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Thermochemistry

Thermochemistry . DO NOW. The specific heat of ethanol is 2.44 J/ g°C . How many kilojoules of energy are required to heat 50.0 g of ethanol from -20.0°C to 68°C?. Objective . Describe how calorimeters are used to measure heat flow . Construct thermochemical equations .

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Thermochemistry

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  1. Thermochemistry

  2. DO NOW • The specific heat of ethanol is 2.44 J/g°C. How many kilojoules of energy are required to heat 50.0 g of ethanol from -20.0°C to 68°C?

  3. Objective • Describe how calorimeters are used to measure heat flow. • Constructthermochemical equations. • Solve for enthalpy changes in chemical reactions by using heats of reaction.

  4. Calorimetry • Calorimetry - the measurement of the heat into or out of a system for chemical and physical processes. • Based on the fact that the heat released = the heat absorbed • The device used to measure the absorption or release of heat in chemical or physical processes is called a “Calorimeter”

  5. Enthalpy • Heat absorbed or released by a reaction at constant pressure H = qq = H = m x C x T Note: We cannot calculate the actual value of enthalpy, only the change in enthalpy

  6. Thermochemical equation • Includes the ΔH as either a product or a reactant CaO(s) + H2O(l)  Ca(OH)2(s) + 65.2 kJ

  7. Heat of Reaction H = Hproducts – Hreactants

  8. Exothermic Reaction 2CO(g) + O2(g)  2CO2(g) H = -566.8 kJ Or 2CO(g) + O2(g)  2CO2(g) + 566.8 kJ Negative sign means energy is released

  9. Energy Change is down ΔH is <0 = Exothermic (heat is given off) Reactants ® Products

  10. 2CO + O2 Energy 2CO2 Reactants ® Products 2CO(g) + O2(g) → 2CO2(g) + 566.8 kJ 566.8kJ given off

  11. Endothermic Reaction 2CO2(g)  2CO(g) + O2(g) H = +566.8 kJ Or 2CO2(g) + 566.8 kJ  2CO(g) + O2(g) Positive sign means energy is absorbed

  12. Energy Change is up ΔH is > 0 = Endothermic (heat is absorbed) Reactants ® Products

  13. 2CO(g) + O2(g) Energy 2CO2(g) Reactants ® Products CaCO3→ CaO + CO2 2CO2(g) + 566.8 kJ →2CO(g) + O2(g) 566.8kJ absorbed

  14. Chemistry Happens in MOLES • An equation that includes energy is called a thermochemical equation • CH4 + 2O2® CO2 + 2H2O + 802.2 kJ • 1 mole of CH4 releases 802.2 kJ of energy. • When you make 802.2 kJ you also make 2 moles of water

  15. The magnitude of ΔH is directly proportional to the amount of reactants or products. • 1A + 2 B ----> 1C ΔH = -100 kJ • 1/2 A + 1B ----> 1/2 C ΔH = -50 kJ

  16. CH4 + 2O2® CO2 + 2H2O; ΔH = -802.2 kJ Rewrite chemical equation as a thermochemical equation. Exothermic or endothermic reaction? If 3 moles of O2 react with excess CH4 how much heat will be produced?

  17. 1CH4(g) + 2 O2(g)® CO2(g) + 2 H2O(l) + 802.2 kJ • If 10. 3 grams of CH4 are burned completely, how much heat will be produced? Convert moles to desired unit Convert to moles Start with known value 1 mol CH4 802.2 kJ 10. 3 g CH4 16.05 g CH4 1 mol CH4 = 514 kJ Ratio from balanced equation ΔH = -514 kJ, which means the heat is released for the reaction of 10.3 grams CH4

  18. 4 NO(g) + 6 H2O(l)  4 NH3(g) + 5 O2(g)    ΔH = +1170 kJ • Based upon the thermochemical equation given, calculate the heat associated with the decomposition of 1.15 g of NO.

  19. 2 C2H6(g) + 7 O2(g)  4 CO2(g) + 6 H2O(l)   ΔH = -3120 kJ • Calculate the mass of ethane, C2H6, which must be burned to produce 100 kJ of heat.

  20. Heat of Reaction CaO(s) + H2O(l)  Ca(OH)2(s) ΔH = -65.2 kJ CaO + H2O Enthalpy (H) ΔH = -65.2 kJ Ca(OH)2

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