Ch. 11 Thermochemistry: Heat and Chemical Change

Ch. 11.1 The Flow of Energy – HeatCh. 11.2 Measuring and Expressing Heat

ChangesCh. 11.3 Heat in Changes of StateCh. 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

H is positive

– Reactions that lose heat to the surroundings are exothermic

• The system loses heat as the surroundings warm up

H 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 T• Specific heat is C, heat is q, mass is m, and

change in temperature is T

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 = H = m x C x T (T = 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 H, 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 (Hfus)

• The heat lost when one mole of substance solidifies at a constant temperature is the molar heat of solidification (Hsolid)

• The quantity of heat absorbed by a melting solid is equal to the quantity of heat lost when a liquid solidifies (Hfus = -Hsolid)

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 (Hvap)

– The amount heat released when 1 mole of vapor condenses is called its molar heat of condensation (Hcond)

– Energy is conserved, Hvapor = -Hcond

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 (Hsoln)

– 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 (H0)

from the standard heats of formation

• The standard heat of formation (Hf0) 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

H0 = Hf0 (products) - Hf

0 (reactants)