AP Chemistry 3.1 Laws of Thermodynamics. What is the change in enthalpy of this reaction?
|AP Chemistry||Law of Thermodynamics|
Laws of Thermodynamics
The Laws of Thermodynamics
|Test Prep||AP Chemistry|
Geez, what does it take around here to get an explosion...
Okay, here's today’s question.
Using Hess's law, what is the change in enthalpy for the following reaction?
And here are your potential answers.
So the question tells us to use Hess’s law, which, unfortunately, has nothing to do with
avocados. [Hess kicks an avocado]
Hess’s law comes from the fact that the overall enthalpy change of a reaction doesn’t
depend on the reaction pathway, or how we get from reactants to products.
For example, we could react one mole of A and 2 moles of B to produce 2 moles of C.
Let’s call this Peter. [Formula example of Hess's law]
What, you think all reactions want to be called "reaction"?
Do you just want to be called "human six billion nine hundred and sixty-two thousand"?
We're calling "reaction 1" Peter.
We could also react one mole of A with one mole of B to produce two moles of D, then
react these moles of D with one mole of B to produce two moles of C.
We’ll call these reactions Sarah and Phil, because they're more than just 2 and 3.
As you can see, reactions Sarah and Phil can be added to produce reaction Peter.
The overall reaction is the same in both cases—one mole of A and two moles of B produce two moles
So in the same way that we added the actual reactions, we can add the enthalpy changes.
Hess’s law says that the overall enthalpy change shouldn’t depend on the reaction
That means that the enthalpy change for Peter should equal the sum of the enthalpy changes [Hess's law for Peter, Sarah, Phil]
for Sarah and Phil.
Okay, so back to the problem at hand! [Man in a rocking chair]
And we’re not referring to the fact we're naming reactions and may be having a mental
breakdown. [Man crying in a rocking chair]
We need to find a way to add the three reactions for which we have ?H to produce the top reaction.
Here’s what we came up with.
If we multiply the first equation by one half, reverse and multiply the second equation by
3/2, and multiply the third equation by one half, then add them all up, we get the first [All equations added together]
We add the ?H values in the same way.
Just don’t forget that if you reverse a reaction, you have to negate the ?H value,
and if you multiply a reaction by a factor, such as 1/2, you should multiply the ?H value [Man discussing ?H value]
by that number too.
So chugga chugga chug... chug through the math machine and…
Looks like our answer is B, 886 kJ.
Now it's time to brainstorm the pilot episode of our new sitcom. [People sitting on a couch watching a sitcom]
It's about three young reactions living together…we're calling it, "Three's Company, but with Reactions".
…We're not great at naming things.