Enthalpy Changes (Hess's Law)

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Hogan Irwin 3A
Posts: 5
Joined: Wed Sep 21, 2016 2:59 pm

Enthalpy Changes (Hess's Law)

Postby Hogan Irwin 3A » Sun Jan 15, 2017 4:39 pm

What are the three different methods we can use to find the change in enthalpy?

Tyler Miao 1A
Posts: 15
Joined: Wed Sep 21, 2016 2:59 pm

Re: Enthalpy Changes (Hess's Law)

Postby Tyler Miao 1A » Sun Jan 15, 2017 4:42 pm

bond enthalpy, Hess's law, and standard reaction enthalpies can be used to find the change in enthalpy

Michelle Kam 1F
Posts: 10
Joined: Wed Sep 21, 2016 2:58 pm

Re: Enthalpy Changes (Hess's Law)

Postby Michelle Kam 1F » Sun Jan 15, 2017 4:47 pm

There are essentially three methods of calculating enthalpy.
First, there is the Hess's Law, which is calculation of standard enthalpy of formation using the give enthalpy of each reactions.
To find t he standard enthalpy of formation of glucose, we use the given equations to form a final equation of 6C + 3O2 + 6H2 -> C6H12O6
we can alter the enthalpies of the given equations by reversing the equation, or multiplying the equation with whole numbers, etc.
then add them all together to finally form the glucose.

Secondly, we have bond enthalpy. We can subtract the sum of bonds formed from the sum of bonds broken.
Summation(bonds broke) - Summation(bonds formed)
the standard bond enthalpy for each bonds are usually given.

Last method is one using the Enthalpy of formation. Here we find the deltaH of the reaction just by using (products - reactants).
Here also, the standard enthalpy of each compound is usually given.

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