The reaction 2 A + B2 -> 2 AB follows this mechanism: Step 1: A + B2 -> AB + B (slow)

Step 2: A + B -> AB (fast)

What is the order with respect to [B2] in the overall reaction?
A. zero
B. first
C. second
D. third
E. it cannot be determined

Yet both compounds and elements are considered pure substances. Pure compounds are created when elements combine permanently, forming one substance. For example, when the elements mercury and oxygen combine, and a chemical reaction takes place, mercury (II) oxide is created.

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