Unit 1: Energy Changes and
Rates of Reaction
Chapters 5: Thermochemistry
Chapter 6: Chemical Kinetics
Different forms of energy
Law of Conservation of Energy
• energy cannot be
created or destroyed,
only transformed from
one form to another
(1st Law of
Thermodynamics)
Thermochemistry
• study of energy changes
in physical, chemical
and nuclear changes
Energy and Molecules
Kinetic Energy: energy of
motion/work (i.e. collision
theory). Substances may have
Thermal Energy (energy from
motion of molecules- amount of
substance matters).
Temperature is the average
kinetic energy of the particle
(amount of substance negated).
Potential Energy:
(due to position and
composition)
stored in molecules because
of the arrangement of
nuclei and electrons in its
atoms, electrical forces b/w
charged particles
Endo vs. Exothermic
• Which process requires energy? releases energy?
breaking bonds or forming bonds
• What is the net E change if:
a) E (required to break bonds) > E (released when bonds are formed)?
net change = E is absorbed (endothermic)
reactants + energy  products
b) E (required to break bonds) < E (released when bonds are formed)?
net change = E is released (exothermic)
reactants  products + energy
IMPORTANT TERMINOLOGY
• Chemical System: substances
(reactants and products) we are
studying, represented by the
chemical equations
Open System: Energy and matter
can move in or out
Isolated System: an ideal system
where no energy or matter can
move in or out
Closed System: only energy can
move in or out, but not matter
• Surroundings: all the matter
around the system that absorbs or
releases heat
Surroundings
System
Exchanges
matter and
energy
Universe
3 TYPES OF SYSTEMS
Identify the systems below:
• Why is E released during an exothermic reaction?
• What is the source of E released during a reaction?
Discuss the combustion of gasoline in terms of PE,
KE, energy absorbed, energy released, net energy
change, exo or endothermic, work.
gasoline + oxygen  carbon dioxide + water + energy
PE of bonds in reactants > PE of bonds in products
KE of reactants < KE of products
E absorbed to break bonds < E released when bonds
are formed  net energy change is release
(exothermic)
High KE of product gases does work on car parts to
make the car move.
Homework:
• p. 282 #1,2,4,5
• p. 285 A-C
• p. 291 #1,2,4,7-9
Follow-up to 5.1
1.
What is the difference between
thermal energy and
temperature of a substance?
2.
Compare the potential energies
of the reactants to that of the
products for the two types of
chemical processes below:
(a) endothermic process
(b) exothermic process
3. Label the following reactions as
“endothermic,” “exothermic,” or
“not enough information.”
(a) O2 → O + O
(b) O2 + O → O3
(c) H2 + Br2 → 2HBr
(d) NaCl → Na+ + Cl−
4. Classify each of the following as an
open system or a closed system.
(a) a pot of boiling water
(b) a sealed bottle of water
(c) a helium balloon
(d)a hot-air balloon
How do we measure energy changes
during a physical or chemical change?
A Simple Calorimeter
Key Assumptions:
1. Any thermal energy transfer
b/w the calorimeter and the
universe is negligible.
2. Any thermal energy
absorbed by the calorimeter
itself is negligible.
3. All dilute, aqueous solutions
have d = 1.00 g/mL and
c = 4.18 J/g°C.
Calculations Involving Thermal Energy Transfer
Understanding the formula:
q=mcΔT
q is thermal energy transferred, in J or kJ
q > 0 exo
m is mass, in g
q < 0 endo
ΔT = T2-T1 the temperature change, in °C
c is the specific heat capacity, in J/(g°C)
(Table 1 p.
292)
Substance
c (J/(g°C))
Ice
2.01
Water
4.18
Steam
2.01
Aluminum
0.900
Given
Required
Analysis
Solution
Paraphrase
p. 295: Tutorial 1
Sample Problem 1:
A student places 50.0 mL of liquid water at
21.00°C into a calorimeter. She places a
sample of gold at 100.00°C into the
calorimeter. The final temp of the water is
21.33°C. Calculate the quantity of thermal
energy, q, absorbed by the water in the
calorimeter.
Sample Problem 2
Using the value of q from #1, calculate the
specific heat capacity of the sample of gold if
its mass is 6.77 g. Assume that the final temp
of the gold sample was the same as the final
temp of the water in the calorimeter.
Sample Problem 3
A 50.0 mL sample of 1.0 mol/L HCl(aq) was
mixed with 50.0 mL of 1.0 mol/L NaOH(aq) at
25°C in a calorimeter. After the solutions were
mixed by stirring, the temperature was 31.9°C.
Determine the quantity of thermal energy
transferred by the reaction to the water and
state whether the reaction was endothermic
or exothermic.
p. 297 #1-3
Mini-Investigation: p. 297
less
stable
Demos
• KMnO4 and glycerol in fume hood
system
• Ba(OH)2(s) + NH4SCN
enthalpy
(s)
change ΔHsys
surroundings
heat (qsurr)
more
stable
Re-draw for an endothermic rxn!
Enthalpy Change
Recall: 2 types of energy
Potential Energy includes:
Protons and neutrons within
nuclei
Electrons in bonds
Kinetic energy includes:
Moving e- within atoms
Vibrational, rotational, and
translational motion
• Internal energy - sum of
all KE and PE of all the
components of the
system
• We can’t measure
internal energy, so…
• measure enthalpy change
– the energy absorbed
from or released to the
surroundings during a
reaction
ENTHALPY CHANGE (ΔH)
aka heat of reaction, enthalpy of reaction, change in heat content
ΔHsystem= - qsurroundings
ΔH > 0 , q < 0
ΔH < 0 , q > 0
Endothermic
Exothermic
ΔH° means @SATP: 100kPa, pure
liquids/solids, 1 M, 25°C or 298K
Calorimetry Calculations:
qsurr = mcΔT
ΔH = -q
enthalpy per gram = ΔH/m
molar enthalpy = ΔH/n
where n = m/M
MOLAR ENTHALPY (ΔHX)
ΔHX (molar enthalpy)
x released/absorbed by system per mole of
• energy
reacting substance
• units of kJ/mol
• good for reference or comparison
ΔH =ΔH
n
ΔH =nΔHx
n(enthalpy
= ΔHchange)
ΔH
• energy released/absorbed by system as R  P
ΔH
• units of kJ x
• depends on amount of substance reacting
Mini-Lab p. 285
• Calculate the molar enthalpy of solution for the salt
used in the expt.
Molar Enthalpy Examples
Ex 1: The molar enthalpy of vaporization of Freon-12 is
34.99 kJ/mol. If 100.0 g of Freon-12 (M = 120.91
g/mol) is vaporized, calculate the expected enthalpy
change.
Ex 2: 50.0 mL of 0.300 mol/L CuSO4(aq) is mixed with
and equal volume of 0.600 mol/L NaOH(aq). The
initial temperature of both solutions is 21.4°C. After
mixing, the highest temperature reached is 24.6°C.
Determine the enthalpy change of the reaction.
Ex 3: A chemist wants to determine the molar enthalpy
of neutralization for the following reaction.
HCl(aq) + NaOH(aq)  NaCl(aq) + H2O(l)
The chemist uses a coffee cup calorimeter to neutralize
61.1 mL of 0.543 mol/L HCl(aq) with 42.6 mL of
NaOH(aq). The initial temperature of both solutions is
17.8 °C. After neutralization, the highest recorded
temperature is 21.6°C. Calculate the molar enthalpy
of neutralization, in kJ/mol HCl.
Ex 4: What mass of KCl must have dissolved if the
temperature of 200.0 g of water increased by 5.5°C?
(ΔHsol’n,KCl = 1.7 x 104 J/mol)
Investigation 5.2.1
• Molar Enthalpy of a Chemical Change
• p. 333
–
–
–
–
–
Title
Purpose
Observations
Analyze and Evaluate (a-g)
Apply and Extend (h,i)
Homework
Practice Problems
• p. 301 #1-4
Section Review
• p. 306 #1,2,4
REPRESENTING ENTHALPY CHANGES
4 Methods1. Include energy value in the thermochemical equation
H2(g) + ½ O2 H2O(l) + 285.8 kJ
Exothermic= product side
Endothermic= reactant side
2. Write the chemical equation and enthalpy change
H2(g) + ½ O2 H2O(l)
ΔH= - 285.8 kJ
Equation must be
balanced
Watch +/- signs
3. State the molar enthalpy for a specific reaction & substance
ΔHForm= - 285.8 kJ/mol H2O(l)
Molar Enthalpy!!! (kJ/mol)
4. Draw a chemical potential energy diagram
Standard molar enthalpies
(ΔH°x ) are @ SATP
REPRESENTING ENTHALPY CHANGES
4 MethodsPotential Energy Diagram for the Formation of Water
4. Draw a chemical potential
energy diagram
Changes to Ep as bonds are broken
and formed
Exothermic Reactions= Energy
released= Ep=products lower
than reactants
Endothermic Reactants= Energy
gained= Ep=products higher
than reactants
(kJ)
Remember:
•Title
•Label axis
•Units
•Reactants on LEFT
•Products on Right
REPRESENTING ENTHALPY CHANGES
4 MethodsWhat about the reverse reaction, the decomposition of water?
1. Include energy value in the thermochemical equation
2. Write the chemical equation and enthalpy change.
3. State the molar enthalpy for a specific reaction.
4. Draw a chemical potential energy diagram.
REPRESENTING ENTHALPY CHANGES…
Ex. What is the thermochemical equation for
the following chemical equation and molar
enthalpy of combustion for butane:
2 C4H10 (g) + 13 O2 (g)  8 CO2 (g) + 10 H2O (l)
ΔHcomb= -2871 kJ/mol
Remember that thermochemical
equations contain ΔH values!
Homework:
p. 304 #1-4
p. 306 #3,5-7