Answer
a. A catalyst decreases the activation energy of the reaction by providing an alternative pathway that has a lower energy of activation.
b. Pure oxygen has a greater number of molecules of oxygen than the same quantity of air in the atmosphere.
With a greater concentration of a reactant ($O_2$), the rate of oxygenation of the blood is increased, and the patient can breathe more easily.
Work Step by Step
a. Page 374, first paragraph of the "Catalysts" topic.
b. Page 374, "Concentrations of Reactants".
