[ramona007]

I'm completely lost on how to do this, please help.
This is what we were given:

Using the following concentration, & rate law data
2NO (g) + 2 H2 (g) ------> N2(g) +2 H2O(g)

Reactant Concentration (mol/L)

[NO]                   [H2]                              rate of apperance N2 (mol/L * s)
0.420                0.122                              0.136
0.210               0.122                               0.0339
0.21                 0.244                               0.0678
0.105               0.488                               0.0339

Using the above info, calculate the values of x and y in the rate expression

x=? y=?

Overall reaction order=?

What is the value of "k" at this temp? Value of k=?


Any help would be greatly appreciated, thanks very much!

[enahs]

Rate = k [NO]^a [H₂]^b
Where [NO] is the concentration of NO (and the same for Hydrogen gas).

You must determine a and b in the rate equation (I am assuming this is the kind of rate equation your instructor is using, you are posting in Gen Chem form so I am guessing you are not using the integrated rate equation (calculus)?).

Once you determine a and b, you can then plug in the values from any one of the 4 lines of your data and solve for k.
The overall reaction order is the sum of the exponents in the rate equation (in this case, a + b), and NO is a^th order and H₂ is b^th order. Note, you can put hydrogen first if you want, it makes no difference.

Do you know how to determine a and b? Can you figure it out mathematically? Or can your look over your notes/text book and find out? It is not that hard, try it.

Edit: Note, my a and b = the instructors x and y. I personally consider it very poor form for him to use x and y in this case, but I also have a degree in math. But in realty it does not matter one bit. I chose to use a and b, because typically in books when you have just two variables in a rate equation of this type they use alpha and beta.

[ramona007]

Rate = k [NO]^a [H₂]^b
Where [NO] is the concentration of NO (and the same for Hydrogen gas).

You must determine a and b in the rate equation (I am assuming this is the kind of rate equation your instructor is using, you are posting in Gen Chem form so I am guessing you are not using the integrated rate equation (calculus)?).

Once you determine a and b, you can then plug in the values from any one of the 4 lines of your data and solve for k.
The overall reaction order is the sum of the exponents in the rate equation (in this case, a + b), and NO is a^th order and H₂ is b^th order. Note, you can put hydrogen first if you want, it makes no difference.

Do you know how to determine a and b? Can you figure it out mathematically? Or can your look over your notes/text book and find out? It is not that hard, try it.

Edit: Note, my a and b = the instructors x and y. I personally consider it very poor form for him to use x and y in this case, but I also have a degree in math. But in realty it does not matter one bit. I chose to use a and b, because typically in books when you have just two variables in a rate equation of this type they use alpha and beta.

I'm actually a little confused as to how to determine what a and b would be, I'm looking through my lecture notes but I'm just not seeing it 

Thanks for your help

[enahs]

Ok, first mathematically it goes like this (the end result is really easy and obvious, but I will show you how to do it mathematically):

You setup a ratio of two equations:

K is a constant, times K divided by K = 1, so that goes away.
You then simplify the exponent like such:

 
Now, notation. The subscripts 1 and 2 just indicate case 1 and 2. They have no impact in calculation, they just emphasize that we are dealing with two different sets of data.

Now pick a data set 2 (the top in how I have it setup), then pick another data set (1, bottom) but selectively chose one where the concentrations stay the same for j. This gives you a ratio of 1, and 1 raised to any power is 1, so your equation then becomes:


Now, you just have to solve for a.
To get b, if you have another set of data where i is now held constant and j varies, then you can solve for b the same exact same way. If not, you can then pick two sets of data, plug in your now known value for a and solve for b (it is just a little harder, but still basic algebra).

In English.
Find two sets of data where the concentration is held constant with respect to one and varies in the other.
Find the change in rate (divide the two rates).
Find the change in concentration (divide the two concentrations).
The change in concentration to what power is the change in the rate?

Example (not with your data)

set      Rate     [fries]      [strawberries]
  1        5           10              10
  2       10          20              10
  3       20          10              20


Change in rate of set 2 with set 1: 10/5 = 2
Change in concentration of set 2 with set 1 of fries: 20/10 = 2
2 (fries) raised to what power = 2 (rate)?, 2¹ = 2, so fries is first order.

Change in rate of set 3 with set 1: 20/5 = 4
Change in concentration of set 3 with 1 of strawberries: 20/10 = 2
2(strawberries) raised to what power = 4 (rate)? 2² = 4, so strawberries is second order.

Rate = k [fries]¹ [strawberries]²




Once you have the exponents (a and b), you can then plug them into the rate equation:
Rate = k ^a [j]^b
Plug in any values from your data (they have to be in the same set, the same line in this case) and solve for k.

So from my fries and strawberries examples, using data set 1

5 = k [10]¹ [10]²
k = 5 / [10]¹ [10]²
k = 0.005


Now, when you do your math make sure to include your units and cancel them properly, as getting the right units on k is very important and your instructor will count of lots of points if you get them wrong (my example was unit less)