How do I calculate the change in enthalpy?

With the given information below find the change in enthalpy:

Given:

1.       H2(g) + (1/2) O2(g) ---> H2O(l)

∆Hf = -285.8 kJ/mol

2.      N2O5(g) + H2O(l) ---> 2HNO3(l)

∆Hf = -76.6 kJ/mol

3.      (1/2) N2(g) + (3/2) O2(g) + (1/2) H2(g) ---> HNO3(l)

∆Hf = -174.1 kJ/mol

 

 

2N2(g) + 5O2(g) ---> 2N2O5(g)

∆H = ??? kJ/mol

H2O(l) à H2(g) + (1/2) O2(g) (you switched these so you now change the sign of the ∆H)

{2HNO3(l)à N2O5(g) + H2O(l)}*2 ( you switched these so you now change the sign of the ∆H)

{(1/2) N2(g) + (3/2) O2(g) (makes it)+ (1/2) H2(g) ---> HNO3 }*4

 

You can use Hee’s Law to solve this, or you can use a formula which is: sum of products (blue)- sum or reactants (in red)

Before you do anything you first need to see what you want everything to equal.  The first step should be to highlight the reaction in which you are trying to make. (in green)

Second step: you should highlight all the elements that you need in order to make this reaction (bold)

Third step: you now have to make sure that they are on the right side of the equation, and if it isn’t than you must switch it.

REMINDER- if you switch the elements you MUST switch the sign of the change in enthalpy

Fourth step: you need to multiply everything out to make sure that you are able to cross cancel everything out, and get the correct number

Fifth step: after you get the numbers to match up with one another you use process of elimination

Sixth step: you add everything up together, and make sure that you multiply the heat of formations by the number that you multiplied by the equation.

Answer:

∆Hf = +571.6 kj/mol (changed positive because you switched the equation around, and multiplied it by 2)

∆Hf = +153.2 kj.mol (positive because you switched the equation around, and multiplied it by 2)

∆Hf = -696.4 kj/mol (negative because you switched the equation around, and multiplied by 4)

 

∆Hf = +28.4 kJ/mol

= answer

 

Determine how much energy (in kJ) we use from gasoline in a year

1)      There is 7.5 billion barrels used by the U.S per year

Click here to get the upload version of the excel sheet conversions are shown*

Everything in red is what cross cancels*

=3.80E16 kj of energy that we use from gasoline in 1 year

Heat of formation

C8H18 + O2 -->  CO2 + H20

C8H18 + 25/2O2--> 8CO2 + 9H20

Look up heat of formations on the sheet shown below: CLICK HERE

Equation that you will use to find the heat of formation: sum of products- sum of reactants

******that equation can be used instead of Hee’s law but you will get the same answer

(8x-393.5)+(9x-285.83)= -5720.47 kj/mol (this is the product side)

(-28.45 kj/mol)+(0)=-28.45 kj/mol (this is the reactant side)

∆Hf = -5512.02 kj/mol which means that for every mole of C8H18 when combusting it gives off 5512.02 kj/mol of energy

The sum is 0 because it is a diatomic element, and with diatomic elements the heat of formation is ALWAYS 0.

2.      Determine how many gallons of E85 would be required to replace all of the energy we currently use in the form of gasoline.

85%= ethanol

15%= gasoline

On excel spreadsheet

=4.05E11 gallons: gallons of ethanol needed to create same amount of energy being 7.5 billion barrels of gasoline

1.      How much land would be required to grow the corn necessary for 15 billion gallons of E85?

On excel spreadsheet

=4.27E7 acres of land are needed

 

Conversion help

BY: Sophie Lasry