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L24-post

# L24-post - Phase Change Lecture 24 Fuelish Choices Heats of...

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1 Lecture 24 Fuelish Choices: Heats of Reaction Key Idea: Energy exchange can be quantified with units of calories or Joules . Phase Change Direction of Energy Transfer Sign of q Type of change H 2 O (l) H 2 O (g) Surr. System q > 0 Endothermic H 2 O (g) H 2 O (l) System Surr. q < 0 Exothermic * How do we measure q? * How large or small are values for q? How do we understand and predict q? * Do we have to measure q for every possible reaction? ENTHALPY : Heat transfer at constant pressure The energy absorbed or released as heat in a phase transition or a chemical reaction at constant pressure is called the “ enthalpy change ,” and is denoted “ H H 2 O (g) Increasing Enthalpy H 2 O (l) H vap = + 44 kJ/mol o H cond = 44 kJ/mol o H : standard enthalpy change P=1 atm, T=25 o C o H 2 O (g) Increasing Enthalpy H 2 O (l) H vap = + 44 kJ/mol o H cond = 44 kJ/mol o H : standard enthalpy change P=1 atm, T=25 o C o * What is H for ½ H 2 O (g) ½ H 2 O (l) ? o * What is H for 2 H 2 O (l) 2 H 2 O (g) ? o C 6 H 12 O 6 (s) + 6 O 2 (g) 6 CO 2 (g) + 6 H 2 O (g) H = 2538 kJ/mol o H = + 2538 kJ/mol o Increasing Enthalpy 6 CO 2 (g) + 6 H 2 O (g) C 6 H 12 O 6 (s) + 6 O 2 (g) 6 CO 2 (g) + 6 H 2 O (l) H = 6 x H cond (H 2 O) = ( 6 mol H 2 O) x ( 44 kJ/mol H 2 O) = 264 kJ o H = 2538 kJ/mol + 264 kJ/mol (of C 6 H

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