**Decomposition of Gaseous Ammonia on a Hot Platinum Surface**

The decomposition of gaseous ammonia (NH3) on a hot platinum (Pt) surface is a zero-order reaction. In a zero-order reaction, the rate of the reaction is independent of the concentration of the reactant(s). This means that the rate of the reaction remains constant regardless of the change in concentration.

**Explanation:**

1. **Introduction to the Reaction**:
- The decomposition of gaseous ammonia on a hot platinum surface is a chemical reaction that occurs when ammonia molecules come into contact with a heated platinum catalyst.
- The reaction can be represented by the following equation: 2NH3(g) ⟶ N2(g) + 3H2(g).

2. **Zero-Order Reaction**:
- In a zero-order reaction, the rate of the reaction is independent of the concentration of the reactant(s).
- This means that increasing or decreasing the concentration of ammonia gas will not affect the rate of the reaction.
- The rate equation for a zero-order reaction can be expressed as: Rate = k, where 'k' is the rate constant.

3. **Role of Platinum Catalyst**:
- The presence of a platinum catalyst is crucial for the decomposition of ammonia to occur at a reasonable rate.
- The platinum surface provides an active site for the adsorption and dissociation of ammonia molecules.
- The dissociation of ammonia on the platinum surface results in the formation of nitrogen (N2) and hydrogen (H2) gases.

4. **Mechanism of the Reaction**:
- The decomposition of ammonia on the platinum surface involves a series of steps.
- Initially, ammonia molecules adsorb onto the platinum surface.
- The adsorbed ammonia molecules then dissociate into nitrogen and hydrogen atoms.
- These atoms recombine to form nitrogen and hydrogen gases, which desorb from the platinum surface.

5. **Effect of Concentration**:
- Since the reaction is zero-order, the concentration of ammonia does not affect the rate of the reaction.
- Increasing the concentration of ammonia will not increase the rate of decomposition.
- Similarly, decreasing the concentration will not slow down the rate.

6. **Temperature and Surface Area**:
- The rate of the reaction can be increased by increasing the temperature of the platinum surface.
- A higher temperature provides more kinetic energy to the ammonia molecules, increasing the likelihood of adsorption and dissociation.
- Additionally, increasing the surface area of the platinum catalyst can also enhance the rate of the reaction by providing more active sites for ammonia adsorption.

In conclusion, the decomposition of gaseous ammonia on a hot platinum surface is a zero-order reaction. The rate of the reaction remains constant regardless of the concentration of ammonia. The presence of a platinum catalyst is essential for the reaction to occur, and factors such as temperature and surface area can influence the rate of the reaction.