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Thermochemistry I

Thermochemistry I. Exothermic, Endothermic, specific heat, heating/ cooling curves. In thermodynamics, the term exothermic is a chemical reaction or process that releases energy in the form of light or heat. Expressed in a chemical equation: reactants → products + energy

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Thermochemistry I

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  1. Thermochemistry I Exothermic, Endothermic, specific heat, heating/cooling curves

  2. In thermodynamics, the term exothermic is a chemical reaction or process that releasesenergy in the form of light or heat. • Expressed in a chemical equation: reactants → products + energy • The opposite is an endothermic reaction. The term endothermic describes a process or reaction that absorbsenergy from its surroundings in the form of heat. • Expressed in a chemical equation: reactants + heat → products • Thus in each term (endothermic & exothermic) the prefix refers to where heat goes as the reaction occurs. • Heat always travels from a hotter source to a colder source.

  3. The specific heat capacity, often called the specific heat, (symbol c) is defined as the amount of heat energy (symbol q) that has to be transferred to or from one unit of mass (gram) to change the system temperature by one degree. The units for the specific heat capacity (c) are: q = joules (J) m = g (or Kg) T = K (or C) The molar heat capacity is the heat capacity per mole of a pure substance. c = q/m DT

  4. Example: It takes 487.5 J to heat 25 grams of copper from 25 °C to 75 °C. What is the specific heat in Joules/g·°C? c = q/m DT c = 487.5 J / 25 g x 50 oC c = 487.5 J / 1250 g oC c = 0.39 J /g oC Example: What is the heat in Joules required to raise the temperature of 25 grams of water from 0 °C to 100 °C? (specific heat of water = 4.18 J/g·°C) q = c m DT q = 4.18 J /g oC x 25g x 100 oC q = 10450 J

  5. The diagram below shows the uptake of heat by 1 kg of water from -50 oCto above 100 oC. A: Rise in temperature as ice absorbs heat.B: Absorption of  heat of fusion.C: Rise in temperature as liquid water absorbs heat.D: Water boils and absorbs  heat of vaporization.E: Steam absorbs heat and thus increases its temperature.

  6. EXAMPLE: Calculate the amount of heat required to completely convert 50 g of ice at -10 ºC to steam at 125 ºC. Heat is taken up in five stages: A. The heating of the ice B. The melting of the ice C. The heating of the water D. The vaporization of the water E. The heating of the steam The specific heat capacity of water is 4.18 J/g ºC The specific heat capacity for Ice and Steam are 2.09 J/g ºC The heat of fusion of ice is 334 J/g The heat of vaporization of water is 2260 J/g

  7. EXAMPLE: Calculate the amount of heat required to completely convert 50 g of ice at -10 ºC to steam at 125 ºC. The specific heat capacity of water is 4.18 J/g ºC The specific heat capacity for Ice and Steam are 2.09 J/g ºC The heat of fusion of ice is 334 J/g The heat of vaporization of water is 2260 J/g Review website

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