Step 1: Evaluate Assertion (A).
Assertion: Order of reaction is applicable to both elementary and complex reactions. This is TRUE. Order is defined as the sum of powers of concentration terms in the experimentally determined rate law, and this definition applies to any type of reaction (elementary or complex).
Step 2: Evaluate Reason (R).
Reason: Order of reaction is an experimental quantity. This is TRUE. Order must be determined from experiments by measuring how rate changes with concentration. It cannot be predicted from the balanced equation alone.
Step 3: Does R correctly explain A?
YES! Because order is experimental (R), it can describe any reaction, elementary or complex (A). For elementary reactions, experimental order happens to equal molecularity. For complex reactions, experimental order reflects the overall rate-determining step and can be any number. The experimental nature of order is precisely why it applies universally.
Step 4: Contrast order with molecularity.
Molecularity applies only to elementary steps, must be a positive integer (1, 2, or 3), and is a theoretical concept. Order applies to all reactions, can be any number (including fractions, zero, negative), and is determined experimentally. This contrast confirms that order (being experimental) has broader applicability.
Step 5: Confirm the logical connection.
The fact that order is experimental directly causes it to be applicable to all reactions (both elementary and complex). R is not just correlated with A; R is the causal reason for A.
Step 6: State the final answer.
Both A and R are true, and R IS the correct explanation of A. \[ \boxed{\text{Both A and R true, and R is the correct explanation of A}} \]