Energetics
Energy changes in chemistry – bond enthalpies
Quick quiz – back page of your book
1. Reactions where the products are at a lower energy than the
FALSE
reactants are endothermic (TRUE/FALSE)
2. Activation energy is theFALSE
amount of energy given out when a
reaction takes place (TRUE/FALSE)
3. A reaction which is exothermic
TRUE transfers heat energy to the
surroundings (TRUE/FALSE)
4. How can we
tell if a reaction
is exothermicchange
or endothermic?
Measure
the temperature
5. Sketch the energy profile for an endothermic reaction.
6. When methane (CH4) burns in oxygen (O2) bonds between which
atoms need to be broken?
C—H bonds and O=O bonds
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.
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.
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).
What happens when the reaction gets the
activation energy?
H H H H
Energy in chemicals
C
O
Bond
Breaking
H
C
H
H
H
O
O
O
Bond
Forming
O
O
O
O
O
H
O
Progress of reaction
C
O
H
O
H
H
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
Bond Energies
reactants
products
H
Bonds form
Endo
Bonds break
Bond forming
products
Energy in chemicals
reactants
Exo
H
Bonds break
•
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
•
Exo or endo (again)
• If the overall energy change is negative is this an exothermic or
endothermic reaction?
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.
Copy this table of bond enthalpies into your book
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
Can you work out the energy change for this
reaction?
What did you get?
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.