Rates of Reaction

Bond Energy Calculations.

In the reaction between hydrogen and chlorine,

hydrogen + chlorine     hydrogen chloride.
H_2(g)    +     Cl_2(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 the 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.

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 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.

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