Ch. 11 Thermochemistry: Heat and Chemical Change Ch. 11.1 The Flow of Energy – Heat Ch. 11.2 Measuring and Expressing Heat Changes Ch. 11.3 Heat in Changes of State Ch. 11.4 Calculating Heat Changes

Ch. 11.1 The Flow of Energy – Heat Energy transformations Thermochemistry is concerned with the heat changes that occur during chemical reactions Energy is the capacity to do work or supply heat Energy can be kinetic or potential Heat (q) is the energy that transfers from one object to another because of a temperature difference between the objects Heat alone cannot be measured Only changes caused by heat can be detected Heat always flows from warmer to cooler

Ch. 11.1 The Flow of Energy – Heat Exothermic and endothermic processes All reactions involve the release or absorption of heat A defined area or system is necessary to study heat The surroundings include the rest of the universe The law of conservation of energy is observed All energy in a system must be accounted for

Ch. 11.1 The Flow of Energy – Heat Exothermic and endothermic processes Reactions that absorb heat from the surroundings are endothermic The system gains heat as the surroundings cool down rH is positive Reactions that lose heat to the surroundings are exothermic The system loses heat as the surroundings warm up rH is negative

Ch. 11.1 The Flow of Energy – Heat Heat capacity and specific heat A calorie is the amount of heat needed to raise the temperature of 1g of H2O 10C A calorie is equal to 4.184 J The amount of heat need to increase the temperature of an object 10C is the heat capacity of that object The greater the mass of the object, the greater the heat capacity The specific heat capacity is the amount of heat it takes to raise the temp of 1g of substance 10C

Ch. 11.1 The Flow of Energy – Heat Heat capacity and specific heat Calculating specific heat capacity C = q / mass x rT Specific heat is C, heat is q, mass is m, and change in temperature is rT

Ch. 11.2 Measuring and Expressing Heat Changes Calorimetry The accurate and precise measurements of heat change for chemical/physical processes For systems at constant pressure, heat content is the enthalpy (H) The terms heat and enthalpy can be used interchangeably q = rH = m x C x rT (rT = Tf – Ti) To measure the heat change for a rxn in aqueous soln in a foam cup calorimeter, you dissolve the reacting chemicals (the system) in known volumes of water (the surroundings) Measure initial and final temperatures Use equation to calculate

Ch. 11.2 Measuring and Expressing Heat Changes Thermochemical equations An equation that includes the change in heat A heat of reaction is the heat change Heats of reaction are reported as rH, which can be positive or negative Problems involving enthalpy are similar to stoichiometry problems The amount of heat released/absorbed depends on the number of moles of reactants involved All chemicals have a heat of combustion for the complete burning of one mole of the substance

Ch. 11.3 Heat in Changes of State Heats of fusion and solidification All solids absorb heat as they melt to become liquids, or lose heat as they solidify from a liquid to a solid The heat absorbed by one mole of substance when it melts from a solid to a liquid at a constant temperature is the molar heat of fusion (rHfus) The heat lost when one mole of substance solidifies at a constant temperature is the molar heat of solidification (rHsolid) The quantity of heat absorbed by a melting solid is equal to the quantity of heat lost when a liquid solidifies (rHfus = -rHsolid)

Ch. 11.3 Heat in Changes of State Heats of vaporization and condensation The amount of heat necessary to vaporize one mole of a liquid substance is called the molar heat of vaporization (rHvap) The amount heat released when 1 mole of vapor condenses is called its molar heat of condensation (rHcond) Energy is conserved, rHvapor = -rHcond

Ch. 11.3 Heat in Changes of State Heat of solution The heat of change caused by the dissolution of of 1 mole of substance is called the molar heat of solution (rHsoln) Some are heat of solution changes are exothermic, some are endothermic

Ch. 11.4 Calculating Heat Changes Hess’s Law Most reactions take place in a series of steps Direct measurements of heats of reaction for a chemical reaction cannot always be made Hess’s law makes it possible to measure a heat of reaction indirectly If you add two or more thermochemical equations to get a final equation, then you can also add the heats of reaction to give the final heat of reaction Known as Hess’s law of heat summation

Ch. 11.4 Calculating Heat Changes Standard heats of formation Sometimes it is difficult to measure the heat change for a reaction Instead, you can calculate the heat of reaction (rH0) from the standard heats of formation The standard heat of formation (rHf0) of a compound is the change in enthalpy that accompanies the formation of 1 mole of compound from its elements In standard states at 250C rH0 = rHf0 (products) - rHf0 (reactants)