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Fuels and Heats of Reactions

Study of heat changes during a chemical reaction, including the measurement of enthalpy changes and the definition of heat of reaction and heat of combustion. Explains the concepts of exothermic and endothermic reactions.

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Fuels and Heats of Reactions

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  1. Fuels and Heats of Reactions

  2. Fuels and Heats of Reactions Study of heat changes during a chemical reaction Enthalpy • a measure of the heat content of a substance at constant pressure • you cannot measure the actual enthalpy of a substance • you can measure an enthalpy CHANGE • written as the symbol DH , “delta H ” Enthalpy change (DH) = Enthalpy of products - Enthalpy of reactants

  3. ENTHALPY REACTION CO-ORDINATE Fuels and Heats of Reactions An exothermic reaction is a chemical reaction that produces heat Enthalpy of reactants > products DH = - ive EXOTHERMICHeat given out Reactants Products

  4. ENTHALPY Products REACTION CO-ORDINATE Reactants Fuels and Heats of Reactions An endothermic reaction is a chemical reaction that absorbs heat Enthalpy of reactants < products DH = + ive ENDOTHERMICHeat absorbed

  5. Heat of Reaction Definition the enthalpy change when the number of moles of reactants indicated in the balanced equation for the reaction, react completely Symbol DH Values exothermic or endothermic Example(s) H2(g) + ½O2(g)  H2O(l) DH = -242 2H2(g) + O2(g)  2H2O(l) DH = -484 Notes units are kJ or kJmol-1 if working in molar quantities

  6. Heat of Combustion Definition the enthalpy change when one mole of a substance is completely burned in excess oxygen Symbol DHc Values always exothermic Example(s) H2(g) + ½O2(g)  H2O(l) DH = -242 C(s) + O2(g)  2CO2(g) DH = -484 Notes Always only one mole of what you are burning on the LHS of the equation

  7. Heat of Combustion • Heats of combustion measured using a container called a bomb calorimeter • Material is put in a small steel container with pure oxygen • The container is put into a container of water • The sample is ignited by electrical wires • The change in temperature is measured • The specific heat capacities of water and the material the bomb is made of are required for calculation purposes

  8. thermometer • stirrer • full of water • ignition wire • steel bomb • sample Heat of Combustion

  9. Heat of Combustion • Bomb calorimeters are also used to measure the efficiency of various fuels known as the Kilogram calorific value • The Kilogram calorific value of a fuel is the heat energy produced when 1 kg of the fuel is completely burned in oxygen

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