Question about the concept of Reaction Rate

[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.

 

[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.

 

[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 don't 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.

 

[Ygggdrasil]

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 don't 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.

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.

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.

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).

 

[DeeNos]

The concept of reaction rate can be a bit confusing, especially when dealing with units. The unit of s^-1, or per second, is often used to describe the rate of a reaction. This refers to the change in concentration of a reactant or product over time, typically in units of moles per liter per second.

In some cases, the rate may be described as s^-1 without a change in property in the numerator because the property being measured is not a concentration. For example, if the reaction rate is being measured in terms of temperature change over time, the units would be degrees Celsius per second.

In the case of the Iodine Clock Reaction Lab, the reaction rate may have been measured in terms of color change over time, rather than a change in concentration. This is a common method for tracking the progress of a reaction and can still provide valuable information about the reaction rate.

It is important to note that the units used to describe reaction rate may vary depending on the property being measured. However, the underlying concept remains the same - the change in a property over time. I hope this helps clarify any confusion about reaction rate.