An Introduction to THERMOCHEMISTRY (5.1)
Heat versus Temperature Demonstration
3 Buckets of water:
Hot
Room temp.
Ice cold
1. Put your hand in the hot water for 30 seconds, then plunge it quickly into the room temp. water
2. Put your hand in the ice water for 30 seconds, then plunge it quickly into the room temp. water
Why did your hand feel cold in the first situation but warm in the next?
3. Put one hand in the hot water and one in the cold water for 30 seconds, then plunge them quickly into
the room temperature water - at the exact same time.
Explain what you felt in the third situation.
Thermochemistry is the study of energy changes that occur during ________________________or
_________________ changes in matter. We usually describe the energy changes in terms of
something losing energy and something gaining energy. When a reaction occurs, heat (q) is transferred
between substances.
Heat:
Temperature:
Bond breaking and bond making
Energy is needed to break a chemical bond (ENDOTHERMIC). On the other hand, the formation of
a chemical bond releases energy (EXOTHERMIC).
Every chemical reaction involves both bond breaking (reactant bonds are broken) and bond making
(product bonds are formed). The energy that is absorbed or released when breaking or making a bond is
called the ____________ ______________.
Endothermic:
Exothermic:
The Language of Thermochemistry
A system is the object of primary concern, or the object being studied.
The surroundings is/are the other object(s) involved, or the environment the object is in.
For example,
Imagine a sample of solid carbon dioxide (dry ice) sitting in the air. The ice will sublimate. The ice is the
_________________, and the atmosphere of the room is the ___________________. Energy, in the
form of heat, is transferred to the ice from the atmosphere, in order for the ice to sublimate.
When we describe a chemical system, we can classify the system based on the exchange of matter and
energy between itself and the surroundings. There are three kinds of systems:
System
Example
An open system, which exchanges both matter
and energy with the surroundings
A closed system, exchanges energy but not
matter with the surroundings
An isolated system is one where ideally, neither
matter or energy are exchanged with its
surroundings
Calorimetry
Calorimetry is the technological process of measuring energy changes in a chemical system. See
Handout: Calorimeters (LSM 5.1-1) to see cross sectional diagrams of a classroom calorimeter and a
“bomb” calorimeter. Calorimetry depends on careful measurements of ________________ and
_______________________ changes. The technique is based on the law of conservation of energy:
heat energy produced is __________________________ from the chemical system to the
_______________________. And, since different substances vary in their ability to absorb heat, a value
called specific ___________ capacity is also important in calorimtery.
Specific Heat Capacity:
To calculate the quantity of heat transferred (“q” in your text, was “Q” in last year’s), we use the following
equation:
q = mc∆T
m = mass(g or kg), c=specific heat capacity (J/g·°C or J/kg·K) and ∆T is change in
temperature (°C or K)
Let’s review some q=mc∆T questions:
1. When 600mL of water in a kettle is heated from 20°C to
85°C to make a cup of tea, how much heat flows into the
water?
2. An unknown metal 1 kilogram mass at 88.0 oC is
submerged in 0.44 kg of water at 6.0 oC. The final
temperature is 20.0 oC. Calculate the specific heat capacity
of this sample and identify the metal.
Introduction to Heat Problems
Part A Q = mc^T problems
Substance
Specific heat
capacity, c (J/g°C)
ice
2.01
water
4.18
steam
2.01
aluminum
0.900
iron
0.444
methanol
2.918
brass
0.380
copper
0.386
glass
.84
#1
Calculate the heat transferred to 0.50- kg of water when it is heated from 30.0 degrees Celsius to 90.0
Degrees
#2
Calculate the heat transferred to 35 kg of lead when it is heated from 36 to 57 degrees C.
#3
An unknown metal absorbs 20.471 kJ of heat energy when it is heated up from 37.5 to 43.2 degrees C.
The mass of the metal is 1.01 Kg
#4
Calculate the heat lost by 345 g of water when it cools from 99 to 1.0 degrees C 141325.8 J
#5
A block of aluminum gains 500 kJ of heat energy as it is heated from 59.12 to 64.15degrees C. Calculate
the mass of the aluminum. 108766.58 J
#6
A piece of copper changes temperature from 125 to 37.5 Degrees C . It loses 900.5 kJ of heat energy.
Calculate the mass of the copper metal. 26728.99
#7
What is the temperature change when 0.200 kg of water gains 8400 J of heat? 10.05 degrees C
116 865 J
Part B Heat loss and Gain problems
1
A student mixes 25.5 kg of cold water at 30.0 Degrees C with some water at 60 degrees Celsius. The final
temperature of the mixture is 40.00 Degrees C. Calculate the mass of the hot water.
2
Calculate the mass of hot copper required to raise 12.5 kg of water from 20.5 to 30.5 degrees C if the
copper was at 60.5 degrees C.
3
A student wants to heat their swimming pool from 25.0 to 37 degrees C.. How much water at 100
degrees C should they add? The mass of the water in the pool is 5.00 X 10 4 kg.
4
What mass of water at 10 degrees C would be required to cool 4.5 kg of water at 1.0 X 102 degrees C to a
final temperature of 35 degrees C.
5
A student places 50.0 g of aluminum at 95 degrees C into an unknown mass of water at 20.0 Degrees C.
The final temperature of the mixture is 30.0 Degrees C. Calculate the initial mass of the water.
6
In Peterborough, Mr Hollandish has his kids heat up their swimming pool by making trips from a hot
water tap carrying cups of water. The mass of the water in the pool was 9.00 X 104 kg. The pool
temperature was 20.0 degrees C. The water poured into the pool was at 80.00 degrees C. Each cup held
0.100 kg of water. Calculate the number of trips required to raise the pool water to 30.0 degrees C.
Molar Enthalpies
Chemical equations are
written to represent the chemicals that are reacting with one another. These equations are usually
balanced with whole numbers indicating the number of moles of substance that reacts with the other
reactants as well as the products.
Thermochemical equations can also be written which represent the reaction however it also includes the
energy associated with that particular reaction.
Example.
1 H2(g) + ½ O2(g) ===> 1 H2O(g) + 241.8 kJ
This equation represents the combustion of 1 mole of hydrogen with 0.5 mole of oxygen gas to form 1
mole of water vapour.
The value of 241.8 kJ represents the Molar Enthalpy for the combustion of 1 mole of hydrogen gas and
is represented by the Delta H symbol.
Definition:
enthalpy change associated with, physical, chemical, or nuclear change involving only
one mole of a particular substance produced or is referenced to.
Molar Enthalpies and Heat Calculations.
Enthaply changes can be measured in many ways such as the vapourization of water.
Molar enthalpy of vapourization of water.
H2O (l) + 40.8 kJ ===> H2O (g)
Sample problem.
What amount of ethylene glycol would vapourize while absorbing 200kJ of heat energy?
Delta H for ethylene glycol = 200 kJ ( energy absorbed)
ethylene glycol = 58.8 kJ /mol (table 2 page 307)
Delta H vap for
Relate MOLAR ENTHALPY to amount of heat actually generated.
The actual number of moles that were used needs to be determined
Since 1
mole = 58.8 kJ of energy, x moles = 200 kJ energy.
Set up your equality and
solve for x.
1/ 58.8 = x / 200
x = 200kJ / 58.8 kJ /mol
x = 3.40
In a nutshell, or burning nutshell.....
A nutshell burning will give off a specific amount of heat which we can determine using Calorimetry.
The question is now and has been, how many moles of “nutshell” is there and thus how much heat is
generated per mole of nutshell.
Sample problem calculating Molar Enthalpies.
100 g of water is placed in a styrofoam cup calorimeter.
Temperature is measured to
be 14.4 degrees C.
0.412g of Calcium metal is placed into the
calorimeter.
When the reaction is complete the temperature was recorded to
be 24.6 degrees C.
Calculate the standard molar enthalpy change for this reaction.
Ca (s) + 2 H2O ( l)
====> Ca(OH)2 ( s) + H2 ( g)
Assume that Calcium is the limiting reagent in this reaction and specific heat of water is 4.18 J/ g.C.
Q = mc^T
( calculations are used done on the calorimeter, not the chunk that caused the change in
temp. ( that comes later)
m = 100 g ( water)
^ T = 24.6 - 14.4 = 10.2
c = 4.18 J/g*C
Q=
(4.18) * (100g) * (10.2)
=
4.26 * 10 3 J or 4.26 kJ
( amount of heat that calorimeter absorbed.)
This heat came from the chemical reaction which was outlined above.
Thus re writing the chemical equation to include the energy, it looks like this:
Ca (s) + 2 H2O ( l)
====> Ca(OH)2 ( s) + H2 ( g) + 4.26 kJ
This shows that heat is part of the products and is listed as the Delta H of reaction as :
Delta H = - 4.26 kJ
This amount of heat was generated by the introduction of only 0.412 g of Calcium
How much heat could be generated by the introduction of 1.00 moles Calcium?
Molar Heat of Fusion
Definition of Molar heat of Fusion
to melt or freeze 1.00 mole of a substance at its melting point.
Amount of heat necessary
Must be 1.00 mole there is no temperature change while change occurs. Melting ice. Temp remains 0
degrees C until all ice melted then temp can increase.
Very important value as it can be used in energy calculations since it can tell us how much energy is
needed to melt each mole of a given substance or cool a substance down.
Every substance has it’s own molar heat of fusion.
Molar heat of fusion for water is 6.02 kJ/mol.
Units are kJ / mol or J/g
Molar heat of fusion Problems.
31.5 g of water is being melted at its melting point of zero degrees C. How many kJ are required?
q = (6.02 kJ / mol) ( 31.5 g/ 18 g/mol)
Enthalpy, Calorimetery Problems
#1
Calculate the enthalpy change for the melting of a 30 g ice cube ( heat of fusion) [10.0 kJ]
#2
A reference value of 39.23 kJ / mol is given for the molar enthalpy of vapourization for methanol. What
enthalpy change occurs in the evaporation of 10.0 g of methanol? [12.26]
#3
An experiment produces evidence that the evaporation of 4.00 g of liquid butane C4H10 (l) requires a
gain in enthalpy of 1.67 kJ. Find the molar enthalpy of vapourization bor butane from this evidence.
[24.3 kJ/mol]
#4
A calorimeter has a heat capacity of 40 .00 kJ/ Degree C. Complete combustion of 1.00 g Hydrogen gas in
this calorimeter causes a temperature increase of 3.54 degrees C. Calculate the molar enthalpy of
combustion for hydrogen from this evidence. [243.2kJ/mol]
#5
Combustion of 3.50 g ethanol , in a calorimeter with a heat capacity of 15.2 kJ/ C degrees C, causes a
temperature increase from 19.88 to 26.18 Degrees C. Find the molar enthalpy of combustion for ethanol
from this evidence. [1260.8 kJ/mol]
Communicating Enthalpy Changes (5.3)
1. Thermochemical Equation with Energy Term
2. Thermochemical Equation with ∆H values
3. Molar Enthalpies of Reaction
4. Potential Energy Diagrams
EXOTHERMIC
Ep
ENDOTHERMIC
Ep
reaction progress
reaction progress
See text page 319 for an excellent summary along with 2 examples.
Representing Enthalpy Changes Exercise
1
Iron (II) sulfide ore is roasted according tot he following chemical equation.
4 FeS (s) + 7 O2 (g) ===> 2 Fe2O3 (s) + 4 SO2 (g)
^ Hc = - 2456 kJ
a
rewrite this chemical equation including the energy as a term in the balanced chemical equation.
b
What is the molar enthalpy for Iron (II) sulfide in this reaction?
c
What is the molar enthalpy for Iron (III) oxide in this reaction?
2
Boron reacts with hydrogen to form diboron hexahydride ( gas) The molar enthalpy of reaction
for boron is + 15.7 kJ /mol
a)
Write the balanced chemical equation using whole number coefficients and including the energy
change as a ^H r
b)
Write the balanced chemical using whole number coefficients and including the energy change
as a term in the balanced chemical equation.
3
The molar enthalpy of combustion for octane, C8H18 ( l) is reported to be - 1.3 MJ/mol.
a
Write the balanced chemical equation using the whole number coefficients and including the
energy change as a ^H r.
b
Write the balanced chemical equation using the whole number coefficients and including the
energy change as a term in the balanced chemical equation.
4
Draw potential energy diagrams to communicate the following chemical reaction. Assume SATP
conditions
a
the formation of chromium (III) oxide
b
the simple decomposition of Silver Iodide
c
the formation of carbon disulfide
Hess's Law (calculating with steps)
Calculating H using Hess's Law
In order to calculate a H overall (net) using Hess's idea we need to be able to manipulate the steps of a
reaction so that they can add up to the overall reaction.
Manipulating Steps
We can manipulate the steps of reactions in only two ways.
a) We can reverse the equations and write it backwards from product to reactant. When we do this the
value of the H for the step changes sign. WHY?
b) We can multiply the equations by any common factor.When we do this we must also multiply the heat
by the same factor.
for example
C(s) + O2(g) -----> CO2 (g) + 393.5 kJ
if two moles of CO2 (g) appeared in the overall reaction we might multiply this reaction by 2 to get:
2C (s) + 2 O2 (g) -----> 2 CO2 (g) + 787 kJ
Calculating Heats of Reaction using Hess's Law
Important
procedure
1) Write the overall equation for the reaction if not given.
2) Manipulate the given equations for the steps of the reaction so they
add up to the overall equation.
3) Add up the equations canceling common substances in reactant and
product.
4) Add up the heats of the steps = heat of overall reaction
Hess's Law (calculating with steps)
Sample calculations using Hess's Law
Calculate the H of the following reactions using Hess's Law and the steps indicated
#1) Calculate the heat released by the burning of sulfur in oxygen given the following steps
2S(s)
+
3O2 (g)
2SO3(g)
H=?
Given Steps
Step 1
S(s) +
O2 (g)
SO2 (g)
H= -297 kJ
Step 2
2SO2 (g) +
O2 (g)
2SO3 (g)
H= - 198 kJ
Answer:
1) The overall reaction is written above.
Note that sulfur (S) is a reactant and sulfur trioxide (SO3) is the product.
Therefore neither reaction in steps 1 or 2 needs to be reversed.
Note that the overall equation has 2 moles of S and 2 moles of SO3.
Therefore the first reaction must be multiplied by two , but the second can be left alone.
2) Manipulate equations
Step 1
2 S(s) +
2O2 (g)
2SO2 (g)
H = - 594 kJ
Step 2
2 SO2(g) +
O2 (g)
2 SO3 (g)
H = - 198 kJ
3) Add equations and heats
Addition: 2S (s) +
Net
2S (s)
+
3O2 (g) +
3O2 (g)
2SO2 (g)
2 SO3 (g)
2SO2 (g) +
2SO3 (g)
H = - 792 kJ
H = - 792 kJ
3) Calculate the heat of reaction for the following equation
C3H8 (g) + 5 O2 (g) -----> 3 CO2 (g) + 4 H2O(g)
given the following steps in the reaction mechanism.
Step #1: 3C (s)+ 4 H2 (g) -------> C3H8 (g)
Step #2: 2 H2(g) + O2 (g) -------> 2H2O (g)
Step #3: C (s) + O2 (g) --------> CO2 (g)
Answer
1) Overall balanced equation is written above. Note that C3H8 is a reactant and there are 3
moles of CO2 and 4 moles of H2O as products.
2) We can manipulate the equations by:
a) Reversing equation #1
b) Multiplying equation #2 by 2
c) Multiplying equation #3 by 3
manipulated equations we get
Manipulated equations
C3H8 (g)
H
4 H2 (g) +
3C (s)
+103.8 kJ
4 H2 (g)
+
2 O2 (g)
4 H2O (g)
-968 kJ
3 C (s)
+
3 O2 (g)
3 CO2 (g)
-1180.5
kJ
the substances in bold print are products
3) Adding the equations we get
H = - 2044.7 kJ
C3H8 (g)
+ 5O2 (g) + 4 H2 (g) + 3 C(s)
3 C (s) + 4 H2 (g) + 4 H2O (g)
+ 3 CO2 (g)
the substances in bold print are common substances and are canceled to produce the net equation
C3H8 (g)
+
5O2 (g)
4 H2O (g)
+ 3 CO2 (g)
H = - 2044.7 kJ
Predicting Delta H Using Hess’s Law Exercises
#1
The enthalpy changes for the formation of two different wolfram bromides are shown below.
W(s) + 2Br2 (l) ==> WBr (s)
Delta H = -146.7 kJ
W(s) + 3Br2 (l) => WBr6(s)
Delta H = - 184.4 kJ
Calculate the standard enthalpy change for the following reaction.
Br2(l) + WBr4(s) ==> WBr6 (s)
#2
Given the following equations:
1
N2O4(g) ==> 2 NO2 (g)
2
NO (g) + ½ O2 ==> NO2 (g)
Delta H = +58 kJ
Delta H = -56 kJ
Calculate the standard enthalpy change for the following reaction.
2NO (g) + O2 (g) ===> N2O4 (g)
#3
Target equation
N2H4(l) + O2 (g) ====> N2(g) + 2H2O (l) Delta H = ?
Given equations
2NH3(g) + 3 N2O (g) ==> 4 N2(g) + 3 H2O (l
Delta H = -1010 kJ
N2O (g) + 3 H2 (g) ===> N2H4 (l) + H2O (l)
Delta H = -317 kJ
2
NH3 (g) + ½ O2 (g) ==> N2H4 (l) + H2O (l)
Delta H = -143 kJ
H2(g) + ½ O2(g) ===>
H2O (l)
Delta H = -286 kJ
Enthalpies of Formation
Thermochemistry is the study of chemical reactions and the energy associated with these
reactions. Calorimetry and Hess’s Law are two methods previously examined which allow us to
determine the enthalpies of the various reactions.
A third method is to use predetermined enthalpies of formation for various substances as they
react with one another in chemical reactions.
Ex.
Enthalpies of formation for the following substances
C + O2 ==> C02
Delta H f = -393 kJ/mol
Delta H f = -74.4 kJ/mol
= - 285.8 kJ/mol
O2
C + H2 ===> CH4
H2 + O2 ==> H20
Delta H f
Delta H f= 0 kJ/mol
The standard enthalpy of formation of an element already in its standard state is zero.
Sample Question:
enthalpy of reaction for the combustion of methane .
CH4(g) + 2 O2 (g) ====> CO2 (g) + 2H2O(l) Delta H Reaction = ?
Use the New Formula ( memorize this) , using Standard Enthalpies of Reaction.
Determine the molar
Delta HR = [ Sum n Delta Hf Products ] - [Sum n Delta Hf Reactants]
= [(1 mol * -393.5 kJ/mol) + ( 2 mol * -285.8 kJ/mol )] ( CO2 (g) )
( 2H2O(l) )
[1 mol*-74.4 kJ/mol + 2 mol * 0 kJ/mol]
(CH4(g) )
(2 O2 (g))
= -965.1 kJ - ( -74.4 kJ)
= -890.7 kJ
Delta H R for 1 mole of methane = -890.7 kJ/mol
Therefore the molar enthalpy for methane combustion is -890.7 kJ/mol
Study Questions
Chapter 5 self Quiz page 355 # 2-18
Chapter 5 review Page 356-357 # 2,4,5,9,10,14,15,16c,d
Predicting Delta H of reactions using
Standard Enthalpies of Formation.
#1
Laboratory quantities of ethylene can be prepared by an elimination reaction of ethanol using an
acid catalyst . Calculate the enthalpy change for the conversion of ethanol into ethylene and
water using standard enthalpies of formation.
#2
Calculate the enthalpy of combustion for acetic acid using standard enthalpies of formation.
#3
The fertilizer urea is produced along with liquid water by the reaction of ammonia and carbon
dioxide. Calculate the enthalpy of this reaction using standard enthalpies of formation
#4
When octane is strongly heated with hydrogen gas, the process of cracking occurs which leads to
the production of 1 mol methane, 2 mol ethane and 1 mol propane from each mole of octane.
a)
write the balanced chemical equation for this reaction
b)
use standard enthalpies of formation to calculate enthalpy change associated with the
cracking of 467 g of octane
Standard Heat of Formation Lab
Problem:
To determine the standard heat of formation and calculate the Delta Hf for NH4Cl(s)) ( remember
definition of Delta H of formation and generate appropriate chemical equation for the formation fo Ammonium
Chloride)
Introduction: To determine and calculate the heat of formation, six intermediate step equations are needed to
analyse and calculate the TARGET EQUATION.
#1
N2(g) + H2(g)===> NH3(g)
^H = -46.15 kJ/mol
#2
H2(g) + Cl2(g) ===> HCl (g)
^H = -92.21 kJ/mol
#3 NH3 (g)
===> NH3 (aq)
^H = -30.47 kJ/mol
#4 HCl (g) ====> HCl (aq)
^H =-74.78 kJ/mol
#5 NH3 (aq) + HCl ====> NH4Cl ( aq)
^H = ?
#6
NH4Cl (s) + H2O (l) ==> NH4Cl ( aq) + H2O(l)
^H = ?
Materials:
1.0 mol/L HCl, , 1.5 mol/L NH4OH(aq) , NH4Cl (s) , water, graduated cylinder, glass stirring rod,
styrofoam calorimeter, electronic balance.
Procedure:
A) Establish a target
equation for the Formation of Ammonium Chloride from its
component elements in their elemental form.
B)
Examine the given equations and conduct reactions for equations # 5 & 6 to determine the heats of
reaction to add to the given equations above.
Reaction #5
Measure 25 mL of 1.0
mol/L HCl ( aq). Measure 25 mL of 1.5 mol/L NH4OH( aq) Record initial temperatures. Mix the two solutions
together in the styrofoam calorimeter and record highest temperature reached.
Reaction #6
Mass
approximately 4.0 g of NH4Cl (s). Measure 50 mL of water and record the initial temperature. Add the
water to the solid in the styrofoam calorimeter and record the highest temperature reached.
Observations: Record qualitative and quantitative observations.
Calculations
1
Calculate the ^ H in kJ/mol of hydrochloric Acid from Reaction #5
2
Calculate
the ^H in kJ/mol of ammonium chloride from reaction #6
3 Combine and manipulate all six
equations to obtain the thermochemical equation for the standard heat of formation for Ammonium
Chloride(s).
4
From the manipulated equations, calculate the value of the standard heat
of formation using the ^H values from given and experimental data.
Discussion:
Compare your experimental value to the value in text and comment on results In your own words
the difference between and endothermic and exothermic reaction.
Conclusion
Multi step Energy Calculations
1
For the following combustion , what mass of carbon dioxide is produced when 1500 kJ of energy is
released?
2 C2H6 (g) + 7 O2 (g) ==> 4 CO2 (g) + 6 H2O (g)
2
+ 2502 kJ
[m= 105.5]
How much energy is released when 1.0 t of sulfur trioxide is produced by the following reaction?
2 SO2 (g)
+ O2 (g)
===> 2 SO3 (g) ^ H = -192.8 kJ
[H = 1.20GJ]
3
In respiration, glucose is oxidized by oxygen gas to produce carbon dioxide gas , liquid water and energy.
What is the energy released when 18.o g of glucose is consumed?
[H = 279.7 kJ]
4
Methanol is burned in a calorimeter. Liquid water is formed as one of the products. If 3.4 g of methanol
reacts, what is the expected temperature change in a calorimeter with a heat capacity of 6.75 kJ/ Degree
C?
[Delta t = 11.4]
5
A waste heat exchanger is used to absorb the energy from the complete combustion of hydrogen sulfide
gas. What volume of water undergoing a temperature change of 64 degrees C is required to absorb all of
the energy from the buring of 15 kg of hydrogen sulfide?
[V= 0.85 L]
Chapter 5 Assignment questions 1-12
A. Determine the amount of heat in Joules absorbed or released (specify) in each of the following
changes:
1) 135.6 g of water cooled from 95.8 °C to 21.6°C
2) 100.0 g of ice melted, with no temperature change
3) 450.0 g of iron cooled from 125.0 °C to 45.0°C
B. Solve the following enthalpy change problems.
4) How much heat is transferred when 5.81 g of graphite reacts with excess H2 as follows? Is the
reaction endothermic or exothermic?
6 C + 3 H2  C6H6
∆H°=49.03 kJ
5) How much heat will be released when 6.44 g of sulphur reacts with O 2 as follows?
2 S + 3 O2  2 SO3
∆H°=-791.4 kJ
C. Solve the following problems involving energy changes.
6) In order to change a particular calorimeter by 1°C, 6485 J of energy is required. The combustion of
2.80 g of ethylene gas (ethene - C2H4) in the same calorimeter causes a temperature rise of 21.4 °C.
From this information, find the heat of combustion , per mole, or ethylene.
7) Which of the following reactions are endothermic?
a) H2(g) + S(s) + 2 O2 (g) ↔ H2SO4 (l) ∆H°=-811.3 kJ
b) 3C (s) + 2 Fe2O3(s) + 463.6 kJ ↔ 4 Fe(s) + 3 CO2 (g)
c) H2O(l) ↔ H2(g) + ½ O2(g) ∆H°= 285.9 kJ
d) C(s) + 2 H2(g) ↔ CH4(g) + 74.9 kJ
8. Given:
a) Na(s) + 1/2 Cl2(g)  NaCl(s) ∆H°=-411kJ
b) H2(g) + S(s) + 2 O2 (g) ↔ H2SO4 (l) ∆H°=-811.3 kJ
c) 2Na(s) + S(s) + 2 O2 (g) ↔ Na2SO4 (s) ∆H°=-1383 kJ
d) 1/2H2(g) + 1/2 Cl2(g)  HCl(g)
∆H°=-92.3 kJ
Find the heat of reaction of the following chemical change: 2NaCl(s) + H2SO4 (l)  Na2SO4 (s) + 2HCl(g)
9. Calculate the enthalpy of combustion for acetic acid using standard enthalpies of formation.
10. What mass of CO2 is produced when 1500 kJ of energy is released from the reaction below?
2 C2H6(g) + 7 O2(g)

4 CO2(g) + 6 H2O (g) +2502 kJ
11. A waste heat exchanger is used to absorb the energy from the complete combustion of hydrogen
sulphide, H2S, gas. What volume of water undergoing a temperature change of 64°C is required to
absorb all of the energy from the burning of 15kg of hydrogen sulphide?
12
Methane can be produced in a biodigester of waste organic matter. 20 kg of methane can be
generated from 508 kg of waste. Methane is used to heat water for homes from 6-42 degrees C. How
many showers can a person take using the hot water if each shower uses 6.8 kg of water?
Review Questions Chapter #5
#1
How much energy was transferred to 67 kg of water as it was heated from 20 degrees Celsius to
98 degrees Celsius. (21.8 GJ)
#2
A 56 kg bar of iron was heated to a temperature of 456 degrees Celsius and immersed into a
bucket of cold water, 3 degrees Celsius to cool the iron bar so it could be handled properly.
The final temperature of the water was 14 degrees Celsius. Calculate the initial mass of
water in the bucket. (238.9 kg)
#3
56 g of Sodium metal was reacted with water in a simple calorimeter. The mass of the water in
the calorimeter which absorbed the heat was 1157 g and had an initial temperature of 18
degrees C. The final temperature was 45 degrees C.
What is the value of Q for this reaction?
What is the
^ HR ?
What is the ^H R /mol Na
(54.4 kJ/mol)
#4
What mass of lithium chloride must have dissolved into 200 g of water if the temperature of
water was initially 14 degrees C and ended up being 22 degrees C. The molar enthalpy of
Lithium Chloride is - 37 kJ/mol (8.5 g)
#5
189 g of benzene is burned in the presence of oxygen gas for a typical combustion reaction.
Calculate the overall energy that is released into the environment. (6522.8 kJ)
#6
When octane burns in a car engine, heat is released. Engines have coolant, usually ethylene
glycol which has a specific heat capacity of 3.5 j/g* C. What mass of octane needs to be
combusted in the engine to cause the heating of 32 Kg of coolant from a base temperature of
8 degrees C to 95 degrees C? What assumptions do you need to make to solve this question?
m = 205.5 g
#7
What is ^H for the combustion of 45 g pentane? ^H = -2104.2 kJ
#8
How much energy is released by the formation of 9.63 g of Sulphur dioxide?
9
Benzoic acid can be burned and it’s enthalpy of combustion measure to be - 3223.6 kJ/mol. Use
this information and values of the standard enthalpies of formation of liquid water, and
carbon dioxide to calculate the standard enthalpy of formation for benzoic acid.
^H f = -388.3 kJ /mol
^H = 44.6 kJ