTHERMIC reactions: Endothermic reactions tend to be less common. Dissolving ammonium nitrate crystals in water is an endothermic reaction. Ammonium + Water  ammonium nitrate – Heat nitrate solution energy Temperature of reactants = 20oC Temperature of products = 13oC

  14. Exothermic and endothermic reactions products Increasing energy Gain in energy For an ENDOTHERMIC reaction, ∆H is positive. This means that heat has been gained in the reaction. reactants Time

  15. Exothermic and endothermic reactions Let’s just recap

  16. Exothermic and endothermic reactions Let’s just recap EXOTHERMIC

  17. Exothermic and endothermic reactions Let’s just recap EXOTHERMIC Heat is given out

  18. Exothermic and endothermic reactions Let’s just recap EXOTHERMIC Heat is given out ∆H is negative

  19. Exothermic and endothermic reactions Let’s just recap ENDOTHERMIC

  20. Exothermic and endothermic reactions Let’s just recap ENDOTHERMIC Heat is taken in

  21. Exothermic and endothermic reactions Let’s just recap ENDOTHERMIC Heat is taken in ∆H is positive

  22. Exothermic and endothermic reactions Another word for the heat energy content of a compound is ENTHALPY

  23. Exothermic and endothermic reactions You need to know that the unit of energy is the JOULE (J).

  24. Exothermic and endothermic reactions You need to know that the unit of energy is the JOULE (J). 1000 joules = 1 kilojoule (kJ)

  25. Calculating enthalpy changes

  26. Calculating enthalpy changes The enthalpy change of a reaction is the enthalpy of the products minus the enthalpy of the reactants.

  27. Calculating enthalpy changes The enthalpy change of a reaction is the enthalpy of the products minus the enthalpy of the reactants. Eg. Enthalpy of combustion of a fuel

  28. Calculating enthalpy changes Eg. Enthalpy of combustion of a fuel The amount of energy released or absorbed in a reaction can be measured using this formula: heat energy = mass of x rise in x specific heat produced water temperature capacity of water (joules) (grams) (oC) J/kg/oC

  29. Calculating enthalpy changes thermometer Measurements: Temperature of water before and after heating Mass of water used Mass of fuel used Draught shield Insulating card water Copper calorimeter Spirit burner

  30. Calculating enthalpy changes EXAMPLE: the temperature of 100g of water was increased by 32.6oC by heating with 0.46 grams of ethanol. Assuming no heat losses from the apparatus, what was the enthalpy of combustion of ethanol, C2H5OH?

  31. Calculating enthalpy changes heat energy = mass of x rise in x specific heat produced water temperature capacity of water (joules) (grams) (oC) J/kg/oC heat energy = 100 x 32.6 x 4.2 produced (joules) heat energy = 13692 (for 0.46g of ethanol) produced (joules)

  32. Calculating enthalpy changes heat energy = mass of x rise in x specific heat produced water temperature capacity of water (joules) (grams) (oC) J/kg/oC So what is the heat energy produced per mole of ethanol? heat energy = 100 x 32.6 x 4.2 produced (joules) heat energy = 13692 (for 0.46g of ethanol) produced (joules)

  33. Calculating enthalpy changes Another word for the heat energy content of a compound is ENTHALPY Remember that a mole of a substance is the relative molecular mass in grams heat energy = mass of x rise in x specific heat produced water temperature capacity of water (joules) (grams) (oC) J/kg/oC So what is the heat energy produced per mole of ethanol? heat energy = 100 x 32.6 x 4.2 produced (joules) heat energy = 13692 (for 0.46g of ethanol) produced (joules)

  34. Calculating enthalpy changes heat energy = 13692 (for 0.46g of ethanol) produced (joules) Ethanol C2H5OH = 24 + 5 + 16 + 1 = 46

  35. Calculating enthalpy changes heat energy = 13692 (for 0.46g of ethanol) produced (joules) Ethanol C2H5OH = 24 + 5 + 16 + 1 = 46 So the mass of one mole of ethanol is 46g

  36. Calculating enthalpy changes heat energy = 13692 (for 0.46g of ethanol) produced (joules) Ethanol C2H5OH = 24 + 5 + 16 + 1 = 46 So the mass of one mole of ethanol is 46g The energy released from the combustion of one mole of ethanol = 13692 x 46/0.46 = 1369200 J = 1369.2 kJ

  37. Calculating enthalpy changes Burning ethanol is an EXOTHERMIC reaction, so heat energy is lost (∆H is –ve) The molar enthalpy of combustion of ethanol is therefore: = -1369.2 kJ/mol

  38. Calculating enthalpy changes Use a simple calorimeter to measure the enthalpy change in reactions such as dissolving, neutralisation and displacement.

  39. Calculating enthalpy changes thermometer Insulating lid Chemical mixture (eg. acid + alkali) Polystyrene cup (insulated)

  40. Calculating enthalpy changes Enthalpy of neutralisation = the heat produced when an acid and alkali react to produce one mole of water.

  41. Calculating enthalpy changes Enthalpy of neutralisation = the heat produced when an acid and alkali react to produce one mole of water. eg. 50cm3 of 2M hydrochloric acid were added to 50cm3 of 2M sodium hydroxide in a polystyrene cup. The temperature rose by 13.7oC.

  42. Calculating enthalpy changes eg. 50cm3 of 2M hydrochloric acid were added to 50cm3 of 2M sodium hydroxide in a polystyrene cup. The temperature rose by 13.7oC. If there were no heat losses, and a final volume of 100cm3, what was the enthalpy of neutralisation?

  43. Calculating enthalpy changes HCl + NaOH  NaCl + H2O

  44. Calculating enthalpy changes HCl + NaOH  NaCl + H2O 1 1 1 1

  45. Calculating enthalpy changes HCl + NaOH  NaCl + H2O 1 1 1 1 Number of moles in 50cm3 of 2M HCl = 50 x 2 = 0.1 1000

  46. Calculating enthalpy changes HCl + NaOH  NaCl + H2O 1 1 1 1 Number of moles in 50cm3 of 2M HCl = 50 x 2 = 0.1 1000 From the equation, 0.1 moles of HCl will produce 0.1 moles of H2O

  47. Calculating enthalpy changes HCl + NaOH  NaCl + H2O 1 1 1 1 Number of moles in 50cm3 of 2M HCl = 50 x 2 = 0.1 1000 From the equation, 0.1 moles of HCl will produce 0.1 moles of H2O Heat released in producing 0.1M = 100 x 13.7 x 4.2 = 5754J of water

  48. Calculating enthalpy changes Heat released in producing 0.1M = 100 x 13.7 x 4.2 = 5754J of water Enthalpy of = 5754 x 1/0.1 J/mol neutralisation

  49. Calculating enthalpy changes Heat released in producing 0.1M = 100 x 13.7 x 4.2 = 5754J of water Enthalpy of = 5754 x 1/0.1 J/mol neutralisation = - 57540 J/mol = - 57.54 kJ/mol (remember: because this is an exothermic reaction, ∆H is negative)

  50. Making and breaking bonds

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