Thermochemistry Some say the world will end in fire, Some say in ice, From what I’ve tasted of desire I hold with those who favor fire. Robert Frost Fire and Ice

Principles of Heat Flow Thermochemistry is the study of heat flow System: the part of the universe on which we are focused Surroundings: exchange energy with system (in close contact with system)

State Properties Composition Temperature Pressure (remind you of stp?) Heat flow (q) q is + when heat flows into the system from the surroundings (heating of a cup of coffee) q is - when heat flows out of the system into the surrounds (cooling of a cup of coffee) if X is a state property, then ∆X = X final -X initial Heat Flow is NOT a state property

Endo- and Exothermic Processes An endothermic process (q>0) in which heat flows form the surroundings into the reaction system. And exothermic process(q<0) in which heat flows from the reaction system into the surroundings system surroundings IN OUT endothermic exothermic

Magnitude of Heat Flow q = C x ∆t ∆t = t final - t initial For a pure substance q = m x c x ∆t m is mass c is specific heat Specific heat is the amount of energy necessary to raise the temperature of 1 gram of a substance 1 degree C. t is temperature q is heat flow C is heat capacity q is measured in joules

Example How much heat is needed to raise the temperature of a 3000 gallon pool from 10 C to 20 C? The specific heat of water is 4.18 J/g o C 1. Q = m x c x ∆t 2. Plug and chug

Calorimetry Calorimeter: device that measures the heat flow in a reaction q reaction = -q calorimeter q reaction = -C cal x ∆t “old” heat unit is the calorie 1 cal = J (c) 1 kcal = kJ (C)

Coffee Cup Calorimeter Heat from the reaction inside the innermost cup flows out to the water in the outer cup. C cal = m water x c water q reaction =-m water x c water x ∆t

Bomb Calorimetry

Example The reaction between H and Cl : H 2(g) + Cl 2(g) --> 2HCl (g) can be studied in a bomb calorimeter. It is found that when a 1.00 g sample of H 2 reacts completely the temperature rises from C to C. Taking the heat capacity of the calorimeter to be 9.33 kj?oC, calculate the amount of heat evolved in the reaction. Which equation to use? q reaction = -C cal x ∆t Plug and chug Answer: kJ exothermic

Example When 1.00 g of CaCl 2 is added to 50.0 g of water in a coffee-cup calorimeter, it dissolves CaCl 2(s) --> Ca 2+ (aq) + 2Cl - ( aq) And the temperature rises from C to C. Assuming that all the heat given off by the reaction is transferred to the water, calculate q for the reaction system. Which equation? exothermic Answer: -734 J Plug and chug q reaction = -m water x c water

Enthalpy (H) Enthalpy is a type of chemical energy (heat content) Under constant P, heat flow for a reaction system is equal to the difference in enthalpy between products and reactants q reaction = ∆H= H products - H reactants

Enthalpy continued For an exothermic reaction q = ∆H< 0 For an endothermic reaction q = ∆H> 0 The enthalpy of a substance, like its volume, is a state property

Thermochemical Equations A chemical equation which shows the enthalpy relation between products and reactants NH 4 NO 3(s) --> NH 4 + (aq) + NO 3 - (aq) ∆H = kJ The sign of ∆H indicates endothermic (+) or exothermic (-) H 2(g) + Cl 2(g) --> 2HCl ∆H = -185kJ The value quoted for H applies when products and reactants are at the same temp (usually 25C) The phases must be specified--significant differences at different phases The coefficients represent numbers of moles

Rules of Thermochemistry The magnitude of ∆H is directly proportional to the amount of reactant or product The ∆H for a reaction is equal in magnitude but opposite in sign to ∆H for the reverse reaction. Heat of fusion : heat absorbed when a solid melts Heat of vaporization: heat absorbed when a liquid vaporizes -185 kJ 1 mol H2 1 mol Cl kJ -185 kJ 2 mol HCl H 2(g) + Cl 2(g) --> 2HCl ∆H = -185kJ 2HCl (g)--> H 2 (g) + Cl 2 (g) ∆H = 185 kJ H 2(g) + Cl 2(g) --> 2HCl(g) ∆H = -185 kJ

Example

Enthalpy of Formation

Example