Ch 17 Review Answers 1-8

1. According to the collision model, what determines whether colliding molecules will

    break bonds and form new products?

     Two substances must collide with sufficient energy to break (Ea) bonds and allow new bonds to form.

2. Higher concentrations of reactants and higher temperatures increase the rate of formation of products. 

    How does the collision model explain these observations? ‘Increase the rate” means the reaction proceeds faster.

1. higher concentrations – higher concentrations lead to more collisions and therefore to more reaction events
2. higher temperatures – the speeds of the molecules increase with temperature. So at higher temperatures, the average collision is more energetic.

3. Sketch a graph of the progress of a chemical reaction showing the activation energy,  E_a, for the

    reaction.  Explain the graph for the decomposition of BrNO into NO and Br₂.

   

See p 539 Figure 17.3 If the energy contained in a collision of two BrNO molecules is greater than E_a, the colliding molecules bounce apart unchanged.

4. How does a catalyst increase the rate of a reaction?  Sketch a graph showing the effect of the catalyst

    on the E_a of the reaction.    See p 539 Figure 17.4

A catalyst provides a new pathway for the reaction with a lower activation energy than the original pathway, because more collisions will have enough energy to allow a reaction. This in turn leads to a faster reaction.

5. What do chemists mean by equilibrium?

The exact balancing of two processes, one of which is the opposite of the other. The rate of the forward reaction is equal to the rate of the reverse reaction. 

6. Why is equilibrium considered to be a dynamic state?

The concentration of the reactants and products do not change, but there is a constant ongoing formation of products and reactants.

7. What happens to the rates of forward and reverse reactions as a chemical system goes from start to

     equilibrium, starting with only reactants (no products).

At first, the rate of the forward reaction decreases and the rate of the reverse reaction increase. Equilibrium is reached when the forward rate and the reverse rate become the same.

8. Observe the following endothermic reaction:  2 NOCl(g)  « 2 NO(g) + Cl₂(g)

    What shift in equilibrium will occur when each of the following occurs?

a. Cl₂ gas is added (the concentration is increased) – left (shift away)

b. The volume is increased – right (larger number of gas moles)

c. NO(g) is removed – right (shift towards)

d. The temperature is decreased – left (endothermic, heat is on left side, shift towards heat)

e. The pressure is increased – left – (shift toward smaller number of gas moles)