CP Chemistry
Mrs. Klingaman
Chapter 16:
Thermochemistry
Name: ________________________
Mods: _____________
Chapter 16: Reaction Energy
Reading Guide
16.1 – Thermochemistry (pgs. 501-514)
1. Thermochemistry is defined as the transfer of energy as _______________ that accompany
chemical reactions and physical changes.
2. See the attached picture of a coffee cup calorimeter.
a) Why is a calorimeter used to measure the heat energy of a reaction?
b) Whys is it important that a calorimeter be insulated?
c) Why is water added to the calorimeter?
d) In an insulated calorimeter the system = __________________________
surrounding = __________________________
3. Define the term temperature:
Important conversion: Kelvin (K) = ºC + ___________
Convert the following: 28 ºC = ________K
303K = _________ ºC
4. Define the term Heat:
5. Define the term specific heat (cp) :
6. Energy lost or gained is defined by the following equation: q = cp x m x ΔT
This equation can be rearranged to describe the specific heat as : cp = q/ m x ΔT
Identify the following variable: q =
cp =
m=
ΔT =
7. Refer to Figure 1.2 and complete the following table:
Substance
Specific heat (J/gK)
Water (l)
Water (s)
Water (g)
Lead (s)
Energy Diagrams and Enthalpy
8. Enthalpy of an exothermic reaction: (Fill in the correct answer)
a)
During an exothermic process energy is _____________ . (absorbed or released)
b)
The enthalpy (ΔH) value is ____________________. (positive or negative)
c) The following energy diagram for an exothermic reaction is shown below.
-
Since the Hproduct < H reactant, the ΔH reaction has a _______ (+/-) value.
9. During an endothermic process energy is a _____________ . (absorbed or released)
a) The enthalpy (ΔH) value is ____________________. (positive or negative)
b) Label the following energy diagram for an endothermic reaction based on figure 1.4
-
Since the Hproduct > H reactant, the ΔH reactions has a ______ (+/-) value
10. Thermochemical equations are balanced chemical equations that include the enthalpy of the
reaction (ΔH).
Identify the following thermochemical equations as either exothermic or endothermic and explain
how you came to realize this.
a) 2H2(g) + O2(g)  2H2O(g)
b)
2H2O(g)  2H2(g) + O2(g)
ΔH = - 483.6 kJ
ΔH = + 483.6 kJ
______________________
(endothermic or exothermic)
______________________
(endothermic or exothermic)
c) Which reaction do you think will be more likely to occur? Why?
Interpreting the Heating Curve of an Unknown Substance
The heating curve (at right) was
developed while heating an
unknown substance
Substance
Melting point
Boiling point
Bolognium
20 °C
100 °C
Unobtainium
40 °C
140 °C
Foosium
70 °C
140 °C
Time (minutes)

Segment A-B: (positive slope)  At the beginning of the heating curve, the substance exists as a
_________________ (single state of matter). Matter in this phase has a(n) _______________________
volume and a(n) ______________________ shape. With each passing minute, ________________ is
added to the substance. This causes a rise in the ___________________________ of the substance. As a
result, the molecules of the substance will _______________ more rapidly, which means an increase in the
molecules’ ____________________ energy.

Segment B-C: (plateau = no slope)  During this segment of the heating curve the temperature is a
constant _______°C. The solid begins to ____________. Therefore, the temperature at point B is known as
the ___________________ ____________ of the substance. Between points B and C, heat continues to be
added over time. This heat is used, to convert the substance from a ________________ to a
________________. This heat energy is known as the latent heat of fusion (fusion = melting).

Segment C-D: (positive slope)  At this point, the substance is completely in the ______________ phase.
Matter in this phase has a(n) _____________________volume and a(n) ______________________ shape.

Segment D-E: (plateau = no slope)  At this point in the heating curve the temperature is a constant
______°C. This is when the liquid begins to _______________________ and the temperature at point D is
known as the ___________________ ____________ of the substance. Between points D and E, the heat
which is added is used to convert the substance from a ___________________ to a ______________.This
heat energy is called the latent heat of vaporization.

Segment E-F: (positive slope)  At this point the substance is completely in the _____________ phase
and the molecules are moving ____________________ as indicated by the high temperatures. Matter in
this phase has a(n) ___________________ volume and a(n) ___________________ shape. Any heat
added after point E will continue to cause an increase in the _______________________ of the substance.
Heating Curve of Water & Relation to Specific Heat
The flat parts (plateaus) on a heating curve represent a phase change. For example, when ice melts,
the energy added to the ice increases the vibrations of the solid molecules until they reach a high
enough temperature, the melting point. At the melting point, the molecules finally have enough kinetic
energy of motion to break free from the attractive forces holding the solid molecules together. Notice
that there is no increase in temperature until all of the ice has been converted to liquid water. This
process is similar for vaporization. Again, the liquid molecules will continue to move faster as heat is
added and the temperature increases. At the boiling point, the liquid molecules have gained enough
energy to escape into the gas phase. Two terms that you will need to know are:
1) Heat of Fusion –
2) Heat of Vaporization –
The slopes of the slanted lines represent the specific heat capacity. The sharper the slope, the
smaller the specific heat capacity of the substance is. This is significant because the sharper the
slope, the more quickly the temperature rises when the substance is heated.
3) Specific Heat Capacity –
It is possible to calculate the quantity of heat is needed (along each segment of a heating curve) to
heat a substance so much that it changes phases.
Heating/Cooling Curve of an Unknown Substance
Directions: Answer the following questions using the heating curve above:
1) What is the freezing point of the substance? _________
2) What is the boiling point of the substance? _________
3) What is the melting point of the substance? _________
4) What letter represents the range where the solid is being warmed? _________
5) What letter represents the range where the liquid is being warmed? _________
6) What letter represents the range where the vapor is being warmed? _________
7) What letter represents the melting of the solid? _________
8) What letter represents the vaporization of the liquid? _________
9) What letter(s) show a change is potential energy? _________
10) What letter(s) show a change in kinetic energy? _________
11) What letter represents condensation? _________
12) What letter represents crystallization (aka: freezing)? _________
The Transfer of Energy as Heat
The 1st Law of Thermodynamics -
Therefore, heat can be transferred between matter as long as the total heat is conserved. Viewing
energy transfer in terms of the “system” and “Surroundings” helps us monitor the direction of heat flow
so that it does not violate the 1st Law of Thermodynamics.
System –
Surroundings –
Two terms, “exothermic” and “endothermic” are often used as a way to describe if heat has moved
into the system or out of the system.
Exothermic (
)–
Diagram:
Endothermic (
Diagram:
)–
Exothermic & Endothermic
Understanding Heat Flow:

Heat always flows from the _____________________ object to the ____________________
object until the temperature of both objects is the ___________________.

Fill in the table below. Remember that the direction of heat flow is always determined from the
perspective of the system, never the surroundings!
Does the
SYSTEM…
Process
1.
Campfire
2.
Heating Soup
3.
Chemical Hot
Pack
4.
Chemical Cold
Pack
5.
Making Ice
6.
Boiling water
7.
Evaporation
of rubbing
alcohol
(think of hand
sanitizer)
Identify the
System
Identify the
Surroundings
release heat
or
absorb heat?
Heat (q) =
Exothermic
or
Endothermic?
positive (+)
or
negative (-)
Phase Changes:
Below is another way to represent the six phase changes. This helps show
that each phase change has an opposite. For example, melting and freezing…
both of these processes require that same quantitative amount of energy,
the only difference is whether the process is exothermic (releases heat)
or endothermic (absorbs heat).
Directions: Label each line with the appropriate phase change and determine
which changes are endothermic and exothermic.
Endothermic
vs.
Exothermic
________________________________________
_______________________________________
________________________________________
_______________________________________
________________________________________
_______________________________________
Enthalpy & Thermochemical Equations – Practice Problems
Chemical and physical equations can be written to express if the reaction which occurred was
endothermic or exothermic. These type of equations are referred to as “thermochemical equations”.
Endothermic: Heat is written as a _____________________ in the equation. This is because
energy must be put into the system for the reaction to occur.
Exothermic: Heat is written as a _____________________ in the equation. This is because
energy is released or “produced” by the reaction.
* Note: Often, the word “energy” is replaced in the equation by
The actual quantity of heat absorbed or released
At constant pressure, the word enthalpy is synonymous with heat (both have the same meaning),
Enthalpy is typically represented by the symbol, ΔH. This is another way to distinguish if a chemical
reaction is endothermic or exothermic. The value of ΔH is listed at the end of a balnced chemical or
physical reaction. Just like with heat, in an endothermic reaction, ΔH = ______ and in an exothermic
reaction, ΔH = ______.
Reaction
Exo or Endothermic?
1) C6H12O6 (s) + O2 (g)  CO2 + H2O + energy
1) _________________
2) H2O (l) + 483.6  H2 (g) + O2 (g)
2) _________________
3) 2 N2 (g) + O2 (g)  2 N2O (g)
H = + 66.4 kJ
4) 2 C2H2 (g) + 5 O2 (g)  4 CO2 (g) + 2 H2O (l) + 2598.8 kJ
5) H2 (g) + Cl2 (g)  2 HCl (g)
H = -183 kJ
3) _________________
4) _________________
5) _________________
* Note: Any reaction can be reversed and the sign of ΔH would become the opposite (ex: if it was
endothermic, the reverse reaction would be exothermic)
Enthalpy Diagrams – Practice Problems
Enthalpy diagrams are used to express if a chemical reaction is endothermic or exothermic by
comparing the energy of the reactants to the products.
1)
2)
Endothermic
Endothermic
or
or
Exothermic
Exothermic
Why?
Why?
Enthalpy Diagrams & Thermochemical Equations – Practice Problems
Directions: Evaluate the following enthalpy diagrams and answer the associated questions
1) H2 (g) + I2 (g)  2 HI (aq)
2) N2 (g) + 2 O2 (g)  2 NO2 (g)
1) Is this reaction endothermic or
exothermic? Explain why.
2) Is this reaction endothermic or
exothermic? Explain why.
Directions: Answer the following questions based on the information given.
3) Are the following reactions endothermic or exothermic?
a. 2 C (s) + H2 (g) + 226.6 kJ  C2H2(g) ________________________
b. 2 C3H6 (g) + 9 O2 (g)  6 CO2 (g) + 6 H2O (l) + 4439.4 kJ _______________________
c. 2 C2H6 (g) + 7 O2 (g)  4 CO2 (g) + 6 H2O (l) ΔH = – 3120 kJ ____________________
4) 2 Ag2O(s) + 122 kJ  4 Ag(s) + O2(g)
a. Is the reaction endothermic or exothermic? _____________________________
b. What is the sign of ΔH for this reaction?
c. Which has a higher enthalpy, the reactant or the products? ______________________
d. Draw the enthalpy diagram for this reaction:
5) S (s) + O2 (g)  SO2 (g)
ΔH= – 297 kJ
a. Is the reaction endothermic or exothermic? ___________________________
b. Which has a higher enthalpy, the reactant or the products? ______________________
c. REVERSE this thermochemical reaction and write it below:
d. Draw the enthalpy diagram for the REVERSE reaction below:
Energy: Unit Conversions – Practice Problems
______ cal = __________ J
;
______ Calorie = _______ kcal = ___________ cal
1. Convert the following from one unit to the other:
a)
1.69 joules to calories
__________________________
b)
3449.6 J to kJ
__________________________
c)
68 calories to kilocalories
__________________________
d)
54 Calories to kilocalories
__________________________
e)
820.1 J to cal
__________________________
f)
3780 cal to kJ
__________________________
2. Convert the following from one unit to the other:
a)
8.50 x 10-2 cal to Calories
__________________________
b)
444 cal to joules
__________________________
c)
1.8 kJ to joules
__________________________
d)
4.5 x 10-1 kJ to calories
__________________________
e)
5.26 kcal to kJ
__________________________
f)
23.5 joules to cal
__________________________
Specific Heat Capacity – Practice Problems
1) Specific Heat Capacity – (Cp)
2) Every substance has a unique Cp value. What does the value of specific heat capacity tell you?
Examples:
3) Specific Heat Capacity of Water =
4) Below, write the equation for specific heat capacity (Cp) and the equation when it is rearranged to
solve for heat (q).
Cp =
q=
Cp = specific heat (units of _______________________)
q = heat energy (units of __________________)
m = mass (units of _________________)
ΔT = change in temperature = Tfinal - Tinitial (units of __________ or __________)
Specific Heat Calculations – Practice Problems
1) How much heat energy (in joules) is required to raise the temperature of 45 g of water from 14°C
to 48°C? (specific heat of water: 4.184 J/g.°C)
Givens
Equation
Work
Answer & Units
2) If a glass contains 78 grams of water, how much heat energy (in calories) must be given off in a
water down from 75°C to 25°C (specific heat of water: 1.0 cal/g.°C)
Givens
Equation
Work
Answer & Units
3) If a 197.5 gram sample of ethanol requires 7279 joules of heat to raise the temperature from 295K
to 310K, what is the specific heat of ethanol?
Givens
Equation
Work
Answer & Units
4) A sample of chloroform is cooled down from a temperature of 79°C to 31°C. If specific heat of
chloroform is 0.96 J/g°C and 348 joules of heat were released during the process, what was the
mass, in grams, of chloroform cooled?
Givens
Equation
Work
Answer & Units
Specific Heat Calculations – Homework
Directions: Determine how much energy (heat) is needed for the following to occur:
1)
How much energy (heat) in joules is required to raise the temperature of 250.0 g of mercury
52°C? (specific heat of mercury: 0.14 J/g.°C)
Givens
Equation
Work
Answer & Units
2)
How much energy (heat) in calories is required to heat 32.0 g of water from 25°C to 80°C?
(specific heat of water: 1.0 cal/g.°C) What is this heat energy also equal to in kilojoules?
Givens
Equation
Work
Answer & Units
3) Calculate the heat capacity of a piece of wood if 1500.0 g of the wood absorbs 6.75×104 joules of
heat, and its temperature changes from 32°C to 57°C.
Givens
Equation
Work
Answer & Units
4) If 1750 joules of heat are required to raise the temperature of aluminum from 8°C to 85°C, what is
the mass of aluminum being heated? (specific heat of aluminum: 0.9 J/g.°C)
Givens
Equation
Work
Answer & Units
5) The temperature of a piece of copper with a mass of 95.4 g increases from 25.0°C to 48.0°C when
the metal absorbs 849 J of heat. What is the specific heat of copper?
Givens
Equation
Work
Answer & Units
6) When 435 J of heat is added to 3.4 g of olive oil at 21°C, the temperature increases to 85°C.
What is the specific heat of olive oil?
Givens
Equation
Work
Answer & Units
7) The specific heat of iron is 0.46 J/g°C. How much heat is absorbed by a 324 g piece of iron to
raise its temperature from 31°C to 74°C?
Givens
Equation
Work
Answer & Units
8) A piece of stainless absorbs 141 calories of heat when its temperature increases by 78°C. If the
specific heat of the stainless steel is 0.50 J/g.°C, what mass of stainless steel was heated?
[Hint: you must convert the unit of heat so that it matches the unit in the specific heat]
Givens
Equation
Work
Answer & Units
Calorimetry
1. Calorimetry –
2. Heat is transferred between two substances when…
(1)
(2)
- Heat is transferred from the __________________ to the _________________ substance
- The transfer of heat stops when both substances are in _____________________
______________________________ (have the same final temperature)
3. Calorimetry must be done using an
INSULATED container known as a
_____________________ (the chemical or
physical reaction takes place inside of this)
4. In calorimetry, ALL the heat that is
___________ by one substance (A) is
___________________ by the other
substance (B).
-
Mathematically,
-
Recall: q = mCpΔT
-
Note: At the end of the reaction,
substances A and B will have the
same final temperature (Tf)
Calorimetry – Practice Problems
1.
A 9.6 g sample of a metal alloy is heated to 102.3°C and then dropped into 27.0 g of water in a
calorimeter. The temperature rises from 23.5°C to 29.2°C. The specific heat of water is 4.184
J/g°C.
a. What information do you know (list all your variables)?
malloy =
mH2O=
∆Talloy =
∆TH2O=
b. Calculate the heat (q) absorbed by the water.
c. What is the value of the heat released from the metal alloy?
d. Calculate the specific heat (Cp) of the metal alloy.
2. A 14.7 g sample of a metal alloy is heated to 96.8C and then dropped into 20 g of water in a
calorimeter. The temperature rises from 22°C to 24.1°C. The specific heat of water is 4.184
J/g°C.
a. What information do you know (list all your variables)?
malloy =
mH2O=
∆Talloy =
∆TH2O=
b. Calculate the heat (q) absorbed by the water.
c. What is the value of the heat released from the metal alloy?
d. Calculate the specific heat (Cp) of the metal alloy.
3. A 23.5 g sample of a magnesium is heated to 93.9°C and then dropped into a calorimeter with
45.0 g of cold water with an unknown initial temperature. The final temperature of the mixture is
32.2°C and the specific heat of magnesium is 1.02 J/g°C. The specific heat of water is 4.184
J/g°C.
a. What information do you know (list all your variables)?
malloy =
mH2O=
∆Talloy =
∆TH2O=
b. Calculate the heat (q) released by the magnesium.
c. What is the value of the heat absorbed from the cold water?
d. Calculate the initial temperature of the cold water.
Enthalpy of Reactions:
1. Enthalpy –
2. Enthalpy is an extensive property which mean the amount of heat released or absorbed by a
reaction is DEPENDANT upon the _________________ of reactants used. The more reactant
used, the ______________ energy absorbed or released.
a. We can calculate the exact amount of enthalpy absorbed/released by using a balance
thermochemical equation and some stoichiometry!
DEMONSTRATION: Thermochemistry Stoichiometry - Calcium Carbide + Ice
Discussion:
Rxn #1:
Rxn #2:
CaC2 (s) + 2 H2O (s)  C2H2 (g) + Ca(OH)2 (s)
2 C2H2 (g) + 5 O2 (g)  4 CO2 (g) + 2 H2O (g)
ΔH = ____2598 kJ
 Does the system (Rxn #2) release or absorb heat? ______________________
 Is this reaction exothermic or endothermic? ______________________________
 If this thermochemical equation was re-written, would heat be a reactant or product?
_____________________________
 Draw an enthalpy diagram below to reflect this reaction:
Data:
 Mass of calcium carbide used in Rxn #1: ____________________ g CaC2
 Mass of acetylene produced in Rxn #1: _____________________ g C2H2
stoichiometry)
(MM of CaC2 = 64.1 g/mol; MM of C2H2 = 26 g/mol)
Calculations:
 Given the mass of acetylene produced, determine the enthalpy of this reaction:
(use
Enthalpy of Reaction Calculations
1) 2 Ag2O(s)  4 Ag(s) + O2(g)
ΔH= + 122 kJ
a. Is the reaction endothermic or exothermic? ________________________
b. Which has a higher enthalpy, the reactant or the products? _________________
c. If 27.4 grams of silver oxide decomposed, what would be the enthalpy of reaction?
2) 2 Na (s) + Cl2 (g)  2 NaCl (s)
ΔH= – 822 kJ
a. Is the reaction endothermic or exothermic? _______________________
b. Which has a higher enthalpy, the reactant or the products? ________________
c. What mass (in grams) of sodium chloride is produced if the reaction emits 479 kJ of
heat?
3) 4 NO (g) + 6 H2O (l) +1170 kJ  4 NH3 (g) + 5 O2 (g)
a. Is the reaction endothermic or exothermic? ___________________________
b. How much heat is absorbed/released if 43.1 grams of nitrogen monoxide reacts?
4) 2 C2H2 (g) + 5 O2 (g)  4 CO2 (g) + 2 H2O (l) + 2598.8 kJ
c. Is the reaction endothermic or exothermic? ___________________________
d. If 1784 kJ of heat were released during the course of this reaction, what mass (in
grams) of oxygen gas was used?
Thermochemistry - Practice Problems #1
Use the reaction below for the decomposition of aluminum oxide to answer the following questions:
2 Al2O3(s)  4 Al(s) + 3 O2(g)
∆H = + 3352 kJ
1. Is the above reaction endothermic or exothermic? _______________________________
2. Draw an enthalpy diagram below to
illustrate the change in enthalpy over
the course of the reaction:
3. I begin the reaction with 2.6 moles of aluminum oxide (Al2O3).
a)
How much heat (in kJ) would evolve? (Hint: Use thermo stoichiometry)
b) How many calories is this equal to?
4. I begin the reaction with 350 g of aluminum oxide (Al2O3).
a) How much heat (in kJ) will be exchanged in this reaction? (Hint: Use thermo stoichiometry)
b) How many Calories is this?
Thermochemistry - Practice Problems #2
Given the following reaction:
14 KMnO4 (g) + 4 C3H5(OH)3 (l)  7 K2CO3 (s) + 7 Mn2O3 (s) + 5 CO2 (g) + 16 H2O (l)
ΔH° = -2067 kJ
1. Determine if the above reaction is endothermic or exothermic? _________________________
2. Draw an enthalpy diagram to illustrate the
change in enthalpy over the course of the
reaction:
3. The reaction above begins with 10 grams of
potassium permanganate, KMnO4.
a.)
How much heat (in kJ) will be exchanged in this reaction? (Hint: Use thermo
stoichiometry)
b.)
How many calories would be given off in this same reaction?
4. If 2.37 moles of glycerin, C3H5(OH)3 (from the reaction above) were used in the reaction, how
much heat (in kJ) would evolve? (Hint: Use thermo stoichiometry)
a.)
How many Calories is this?
Thermochemistry- Chapter Review
1) Complete the following table:
Sample
System
Surrounding
Endothermic
or
Exothermic?
q = +/-
Adding ice to your
soda
Toasting a
marshmallow
Churning ice cream
Warming your toes
by the fire
2) Complete the following conversions:
a) 47,500 cal  Joules
b) 105 Joules  Calories
c) 0.251 kJ  calories
3) Draw an energy diagram for the following examples:
a) NaOH (s)  NaOH (aq) + heat
b) NH4NO3 (s) + heat  NH4NO3 (aq)
4) Are the following reactions endothermic or exothermic?
a) 2 C (s) + H2 (g) + 226.6 kJ  C2H2(g) _______________
b) 2 C3H6 (g) + 9 O2 (g)  6 CO2 (g) + 6 H2O (l) + 4439.4 kJ ________________
c) 2 C2H6 (g) + 7 O2 (g)  4 CO2 (g) + 6 H2O (l) ΔH = – 3120 kJ ____________
5) Complete the following calculations:
a) How many kJ of energy are needed to raise the temperature of 165 g water from 10 ˚C to 47
˚C (the specific heat capacity of water is 4.184 J/g ˚C, 1kJ = 1000J)?
b) When 120 g aluminum absorbs 9612J of energy, the temperature increases from 25 ˚C to 115
˚C. Find the specific heat of aluminum.
c) The specific heat of lead is 0.129 J/g ˚C. Find the amount of heat released when 120 grams of
lead are cooled from 37.2 ˚C to 22.5˚C
6) Solve the following problems based on the thermochemical equations given:
a) C2H4 (g) + 3O2 (g)  2CO2 (g) + 2H2O (l)
ΔH = -1.39 x 103 kJ
Calculate the amount of heat released when 4.79 g C2H4 reacts with excess oxygen.
b) 4NH3 (g) + 3O2  2N2 (g) + 6H2O
ΔH = -1.53 x 10 3 kJ
Calculate the enthalpy of the reaction when 146.3 g NH3 g reacts with excess oxygen.