Topic:
Thermochemistry
Essential Question:
How does energy
flow in chemical
Name: ___________________________________________________________________________
Class: ____________________________________________________________________________
Date: _____/_____/______
Period: ______________
reactions?
All chemical reactions involve changes in energy.
Typically this energy comes in the form of heat.
17.1 THE FLOW OF ENERGY
Objectives:
1. Differentiate between potential & kinetic energy, and between
heat & temperature.
2. Convert between calories and joules.
3. Calculate heat flow between objects using changes in
temperature.
Energy – the ability to do work or produce heat
Thermochemistry - the study of energy changes that occur during
chemical reactions and changes in state.
 Potential energy – stored
 Kinetic energy – motion
 Law of Conservation of Energy
 Energy is neither created nor lost in a chemical
reaction.

Heat v. Temperature
 Heat (q) = energy in the process of flowing from a
higher kinetic energy to an area of lower kinetic energy.
 Temperature (T) = average kinetic energy in a sample
of matter

Calculating Heat
 calorie (cal) = q required to raise 1g H2O by 1oC
 Calorie (kcal) = 1000 calories (“a nutritional Calorie”)
 Joule (J) = SI unit of energy (4.184 J = 1 cal)
Endothermic and Exothermic Processes
 Chemical reactions and changes in physical state generally
involve either the absorption or release of heat.
 System = the part of the universe on which you focus your
attention.


Surroundings = everything else in the universe.
If the energy of the system increases during that process, the
energy of the surroundings must decrease by the same amount.
 If the energy of the system decreases during that process, the
energy of the surroundings must increase by the same amount.
 Endothermic process = heat is absorbed from the surroundings
(system is gaining heat; q is positive).
 Exothermic process = releases heat into its surroundings
(system is losing heat; q is negative).
Specific Heat
 Calculating Heat Changes
 Heat capacity = amount of heat required to raise the
temperature of a substance 1oC. The heat capacity of an object
depends on both its mass and its chemical composition.

Specific heat = amount of heat required to raise the
temperature of 1 gram of a substance 1oC.
o
o
 Specific heat of water = 4.184 J/(g  C) or 1 cal/(g  C)
Measuring Heat
 Finding Specific Heat using Water
 The specific heat of substance may be found using
calorimetry with water as the surrounding media.
 Find the heat gained by the water using
qwater = mwater  cwater  Twater .
 Set the heat lost by the substance equal to –qwater
qsubstance = -qwater .
 Using the temperature change of the substance and its
mass, calculate csubstance.
csubstance = qsubstance /(msubstance  Tsubstance) .
17.2 MEASURING AND EXPRESSING ENERGY CHANGES
Objectives:
1. Calculate the specific heat of a substance.
2. Define enthalpy.
3. Describe enthalpy changes in endothermic and exothermic
reactions.

Calorimetry – study of heat
flow
 Heat flow can’t be
measured directly, it must
be calculated by observing
changes in temperature
 Basics of calorimetry
o
Heat change from a
reaction is equal,
but opposite in sign
to heat change in surroundings
qreaction = -qsurrounding .
o
One can determine the heat absorbed by the
surroundings by using the formula
q = m  c  T .
where: m = mass of surrounding media
c = specific heat of the surrounding media
T = change in temperature of surrounding
media
(T = Tfinal – Tinitial)
Usually water is used as the surrounding media
because it is easy to work with and has a well known
specific heat.
qwater = mwater  4.184J/(g  oC)  T
o
Once the energy change of the water is known, the
energy change of the sample is calculated by changing
the sign (“+”  “−“) or (“−“  “+”) of q.
o
Thermochemistry
 The study of heat changes in chemical reactions
 Enthalpy (H)- heat content of a system at constant
pressure
 Exothermic Reactions
o Reactions that release energy
 Heat is a product of the reaction
 Hreactants > Hproducts
C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(g) + 2043kJ

Endothermic Reactions
o Reactions that absorb energy
 Heat is a reactant in the reaction
 Hreactants < Hproducts
C(s) + H2O(g) + 113kJ  CO(g) + H2(g)
Heats of Reaction and Heats of Combustion
 These are enthalpy changes.
 Heat of Reaction = enthalpy change for the chemical equation
exactly as written. Heats of reaction are reported as H. The
physical state of reactants and products must be given,

Heat of Combustion =heat of reaction for the complete
burning of one mole of a substance.

Enthalpy Change (H)
Hrxn = Hproducts – Hreactants
 Endothermic Reactions
o Since Hproducts > Hreactants, Hrxn will be positive
(system adds heat)



Exothermic Reactions
o Since Hproducts < Hreactants, Hrxn will be negative
(system loses heat)
The H for a reaction is typically listed at the end of a
thermochemical equation.
C(s) + H2O(g)  CO(g) + H2(g) ; H = +113kJ
Standard Enthalpy Changes (H0) – change in enthalpy
when a reaction occurs with reactants and products in
their standard states.
 Standard enthalpy state is the most stable pure form of
the element at 1 atm & and 25oC
 Enthalpy of elements is arbitrarily set to zero
17.3 HEAT IN CHANGES OF STATE
Objectives:
1. Calculate enthalpy changes for a chemical reaction.
2. Calculate enthalpy changes for a change of phase.
Using Thermochemical Equations
 Calculating Enthalpy Changes

In a chemical reaction, the enthalpy change applies
proportionally to the amounts of reactants & products.

For example:
C(s) + H2O(g)  CO(g) + H2(g) ; H = +113kJ
If 2 moles of C are used and react completely, the energy absorbed
(because H is positive) would be 226kJ.
H can be set equivalent to any formula in a balanced
equation and calculated for a reaction using stoichiometry.
 Heat Flow in Changes of State

Heat flow also occurs in phase changes.
 Molar Enthalpy of Vaporization (aka, Heat of
Vaporization)
o Heat required to vaporize 1 mol of a liquid
H2O(l)  H2O(g) ; Hvap = 40.7 kJ/mol
 Molar Enthalpy of Fusion (aka, Heat of Fusion)
o Heat required to melt 1 mol of a solid
H2O(s)  H2O(l) ; Hfus = 6.01 kJ/mol
 Molar Enthalpy of Solidification (aka, Heat of
Solidification)
o Heat lost when 1 mol of a liquid solidifies



H2O(l)  H2O(s) ; Hsolid = -6.01 kJ/mol
Molar Enthalpy of Condensation (aka, Heat of
Condensation)
o Heat released when 1 mol of a vapor condenses
H2O(g)  H2O(l) ; Hcond = -40.7 kJ/mol
Molar Enthalpy of Solution (aka, Heat of Solution)
o Heat required to melt 1 mol of a solid
CaCl2(s)  Ca2+(aq) + 2Cl-(aq) ; Hsoln = -82.8
kJ/mol
17.4 CALCULATING HEATS OF REACTION
Objectives:
1. Describe the basis for Hess’s Law
Hess’s Law
 If you add two or more thermochemical equations to give a final
equation, then you can also add the heats of reaction to give the
final heat of reaction. Hess’s law allows you to determine the heat
of reaction indirectly by using the known heats of reaction of two
or more thermochemical equations.
Hnet = H1 + H2 …
SUMMARY: