HELP!!
I cannot find the value of ÄH, for the following reactions.
A + 2B --> C + 3D , ÄH = 154 kJ
1/2C + 3/2D --> 1/2A + B, ÄH= ?
I have tried the answers -302 and 154 and neither were correct.
Have any suggestions?? Thank You!
Isnt the second reaction the first reaction backwards, or 1/2 of it backwards, so deltaH=-154/2 kJ
Yes, the 2nd reaction is the 1st reaction reversed.
The book says when you have a reaction that is reversed, take the opposite sign of the ÄH... which should be -154 by that definition. I'm not sure if I am suppose to divide 154 by 2??
yes, you are supposed to divide. It is 1/2 of the first reaction backwards.
To find the value of ÄH for the second reaction, you can use Hess's Law. Hess's Law states that the enthalpy change of a reaction is the same regardless of the route taken, as long as the initial and final conditions are the same. In this case, we can manipulate the given reactions and their enthalpy changes to obtain the desired reaction.
First, reverse the first reaction:
C + 3D --> A + 2B, ÄH = -154 kJ
Next, multiply the second reaction by 2 to match the coefficients of A and B in the reversed first reaction:
C + 3D --> A + 2B, ÄH = 2 * ÄH (second reaction)
Now, the coefficients of A and B in both reactions match.
Add the reversed and multiplied reactions together:
C + 3D + C + 3D --> A + 2B + A + 2B
Simplify:
2C + 6D --> 2A + 4B
The ÄH of the combined reaction is the sum of the individual ÄH values:
ÄH(combined) = -154 kJ + 2 * ÄH (second reaction)
Since we don't have the specific value for ÄH in the second reaction, let's denote it as x:
ÄH(combined) = -154 kJ + 2x
You can now solve for x, given that ÄH(combined) = ? kJ. Substitute the known values and solve for x.
Once you have the value of x, you can calculate the ÄH for the second reaction (1/2C + 3/2D --> 1/2A + B).