15. Energy Changes
A nuclear explosion releases a huge amount of energy in a very short period of time. This energy is
released in forms such as heat, light, electromagnetic and sound. The destructive force comes from
the energy released, annihilating anything within a few hundred metres, and causing devastating
destruction kilometres from where the bomb was detonated.
Learning Outcomes
1.
2.
3.
4.
5.
6.
7.
8.
9.
Recognise that reactions can give out or take in energy.
State that during a chemical reaction, bonds are broken and new bonds are formed.
Describe the term exothermic and endothermic in relation to processes or reactions in which energy is transferred to
or from the environment.
Describe the meaning of enthalpy change in terms of exothermic and endothermic reactions.
Represent energy changes by energy profile diagrams.
Describe bond breaking as an endothermic process and bond making as an exothermic process.
Explain overall enthalpy changes in terms of the energy changes associated with the breaking and making of covalent
bonds.
Construct energy profile diagrams to include activation energy.
Describe hydrogen, derived from water or hydrocarbons, as a potential fuel, reacting with oxygen to generate electricity
directly in a fuel cell.
Energy Changes
Start of Pre-Lesson Reading
1. What are Exothermic and Endothermic Changes?
Meaning of
Exo: Out; Thermic: Heat
the word
Definition
An exothermic reaction is a reaction
where energy is given out to the
surroundings.
Examples
Condensation
Freezing
Combustion
Acid-base reaction (neutralisation)
Temperaturetime graph
Temperature/ oC
Endo: In; Thermic: Heat
An endothermic reaction is a reaction
where energy is absorbed from the
surroundings.
Boiling
Evaporation
Photosynthesis
Dissolving of ammonium compounds in
water
Thermal decomposition
Electrolysis
Temperature/ oC
room
room
Time/ s
Time/ s
•
Energy changes in a reaction can be explain by:
(a) Energy level diagrams
(b) Bond breaking and bond forming
•
Unit for energy: Joules (J).
2. How can Energy Changes be Represented?
Equation (1): 2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l)
H = -114 kJ
Interpretation of equation (1) above
- The temperature of the reaction mixture will rise as the reaction proceeds.
- Neutralisation is an exothermic reaction (because H < 0)
Equation (2): CuCO3(s) → CuO(s) + CO2(g)
H = +46 kJ
Interpretation of equation (2) above
- The temperature of the reaction mixture will fall as the reaction proceeds.
- Thermal decomposition is an endothermic reaction (because H > 0)
2
Energy Changes
•
The amount of heat energy given out or taken in during a chemical reaction is known as the
enthalpy change.
•
The symbol for enthalpy change is H. The units are kJ.
•
Exothermic: - H
•
Endothermic: + H
Pre-Check 1
1.
Which of the following reactions is endothermic?
A
B
C
D
the mixing of dilute sulfuric acid with sodium hydroxide solution
the mixing of concentrated sulfuric acid with water
the formation of sugars by photosynthesis
the reaction of methane with oxygen.
Watch this video: http://bit.ly/OwwSw0
End of Pre-Lesson Reading
3. What are Energy
Level Diagrams?
Exothermic Reaction
Endothermic Reaction
Energy/ kJ
Energy/ kJ
Products
Reactants
H>0
H<0
Reactants
Products
progress of reaction
progress of reaction
Energy is given out to the surroundings during
an exothermic reaction. Hence the energy
level of the products is less than that of the
reactants.
Energy is taken in from the surroundings
during an endothermic reaction. Hence the
energy level of the products is more than
that of the reactants.
3
Energy Changes
Checkpoint 1
1. The reaction between magnesium and dilute hydrochloric acid is exothermic.
(a) Write an equation to represent the reaction;
(b) Draw an energy level diagram to show the changes in energy levels of the reactants and
products.
4. Activation Energy and Energy
Profile Diagrams
Fig 1: A match is surrounded by air, which contains oxygen. Why does the
match not burst into flames spontaneously?
•
For all reactions, bonds in the reactant particles must be broken before new bonds in the product
particles are made.
•
A simplified kinetic particle theory description of a chemical reaction:
no reaction
reaction
Fig 2: A collision does not always result in a chemical reaction. The
particles must collide with the right orientation and sufficient energy.
4
Energy Changes
•
Activation energy (Ea) is the minimum energy required to be possessed by the reactant
particles in order for a chemical reaction to occur.
Exothermic Reaction
Endothermic Reaction
Energy/ kJ
Energy/ kJ
Activation
energy, Ea
Reactants
Products
H>0
H<0
Products
Activation
energy, Ea
Reactants
progress of reaction
progress of reaction
Energy is given out to the surroundings during
an exothermic reaction. Hence the energy
level of the products is less than that of the
reactants.
Energy is taken in from the surroundings
during an endothermic reaction. Hence the
energy level of the products is more than
that of the reactants.
5. Energy Changes During Bond Breaking and Bond Forming
•
A chemical reaction involves the breaking of chemical bonds in reactant particles and the
formation of new bonds in product particles. This involves energy changes.
•
Chemical reactions usually involve bonds breaking in reactant particles first, followed by bonds
forming to form product particles.
Bond Breaking
1. Energy is required to break bonds.
2. Bond breaking is endothermic.
Overall enthalpy
change ( H)
=
Bond Forming
1. Energy is released during bond formation.
2. Bond forming is exothermic.
total energy absorbed for bond
breaking
5
-
total energy released during
bond forming
Energy Changes
•
If more energy is absorbed than released (e.g., 500 kJ – 1000 kJ), overall enthalpy change < 0,
i.e., exothermic reaction.
•
If more energy is released than absorbed (e.g., 1000 kJ – 500 kJ), overall enthalpy change > 0,
i.e., endothermic reaction.
Worked Example
Find the value of H for the reaction between hydrogen and oxygen, and (i) conclude whether the
reaction is exothermic or endothermic, and (ii) draw the energy level diagram for the reaction. Some
bond energy values are provided in the table below.
Bond
H–H
Bond Energy per mol/ kJ
436
O–H
464
O=O
497
Step 1: Write the balanced chemical equation.
2H2(g) + O2(g) → 2H2O(g)
Step 2: Determine the number of bonds to be broken and formed.
2H-H + O=O → 2H-O-H
Bonds Broken
Bonds Formed
2 H-H
4 O-H
1 O=O
Step 3: Calculate based on the bond energies
Overall enthalpy change for the reaction = [(2 × 436) + (1 × 497)] – (4 × 464) = -487 kJ
The reaction is exothermic.
Energy/ kJ
2H2(g) + O2(g)
reactants
H = -487 kJ
products
2H2O(g)
progress of reaction
6
Energy Changes
Checkpoint 2
What information does the energy level diagram in the worked example on page 6 tell you?
1. The reaction is exothermic. More energy is released during formation of O-H bonds than absorbed
to break the H-H and O=O bonds.
2. The energy level of the reactants is higher than that of the products.
Checkpoint 3
The table below shows some bond energies, measured in kilojoules per mole.
bond
H-H
H-Cl
C=C
N=N
bond energy in
kJ/mol
436
431
612
409
bond
Cl-Cl
C-H
C-C
N-N
bond energy in
kJ/mol
242
412
348
163
Bond energy is the energy required to break the bonds between pairs of atoms.
(a) Which of the bonds listed above is the strongest?
(b) Is a double bond between two carbon atoms twice as strong as a single bond? Use the
information given in the table to explain your answer.
(c) Use the information given to calculate the total energy required to break one mole of methane
into atoms.
7
Energy Changes
CH4 → C + 4H
Final answer:
To break a mole of methane into atoms, we need to consider the bond dissociation energy
of the molecule. The bond dissociation energy for the carbon-hydrogen (C-H) bond in
methane is approximately 413 kJ/mol. Since methane has four C-H bonds, the total energy
required to break a mole of methane into atoms is 1652 kJ/mol.
Explanation:
In order to break a mole of methane into atoms, we need to consider the bond
dissociation energy of the molecule. Methane (CH4) consists of one carbon atom
bonded to four hydrogen atoms. To break this molecule into atoms, we need to break the
carbon-hydrogen (C-H) bonds. The bond dissociation energy for the C-H bond is around 413
kilojoules per mole (kJ/mol).
Since methane has four C-H bonds, we need to multiply the bond dissociation energy by the
number of bonds to find the total energy required to break a mole of methane into atoms.
Therefore, the energy needed would be:
Energy needed = C-H bond dissociation energy x Number of C-H bonds =
413 kJ/mol x 4 = 1652 kJ/mol
(d) Use the information given to calculate the overall enthalpy change for the following reaction:
H2 + Cl2 → 2HCl
Bonds Broken
H-H
Cl-Cl
Bonds Formed
2 H-Cl
Overall enthalpy change for the reaction = [(436) + ( 242)] – (2 × 431) = -184 kJ
8
Energy Changes
6. Hydrogen as a Fuel
•
•
Advantages
Efficient source of fuel as it releases twice as •
much heat energy compared to other
common fuels.
Renewable source of energy – can be
obtained from electrolysis of water.
•
Hydrogen is extremely flammable and
explosive, and special care has to be taken
to transport and store hydrogen.
•
Electrolysis is an expensive process.
Currently, hydrogen is obtained from
petroleum and natural gas (CH4).
2H2O(l) → 2H2(g) + O2(g)
•
Burns cleanly, does not cause pollution as
the product of the reaction is water.
Disadvantages
Hydrogen exists as a gas. To be used
effectively, it needs to be compressed into a
liquid or stored in forms which will not occupy
a large amount of space.
2H2(g) + O2(g) → 2H2O(l)
Ni
CH4(g) + H2O(g) → CO(g) + 3H2(g)
CO(g) + H2O(g) → CO2(g) + H2(g)
7. Fuel Cells
Fig 3: Hydrogen-oxygen fuel cell.
At the positive electrode
Oxygen is reduced to form hydroxide ions
At the negative electrode
Hydrogen is oxidised to form water
O2(g) + 2H2O(l) + 4e- → 4OH-(aq)
H2(g) + 2OH-(aq) → 2H2O(l) + 2eOverall reaction is the conversion of hydrogen and oxygen into water with the release of electrical
energy.
2H2(g) + O2(g) → 2H2O(l)
Advantages
Disadvantages
9
Energy Changes
•
•
•
Fuel
cells
can
produce
electricity •
continuously provided there is a continuous
supply of fuel.
•
They convert chemical energy into electrical
energy very efficiently.
Water, a by-product of the fuel cell can be
consumed. Fuel cells used in spacecrafts •
provide electricity and water.
Fuel cells are expensive, so they cannot be
used widely.
Hydrogen exists as a gas. To be used
effectively, it needs to be compressed into a
liquid or stored in forms which will not occupy
a large amount of space.
Hydrogen is extremely flammable and
explosive, and special care has to be taken
to transport and store hydrogen.
Checkpoint 4
1. When liquid hydrogen peroxide (H2O2) is mixed with liquid hydrazine (N2H4), a very fast
exothermic reaction takes place which can propel a rocket. The reaction products are nitrogen
and steam.
(a) What is meant by an ‘exothermic reaction?’
(b) Suggest a reason why this reaction needs to be very fast.
(c) Construct the equation, including the state symbols, for this reaction between hydrogen
peroxide and hydrazine.
(d) If the rocket is loaded with 680 kg of hydrogen peroxide, calculate the mass of hydrazine
which will be need to react with it.
10
Energy Changes
2. One type of fuel cell produces electricity from the controlled reaction of hydrogen with oxygen
in the presence of an electrolyte. The diagram below shows the construction of such a fuel cell.
?
The reaction at the anode is: H2(g) + 2OH-(aq) → 2H2O(l) + 2eThe reaction at the cathode is: O2(g) + 2H2O(l) + 4e- → 4OH-(aq)
(a) Write down the names and formulae of the ions present in the electrolyte.
(b) At which electrode does oxidation take place? Give a reason for your answer.
(c) Normally, hydrogen and oxygen react together to release heat energy.
(i)
What is the name given to such a reaction?
(ii)
Draw an energy profile diagram for this reaction.
11
Energy Changes
(d) What volume of oxygen, measured at r.t.p., is needed to react completely with 1.2 dm3 of
hydrogen under the same conditions?
12