Hess's Law and change of enthelpy

  • Thread starter Integral0
  • Start date
  • Tags
    Change Law
In summary, the enthalpy of combustion for solid carbon to form carbon dioxide is -393.7 kj/mol, and for carbon monoxide to form carbon dioxide is -283.3 kj/mol CO. By using Hess's Law, the change in enthalpy for the reaction 2C(s) + O2(g) -> 2CO(g) is calculated to be 221 kj/mol.
  • #1
Integral0
49
0
The enthalpy of combustion of solid carbon to form carbon dioxide is -393.7 kj/mol carbon, and the enthalpy of combustion of carbon monoxide to form carbon dioxide is -283.3 kj/mol CO. Use these data to calculate change of enthalpy for the reaction

2C(s) + O2(g) -> 2CO(g)

----------------------

I'm quite lost . . . after trying this problem for about 20 mins now . . . I still don't see how to use Hess's Law to formulate this answer

What i tried to do was to "form" the equations by the words in the problems.

He are my formulations below; however, I can't seem to get the equation mentioned above.

2CO + O2 -> 2CO2
C + O2 -> CO2

when I rearrange these . . . I can't seem to get 2C(s) + O2(g) -> 2CO(g)

the answer is -220 Kj/mol

thanks
 
Last edited:
Chemistry news on Phys.org
  • #2


EUREKA! I GOT IT :) :)

2 (C + O2 -> CO2) change in Enthalpy = 2(-394) kj/mol
2CO2 -> O2 + 2CO change in Enthalpy = 2(239.3) kj/mol

= 2C + O2 -> 2CO change in Enthalpy = 221 kj/mol + or - 1 kj/mol

:D ;) :)
 
  • #3


Hess's Law states that the enthalpy change for a reaction is independent of the pathway taken, and only depends on the initial and final states of the reaction. This means that we can use the enthalpy values for different reactions to calculate the enthalpy change for a specific reaction.

In this case, we can use the enthalpy values given for the combustion reactions of solid carbon and carbon monoxide to calculate the enthalpy change for the reaction 2C(s) + O2(g) -> 2CO(g).

First, we can write out the balanced equation for the reaction:

2C(s) + O2(g) -> 2CO(g)

Next, we can use the enthalpy values provided to write out the enthalpy change for each step of the reaction:

Step 1: Combustion of solid carbon to form carbon dioxide
C(s) + O2(g) -> CO2(g)
Enthalpy change = -393.7 kJ/mol

Step 2: Combustion of carbon monoxide to form carbon dioxide
CO(g) + 1/2 O2(g) -> CO2(g)
Enthalpy change = -283.3 kJ/mol

Since we want to calculate the enthalpy change for the reaction 2C(s) + O2(g) -> 2CO(g), we need to multiply the enthalpy change for step 1 by 2, and the enthalpy change for step 2 by 2:

Step 1: 2C(s) + 2O2(g) -> 2CO2(g)
Enthalpy change = -2 * 393.7 kJ/mol = -787.4 kJ/mol

Step 2: 2CO(g) + O2(g) -> 2CO2(g)
Enthalpy change = 2 * (-283.3 kJ/mol) = -566.6 kJ/mol

Now we can add these two enthalpy changes together to get the overall enthalpy change for the reaction:

Enthalpy change = -787.4 kJ/mol + (-566.6 kJ/mol) = -1354 kJ/mol

However, we need to remember that the enthalpy change for step 2 is for the formation of 2 moles of CO(g), while the reaction only produces 1 mole of CO(g). Therefore,
 

What is Hess's Law?

Hess's Law states that the total enthalpy change of a chemical reaction is independent of the pathway between the initial and final states. In other words, the change in enthalpy is the same whether the reaction takes place in one step or multiple steps.

How is Hess's Law related to the change of enthalpy?

Hess's Law is directly related to the change of enthalpy because it states that the total enthalpy change of a reaction is constant, regardless of the pathway taken. This means that the change in enthalpy for a reaction can be calculated by using the enthalpy changes of intermediate reactions, rather than directly measuring the change in enthalpy of the overall reaction.

How is Hess's Law used in practical applications?

Hess's Law is commonly used in chemistry and thermodynamics to calculate the change in enthalpy for reactions that are difficult or impossible to measure directly. It is also used in industries such as energy production and manufacturing to optimize processes and improve efficiency.

What is the significance of the change in enthalpy?

The change in enthalpy of a reaction represents the amount of heat that is either absorbed or released during the reaction. This information is important in understanding the thermodynamics of a system and can be used to predict the direction and extent of a reaction.

Are there any limitations to using Hess's Law?

While Hess's Law is a useful tool for calculating enthalpy changes, it is based on the assumption that enthalpy is a state function. This may not be true for all reactions, as some may involve changes in temperature, pressure, or other variables. Additionally, Hess's Law is not applicable for reactions that involve a change in phase.

Similar threads

  • Chemistry
Replies
4
Views
4K
Replies
131
Views
4K
  • Biology and Chemistry Homework Help
Replies
6
Views
1K
  • Chemistry
Replies
1
Views
2K
Replies
9
Views
2K
Replies
1
Views
3K
  • Biology and Chemistry Homework Help
Replies
7
Views
2K
  • Chemistry
Replies
2
Views
2K
Back
Top