Photochemical combination of hydrogen and chlorine is a `underline("first order reaction.")`

Explanation

To determine whether the photochemical combination of hydrogen and chlorine is a first-order reaction, we can analyze the reaction step by step.

### Step 1: Understanding the Reaction
The reaction between hydrogen (H₂) and chlorine (Cl₂) to form hydrogen chloride (HCl) can be represented as:
\[ \text{H}_2(g) + \text{Cl}_2(g) \rightarrow 2 \text{HCl}(g) \]

### Step 2: Identifying the Order of Reaction
The order of a reaction is determined experimentally and is based on the rate law expression. The rate law relates the rate of the reaction to the concentration of the reactants.
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