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Energy calculations

Energy calculations. Be careful in your use of units when doing these calculations. Energy is measured in J or kJ. All energy values are positive – you cannot have negative energy.

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Energy calculations

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  1. Energy calculations

  2. Be careful in your use of units when doing these calculations. Energy is measured in J or kJ. All energy values are positive – you cannot have negative energy. Enthalpy, ΔH, is measured in kJ mol–1 and can be either positive or negative depending on whether the reaction is endothermic or exothermic.

  3. The energy change in a reaction can be treated in the same way as any other reactant or product. N2(g) + 2O2(g) → 2NO2(g) ΔH = +66 kJ mol–1 For every mol of N2 burnt, 2 mol of NO2 are formed, and 66 kJ of energy is absorbed. So if 5 mol of N2 is burnt, 5 2 = 10 mol of NO2 are formed and 5 66 kJ = 330 kJ of energy is absorbed. 2 66 kJ How many moles of NO2 must be formed to absorb 200 kJ of energy? 200 kJ Think first: 200 kJ is more than 66 kJ, so there must be more than 2 mol of NO2 formed. Put the 200 kJ on top.

  4. What mass of B2O3 is formed if the combustion of B2H6 produced 510 kJ of energy? B2H6(g) + 3O2(g) → B2O3(s) + 3H2O(l) ΔH = –2040 kJ mol–1 M(B2O3) = 69.6 g mol–1 510 kJ 1 2040 kJ 69.6 g mol–1 First calculate the amount, in moles, of B2O3 formed. Think first: 510 kJ is less than 2040 kJ, so less than 1 mol of B2O3 is formed. Put the 510 kJ on top. 0.250 mol Then calculate the mass of B2O3 formed.

  5. Water and energy are both produced when propane burns. C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(l) ΔH = –2217 kJ mol–1 How much energy is produced if 100 g of water are formed? M(H2O) = 18 g mol–1. 4 2217 kJ 100 g 18 g mol–1 First calculate the amount of water formed in moles. 5.56 mol Then calculate the energy produced:

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