Teacher Lecture Notes
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 what __________ __________? 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,
__________ is released. Therefore the products have less energy than
the reactants.
Student Notes
C3.4A
Use the terms
endothermic and
exothermic
correctly to
describe chemical
reactions in the
Laboratory
Endothermic Reactions-absorb heat (endo=inside, thermic=heat). The
chemical bonds formed store the heat as energy. Energy must be supplied
to make these reactions occur. The products have more energy than the
reactants because of the energy stored in their bonds.
12-1 Section Review
1. What is an exothermic reaction? On which side of the chemical
equation would you write the term energy?
2. What is an endothermic reaction? On which side of the chemical
equation would you write the term energy?
4. When clouds form water vapor (gas) turns back to liquid water. 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? Explain. Hint. Who has more kinetic energy solids,
liquids or gases, so is energy being lost or gained in each phase change?
5. Why do you think that wonderful system in 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 H) the heat gained or lost during a chem. Rxn.
The formula is Delta H = enthalpy of products – enthalpy of reactants
Delta  = products   reactants
C3.4c Write
chemical
equations
including the heat
term as a part of
equation or using
ΔH notation.
 Delta H for endothermic rxns is positive
Who has
the higher
energy the
reactants or
the
products?
Write the formula for this rxn
 Delta H for exothermic rxns is negative
Who has
the higher
energy the
reactants or
the
products?

Write the formula for this reaction
Temp and pressure must be kept constant since they influence enthalpy,
therefore 1atm and 25C is used (standard state) and product must be pure
Standard Enthalpy – delta H prime- the measured change in enthalpy
when reactants change to products in standard state. Solving enthalpy
problems requires stoich. Observe the following reaction.
2H2O2(l) --> 2H2O(l) + O2(g)   = -190kJ
Is this rxn endo or exo thermic?
How much heat would be lost if 1.0 g of H2O2 decomposes?
Steps
 First convert the grams of peroxide to moles using the
molar mass (M)
 Then use the mole ratio of peroxide for the energy
produced
C5.4d Explain why
freezing is an
exothermic change
of state.
.
C2.3a
Explain how the rate
of a given chemical
reaction is
dependent on the
temperature and the
activation energy.
C2.3b
Draw and analyze a
diagram to show the
activation energy for
an exothermic
reaction that is very
slow at room
temperature.
C3.1b Draw enthalpy
diagrams for
exothermic and
endothermic
reactions.
C3.4B Explain why
chemical reactions
will either release or
absorb energy.
C3.4d Draw enthalpy
diagrams for
reactants and
products in
endothermic and
exothermic
reactions.
.Lets try another. 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  = -kJ
 Convert the grams of glucose to moles using the molar mass
 Convert the moles to kJ given the information from above
C3.2a
Describe the
energy changes in
photosynthesis
and in the
combustion of
sugar in terms of
bond breaking and
bond making.
Again. 147g of Nitrogen dioxide is dissolved in water to produce nitric
acid and nitrogen monoxide. 3mols nitrogen dioxide delta H = –138kJ
12-3 Hess’s Law
Hess’s Law –The enthalpy change for the net rxn 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
 If coefficients are multi or divided the delta H must be also
 If an eq. is reversed so is the sign on the delta H
Observe the 2 step formation of nitrogen dioxide (air pollutant)
Delta Hnet = delta H1 + delta H2
C3.1d
Calculate the
amount of heat
produced for a
given mass of
reactant from a
balanced chemical
equation.
Let’s try a few together
Calculate the delta Ho of S + O2  SO2.
From the following enthalpy changes
2SO2 + O2  2SO3
delta H = –196kJ
2S + 3O2  2SO3
delta H = -760kJ
Steps
1. Flip equations and signs of the delta H to get the reactants
and products on correct side of final equation. Make sure
your yields signs line up on both eq. Cancel out as needed
2. Mult or divide the coefficients and the delta H to get the
correct coefficients of the final equation.
3. Now calculate the Net delta H from the corrected delta H 1
and 2
Again
Calculate the delta H prime of PCl3 + Cl2  PCl5
From the following enthalpy changes
2P + 3Cl2  2PCl3
delta H = -640kJ
2P + 5Cl2  2PCl5
delta H = -760kJ
Step One
Step Two
Step Three
Again Again 
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
Work the practice problems on pg 391
delta H = -277kJ
delta H = -828kJ
delta H = -297kJ
C3.1a Calculate the
ΔH for a given
reaction using
Hess’ Law.
Section Review 12-3
1. Explain how Hess’s Law can be used to find the delta H prime of a rxn
2. Identify the following reactions as endothermic or exothermic.
Explain your answers
a. Br2(s) --> Br2(l)
b. Br2(g) --> Br2(l)
3. Explain how Hess's Law is really a statement about the Law of
Conservation of Energy.
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 calorimeter.
Heat Capacity-the amt 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.
Specific heat- the heat capacity of 1g of a substance. (Phy property).
The amt of heat needed to raise the temperature of 1g of water by 1 Co.
You need 4.184J (sound familiar???) 4.184J = 1 calorie which raises
water by 1 Co
1000cal =1kcal=1Cal Oh ya, that's right...we did
this before! Sooo the specific heat of water is 4.184J/g*Co Remember
Chemists very rarely write the number one! Cool. The specific heat for a
substance is unique. Water has a high specific heat....the old saying "A
watched pot never boils" comes from that idea. It takes a lot of energy to
raise the temperature of 1g of water by 1Co. What about metal? Why
does a metal cooking spatula have a plastic or wood handle? So
remember how Ms B always says temp is a measure of.___________
_________. Well you can add a lot of heat (energy) to water before you
change the temp because it has a high specific heat!
You know we're going to use this newfound knowledge in calculations
right? Here goes.
The symbol q = heat transfer measurement made in a calorimeter. Heat
transfer from a reaction qrxn is equal to but opposite in sign from the q
surroundings, which is the water! Don't freak out. You knew if the energy
in the products went down (exo) the surrounding temp went up right? So
what is lost by the chemicals is gained by the water (and visa versa)
Soooo
qrxn = -qsur
m = mass of water and C = specific heat of water(constant) and (Tf-Ti) is
the final temp of the water minus the initial (starting) temp of the water.
The full equation is:
qsur = m x C x (Tf-Ti)
The last step would be to convert the qsur for the mass used to the moles
for a balanced equation.
 moles in balanced eq x (qsur /moles of substance)
Try this:
4.25 g NH4NO3 dissolves in 60.0 g of water the temp drops from 21.0 oC
to 16.9oC. Calculate 
C3.1c Calculate
the ΔH for a
chemical reaction
using simple
coffee cup
calorimeter.
NH4NO3   NH4  NO3   
qsur =m x C x (Tf-Ti)
qsur = (60.0g)(4.184J/goC)(16.9-21.0oC)
qsur = -1.03 x 103 J therefore the qrxn is 1.03 x 103 J
Now you must figure out how many moles of that substance released this
much energy, then convert that energy to the energy you would release
from the balanced equation!
4.25g NH4NO3 (1mol NH4NO3/80.0gNH4NO3 )= 0.0531 mol NH4NO3
Delta H = 1mol NH4NO3 x 1.03 x103 J/0.0531 mol NH4NO3 .
Delta H = +19400J = +19.4kJ. 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
C5.4A
Compare the
energy required to
raise the
temperature of one
gram of aluminum
and one gram of
water the same
number of degrees.
Section Review 12-4
1. What is a calorimeter? How does it determine the heat
transferred in a chemical rxn?
2. What is the difference between heat capacity and specific heat?
3. Fats are able to supply twice the energy as carbohydrates. Give a
possible explanation for this.
4. What error is introduced by including only the raise in
temperature of water in the calorimeter as the heat of a reaction?
12-5 What is Heat?
The Caloric Theory- scientists like Lavoisier of the 1700's believed heat
was fluid. That it flows. Count Rumford and James Joule did experiments
indication 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 Ms B always says temp is a measure
of ___________ ____________
Section Review 12-5
1. How does the Kinetic theory explain heat?
2. Describe some of the experiments that led scientists to the current
concept of heat. (yes.....you must crack you text!)
3. Give three examples of heat being produced from the
conversion of one type of energy to another.
Special Notes
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