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MASS-ENERGY STOICHIOMETRY

MASS-ENERGY STOICHIOMETRY. A chemical equation which contains the energy change associated with the reaction is called a thermochemical equation. If a thermochemical equation contains either the energy gain or loss or the mass of one reactant or product, it is

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MASS-ENERGY STOICHIOMETRY

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  1. MASS-ENERGY STOICHIOMETRY A chemical equation which contains the energy change associated with the reaction is called a thermochemical equation. If a thermochemical equation contains either the energy gain or loss or the mass of one reactant or product, it is possible to calculate the other using mass-energy stoichiometry. In an exothermic reaction energy is given off and the heat of reaction “q” is negative. The energy will be on the right side of the equation. In an endothermic reaction, energy is put into the system and the “q” is positive. The energy will be shown on the left side of the equation.

  2. Sample Problem: Using the following equation, how many kilojoules would be produced by the combustion of 6.95 g of glucose, C6H12O6? 6.95 g ? C6H12O6 + O2 CO2 + H2O + 157.3 kJ 6 6 6 Step 1. Balance the equation. Step 2. Using dimensional analysis convert grams of given substance to moles. Step 3. Insert the ratio of energy (kJ) to moles of given substance. If the energy is on the left side it is positive, if the energy is on the right side then it is negative. Step 4. Cancel and solve 6.95 g C6H12 O6 1 mol C6H12O6 180.1 g C6H12O6 -157.3 kJ 1 mol C6H12O6 = -6.07 kJ

  3. Practice Problems Try to work the next problems on the paper first. Then use the power point to check your work.

  4. Practice Problem 1: How much energy, according to the following equation, is produced by the combustion of 64.2 g of propane (C3H8)? 64.2 g ? C3H8 + 5O2 3CO2 + 4H2O + 68.4 kJ Solve and then check your answer below. 64.2 g C3H8 1mol C3H8 44.1 g C3H6 -68.4 kJ 1 mol C3H8 = -99.6 kJ

  5. Practice Problem 2: How much energy is produced according to the following equation by the reaction of 78.9 g of sodium peroxide (Na2O2) with a excess of water? 78.9 g ? 2Na2O2 + 2H2O  4NaOH + O2 + 215.76 kJ Solve and then check your answer below. 78.9 g Na2O2 1 mol Na2O2 78.0 g Na2O2 -215.76 kJ 2 mol Na2O2 = -109.1 kJ

  6. Practice Problem 3: According to the equation below, how much energy would be required to produce 9.21 grams of cobalt (II) oxide, CoO? Note: This is an endothermic reaction so the q is positive. ? 9.21 g CoCO3 + 81.6 kJ  CoO + CO2 Solve and then check your answer below. 9.21 g CoO 1 mol CoO 74.9 g CoO 81.6 kJ 1 mol CoO 10.0 kJ =

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