The type of reaction that is typically found when a material that is required for the reaction to proceed, such as a surface or a catalyst, is saturated by the reactants is called a __________ order reaction.

Correct — D. When the surface or catalyst a reaction needs is saturated with reactant, adding more reactant changes nothing, because every active site is already occupied.

The rate therefore stops depending on concentration:
rate = k[A]⁰ = k

A rate that stays constant and independent of reactant concentration is a zero order reaction — option (d).

Why the others are wrong

• (a)
  third — Third order means the rate varies with the cube of concentration, the opposite of a saturated surface where concentration has stopped mattering. Genuine third order reactions are rare.
• (b)
  first — First order gives rate = k[A], so the rate falls steadily as the reactant is consumed. A saturated catalyst removes exactly that dependence.
• (c)
  second — Second order gives a rate proportional to [A]² or to [A][B], so doubling a reactant would change the rate. On a saturated surface it does not.

The order of a reaction is the power to which concentration is raised in the experimental rate law. It is measured, never read off the balanced equation.

A zero order reaction has rate = k. Concentration falls linearly with time, the units of k are mol L⁻¹ s⁻¹, and the half-life is [A]₀/2k, so it depends on the starting concentration.

Zero order behaviour appears when something other than concentration limits the reaction — a saturated catalyst surface, a fixed light intensity, or an enzyme already working at its maximum rate.

Key facts

• For a zero order reaction the rate law is rate = k, independent of reactant concentration.
• The units of the rate constant k for a zero order reaction are mol L⁻¹ s⁻¹.
• The half-life of a zero order reaction is [A]₀/2k, so it depends on the initial concentration.
• Decomposition of ammonia on a hot platinum surface at high pressure is a standard zero order example.

Common traps

• Reading the order off the balanced equation instead of the experimental rate law
• Assuming adding more reactant always makes a catalysed reaction faster

No directly related past PYQ was found.