Hess's Law and Thermochemical Equations

• djfrankym
In summary, the standard heat of formation of gaseous hydrogen bromine was evaluated using standard enthalpy values from various reactions, including the formation of HBr from its elements. After accounting for states of compounds, the thermochemical equation for the formation of 1 mol of HBr(g) is 1/2H2(g) + 1/2Br2(g) --> HBr(g) with a value of Delta H final = -36.8kJ/mol.
djfrankym
[SOLVED] Hess's Law and Thermochemical Equations

Homework Statement

Question: 5.63:
The standard heat of formation of gaseous hydrogen bromine was first evaluated by means of the standard enthalpy values measured for the following reactions. The last three are standard heats of solution.

Cl2(g) + 2KBr(aq) --> Br2(aq) + KCl(aq) Delta H = -96.2kJ
H2(g) + Cl2(g) --> 2KCl (g) Delta H = -184kJ
HCl(aq) + KOH(aq) --> KCl(aq) + H2O Delta H = -57.3kJ
HBr(aq) + KOH(aq) --> KBr(aq) + H2O Delta H = -57.3kJ
HCl(g) --> HCl(aq) Delta H = -77.0kJ
Br2(g) --> Br2(aq) Delta H = -4.2kJ
HBr(g) --> HBr(aq) Delta H = -79.9kJ

Write the thermochemical equation for the formation of 1 mol of HBr(g) from its elements, including its value for Delta H final.

Homework Equations

(Included in question)

The Attempt at a Solution

I assumed you would have to isolate 1/2H2 and 1/2Br2 to make the final equation of 1/2H2 + 1/2Br2 --> HBr(g). No matter which way I work the equations I end up without H2 or Br2 depending on my method. However, I am wondering if I was on the right track. Is that what I am looking for?

EDIT: Realized my mistake. Disregarded the states of compounds. SOLVED

Last edited:
Looks good, slight typo in the HCl formation equation (not KCl).

What is Hess's Law?

Hess's Law states that the enthalpy change of a reaction is independent of the pathway taken from the initial to final state. In other words, the overall energy change of a reaction is the same regardless of the intermediate steps.

How is Hess's Law used in thermochemistry?

Hess's Law is used to calculate the enthalpy change of a reaction, even if it cannot be directly measured. This is done by breaking down the reaction into smaller, measurable steps and then using the law to add or subtract the enthalpy changes of each step to find the overall change.

What are thermochemical equations?

Thermochemical equations are balanced chemical equations that include the enthalpy change of the reaction. This allows for the calculation of the enthalpy change of a reaction using Hess's Law.

What is the difference between enthalpy and energy?

Enthalpy is a measure of the total heat content of a system at a constant pressure. It includes the internal energy of the system as well as the energy required to push back the surroundings. Energy, on the other hand, is a more general term that refers to the ability to do work or cause change.

How is Hess's Law related to the first law of thermodynamics?

Hess's Law is a direct application of the first law of thermodynamics, which states that energy cannot be created or destroyed, only converted from one form to another. This law is reflected in Hess's Law, as the total energy of a reaction remains constant even when broken down into smaller steps.

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