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Thermochemistry

Thermochemistry. Energy change that occurs during a chemical reaction and/or changes in state. Heat. A form of energy Flows from warmer objects to cooler objects Represented by the variable “q” Measured by units: calorie or joule 1 J = 0.2390 cal 4.184 J = 1 cal

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Thermochemistry

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  1. Thermochemistry Energy change that occurs during a chemical reaction and/or changes in state

  2. Heat • A form of energy • Flows from warmer objects to cooler objects • Represented by the variable “q” • Measured by units: calorie or joule 1 J = 0.2390 cal 4.184 J = 1 cal 1000 cal = 1 kilorcalorie = 1 Cal

  3. Exothermic and Endothermic • Exothermic Process • Energy flows from the system into the surroundings • Endothermic Process • Energy flows from the surroundings into the system • Conservation of Energy • Energy is not created or destroyed in a physical or chemical process • If energy in a system decreases, then the energy of the surroundings increases by the same amount

  4. Heat Capacity • Amount of heat needed to increase the temperature of an object exactly 1 ˚C • Depends on mass and chemical composition • Greater mass = greater heat capacity • Specific Heat Capacity, C (Specific Heat) • Amount of heat needed to raise 1 g of a substance by 1 ˚C • Metals have low specific heats • Water has a relatively high specific heat

  5. Calculating Specific Heat • C = q / (m x ΔT) • C is the variable for specific heat • Could be in units of J/(g ˚C) or cal/(g ˚C) • q is the variable for heat (joules or calories) • M is the variable for mass (grams) • ΔT (Tf – Ti) is the variable for change in temperature (˚C) *become familiar with table 17.1 on page 508

  6. Measuring Enthalpy Changes • Enthalpy (H) is the heat content of a system at constant pressure • Heat absorbed or released by a system (constant pressure) is the change in enthalpy, ΔH • We will only work with a constant pressure scenario…therefore, q = ΔH • Calorimetry • Measurement of heat flow into/out of a system • Calorimeter is the device used to measure heat flow

  7. Put it all together… • If a system gains heat, that amount of heat was lost by the surroundings… • qsys = - qsurr(the negative sign represents loss) • ΔH = qsys= - qsurr= - m x c x ΔT • Exothermic reactions have a negative value for ΔH • Endothermic reactions have a positive value for ΔH

  8. Thermochemical Equation • A chemical equation that includes enthalpy change • Can be either a “reactant” • Endothermic 2NaHCO3 + 128 kJ  Na2CO3 + H2O + CO2 or 2NaHCO3 Na2CO3 + H2O + CO2 ΔH = 129 kJ • Or a “product” • Exothermic CaO + H2O  Ca(OH)2 + 65.2 kJ or CaO + H2O  Ca(OH)2 ΔH = -65.2 kJ

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