# Hess's Law Calculations (Enthelpy)

• Race
In summary: The first one is NH4Cl(aq) → NH4(aq) + Cl (aq), while the second one is NH3(aq) + HCL (aq) → NH4Cl(aq). The formation reactions for these processes can be calculated by adding the corresponding ΔH values from the given reactions. The neutralization reaction ΔH is -45.77kJ + (-92.31kJ) = -138.08kJ, and the solution reaction ΔH is (-35.40kJ) + (-72.84kJ) = -108.24kJ. Therefore, the formation reaction ΔH is -138.08kJ + (-108.24kJ) = -246.32kJ. In summary, the lab
Race
We studied the reactions NH4Cl(aq) → NH4(aq) + Cl (aq) and NH3(aq) + HCL (aq) → NH4Cl(aq) in lab today, and part of our assignment was to give the formation reaction by first calculating the neutralization part ΔH of our lab and the solution reaction part ΔH of our lab.

We were given this:

1/2N2(g) +3/2H2(g) → NH3(g) -45.77kj
1/2H2(g)+ 1/2Cl2(g) → HCl (g) -92.31kj
NH3(g) → NH3(aq. 1.50M) -35.40kj
HCl(g) → HCl(aq, 1.50M) -72.84 kj

And were asked to add the reactions we did in class to them. Then we were asked for Neutralization reaction ΔH and Solution reaction ΔH. After we found this we were asked for Formation reaction ΔH.

My attempt at a solution:

My lab partner did everything with the professor while I was cleaning up from the actual experiment, and they went to the office and since lab went over time I had to rush to get to work, so whatever was done I didn't get to see. So I'm trying to work through this on my own until my professor e-mails me back!

I have done several Hess's Law problems in the past, but was never as confused by the wording of the question as I am now. If someone could just figure that out, I would appreciate it so much!

Probably the best starting point is to write reaction equations for both processes in question.

## 1. What is Hess's Law and how is it used in Enthalpy calculations?

Hess's Law states that the total enthalpy change of a reaction is independent of the pathway taken, as long as the initial and final conditions are the same. This allows us to use known enthalpy values for different reactions to calculate the overall enthalpy change of a reaction.

## 2. How do you determine the enthalpy change of a reaction using Hess's Law?

To determine the enthalpy change of a reaction using Hess's Law, you must first write out the balanced chemical equation for the reaction. Then, identify any intermediate steps or reactions that can be used to break down the overall reaction. Finally, use the known enthalpy values for these intermediate reactions to calculate the overall enthalpy change using the equation: ∆H = ∑∆Hproducts - ∑∆Hreactants.

## 3. What is the difference between standard enthalpy of formation and standard enthalpy of combustion?

The standard enthalpy of formation (∆Hf) is the enthalpy change when one mole of a compound is formed from its elements in their standard states. The standard enthalpy of combustion (∆Hc) is the enthalpy change when one mole of a substance is completely burned in excess oxygen under standard conditions. ∆Hf is used in Hess's Law calculations, while ∆Hc is used to determine the energy content of fuels.

## 4. Can Hess's Law be applied to reactions that involve changes in state?

Yes, Hess's Law can be applied to reactions that involve changes in state (e.g. solid to gas, liquid to gas). The enthalpy values for these state changes can be obtained from thermodynamic tables and used in the overall enthalpy calculation.

## 5. What are some limitations of using Hess's Law in enthalpy calculations?

One limitation of using Hess's Law is that it assumes all reactions are carried out under standard conditions (25°C and 1 atm). In reality, reactions may occur under different conditions which can affect the enthalpy change. Additionally, Hess's Law is based on the assumption that the enthalpy change is only dependent on the initial and final states, which may not always be the case.

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