UNIT 18:ENERGY CHANGES AND ENERGY PROFILE DIAGRAM
UNIT 18: ENERGY CHANGES AND ENERGY PROFILE
DIAGRAMS
Key unit Competence: Explain the concept of energy changes and energy profile
diagrams for the exothermic and endothermic processes.
Learning Objectives
By the end of this unit, student should be able to:
• Define the term Thermochemistry.
• Explain the concept of system and distinguish between the types of systems.
• Distinguish between Temperature and heat.
• Explain the concept of Exothermic and endothermic reactions and represent
them using energy profile diagrams.
• Carefully deal with reactions that produce a lot of energy.
• Appreciate the use of chemical energy in daily life.
• Respect the experimental protocol during chemistry practicals.
• Relate the type of reaction to its energy profile diagram.
• Interprete the experimental results about energy changes occurring during
chemical reactions.
• Explain the energy change as a function of the breaking and formation of
chemical bonds.
18.1. Concept of a system
18.1. Concept of a system
Activity 18.1
Topic: Energy transfer between a system and surroundings.
Apparatus and equipment (per group)
• Eye protection
• Four test-tubes or four expanded polystyrene cups with lids to act as
calorimeters
• Spatula
• Teat pipette or small measuring cylinder
• Thermometer
• Access to a balance
Experiment 4
Repeat the same procedure as in experiment 1 but use copper (II) sulphate
solution and Zinc powder instead of water and anhydrous copper (II) sulphate respectively.
Safety
• Wear eye protection.
• Anhydrous copper (II) sulfate is harmful.
• Zinc powder is flammable.
Introduction
Instant hot and cold packs are available for use in first aid. This experiment
illustrates the types of chemical reaction that occur in these packs.
Study questions
1. Identify the reactions that are exothermic and those that are endothermic.
2. Write symbol equations to represent the chemical reaction taking place in Experiment 3.
3. Which two substances could be put in a cold pack?
4. Golfers need a hand warmer to keep their hands warm on a cold day. Which
chemicals could be put in these warmers?
All chemical reactions involve the breaking of bonds in the reactants and the
formation of new bonds in the products. The breaking of bond requires energy,
whereas the formation of bond releases energy.
Thermochemistry is the study of heat and energy associated with a chemical
reaction or a physical transformation. Thermodynamics is the study of the
relationship between heat, work, and other forms of energy. A reaction may release
or absorb energy, and a phase change may do the same, such as in melting and
boiling. Energy is exchanged between a closed system and its surroundings during
the heating and cooling processes.
A system is a part of the universe which is studied using laws of thermodynamics.
Everything outside the system is the surroundings. An infinitely small region
separating the system from the surroundings is called boundary. In Chemistry the
chemical system consists of reactants and products. The systems are classified
according to the number of factors including the composition and the interaction
with the surroundings. A system can be homogeneous or heterogeneous. It can
be in gaseous, liquid or solid state. A system is said to be in equilibrium when its
properties do not change with time. The state of a system is described using its
composition, temperature and pressure.
Three types of systems can be distinguished according to the exchange between
the system and the surroundings in terms of matter and/ or energy.
1. An open system is a system that can exchange both matter and energy with the
surroundings (Figure 18.1).
lid prevents the exchange of matter between the system and the surroundings. An
isolated system is a system which is both sealed and insulated. It can exchange
neither matter nor energy with its surroundings.
Examples
Hot coffee in a thermos flask (Figure 18.3).The latter is a closed system. The outer
surface is insulated and thus neither heat nor matter transfer take place between
the system and the surrounding.
2. Indicate the direction of heat (from one compartment to another) and explain
your answer for the following phenomenon
a) When you touch water in a saucepan on top of a stove with your hand and you
fill it is warm
b) When you touch water from the tap with your hand and you fill it is cold
c) When you mix cold water and warm water
18.2.1. Internal energy
The first Law of Thermodynamics deals with energy that is transferred between a
given system and its surroundings in form of heat. The exchange of energy is related
to the energy that is stored in the system called internal energy E. The internal
energy is the sum of the kinetic and potential energies of the particles that form a
system.
18.2.2. Heat energy and temperature
The heat or thermal energy of an object is the total energy of all the molecular
motion inside that object. When two bodies are in contact, heat always flows from
the object with the higher temperature to that of lower temperature. Heat transfer
ceases when a thermal equilibrium is attained. The heat content of a body will
depend on its temperature, its mass, and the material it is made of. Because heat is a
form of energy, it is measured in Joules (J) or kilojoules (kJ) or calorie (cal). A calorie
is defined as the amount of energy needed to raise the temperature of one gram of
water by one degree Celsius.
1 calorie (cal) = 4186 joules (J); 1000 cal = 1 kcal = 4.186 kJ.
The temperature is a measure of the average heat energy (thermal energy) of the
molecules in a substance. When an object has a temperature of 100 °C, for example,
it does not mean that every single molecule has that exact thermal energy. In any
substance, molecules are moving with a range of energies, and interacting with
each other. The temperature is a physical measure expressing how an object is hot
or cold.The temperature is measured using a variety of temperature scales. The most
commonly used are degree Celsius (°C) and degree Kelvin (K):
K = °C + 273
N.B: In thermodynamic calculation, degree Kelvin, not degree celcius, is used.
First Law of Thermodynamics
Thermodynamics is part of physical chemistry that deals with the relationships
between heat and other forms of energy. In particular, it describes how thermal
energy is converted to and from other forms of energy and how it affects matter.
The first Law of thermodynamics is a statement about conservation of energy and it
categorizes the method of energy transfer into two basic forms: work (W) and heat
(Q). The First Law of Thermodynamics states that energy can be converted from
one form to another with the interaction of heat, work and internal energy,
but it cannot be created or destroyed, under any circumstances. Internal energy
refers to all the energies within a given system, including the kinetic energy of
molecules and the energy stored in all of the chemical bonds between molecules.
For a closed system (without mass input and output), the internal energy is the sum
of the heat energy and the work done by the system or the surroundings
∆U = Q + W
Where W is the energy transferred to the system by doing work and Q is the energy
transferred to it by heating.
The work done by the system on the surroundings is negative. Therefore, the first
law of Thermodynamics is written as:
ΔU = Q – W
Work (W) is also equal to the negative external pressure on the system multiplied by
the change in volume. It can be expressed as:
W = −P∆V
Where P is the external pressure on the system, and ΔV is the change in volume.
This is specifically called pressure-volume work. Therefore, the Fist Law of
Thermodynamics is expressed using equation:
ΔU = Q -P∆V
Glasses P and Q have the same amount of water. Glasses R and S have the same
amount of water.
The water in Glasses P and R are at the same temperature. The water in Glasses Q
and S are at the same temperature.
1. Fill in the blanks below with the correct answers.
a. The water in Glass……..has the most heat.
b. The water in Glass……..has the least heat.
2. Ari touched a metal spoon. The metal spoon felt cold. Choose the best answer.
a. Heat flows from hand to spoon
b. Heat flows from spoon to hand
c. Heat does not flow
d. Heat flows in both directions
3. Tom placed a metal spoon in a mug of hot coffee as shown below. The metal
spoon got hot. Choose the best answer
a. Heat flows from hand to spoon
b. Heat flows from spoon to hand
c. Heat does not flow
d. Heat flows in both directions
4. Complete the statement below.
If two objects are near each other and one object is hotter than the other, then
heat will flow from the …………………….object to the…………………..
object.
5. Complete the crossword puzzle using the clues given below.
Down
1. Our sense of ………………….cannot measure temperature accurately.
3. Wood is a …………………….conductor of heat.
4. Heat is a form of ………………………..
6. ………………………….is a measure of how hot or cold an object is.
10. Metals can ………………………………when heated.
Across
2. Heat is used to …………………………… food.
5. When two objects of different temperatures are in contact, heat will travel from
the ………… object to the other object.
6. What does the first law of thermodynamics have to do with systems?
7. The instrument used to measure temperature accurately is a
……………………………..
8. Temperature is measured in the unit ……………………….Celsius (°C).
9. A……………, when used with a temperature sensor, can be used to measure
and record temperatures.
10. The Sun is an important ………………………….of heat.
11. A hotter object will has a ……………………….temperature.
12. A gas is compressed and during this process the surroundings does 462 J of
work on the gas. At the same time, the gas loses 128 J of energy to the surroundings
as heat. What is the change in the internal energy of the gas?18.3. Standard Enthalpy changes
Activity 18.3
1. What is meant by standard conditions of temperature and pressure?2. Which term describes the sum of kinetic energy and potential energy?
The standard enthalpy of Hydration also called Standard enthalpy of solvation
is the amount of heat released when one mole of isolated gaseous ions dissolve in
water forming one mole of aqueous ions under standard conditions. The positive
terminal of the water molecule is attracted to the anion while its negative terminal
is attracted to the cation. This is an ion-dipolar attraction which is typically anelectrostatic interaction. This latter is accompanied by the release of heat energy.
1. Discuss the type of energy form present in points A, B and C of the pathway
followed by the vehicle.
2. Discuss how each form of energy changes from point A to point C.
3. Which points corresponds to maximum stability and minimum stability,
respectively? Relate your answer to energy concept.
When a chemical reaction happens, the energy is transferred to or from the
surroundings and often there is a temperature change. For example, when a bonfire
burns, it transfers the heat energy to the surroundings. The objects near the bonfire
become warmer and the temperature rise can be measured with a thermometer.
There are some chemical reactions that must absorb energy in order to proceed.
These are endothermic reactions. Some other chemical reactions release energy to
the surroundings. The energy released can take the form of heat, light, or sound.
These are exothermic reactions.
1. Exothermic reactions
They are characterized by an increase in the temperature of the surroundings, i.e.
energy is given up. Heat is lost to the surroundings and by convention it is negative
and represented as: ΔH < 0
For exothermic reaction (Figure 18.8), total energy of the reactants is higher than in
the product, because the heat energy absorbed during bond breaking is lower thanthe heat energy released during bond formation.
2. Endothermic reactions
These are reactions that take place by absorbing the energy from the
surroundings. The energy is usually transferred as heat energy; in this case the
surroundings loses energy to the reactants causing the surroundings to get colder.
Endothermic reactions cannot occur spontaneously. Work must be done in order
to get these reactions to occur. When endothermic reactions absorb energy, a
temperature drop in the surroundings is observed during the reaction. Endothermic
reactions are characterized by positive heat flow (into the reaction) and an increase
in enthalpy, by convention it is represented by: ΔH > 0
For endothermic reaction (Figure 18.9), the total energy of the reactants is lower
than the product, because the heat energy absorbed during bond breaking is higher
than the heat energy released during bond formation.
You have certainly experienced this effect when you put a drop of methanol or any
other volatile substance on your skin; you feel cold because that part of your skin issupplying energy to evaporate the volatile liquid
3. Activation energy, Ea
The activation energy is the minimum energy required for a chemical reaction to
take place. It is the energy barrier that has to be overcome for a reaction to proceed.
Without that minimum energy, the reaction will not take place. That is why, for
example, the only fact that a dry wood is in contact with oxygen of air will not
start burning; there is a need of supplying the minimum energy to overcome the
activation energy barrier, this is done by using a burning match.
4. Activated complex
The activated complex is the intermedicate species, where former chemical bonds
are being broken, whereas new chemical bonds are being formed. In term of energy,it corresponds to the activation energy.
5. Determine the activation energy for the reverse reaction.
6. Determine the enthalpy change of reaction for the forward reaction.
7. Determine the enthalpy change of reaction for the reverse reaction.
8. Fill in using exothermic or endothermic.
a. The forward reaction is ……………………..
b. The reverse reaction is ………………………
9. Which chemical species or set of chemical species represent the activated
complex?
10. Which one of the chemical bonds A-X and M-X is stronger? Explain.
11. State the chemical species whose particles move the fastest. Explain your
answer.
12. State the chemical species whose particles move the slowest. Explain your
answer.
13. The compound AX and the element M are in gaseous and solid states,
respectively.
What effect would grinding M into a fine powder have on this energy profilediagram?
18.5. End unit Assessment
Regarding the absorption or release of energy, what is the nature of the
overallreaction?
b. What is the activation energy for the forward reaction?
c. What is the activation energy for the reverse reaction?
d. Determine the enthalpy change of reaction for the forward reaction?
e. Is the reverse reaction endothermic or exothermic?
f. Which chemical species constitute the activated complex?
g. Which chemical species or set of chemical species have the maximum potential
energy?
h. Which chemical species or set of chemical species have the maximum kineticenergy?
REFERENCES
Ajikumar, A. (2006). The MCAT chemistry book. A comprehensive review of both general and organic chemistry. Los Angeles: Nova Press.
Allaby, M., Brawell, M., Daintith, J., Day, T., Haywood, J., Henderson, J., et al. (2006).
The facts on File chemistry handbook (2nd ed.). New York, USA: Diagram
Visual Information Ltd.
Arthur, E., & Gary, F. (2003). Active chemistry.
Cann, P. H. (2002). Chemistry for Advanced level. Landon: John Murray publishers
Ltd.
David, B. (n.d.). Cambridge international AS and A level Chemistry revised guide.
Goldberg, D. E. (2007). Fundamentals of Chemistry (5th ed.). Mc-Graw Hill Companies.
Graham Hill, J. H. (1995). Chemistry in context (Vol. Fourth edition). Surrey: Thomas
Nelson and Sons Ltd.
Jim, C. (2017). chemguide. Retrieved February 23, 2018, from https://www.chemguide.
co.uk/atoms/bondingmenu.html
Jim, C. (2017). Chemguide. Retrieved February 6, 2018, from http://www.chemguide.
co.uk/analysis/masspec/hw it works.html
Jim, C. (2017). Chemguide. Retrieved February 20, 2018, from https://www.chemguide.
co.uk/atoms/structures/metals.html
Kasangandjo, L. (2013). Advanced Level Chemistry. Edinburgh: Pearson Education
Limited.
Kenneth, W. W., Raymond, E. D., Peck, M. L., & George, G. S. (2014). Chemistry
(10th ed.). Belmont, USA: Brooks/Cole, Cengage Learning.
Kotz, J. C., Treichel, P. M., & Townsend, J. R. (2012). Chemistry and chemical reactivity (8th ed.).
Lister, T. R. (1995). Understanding Chemistry for Advanced Level. London: Cheltenham.
Lwishi, K. (2013). Chemistry S4 (Vol. First Edition). Essex, England: Pearson Education Limited.
Martin, S. S. (1996). The molecular nature of matter and change, fifth edition . New
York: Published by McGraw-Hill.
Mukama, D. O. (2013). Chemistry for Rwanda Secondary Schools, Advanced level.
Kigali: Fountain
Manish, B. (n.d.). Deep Dive @ Sanfoundry. Retrieved February 9, 2018, from http://
www.sanfoundry.com/analytical-instrumentation-questions-answers-components-mass-spectrometer/
Miloslav, N. (2009). International Union of Pure and Applied Chemistry. 2005-2009.
“IUPAC Gold Book. “ Online Book.
Nivaldo, T. J. (2016). Principles of Chemistry. A molecular Approach (3th ed.). Pearson
Education.
S.Obonyo, D. (2013). Chemistry for Rwanda Secondary Schools (Vol. Advanced Level
Senior 4). Kigali: Fountain Publishers Rwanda lTD.
Peter Cann. Peter Hughes. (2002). Chemistry for advanced level. London: John Murry
publishers Ltd.
Peter, C., & Peter, H. (2015). Chemistry for advanced level. John Murray publishers
Ltd.
Petrucci, & Ralph, H. (2007). General Chemistry: Principles & Modern Applications
(9th ed.). Pearson Prentice Hall.Upper Saddle River, NJ.
Petrucci, R. H., Herring, F., Madura, J. D., & Bissonnette, C. (2011). General Chemistry: Principles and Modern Applications (10th ed.). Toronto, Ontario, USA:
Pearson Prentice Hall.
Ramsden, E. N. (2000). Advanced Level Chemistry (4th ed.). Cheltenham, United
Kingdom: Nelson Thornes Ltd.
Rasool, H. B. (2012). Mass Spectrometry (Importance and Uses). Pharmaceut Anal
Acta, 3(10), doi:10.4172/2153-2435.1000e138.
Ryan, L., & Norris, R. (n.d.). Cambridge international A S and A Level Chemistry
course book (2nd ed.).
Satya, P., G.D., T., S.K., B., & R.D., M. (1999). Advanced Inorganic Chemistry (16 ed.,
Vol. 1). New Delhi, India: S. CHAND& COMPANY LTD.
Shriver, D., & Atkins, P. (1999). Inorganic Chemistry. Oxford University Press, Oxford
292.
Silberberg, M. S. (2006). CHEMISTRY: The Molecular Nature of Matter and Change
(4th ed.). New York, USA: The McGraw-Hili Companies, Inc.
Wade, B. (2015). CK-12 Chemistry-Intermediate. USA: CK-12 Foundation.Zumdahl, S. S., & Zumdahl, S. A. (2007). Chemistry (7th ed.). Boston, USA/ University of Illinois: Houghton Miffin Compan