Enthalpy changes- endothermic and exothermic reactions

1. Energy Changes In Reactions 11 April 2024
Reaction Start Temp (0C) End Temp (0C)
A+B 19 27
C+D 20 25
E+F 19 17
a) Decide which is reaction is exothermic or
endothermic. How can you tell?
b) The volume of solution was the same in each
reaction. Which had the largest energy change?

2. Learning Intention - To be able to complete bond energy
calculations
1. How can exothermic and
endothermic reactions be
explained in terms of bonds?
2. How are exothermic reactions
modelled?
3. How are energy changes in
reactions calculated?
Endothermic – More energy is taken in to
break bonds, than is given out to make
bonds.
Exothermic – More energy is given out to
make bonds, than is taken in to break bonds.
Today’s Learning Questions Today’s Language of Learning
Energy Changes In Reactions 11 April 2024

3. Temperature changes
• EXOTHERMIC REACTION
• ENDOTHERMIC REACTION

5. What happens when the reaction gets
the activation energy?
Bond
Forming
Bond
Breaking
Progress of reaction
Energy
in
chemicals
O
O
O
O
H
C
H
H
H
O O
O
O
C H H H H
O C O
O
O
H H
H H

6. During a chemical reaction, covalent bonds in the
molecules of the reactant break and new bonds are
made in the molecules of the product.
Energy is transferred to the reactants to break their
bonds. Breaking bonds is endothermic.
Energy is transferred to the surroundings as bonds
form. Making bonds is exothermic.

7. Each type of covalent bond has a different amount of
energy associated with it.
A reaction is exothermic overall if more energy is given
out when making bonds than is needed to break
bonds.
Describe what happens in an endothermic reaction in
terms of the energy of breaking and making bonds.
More energy is needed to break bonds than is released
in making bonds.

8. Hydrogen reacts with chlorine to form hydrogen
chloride. The balanced equation is:
H2 + Cl2 → 2HCl
This can be re-written to show the covalent bonds.
H−H + Cl–Cl → H–Cl + H–Cl
Bonds broken in reactants: H−H and Cl−Cl
Bonds made in products: 2 × H−Cl

9. Hydrogen reacts with chlorine to form hydrogen
chloride. The balanced equation is:
H2 + Cl2 → 2HCl
Overall the reaction is exothermic. What does this tell
you about the relative energy needed to break and
make bonds?
More energy is released when two H−Cl bonds are
made than the amount of energy needed to break the
H−H and Cl−Cl bonds.

10. Hydrogen reacts with oxygen to form water.
The equation showing all the bonds is:
2H–H + O=O → 2H–O–H
List the number and type of bonds:
a) broken
b) made.
a) Bonds broken: 2H−H and O=O
b) Bonds made: 4O−H (or 4H−O)

11. Using bond enthalpies
By using the energy that it takes to break/make a
particular bond we can work out the overall
enthalpy/energy change for the reaction.
Sum (bonds broken) – Sum (bonds made) = Energy
change
ΔH= (bond enthalpies of bonds broken) - (bond enthalpies of bonds made)
S S

12. Bond energy calculations
The energy needed to break one mole of a particular
covalent bond is known as its bond energy.
Energy is measured in kilojoules and bond energy is
measured in kilojoules per mole (kJ mol−1).
The bond energy is different for every type of covalent
bond. For example, the bond energy for a C−H bond is
413 kJ mol−1 but the bond energy for a C−O bond is
358 kJ mol−1.

13. Calculate the energy change when hydrogen and oxygen
react to form water.
2H–H + O=O → 2H–O–H
Bond energies:
H−H 436 kJ mol−1 O=O 498 kJ mol−1 O−H 464 kJ mol−1
Step 1 Calculate the energy taken in (bonds broken)
2 × (H−H) = 2 × 436 = 872 kJ mol−1
1 × (O=O) = 1 × 498 = 498 kJ mol−1
Total energy in = 872 + 498 = 1370 kJ mol−1

14. Calculate the energy change when hydrogen and
oxygen react to form water.
2H–H + O=O → 2H–O–H
Bond energies:
H−H 436 kJ mol−1 O=O 498 kJ mol−1 O−H 464 kJ
mol−1
Step 2 Calculate the energy out (bonds made)
4 × (O−H) = 4 × 464 = 1856 kJ mol−1

15. Calculate the energy change when hydrogen and oxygen
react to form water.
2H–H + O=O → 2H–O–H
Bond energies:
H−H 436 kJ mol−1 O=O 498 kJ mol−1 O−H 464 kJ mol−1
Step 3 Energy change = energy in − energy out
= 1370 − 1856
= −486 kJ mol−1
The negative sign shows
that more energy is given
out from this reaction
than taken in.
This means that the
reaction is
exothermic. An
endothermic reaction
would have a positive
sign.

16. Calculate the energy change when hydrogen and chlorine
react to form hydrogen chloride.
H−H + Cl–Cl → H–Cl + H–Cl
Bond energies:
H−H 436 kJ mol−1 Cl−Cl 243 kJ mol−1 H−Cl 432 kJ mol−1
Step 1 Calculate the energy in (bonds broken)
1 × (H−H) = 1 × 436 = 436 kJ mol−1
1 × (Cl−Cl) = 1 × 243 = 243 kJ mol−1
Total energy in = 436 + 243 = 679 kJ mol−1

17. Calculate the energy change when hydrogen and
chlorine react to form hydrogen chloride.
H−H + Cl–Cl → H–Cl + H–Cl
Bond energies:
H−H 436 kJ mol−1 Cl−Cl 243 kJ mol−1 H−Cl 432 kJ
mol−1
Step 2 Calculate the energy out (bonds made)
2 × (H−Cl) = 2 × 432 = 864 kJ mol−1

18. Calculate the energy change when hydrogen and chlorine
react to form hydrogen chloride.
H−H + Cl–Cl → H–Cl + H–Cl
Bond energies:
H−H 436 kJ mol−1 Cl−Cl 243 kJ mol−1 H−Cl 432 kJ mol−1
Step 3 Energy change = energy in − energy out
= 679 − 864
= −185 kJ mol−1
This reaction is exothermic.
An endothermic reaction
would have a positive sign.

19. C3H8 + 5O2  3CO2 + 4H2O
C C C H
H
H H H
H H H
O O
Bonds Broken
8 X C-H
2 X C-C
5 X O=O
O O
O O
O O
O O
Bond Energy
(kJ/mol)
C-H 412
C-C 348
O=O 498
O-H 463
C=O 743
Bonds Broken
8 X 412
2 X 348
5 X 498 = 6482kJ
C O
O
C O
O
C O
O
H H
O
H H
O
H H
O
H H
O
Bonds Made
6 X C=O
8 X O-H
Bonds Made
6 X 743
8 X 463 = 8162kJ
6482-8162 = -1680kJ mol-1
Exothermic (-ve)

20. Exo or endo (again)
• If the overall energy change is negative is this an
exothermic or endothermic reaction?

21. Calculating bond energies
What are the energy changes in the reaction between hydrogen and chlorine?
H2 2HCl
+ Cl2 
hydrogen hydrogen chloride
+ chlorine 
energy for bond-breaking
= H – H + Cl – Cl
= 432 kJ + 240 kJ
= 672 kJ
= H – Cl + H – Cl
= 428 kJ + 428 kJ
= 856 kJ
energy from bond-making
= energy out – energy in
=672 Kj - 856 kJ
= -184 kJ
total energy change

22. You will be given these values
Bond Bond enthalpy (kJ) Bond Bond enthalpy (kJ)
C—H 435 Cl—Cl 243
C—C 348 C—Cl 346
H—H 436 H—Cl 452
H—O 463 O=O 498
C=O 804
C=C 614
Definition
Average bond enthalpy is the average enthalpy change that takes place when
breaking by homolytic fission 1 mol of a given type of bond in the molecules of a
gaseous species.

25. A Level Past paper question

26. Mark scheme