Rate question

The concentration of A was increased by a factor of 3 and the concentration of B was halved. This caused the initial rate to increase by a factor of nine. Deduce the order of reaction with respect to A and that of B. How would people go about this?

Reply 1

charco

Study Forum Helper

assuming that the maximum order possble is 2 for an individual component you can see that the effect of A alone causes a squared effect on the rate. Therefore the order wrt [A] = 2

As there is no effect from the order is 0

If B were to have an effect then the order of [A] would have to be greater than 2 - highly improbable

Reply 2

Tabris

Charco took the words out of my mouth.

Reply 3

Riddhy

then the ORDER IS 2 right?

Reply 4

heeeel

Charco is one of the people who replied

Reply 5

silent ninja

heeeel

Charco is one of the people who replied

But did he mean he didnt understand what charco means or what charco posted... :confused:

Reply 6

dimz

silent ninja

But did he mean he didnt understand what charco means or what charco posted... :confused:

exactly

Reply 7

charco

Study Forum Helper

How dare he not understand what I posted!

so confused

When A is multiplied by 3, the rate increases by a factor of 9. Find the relationship between 3 and 9:

9 = 32 (squared)

Therefore the order is 2

Reply 10

dairymasta

Increasing A causes rate to become squared...because rate squared, [A] = second order...halving has no effect, so rate for = 0...order of [A] in relation to = second order.

Or something to that effect~

Reply 11

hopefulloser

so basically u do some sort of division...and at the end of the day...its a third chance ur right...right?

I just hope it dnt come up difficult...it wont hurt them if they give us a straight forward one!! :frown:

Reply 12

DartsOfPleasure

You've got to calculate the relationship between all the numbers - what's actually happened and what should have happened. I find these tricky too, but in my book it says you can work out k for one of the reactions and use that to find out the blanks.

Reply 13

jellytotz

DartsOfPleasure

lol...noo make sure you do learn how to do these..as i went in to the exam in jan thinking il just find the k and work out the blanks..but the very first question i remember was to find the orders to the reactions and so you wern't given a rate equation...therefore meaning you couldn't work out k!! :mad:

Reply 14

hopefulloser

aaaaaaaaahhhhhhhhhhhhhhh :'(

Reply 15

Tabris

Since we're talking about rate, I've got a question.

How do you figure out the units for the rate equation.

I always get an answer for the rate equation, but I never know what units to put. Or is it just moldm^-3s^-1

Reply 16

charco

Study Forum Helper

The rate equation is an equation and nothing more ... ok - it can only be determined (its form) experimentally

It has different forms depending on the orders of the reaction with respect to the individual component concentrations..

eg:

Rate = k[A]⁰
Rate = k[A]¹
Rate = k[A]²
Rate = k[A]¹¹

etc etc

the units for rate are always the same as the definition of rate is the change of concentration with time (ok the time units can change as well but this shouldn't be an issue)
The units of [A] and/or are always the same = mol dm^-3

So its a question of looking at the particular version of the rate equation that you have, rearranging it to give 'k =' and putting the units into all of the parts to see what k is left in.

For example, if the rate equation is rate = k[A]²⁰

rearrange to get:

k= rate/[A]²⁰

Now put in the units for the rate and [A] and (as is raised to the power of zero it becomes =1 and therefore has no units)

k= mol dm^-3 s^-1/(mol dm^-3)²

now just multiply out and cancel out and finallly move everything to the top line (th enumerator)

k = dm³ mol^-1 s^-1

Reply 17

silent ninja