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AS MODULE CHEM2 ENERGETICS

AS MODULE CHEM2 ENERGETICS. Enthalpy change (ΔH ) know that reactions can be endothermic or exothermic understand that enthalpy change (Δ H ) is the heat energy change measured under conditions of constant pressure

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AS MODULE CHEM2 ENERGETICS

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  1. AS MODULE CHEM2 ENERGETICS

  2. Enthalpy change (ΔH ) • know that reactions can be endothermic or exothermic • understand that enthalpy change (ΔH ) is the heat energy change measured under conditions of constant pressure • know that standard enthalpy changes refer to standard conditions, i.e. 100 kPa and a stated temperature (e.g. ΔH298) • be able to recall the definition of standard enthalpies of combustion (ΔH̳) and formation (ΔH̲) • Calorimetry • be able to calculate the enthalpy change from the heat change in a reaction using the equation q = mc ΔT • Enthalpy change (ΔH ) • know that reactions can be endothermic or exothermic • understand that enthalpy change (ΔH ) is the heat energy change measured under conditions of constant pressure • know that standard enthalpy changes refer to standard conditions, i.e. 100 kPa and a stated temperature (e.g. ΔH298) • be able to recall the definition of standard enthalpies of combustion (ΔH̳) and formation (ΔH̲̲) • Calorimetry • be able to calculate the enthalpy change from the heat change in a reaction using the equation q = mc ΔT

  3. Energetics - Terms System The methane and oxygen, and the carbon dioxide and water produced. CH4 (g) + 2O2 (g)CO2 (g) + H2O (l) Surroundings Everything that is not the chemical system.

  4. Energetics - Terms Calculating energy transferred during reactions Enthalpy CH4 (g) + 2O2 (g)CO2 (g) + H2O (l) Reactions carried out OPEN to the atmosphere, so… …any gases produced can escape, so… …measurements involving the system are carried out at… …CONSTANT PRESSURE. The heat energy transfer of a reaction when measured under CONSTANT PRESSURE is known as the ... ΔH ENTHALPY CHANGE,

  5. Energetics - Terms Calculating energy transferred during reactions CH4 (g) + 2O2 (g)CO2 (g) + H2O (l) ΔH (reaction) H (reactants) H (products) …only the changes in enthalpy when a reaction takes place. H(products) - H(reactants) but... The actual enthalpy content of a compound CANNOT be measured…

  6. Absolute v Relative Enthalpy What is the difference in height between the two individuals? If we can measure each person’s height: height difference = (x – y) cm 2. If we can’t measure each height: height difference = (x – y) cm y cm x cm

  7. Absolute v Relative Enthalpy What is the difference in height between the two individuals? If we can measure each person’s height: height difference = (x – y) cm Height has been calculated using absolute measurements. z cm 2. If we can’t measure each height: height difference = z cm Height has been calculated using relative measurements. Both measurements give the same value!

  8. Exothermic SURROUNDINGS Temperature of surroundings rises ΔH = -ve SYSTEM EXOTHERMIC reaction System loses energy to the surroundings

  9. Endothermic SURROUNDINGS Temperature of surroundings falls ΔH = +ve SYSTEM ENDOTHERMIC reaction System gains energy from the surroundings

  10. Enthalpy Changes - Examples Exothermic: ΔH = -ve combustion of fuels respiration (oxidation of carbohydrates) Endothermic: ΔH = +ve photosynthesis thermal decomposition of calcium carbonate

  11. Enthalpy Changes - Definitions Standard Enthalpy Changes Enthalpy change values depend on... 298K (25ºC) TEMPERATURE 100kPa (1 atm) PRESSURE at which the reaction takes place. To enable comparisons to be made, we measure enthalpy changes under STANDARD CONDITIONS.

  12. Enthalpy change (ΔH ) • know that reactions can be endothermic or exothermic • understand that enthalpy change (ΔH ) is the heat energy change measured under conditions of constant pressure • know that standard enthalpy changes refer to standard conditions, i.e. 100 kPa and a stated temperature (e.g. ΔH298) • be able to recall the definition of standard enthalpies of combustion (ΔH̳) and formation (ΔH̲) • Calorimetry • be able to calculate the enthalpy change from the heat change in a reaction using the equation q = mc ΔT • Enthalpy change (ΔH ) • know that reactions can be endothermic or exothermic • understand that enthalpy change (ΔH ) is the heat energy change measured under conditions of constant pressure • know that standard enthalpy changes refer to standard conditions, i.e. 100 kPa and a stated temperature (e.g. ΔH298) • be able to recall the definition of standard enthalpies of combustion (ΔH̳) and formation (ΔH̲̲) • Calorimetry • be able to calculate the enthalpy change from the heat change in a reaction using the equation q = mc ΔT

  13. Enthalpy Changes - Definitions Standard Enthalpy Changes Symbolic representation: ΔHʅ ʅ implies ‘standard conditions’ Two important enthalpy changes you must be able to define: Standard Enthalpy of Combustion Standard Enthalpy of Formation

  14. Plimsoll Lines

  15. Plimsoll Lines

  16. Plimsoll Lines

  17. Plimsoll Lines

  18. Enthalpy Changes - Definitions DHʅc Standard Enthalpy of Combustion ..is the enthalpy change... when 1 mole of substance, (the amount)of substance in its standard state, (its physical state under standard conditions) burns completely in oxygen (enough oxygen for complete combustion) to form products (the substances made during the reaction) in their standard states, (their physical states under standard conditions) under standard conditions. Temperature: (298 K (25ºC), Pressure:100 kPa (1 atm)

  19. Enthalpy Changes - Definitions Standard Enthalpy of Combustion ..is the enthalpy change when 1 mole of substance, in its standard state,burns completely in oxygen to form products in their standard states, under standard conditions. Values: always exothermic Examples: C(graphite)+ O2(g)  CO2(g) H2(g)+ ½O2(g)  H2O(l) C2H5OH(l)+ 3O2(g) 2CO2(g)+ H2O(l) NB. always only ONE MOLE of what is being burned on the LHS of the equation

  20. Enthalpy Changes - Definitions DHʅf Standard Enthalpy of Formation ..is the enthalpy change... when 1 mole of a compound, (the amount)of a compound* in itsstandard state, (its physical state under standard conditions) is formed from its elements (the elements the compound contains) in their standard states, (their physical states under standard conditions) under standard conditions. Temperature: (298 K (25ºC), Pressure:100 kPa (1 atm) * Enthalpy of formation of an element is defined as ZERO

  21. Enthalpy Changes - Definitions Standard Enthalpy of Formation ..is the enthalpy change when 1 mole of substance, in its standard state,is formed from its elements in their standard states, under standard conditions. Values: usually (but not exclusively) exothermic Examples: C(graphite)+ O2(g)  CO2(g) H2(g)+ ½O2(g)  H2O(l) 2C(graphite)+ ½O2(g)+ 3H2(g) C2H5OH(l) NB. always only ONE MOLE of product on the RHS of the equation

  22. Enthalpy Changes - Definitions Remember! Standard Enthalpy of Formation ...of an element ..is defined as ZERO

  23. Enthalpy Changes - Definitions Standard Enthalpy of Combustion ..is the enthalpy change when 1 mole of substance, in its standard state,burns completely in oxygen to form products in their standard states, under standard conditions. Standard Enthalpy of Formation ..is the enthalpy change when 1 mole of substance, in its standard state,is formed from its elements in their standard states, under standard conditions.

  24. Note! per 1 mole of the substance Note! per 1 mole of the compound Enthalpy Changes - Definitions Standard Enthalpy of Combustion – example… Combustion of methane: ΔHʅ = -890 kJmol-1 CH4 (g) + 2O2 (g)CO2 (g) + H2O (l) Standard Enthalpy of Formation – example… Formation of sulphuric acid: Δ Hʅ = -814 kJmol-1 H2 (g) + S(s) + 2O2(g) H2SO4(l) Note! The reaction equation for the enthalpy of formation is very rarely a viable way of making the compound.

  25. Enthalpy Changes - Definitions Write an equation showing the enthalpy of combustion of… C2H6 (g) + 3½O2 (g)2CO2 (g) + 3H2O (l) ethane CH3OH+ O2CO2 + 2H2O methanol CH3OH(l) + O2 (g)CO2 (g) + H2O (l) CH3OH(l) + 1½O2 (g)CO2 (g) + 2H2O (l) Write an equation showing the enthalpy of formation of… ½ N2 (g) + 1½H2 (g) NH3 (g) ammonia Pb(s) + N2 (g) + 3O2(g) Pb(NO3)2(s) lead nitrate

  26. Enthalpy Changes - Calorimetry Calorimetry… …the process of measuring enthalpy changes of reaction... …by measuring the temperature changes of the surroundings...

  27. Enthalpy Changes - Calorimetry 24ºC 18ºC It takes 50 kJ of energy to make this amount of substance go up 1 degree C 6 deg C specific heat capacity the heat needed to raise the temperature of 1 gram of it by 1 kelvin (1 deg C) x kJ of energy = 6 x 50 = 300 kJ

  28. Enthalpy Changes - Calorimetry Calorimetry… …the process of measuring enthalpy changes of reaction... …by measuring the temperature changes of the surroundings... …and using specific heat capacities to calculate energy transfers. The specific heat capacity of a substance is the heat needed to raise the temperature of 1 gram of it by 1 kelvin (1 deg C) J g-1 K-1 Unit:

  29. Enthalpy Changes - Calorimetry Calorimetry… If the energy change in a reaction can be used to change the temperature of a known mass of water… …then, since 1g of it needs 4.18J to raise it by 1K (its specific heat capacity)… …the energy, q, transferred to it would be… mass of water x specific heat capacity of water x temperature change m x c x ΔT q = m x c x ΔT

  30. Thermometer Glass stirrer Enthalpy Changes - Calorimetry Calorimetry: To determine enthalpy change in neutralisation reactions A well-insulated container for the reaction is needed… Heat loss must be minimised! Cork lid Two polystyrene cups together containing reactants

  31. Enthalpy Changes - Calorimetry Calorimetry: To determine enthalpy change in neutralisation reactions 1. Measure out 50cm3 of 1.0M sodium hydroxide solution and put it in the polystyrene cup.. 2. Measure 50cm3 of 1.0M hydrochloric acid solution into a glass beaker. 3. Measure the temperature of the sodium hydroxide solution, and then every minute for three minutes. 21.0 0 1 20.5 2 20.0 20.0 3 HCl NaOH

  32. Enthalpy Changes - Calorimetry Calorimetry: To determine enthalpy change in neutralisation reactions 4. At minute 4, add the 50cm3 of hydrochloric acid to the sodium hydroxide… 5. Continue taking the temperature every minute until the tenth minute… 21.0 0 1 20.5 2 20.0 20.0 3 4 acid added 20.0 5 6 25.6 25.1 7 8 24.8 HCl NaOH 24.7 9 10 24.6

  33. Enthalpy Changes - Calorimetry Calorimetry: To determine enthalpy change in neutralisation reactions 4. At minute 4, add the 50cm3 of hydrochloric acid to the sodium hydroxide… 5. Continue taking the temperature every minute until the tenth minute… 21.0 0 1 20.5 2 20.0 20.0 3 4 acid added 20.0 5 6 25.6 25.1 7 8 24.8 HCl 24.7 9 10 24.6

  34. Enthalpy Changes - Calorimetry Calorimetry: To determine enthalpy change in neutralisation reactions 4. At minute 4, add the 50cm3 of hydrochloric acid to the sodium hydroxide… 5. Continue taking the temperature every minute until the tenth minute… and interpolate to get the temp at 4 minutes when 6. Graph the results… the acid was added… 21.0 0 1 20.5 2 20.0 20.0 3 4 acid added 20.0 5 6 25.6 25.1 7 8 24.8 24.7 9 10 24.6

  35. 50 cm3 HCl + 50 cm3 NaOH Enthalpy Changes - Calorimetry An assumption will be made that since the solution is almost entirely water, its mass and specific heat capacity can be taken as being that of water Calorimetry: To determine enthalpy change in neutralisation reactions q = m x c x ΔT 7. Calculate the heat energy given out… 4.2Jg-1K-1 100 g Sp ht cap of soln = mass of solution = temperature change = 6.3 K density of solution = 1 gcm-3 26.3 100 cm3 aqueous soln. Temperature rise = 6.3 K 20.0 100 g sp. ht. cap. = 4.2 Jg-1K-1

  36. volume (cm3) x Molarity moles = 1000 50 x 1.0 0.05 moles moles = = 1000 Enthalpy Changes - Calorimetry Calorimetry: To determine enthalpy change in neutralisation reactions q = m x c x ΔT 7. Calculate the heat energy given out… 4.2Jg-1K-1 100 g Sp ht cap of soln = mass of solution = temperature change = 6.3 K q = m x c x ΔT = 100 x 4.2 x 6.3 = 2646 J = 2.646 kJ 8. Calculate the number of moles of reactants… acid and alkali… ΔH(neutralisation) = q / no. of moles 9. Calculate the molar heat of neutralisation… = 2.646 / 0.05 = -52.92 kJmol-1 (- sign shows that energy was given out)

  37. shields – minimise heat loss / draughts thermometer – to record temperature before and after experiment known mass of water spirit lamp, weighed before and after to find the mass of alcohol burned Enthalpy Changes - Calorimetry Calorimetry: To determine enthalpy change in combustion reactions (e.g. of an alcohol, ethanol) - equipment required…

  38. Enthalpy Changes - Calorimetry Calorimetry: To determine enthalpy change in combustion reactions (e.g. of an alcohol, ethanol) - equipment required… The equation for the reaction is... C2H5OH(g) + 3O2 (g)2CO2 (g) + 3H2O (l) 1. Weigh the spirit lamp containing ethanol… 2. Set up the apparatus as shown with 100 ml of water in the calorimeter… 3. Take the temperature of the water … and stir the water. 4. Light the wick… 5. Keep noting the temperature and after it has increased by about 30 deg C, record it and extinguish the burner… 6. Reweigh the spirit lamp to determine the mass of ethanol burnt.

  39. Enthalpy Changes - Calorimetry Calorimetry: To determine enthalpy change in combustion reactions (e.g. of an alcohol, ethanol) - typical results…

  40. moles = Mr mass Enthalpy Changes - Calorimetry Calorimetry: To determine enthalpy change in combustion reactions (e.g. of an alcohol, ethanol) - calculation… q = m x c x ΔT 1. Calculate the heat energy given out… 100 g Mass of ethanol = mass of water = temperature change = 27.6 K 0.54 g q = m x c x ΔT = 100 x 4.2 x 27.6 = 11592 J = 11.59 kJ 2. Calculate the number of moles of ethanol burnt… 0.54 0.0117 moles moles = = 46 ΔH(combustion) = q / no. of moles 3. Calculate the molar enthalpy of combustion… = 11.59 / 0.0117 = -990.6 kJmol-1 (- sign shows that energy was given out)

  41. Enthalpy Changes - Calorimetry Calorimetry: To determine enthalpy change in combustion reactions (e.g. of an alcohol, ethanol) - Improving the accuracy… Not all of the energy produced by the burning ethanol is absorbed by the water • the calorimeter itself absorbs some heat energy • heat is lost to the air

  42. Enthalpy Changes - Calorimetry Calorimetry: To determine enthalpy change in combustion reactions (e.g. of an alcohol, ethanol) - Accounting for the heat absorbed by the calorimeter… Assume… sp ht cap of copper = 0.387 Jg-1K-1 mass of calorimeter = 78 g temperature rise = 27.6 K q = m x c x ΔT = 78 x 0.387 x 27.6 = 833.1 J = 0.833 kJ Add this to the previously calculated value… = 991.4 kJmol-1 + 0.833 990.6

  43. Enthalpy Changes - Calorimetry Calorimetry: To determine enthalpy change in combustion reactions (e.g. of an alcohol, ethanol) - Improving the accuracy… Flame calorimeter The fuel is burned and the heat is passed through a copper coil that is surrounded by water.

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