Everything Science Grade 11

CHAPTER 7. ENERGYCHANGES IN CHEMICAL REACTIONS 7.6

DEFINITION: Activated complex

The activated complexis a transitional structure in a chemical reaction that results

from the effective collisions between reactant molecules, and which remains while

old bonds break and new bonds form.

Tip

Enzymes and activation

energy

An enzyme is a cata-

lyst that helps to speed

up the rate of a reac-

tion by lowering the ac-

tivation energy of a re-

action. There are many

enzymes in the human

body, without which lots

of important reactions

would never take place.

Cellular respiration is

one example of a reac-

tion that is catalysed by

enzymes. You will learn

more about catalysts in

Grade 12.

In endothermic reactions, the final products have a higher energy than the reactants. An energy
diagram is shown below(Figure 7.2) for the endothermic reaction XY + Z→ X + YZ. In this example,
the activated complex has the formula XYZ. Notice that the activationenergy for the endothermic
reaction is much greaterthan for the exothermicreaction.

[XYZ]

X + YZ

products

XY + Z

reactants

ΔH > 0

Time

Potential energy

activation

energy

Figure 7.2: The energy changes that take place during an endothermic reaction

FACT

The reaction between

H and F was considered

by NASA (National Aero-

nautics and Space Ad-

ministration) as a fuel

system for rocket boost-

ers because of the en-

ergy that is released dur-

ing this exothermic reac-

tion.

Exercise 7 - 2

1. Carbon reacts with water according to the following equation:

C + H 2 O→ CO + H 2 ΔH > 0

(a) Is this reaction endothermic or exothermic?

(b) Give a reason for your answer.

2. Refer to the graph below and then answer thequestions that follow: