Order of Reaction

The order of a chemical reaction refers to the relationship between the rate of the reaction and the concentration of the reactants. It indicates how the concentration of a reactant affects the rate of the reaction. The order of a reaction can be determined by analyzing the change in concentration of a reactant over time.

Given Information:
Initial concentration of substance B = 1 mM
Concentration of substance B at different times:
- Time 1: 0.83 mM
- Time 2: 0.72 mM
- Time 4: 0.56 mM
- Time 8: 0.38 mM
- Time 16: 0.24 mM

Determination of Order of Reaction:
To determine the order of the reaction, we need to analyze how the concentration of substance B changes with time. We can do this by comparing the rate of change at different time intervals.

Step 1: Calculate Rate of Reaction
The rate of reaction can be calculated by dividing the change in concentration of substance B by the corresponding change in time. Let's calculate the rates at different time intervals:

- Rate from time 1 to time 2 = (0.72 mM - 0.83 mM) / (2 - 1) = -0.11 mM
- Rate from time 2 to time 4 = (0.56 mM - 0.72 mM) / (4 - 2) = -0.08 mM
- Rate from time 4 to time 8 = (0.38 mM - 0.56 mM) / (8 - 4) = -0.045 mM
- Rate from time 8 to time 16 = (0.24 mM - 0.38 mM) / (16 - 8) = -0.02 mM

Step 2: Determine Order of Reaction
The order of reaction can be determined by comparing the rates at different time intervals. If the rate of reaction remains constant, the reaction is zero order. If the rate of reaction decreases with decreasing concentration, the reaction is first order. If the rate of reaction decreases exponentially with decreasing concentration, the reaction is second order.

Comparison of Rates:
- Rate from time 1 to time 2: -0.11 mM
- Rate from time 2 to time 4: -0.08 mM
- Rate from time 4 to time 8: -0.045 mM
- Rate from time 8 to time 16: -0.02 mM

Observation:
The rates of reaction decrease as the concentration of substance B decreases over time. However, the rate decreases in a non-linear manner. The rate decreases by a larger amount from time 1 to time 2 compared to the decrease from time 8 to time 16. This suggests that the reaction is not first order.

Conclusion:
Based on the given concentration data and the analysis of the rates of reaction, it can be concluded that the order of the reaction is second order. The reaction rate decreases exponentially with decreasing concentration, indicating a second-order relationship between the concentration of substance B and the rate of the reaction.