hydrogen
chloride.H2(g) + Cl2(g)
2HCl(g)Bond Energy Calculations.
In the reaction between hydrogen and chlorine,
hydrogen
+ chlorine
hydrogen
chloride.
H2(g) + Cl2(g)
2HCl(g)
The reactants are hydrogen and chlorine,
the product
is hydrogen chloride.
You must put energy
in to break the bonds in
the reactants
before energy is given out by forming new bonds in the products.
The minimum
amount of energy you need to
put in
to make the reaction happen is called the activation energy.
When the bonds between the hydrogen and
chlorine
molecules (the reactants) have been broken,
then both
hydrogen and chlorine exist as atoms.
When an atom of hydrogen and
chlorine react
with each other they form a new bond,
give energy
out, and the product is
hydrogen chloride.
The table below shows bond energies.
Bond energies are given in kilo Joules
(thousand Joules)
per mole of
bonds.
| Bond | Bond Energy (in kJ per mole) |
| H—H | 436 |
| Cl—Cl | 242 |
| H—Cl | 431 |
The bond energy is the
amount of energy which
1) you must put in to break the bond,
or
2) is given out when the new
bond is formed.
In the reaction between hydrogen and chlorine, the big numbers
used to balance the equation
(1H2
+ 1Cl2 2HCl)
tell you how many
moles there are of reactant
and product.
You must break
1 mole of H—H bonds and
1 mole of Cl—Cl bonds.
The amount of energy needed is
436 + 242
kJ
= 678 kJ.
When 2
moles of H—Cl bonds are formed,
the amount of energy given out
is
2 x 431 kJ
= 862 kJ.
The
difference between these two energies
(reactants minus products) is the overall
amount of energy given
out per mole of reactants.
678 - 862
kJ =
-184 kJ.
The
change in energy during a reaction is given
the symbol DH.
In the reaction between hydrogen and chlorine,
DH =
-184 kJ per mole.
For an exothermic reaction, DH is negative.
Rates of
Reaction Catalysts and
Energy Search Questions 
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