1. Energy changes in chemistry – bond enthalpies
Energetics
Energy changes in chemistry – bond enthalpies

2. Quick quiz – back page of your book
Reactions where the products are at a lower energy than the reactants are endothermic (TRUE/FALSE)
Activation energy is the amount of energy given out when a reaction takes place (TRUE/FALSE)
A reaction which is exothermic transfers heat energy to the surroundings (TRUE/FALSE)
How can we tell if a reaction is exothermic or endothermic?
Sketch the energy profile for an endothermic reaction.
When methane (CH4) burns in oxygen (O2) bonds between which atoms need to be broken?
FALSE
FALSE
TRUE
Measure the temperature change
C—H bonds and O=O bonds

3. Objectives
Must Recall that energy is needed to break chemical bonds and energy is given out when chemical bonds form Should Identify which bonds are broken and which are made when a chemical reaction takes place. Could Use data on the energy needed to break covalent bonds to estimate the overall energy change for a reaction.

4. Activation energy revisited
What is the activation energy of a reaction?
The energy needed to start a reaction.
BUT what is that energy used for and why does the reaction need it if energy is given out overall?
The activation energy is used to break bonds so that the reaction can take place.

5. Burning methane
Consider the example of burning methane gas. CH4 + 2O2  CO2 + 2H2O This reaction is highly exothermic, it is the reaction that gives us the Bunsen flame. However mixing air (oxygen) with methane is not enough. I need to add energy (a flame).

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

7. 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

8. Bond Energies
Where the energy from bond forming exceeds that needed for bond breaking the reaction is exothermic.
Where the energy for bond breaking exceeds that from bond forming the reaction is endothermic.
Energy in chemicals
reactants
products H
Bonds break
Bond forming
Bonds form
Exo
Endo

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

10. Copy and complete
_______ ______is the energy needed to start the reaction by __________ chemical _____ in the reactants. Energy is given out when ______ chemical bonds form. The overall energy change for a reaction can be worked out as the energy required for all bonds _______ minus the energy gained by ______ bonds. [Activation energy, new, bonds, broken, breaking, making]

11. Copy this table of bond enthalpies into your book
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

12. Can you work out the energy change for this reaction?
CH4 + Cl2  CH3Cl + HCl Tip: Draw the reactants and products and work out the bonds you are breaking and the ones you are making.

13. What did you get?

14. Summary
Recall that energy is needed to break chemical bonds and energy is given out when chemical bonds form Identify which bonds are broken and which are made when a chemical reaction takes place. Use data on the energy needed to break covalent bonds to estimate the overall energy change for a reaction.