# Homework Help: Question about the concept of Reaction Rate

1. Feb 11, 2010

### Senjai

This is not a homework question.

Im curious as to why in some situations rate is described as $$s^{-1}$$ without a change in property in the numerator.

As i understand, rate is defined as a change of a property over the time elapsed over that change.. How can we have a reaction rate without having a change in property?]

More specifically, this came to my notice in a Iodine Clock Reaction Lab. When graphing the reaction rate vs the concentration of the reacting species.

Thanks much,
Richard.

2. Feb 11, 2010

### Ygggdrasil

Are you looking at the rates at which a certain chemical reaction is occurring or the rate constant (k) for a particular reaction? Rates of chemical reactions (usually expressed as v) are almost always in the units of concentration over time. Rate constants, which are different from rates, come from the rate equation, an expression that relates the concentration of reactants and products to the overall rate of a chemical reaction. For example in a hypothetical reaction of A --> B, the rate equation might look like:

v = k[A]

Meaning that the rate of the reaction is proportional to the concentration of species A. Here the rate constant would have units of s-1.

3. Feb 13, 2010

### Senjai

Yes we were using the rate equation..

However when we experimentally determined the rates, we just took the times for the reaction to change color, and calculated the rates as 1 / time for reaction completion..

And i dont understand how we can do that for the rate of the equation..

Someone told me it had to do with the 1 symbolizing one "complete reaction" over that time.

We then used the rates, orders, and concentrations to find k..

Our eqn was: $$Rate = 2.1 x 10^6[H^+]^1[IO_3^-]^1[HSO_3^-]^1$$

thanks again, senjai.

4. Feb 14, 2010

### Ygggdrasil

This approach is valid only for zeroth-order reactions. For more complicated reactions, you would need to plot our the concentration of product or reactant versus time and fit the curve you generate to a specific function (whose form depends on the rate equation you think the reaction follows). For example, for first order reactions, the amount of reactant will exponentially decay over time.

In this case, your rate constant would have units of M-2 s-1 in order to make the units work out (so that the rate is in units of M s-1).