Define rate constant (k) ?

Explanation

**Step-by-Step Solution:**

1. **Definition of Rate Constant (k)**: The rate constant, denoted as \( k \), is a proportionality constant in the rate law of a chemical reaction. It relates the rate of the reaction to the concentrations of the reactants.

2. **Understanding Rate Law**: The rate law is an experimentally determined equation that expresses the rate of a chemical reaction in terms of the molar concentrations of the reactants. It can be represented as:
\[ \text{Rate} = k [A]^x [B]^y \]
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