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Measuring Heat

reaction. reaction. Measuring Heat. Exothermic reaction, heat given off & temperature of water rises. Endothermic reaction, heat taken in & temperature of water drops. Calorimetry : the accurate and precise measurement of heat change for chemical and physical processes.

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Measuring Heat

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  1. reaction reaction Measuring Heat Exothermic reaction, heat given off & temperature of water rises Endothermic reaction, heat taken in & temperature of water drops

  2. Calorimetry: the accurate and precise measurement of heat change for chemical and physical processes. Heat released by the system = heat absorbed by its surroundings Calorimeter: the insulated device used to measure the absorption or release of heat in chemical or physical processes

  3. ENTHALPY (H) Used to quantify the heat flow into or out of a system in a process that occurs at constant pressure H = H (products) – H (reactants) • H = q • this relation is only valid at constant pressure • q = H = m x C x T As most reactions in chemistry take place at constant pressure we can say that: A change in enthalpy = heat supplied

  4. Burning fossil fuels is an exothermic reaction EXOTHERMIC & ENDOTHERMIC REACTIONS Exothermic process: a change (e.g. a chemical reaction) that releases heat. A release of heat corresponds to a decrease in enthalpy Exothermic process: H < 0 (at constant pressure)

  5. Endothermic process: a change (e.g. a chemical reaction) that requires (or absorbs) heat. An input of heat corresponds to an increase in enthalpy Endothermic process: H > 0 (at constant pressure) Forming Na+ and Cl- ions from NaCl is an endothermic process Photosynthesis is an endothermic reaction (requires energy input from sun)

  6. Constant-Pressure Calorimeter: used for reactions occurring in solution and in open air No Heat enters or leaves

  7. Constant- Volume chamber: measures the heat released from burning a compound The mass of the system is constant Bomb Calorimeter

  8. H2O (s) H2O (l) DH = 6.01 kJ Thermochemical Equations Is ΔH negative or positive? System absorbs heat Endothermic ΔH > 0 6.01 kJ are absorbed for every 1 mole of ice that melts at 00C and 1 atm.

  9. DH = -890.4 kJ CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) Thermochemical Equations Is ΔH negative or positive? System gives off heat Exothermic ΔH < 0 890.4 kJ are released for every 1 mole of methane that is combusted at 250C and 1 atm.

  10. H2O (s) H2O (l) 2H2O (s) 2H2O (l) H2O (l) H2O (s) DH = -6.01 kJ DH = 6.01 kJ/molΔH = 6.01 kJ DH = 2 mol x 6.01 kJ/mol = 12.0 kJ Thermochemical Equations • The stoichiometric coefficients always refer to the number of moles of a substance • If you reverse a reaction, the sign of ΔH changes • If you multiply both sides of the equation by a factor n, then ΔH must change by the same factor n.

  11. How much heat is evolved when 266 g of white phosphorus (P4) burn in air? x H2O (l) H2O (g) H2O (s) H2O (l) 3013 kJ 1 mol P4 x DH = 6.01 kJ DH = 44.0 kJ 1 mol P4 123.9 g P4 Thermochemical Equations • The physical states of all reactants and products must be specified in thermochemical equations. P4(s) + 5O2(g) P4O10(s)ΔHreaction = -3013 kJ = 6470 kJ 266 g P4

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