# Chemistry Help Needed-Reaction Rates (Not for homework, for understanding).
**URL:** https://boards.straightdope.com/t/chemistry-help-needed-reaction-rates-not-for-homework-for-understanding/653067
**Category:** Factual Questions
**Created:** [March 16, 2013, 1:29am UTC](https://boards.straightdope.com/t/chemistry-help-needed-reaction-rates-not-for-homework-for-understanding/653067 "2013-03-16T01:29:37Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Sue\_Duhnym](https://boards.straightdope.com/u/Sue_Duhnym)
#### Post date: [March 16, 2013, 1:29am UTC](https://boards.straightdope.com/t/chemistry-help-needed-reaction-rates-not-for-homework-for-understanding/653067/1 "2013-03-16T01:29:37Z")
My book offers the following example:
H[sub]2[/sub]O[sub]2[/sub] + 3I[sup]-[/sup] + 2H[sup]+[/sup] ----\> I[sub]3[/sub][sup]-[/sup] + 2H[sub]2[/sub]O
In the first 10 seconds of the reaction the concentration of I[sup]-[/sup] dropped from 1.000M to 0.868M. Calculate the average rate of reaction . Got that, it’s fine, totally agree with the answer of 4.40 E-3 M/s
But then, they ask you to calculate the rate of reaction in respect to H[sup]+[/sup], which they give as -8.80 E-3 M/s.
Not only that, but the book goes on to give the reaction rates of H[sub]2[/sub]O[sub]2[/sub] and I[sub]3[/sub][sup]-[/sup] as +/- (respectively) 4.40 E-3 M/s.
What am I missing here? You always have one constant (delta concentration/delta t), which is divided by the stoichiometry coefficient of the reactant or product. How the fuck can the reaction rate of the iodide ion equal double the reaction rate of hydronium? Someone, please, explain it to me like I’m five.
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### Author:  [@Nava](https://boards.straightdope.com/u/Nava)
#### Post date: [March 16, 2013, 6:36am UTC](https://boards.straightdope.com/t/chemistry-help-needed-reaction-rates-not-for-homework-for-understanding/653067/2 "2013-03-16T06:36:56Z")
> [@Sue\_Duhnym](#):
>
> How the fuck can the reaction rate of the iodide ion equal double the reaction rate of hydronium? Someone, please, explain it to me like I’m five.
Oh, that’s the easy part of the question. Look at the reaction again. For every I[sub]3[/sub]- you’re using 2 H+: you’re using H+ exactly twice as fast as you’re getting I[sub]3[/sub]-
If you had something in the equation that was 5 X, the reaction rate for X would be exacty 5 times that of I[sub]3[/sub]- and 2.5 times that of H+ (with appropriate signage depending on which side of the equation 5 X was in)
Now, Q: what’s the rate for I-
3 times that of I[sub]3[/sub]-
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### Author:  [@Sue\_Duhnym](https://boards.straightdope.com/u/Sue_Duhnym)
#### Post date: [March 16, 2013, 11:15pm UTC](https://boards.straightdope.com/t/chemistry-help-needed-reaction-rates-not-for-homework-for-understanding/653067/3 "2013-03-16T23:15:12Z")
> [@Nava](#):
>
> Oh, that’s the easy part of the question. Look at the reaction again. For every I[sub]3[/sub]- you’re using 2 H+: you’re using H+ exactly twice as fast as you’re getting I[sub]3[/sub]-
You’re mistaking the reactant and the product. The reaction rate given (that I don’t disagree with) is for the reactant, the 3I-
But don’t worry about it, you explained it in a way that got me to figure it out. Thanks!
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### Author:  [@Nava](https://boards.straightdope.com/u/Nava)
#### Post date: [March 17, 2013, 5:49pm UTC](https://boards.straightdope.com/t/chemistry-help-needed-reaction-rates-not-for-homework-for-understanding/653067/4 "2013-03-17T17:49:58Z")
> [@Sue\_Duhnym](#):
>
> Not only that, but the book goes on to give the reaction rates of H[sub]2[/sub]O[sub]2[/sub] and I[sub]3[/sub][sup]-[/sup] as +/- (respectively) 4.40 E-3 M/s.
See the underlined part. I was basing my information on reading what you wrote.