12-1 Chemical Reactions That
Involve Heat
Law of conservation of
energy - energy is neither
created or destroyed.
 Most chemical reactions
involve changes in
energy.
 The making and breaking
of chemical bonds either
absorb or release energy.

Heat is the transfer of
energy from one object to
another.
 Temperature is a measure
of kinetic energy
 Thermochemistry- the
study of changes in heat in
chemical reactions.

Exothermic Reactions
 Release
heat (exo=outside, thermic=heat)
 Combustion is an exothermic reaction.
 When you burn things heat is put out, you feel it!
 When reactions are exothermic, release heat,
ENERGY is released.
 Therefore, the products have less energy than the
reactants.
Endothermic Reactions
 Reactions
that absorb heat (endo=inside,
thermic=heat).
 Chemical bonds formed store heat as energy.
 Energy must be supplied to make these reactions
occur.
 Products have more energy than reactants because
of energy stored in bonds.
GUMMY BEAR SACRIFICE!
http://vimeo.com/24012128
Is this an example of an endothermic
or exothermic reactions?
How do you know???
12-1 Section Review Questions…
1. In an exothermic
reaction, which side
of the chemical
equation would you
write the term energy?
2. In an endothermic
reaction, which side
of the chemical
equation would you
write the term energy?
3. Is cloud formation exothermic
or endothermic? Why do
you feel cooler on a hot
summer day when you mist
yourself with water and let it
evaporate off you? Hint: Who
has more kinetic energysolids, liquids or gases? So is
energy being lost or gained in
each phase change?
4. Why do you think your body
produces sweat? Why are you
in trouble when you stop
sweating (2 parts)?
12-2 Heat and Enthalpy Changes



Energy is measured in
kilojoules (kJ)!
Enthalpy-depends on
temperature, phase (state of
matter), and composition.
When pressure remains
constant, the heat released or
absorbed in a chemical
reaction is equal to the
enthalpy change.
Enthalpy Change (delta
ΔH) the heat gained or
lost during a chemical
reaction.
 The formula is:

ΔH = enthalpy of products –
enthalpy of reactants
Enthalpy Change for Endothermic
Reactions…
ΔH for endothermic = positive
 Who has the higher energy the reactants or the products?
 ΔH for exothermic = negative

Stoichiometry & Enthalpy!
 Solving
enthalpy problems requires stoichiometry
 Observe the following reaction:
2H2O2(l)  2H2O(l) + O2(g)  = -190kJ
 Is
this rxn endo or exo thermic?
Sample Enthalpy Problem


How much heat is transferred when 9.22g of
glucose in your body reacts with oxygen in
respiration, which produces carbon dioxide and
water.
1 mole of glucose delta H  = -2803 kJ
9.22g glucose 1 mol glucose -2803kJ
180g glucose 1mol glucose
= - 144 kJ
Sample #2
 147g
of Nitrogen dioxide is dissolved in water to
produce nitric acid and nitrogen monoxide.
 3mol nitrogen dioxide delta H = –138kJ
3NO2 + H2O  2HNO3 + NO
147g NO2
1mol NO2
46g NO2
-138 kJ
3mol NO2
= - 147 kJ
12-3 Hess’s Law
Hess’s Law –The enthalpy change for the net rxn
(rxn=REACTION) is the sum of the enthalpy’s of the
individual rxns.
 An indirect method of measuring enthalpy given a series
of rxns.
 Rules for manipulating the equations:

If coefficients are multiplied or divided, ΔH must be also
 If an equation is reversed, so is the sign on ΔH

Hess’s Law Problems
1.
The brown haze of smog over large cities is often due to
the production of NO2 from N2 and O2. Given the
information below, what is the net enthalpy change for
this reaction?


N2 + O2  2NO ΔH= +181 kJ
2NO + O2  2NO2 ΔH= - 113kJ
If reactants and products are on proper sides of arrows, add
equations together.
N2 + O2 + 2NO + O2  2NO + 2NO2
N2 + 2O2  2NO2
ΔH = +118 + -113 = +68kJ
More Practice
2. Calculate ΔH of
S + O2  SO2
From the following enthalpy changes:
 2SO2
 2S
+ O2  2SO3 ΔH = –196kJ
+ 3O2  2SO3
ΔH = -760kJ
3. Calculate the ΔH of PCl3 + Cl2  PCl5
 From the following enthalpy changes
2P + 3Cl2  2PCl3
delta H = -640kJ
2P + 5Cl2  2PCl5
delta H = -760kJ
One More 
4. Calculate the delta H prime for
2S + 2OF2  SO2 + SF4
From the following rxns:
 OF2 + H2O  O2 + 2HF
 SF4 + 2H2O  SO2 + 4HF
 S + O2  SO2
delta H = -277kJ
delta H = -828kJ
delta H = -297kJ
12-4 Calorimetry
Calorimetry- the study of
heat flow and heat
measurement.
 These experiments
determine the heats
(enthalpy changes) of rxns
by measuring the changes
of temperature in a device
called a calorimeter (we’ll
make one in lab…)

Heat Capacity-the amount
of heat needed to raise the
temperature of the object
by 1 degree Celsius.
 This is dependent on the
mass and the composition
of the substance.

Heating lots of water vs. a
drop…
 Heating water vs. metal…

SPECIFIC HEAT
Specific
The
heat-
amount of heat needed to raise the
temperature of 1g of water by 1 Co.
Water’s specific heat is 4.184 J
TRUE OR FALSE?



A small temperature
change means that a small
quantity of heat was
transferred….



FALSE!
A small change in temperature
may be produced by a large
quantity of heat…
…in an object that has a very
large heat capacity!
HEAT & TEMP ARE
RELATED, BUT NOT THE
SAME!
Specific heat is the amount of
heat needed to raise the
temperature
The Equation
qsur = m x C x (Tf-Ti)
&
qrxn = - qsur






q = heat transfer
measurement made in a
calorimeter.
Sur = surrounding solution
Rxn = overall reaction
m = mass of water
C = specific heat
(Tf-Ti) is final temperature
minus initial temperature
Sample Problem
1. 4.25 g NH4NO3 dissolves in 60.0 g of water and the temp
drops from 21.0 oC to 16.9oC. Calculate .
NH4NO3  NH4  NO3
qsur = m x C x (Tf-Ti)
qsur = (60.0g)(4.184J/goC)(16.9-21.0oC) = -1.03 x 103 J (heat
transferred to the surroundings)
qrxn = 1.03 x 103 J (heat transferred in the reaction)
But this is not  yet…
Everything Goes Through Moles!
4.25g NH4NO3
1mol NH4NO3
1.03 x103 J
80.0gNH4NO3
1 mol NH4NO3
Is this reaction endo or exo thermic????
Again!

13.7g of solid lead II nitrate dissolves in 85.0g of water in
a calorimeter, the temp drops from 23.4C to 19.7 C.
Calculate the delta H for this rxn

Fill in the correct numbers and solve

Convert the lead II nitrate grams to mols

Convert the mols from above to J for 1 mol (or the given
balanced eq.)
Do problem #6 on page 397 text
It’s a good one…
12-5 What is Heat?
The Caloric Theoryscientists like Lavoisier of
the 1700's believed heat
was fluid. That it flows…
 Count Rumford and James
Joule did experiments
indicating that heat was a
form of energy.




The Kinetic Theory -In the
1900 century scientists began
to understand that heat was the
result of motion and vibration
of particles of matter.
Heat-the transfer of kinetic
energy from a hotter object to a
cooler one.
Now, hopefully you
understand why temperature is
a measure of ___________
____________!
Entropy
– randomness in a reaction with regard
to energy that cannot be reharnessed (heat!)
 ΔS
 Disorder
A reaction is ALWAYS spontaneous if
ΔS + and ΔH –
A reaction is NEVER spontaneous if
ΔS - and ΔH +
If both signs are the same, spontaneity
depends on temperature