Enthalpy ( Δ H)

1. Enthalpy (ΔH)

2. Calorimetry • the measurement of heat flow in/out of a system • performed in a calorimeter • the heat released by the system is equal to the heat absorbed by the surroundings • enthalpy (ΔH) is the heat content in a system • heat (q) = enthalpy change (ΔH)

3. ΔH = m · C · ΔT • same formula as before, just substituting ΔH for q • When aqueous solutions of 25 ml of HCl and 25 ml of NaOH are mixed in a calorimeter at 21ºC, the temperature rises to 33ºC. What is the enthalpy change? Water’s specific heat is 4.18 J/(g ·ºC). • ΔH = 50g · 4.18 J/(g ·ºC) · 12ºC • 2,508 J

4. Heat in Changes of State • whenever a substance undergoes a state change, the temp. remains constant as it gains or loses heat • example- an ice cube melting gains heat but does not increase in temp. as it melts

5. molar heat of fusionΔHfus • the heat absorbed by one mole of a substance as it melts to a liquid • molar heat of solidification ΔHsolid • the heat lost by one mole of a substance as it solidifies • molar heat of vaporization ΔHvap • the heat necessary to vaporize one mole of a liquid • molar heat of condensation ΔHcond • the heat lost when one mole of a vapor condenses

6. How many grams of ice will melt if 3.2 kJ of heat are added? The ΔHfus of ice is 6.01kJ/mol. • 3.2 kJ x 1 mol ice x 18.0g ice 6.01 kJ 1 mol ice • 9.56 g ice

7. How much heat is absorbed when 24.8 g of water is converted to steam? The ΔHvap of water is 40.7kJ/mol. • 24.8.2 g H2O x 1 molH2O x 40.7 kJ 18.0 g H2O 1 mol H2O • 56.1 kJ