Given the balanced equation representing a reaction:

H2 (g) + Cl2 (g) = = >2HCl (g) + energy

Which statement describes the energy changes in this reaction?

(1) Energy is absorbed as bonds are formed, only.

(2) Energy is released as bonds are broken, only.

(3) Energy is absorbed as bonds are broken, and energy is released as bonds are formed.

(4) Energy is absorbed as bonds are formed, and energy is released as bonds are broken.

Answer: (3) Energy is absorbed as bonds are broken, and energy is released as bonds are formed.

Explanation:

1) The equation given is H₂ (g) + Cl₂ (g) → 2HCl (g) + energy

2) The energy is shown in the product side, so energy is a product of the reaction, so energy is release.

But this is just the net energy of the process. You need more insight to deal with the energy changes in the reaction.

3) Chemical bonds store energy; this stored energy is the potential chemical energy of the molecules.

When a chemical reaction occurs, the first stage is to brake bonds.

Braking chemical bonds requires energy to overcome the bond energy. Braking bonds always absorbs energy.

On the other hand, forming bonds always release energy.

The neat energy of the reaction is the difference between the energy needed to break bonds and the energy released when new bonds form.

So, regardless the fact that, in the chemical equation that represents the reaction a net energy release is shown, you know that energy is absorbed as bonds are broken, and energy is released as bonds are formed (option 3).

4) Energy is absorbed as bonds are formed, and energy is released as bonds are broken.