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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: bg19 on November 28, 2004, 05:37:20 PM

Title: Energy Calculations
Post by: bg19 on November 28, 2004, 05:37:20 PM

Hi,

I having troubles with the following problems.

1) Calculate the energy change which will occur when 30g of nitrogen(II) oxide is converted to nitrogen(IV) oxide by reacting with oxygen in the air.

2)Copper(II) oxide can be reduced to copper metal and water by using hot hydrogen gas. Calculate the energy change when 54g of water is formed by this process.

I think my problem with these problems are I dont fully understand the concepts. If you could explain to me which formula I would use and why, I would really appriciate it.

Thanks so much

Title: Re:Energy Calculations
Post by: Demotivator on November 28, 2004, 06:00:15 PM

Use Hess's law utilizing heats of formation (Hf)  of compounds:
 energy = sum of Hf of products - sum Hf reactants.

and keep track of the units invoved.

Title: Re:Energy Calculations
Post by: Donaldson Tan on November 29, 2004, 08:20:09 PM

I try to explain Hess' Law to you:

for example, a chemical reaction takes the route below:
eg. sodium + water
1. Na -> Na+ + e
2. H2O + e -> OH- + H-
3. H- + H2O ->H2 + OH-
overall: Na + 2H2O -> NaOH + OH-

but hess' law tell you, that regardless what the reaction pathway is, each hypothetical one computes the same value for change in enthalpy, so you can assume the reaction uses the most simple mechanism to achieve the products, and you can use that to calculate the delta H.

below is the simplest mechanism:
sodium + water
1. Na (s) -> Na (g) -> Na+(g) + e
2. H2O(l) -> H2O(g) -> 2H(g) + O(g)
3. 2H(g) -> H2(g)
4. O(g) + H(g) + e -> OH- (g)
5. Na+ + OH- -> NaOH (s)
these steps consist of the most fundemental steps, ie. all bonds in the reactants are broken and new bonds are form among the atoms in their elemental state to form the product, ie. delta H = BE(product) - BE(reactant) where BE: bond energy

Title: Re:Energy Calculations
Post by: jdurg on November 30, 2004, 05:09:53 PM

Quote from: geodome on November 29, 2004, 08:20:09 PM

I try to explain Hess' Law to you:

for example, a chemical reaction takes the route below:
eg. sodium + water
1. Na -> Na+ + e
2. H2O + e -> OH- + H-
3. H- + H2O ->H2 + OH-
overall: Na + 2H2O -> NaOH + OH-

but hess' law tell you, that regardless what the reaction pathway is, each hypothetical one computes the same value for change in enthalpy, so you can assume the reaction uses the most simple mechanism to achieve the products, and you can use that to calculate the delta H.

below is the simplest mechanism:
sodium + water
1. Na (s) -> Na (g) -> Na+(g) + e
2. H2O(l) -> H2O(g) -> 2H(g) + O(g)
3. 2H(g) -> H2(g)
4. O(g) + H(g) + e -> OH- (g)
5. Na+ + OH- -> NaOH (s)
these steps consist of the most fundemental steps, ie. all bonds in the reactants are broken and new bonds are form among the atoms in their elemental state to form the product, ie. delta H = BE(product) - BE(reactant) where BE: bond energy

For number 5 there it should be Na+ + OH- -> NaOH(aq)  The Delta hf of NaOH(aq) is slightly different than the Delat hf of NaOH(s).