Need Tips for doing Hess's Law

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In summary, the question is asking for the standard enthalpy change for the production of hydrogen from methane and steam. Several equations are provided, but only three are necessary to solve the problem. By manipulating the given equations, the desired result can be obtained. There are also multiple ways to solve the problem, with the simplest route yielding a value of 205.7 kJ.
  • #1
nobb
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Hey. I am having trouble with this question:

What is the standard enthalpy change for the production of hydrogen from methane and steam: CH4(g) + H2O(g) -> CO(g) + 3H2(g)

2C(s) + O2(g) -> 2CO(g) Hc= -221.0 kJ
CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g) Hc= -802.7 kJ
CO(g) + H2O(g) -> CO2(g) + H2(g) Hr= -41.2 kJ
2H2(g) + O2(g) -> 2H2O(g) Hc= -483.6 kJ
C(s) + 2H2(g) -> CH4(g) Hf= -74.4 kJ
C(s) + H2O(g) -> CO(g) + H2(g) Hr= 131.3 kJ
2CO(g) + O2(g) -> 2CO2(g) Hc= -566.0 kJ
CO(g) + H2(g) + O2(g) -> CO2(g) + H2O(g) Hr= -524.8 kJ

This question is really complex and I do not know which equations to switch. I don't really know where to start. Could someone please offer some tips on how to do this question and others?
 
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  • #2
Add all reactions to one another to obtain the final (desired) one. Remember that switching a reaction from left to right also changes its sign; if it is negative, treat this as a positive one.
 
  • #3
Yes I know that. I am confused because I do not know which ones to switch. I tried this question many times and I still can't get the answer
 
  • #4
Switch #5 and add with #6. You'll find a value about 208 kJ.
 
  • #5
Actually, they have provided more reactions than you need. That's what makes it look complex, while in fact, the problem can be solved easily using only 3 of the above equations.

2C(s) + O2(g) -> 2CO(g) Hc= -221.0 kJ
CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g) Hc= -802.7 kJ
CO(g) + H2O(g) -> CO2(g) + H2(g) Hr= -41.2 kJ
2H2(g) + O2(g) -> 2H2O(g) Hc= -483.6 kJ
C(s) + 2H2(g) -> CH4(g) Hf= -74.4 kJ
C(s) + H2O(g) -> CO(g) + H2(g) Hr= 131.3 kJ
2CO(g) + O2(g) -> 2CO2(g) Hc= -566.0 kJ
CO(g) + H2(g) + O2(g) -> CO2(g) + H2O(g) Hr= -524.8 kJ

Remember, there are probably several ways to solve this using any number of the given equations. It may even be possible to use them all, but I'm certainly not going to try that when I've found a simpler way.

Look at the bolded equations and figure out how to manipulate them to get the desired result.

Having done this, try to come up with at least one other alternative route (there's one using only 2 equations). If you do these two things, you'll have little trouble with such problems in the future.

Edit : chem_tr has given away the simpler solution too, so try and look for a third one : I get 205.7 kJ, both ways.
 
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  • #6
Ok thanks. That makes much more sense now.
 

1. What is Hess's Law and why is it important for chemistry?

Hess's Law is a fundamental principle in chemistry that states the enthalpy change of a chemical reaction is independent of the pathway taken to achieve the products. This means that the total enthalpy change for a reaction can be calculated using the enthalpy changes of the individual steps involved. Hess's Law is important because it allows us to predict and calculate enthalpy changes for reactions that are not easily measured in a laboratory setting.

2. How do I use Hess's Law to calculate enthalpy changes?

To use Hess's Law, you need to first determine the individual enthalpy changes for each step of the reaction. Then, add or subtract these values according to the stoichiometric coefficients of the reaction to calculate the overall enthalpy change. It is important to note that the enthalpy changes for each step must be measured under the same conditions for the calculation to be accurate.

3. Can Hess's Law be applied to all chemical reactions?

Yes, Hess's Law can be applied to all chemical reactions as long as the enthalpy changes for each step are known or can be calculated. However, it is important to remember that this law only applies to reactions in the gas phase or in solution, and it is not applicable to reactions involving solids or pure liquids.

4. What are the limitations of Hess's Law?

One limitation of Hess's Law is that it assumes that the overall enthalpy change is equivalent to the sum of the enthalpy changes for each step. However, this may not always be the case due to factors like non-ideal conditions and heat loss. Another limitation is that it does not take into account the effects of temperature and pressure on the enthalpy change.

5. How can I use Hess's Law to solve numerical problems?

To solve numerical problems using Hess's Law, you need to first write out the chemical equation for the reaction and identify the steps involved. Then, determine the individual enthalpy changes for each step and use the law to calculate the overall enthalpy change. Finally, make sure to convert any units if necessary and round the final answer to the appropriate number of significant figures.

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