Why are reaction orders integers?

In summary, the reaction orders are integers because they reflect the reaction mechanism. The experiment results showed that the reaction order is about 1.2 with respect to both persulfate and iodine ions.
  • #1
george_ong
1
0
I have just done an experiment on the clock reaction between iodine and persulfate ions. Using my experiment result, I have determined that the reaction orders are about 1.2 with respect to both persulfate and iodine ions.

There is this question ' Explain why the reaction orders should be integers' in the exercise section of my lab manual which I am supposed to answer. Can anyone give me a reason why the reaction orders can be rounded off to the nearest integer?

Thanks.
 
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  • #2
It is not about rounding, in most cases reaction orders ARE integers. They reflect reaction mechanism.
 
  • #3
Yes, often the reaction orders are integral and observed deviation is due to experimental error.

However integral reaction orders reflecting mechanism are necessarily true only for elementary reactions. That is when A -> B or A + B -> C (or C + D) is the mechanism , not just the overall result of a more complicated mechanism. For elementary mechanisms moreover, the reaction order is 1 or for each participant (or 2 if the reactants are reacting with themselves A + A -> A2).

But if the reaction mechanism is a series of steps (with or without back reaction in some of the steps) the overall result even though each step its first order may be a concentration dependence that is more complicated than simple first order. Which gives chemists one possibility or necessity for disentangling to resolve just what is the mechanism.

Then, guessing, what were the observations in this experiment? Was it just the time taken for the solution to suddenly change colour? If so the reciprocal of that time is not, or not self-evidently, the velocity of any reaction. It is the result of a moderately complex process , see e.g. http://en.wikipedia.org/wiki/Iodine_clock_reaction

In principle you'd have to derive the rate equations for that reaction, solve (integrate) them, and then find time for [I2] to exceed a certain threshold level.

In principle. Maybe there are simplifications possible - look up the references in the link.
 

1. Why do reaction orders have to be integers?

The order of a reaction refers to the dependence of the reaction rate on the concentration of reactants. It is determined experimentally and can only take on whole number values. This is because the reaction rate is a function of the concentration of reactants, which can only be measured in discrete units. Therefore, the reaction order, which is a measure of the relationship between the reaction rate and reactant concentration, must also be an integer.

2. What does it mean if a reaction order is fractional?

If a reaction order is fractional, it means that the reaction rate does not have a direct relationship with the concentration of the reactant. This can occur in complex reactions where multiple steps are involved, or when the rate of one step is much faster than the others. In these cases, the reaction order is determined by the slowest step and may not be a whole number.

3. Can the reaction order change over time?

No, the reaction order is a constant value that is determined at the beginning of the reaction. It is not affected by the progress of the reaction or changes in reactant concentrations. However, the reaction order can change if the reaction conditions, such as temperature or catalyst, are altered.

4. Why is the reaction order important?

The reaction order provides valuable information about the rate of a reaction and how it is affected by changes in reactant concentrations. It also helps to determine the rate law, which is an equation that describes the relationship between the rate of a reaction and the concentrations of reactants. This information is crucial in understanding and predicting the behavior of chemical reactions.

5. Can a reaction order be negative?

No, a reaction order cannot be negative. A negative reaction order implies that an increase in the concentration of a reactant results in a decrease in the reaction rate, which is not possible. However, the reaction order can be zero, which means that the rate of the reaction is independent of the concentration of that particular reactant.

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