B. The forward reaction is exothermic, since ΔE is negative.
C. The reverse reaction is endothermic.
D. The enthalpy change, ΔE, of the reaction is −222 kJ.
8 . D
In order for products to form, the reactant atoms or molecules need to collide at an
orientation that allows them to react, and with sufficient energy to surmount the activation
barrier as it goes through the transition state.
9 . C
If the number of nuclei decaying in a sample has decreased by a factor of 4, the sample has
been through 2 half-lives, and the half-life will be
The equation to determine the decay constant for the first-order reaction is
Thus, given that the half-life is 56 sec, the decay constant will be
10 . C
Reducing the activation energy makes it easier for molecules to overcome the barrier to
reaction: At any temperature, there will now be more molecules that have sufficient energy to
react. The rate of reaction will therefore be faster. All the other choices tend to decrease the
reaction rate: Lowering the temperature would mean decreasing the energy of the molecules;
it will be more difficult for them to overcome the energy barrier. Increasing the volume of the
reaction vessel, with all else remaining the same, would decrease the density (and hence
concentration) of the reactant molecules, causing collisions among them to be less frequent.
