Everything Science Grade 12

3.6 CHAPTER 3. REACTION RATES

calcium carbonate &

hydrochloric acid

limewater

delivery tube

rubber stopper

rubber stopper

Discussion:

• If you look at the last two equations you will seethat the one is the reverse
  of the other. In other words, this is a reversible reaction and can be written
  as follows:
  CaCO 3 (s) + H 2 O + CO 2 � Ca(HCO 3 ) 2


• Is the forward reactionendothermic or exothermic? Is the reverse reac-
  tion endothermic or exothermic? You shouldhave noticed that the re-
  verse reaction only tookplace when the solutionwas heated. Sometimes,
  changing the temperature of a reaction can change its direction.

Chemical equilibrium ESCAY

Using the same reversible reaction that we usedin an earlier example:

H 2 (g) + I 2 (g)� 2 HI (g)

The forward reaction is:

H 2 + I 2 → 2 HI

The reverse reaction is:

2 HI→ H 2 + I 2

When the rate of the forward reaction and thereverse reaction are equal, the system
is said to be in equilibrium. Figure 3.6 shows this.Initially (time = 0), therate of the