# Enthalpy Calculation for 4B+3O2-->2B2O3 Reaction Using Hess's Law

• Evis
In summary, to calculate the enthalpy of the reaction 4B(s)+3O2(g)→2B2O3(s), we must use Hess' Law and add the enthalpies of multiple reactions. This includes adding the reactions B2O3(s)+3H2O(g)→3O2(g)+B2H6(g) with ΔH∘A=+2035kJ, 2B(s)+3H2(g)→B2H6(g) with ΔH∘B=+36kJ, H2(g)+1/2O2(g)→H2O(l) with ΔH∘C=−285kJ, and H2O(l)
Evis

## Homework Statement

Calculate the enthalpy of the reaction

4B(s)+3O2(g)→2B2O3(s)

given the following pertinent information:
1. B2O3(s)+3H2O(g)→3O2(g)+B2H6(g), ΔH∘A=+2035kJ
2. 2B(s)+3H2(g)→B2H6(g), ΔH∘B=+36kJ
3. H2(g)+1/2O2(g)→H2O(l), ΔH∘C=−285kJ
4. H2O(l)→H2O(g), ΔH∘D=+44kJ

## Homework Equations

ΔH=Sum of all ΔH of each reaction. (Hess' Law?)

## The Attempt at a Solution

2[2B(s)+3H2(g)→B2H6(g)] ΔH=72 J
2[3O2(g)+B2H6(g)→B2O3(s)+3H2O(g)] ΔH=-4070kJ

ΔH=3998 kJThis incorporates the correct reactants and products. We still need to add H2, but there's no way to react only the given reactants, B and O2, into H2 at all. Also, there is no way to react out the H2O from the given equations.

The actual solution

6[H2O(g)→H2O(l)] ΔH=-264 kJ
6[H2O(l)→H2(g)+1/2O2(g)] ΔH=1710J
2[2B(s)+3H2(g)→B2H6(g)] ΔH=72 kJ
2[3O2(g)+B2H6(g)→B2O3(s)+3H2O(g)] ΔH=-4070kJ

ΔH=-2552

Colours correspond to canceled products.

I understand the process given, but not how it relates to the goal equation. This process requires water to be added as a reactant and for 3H2 and 3H2O to be products, which are not included in the goal equation. Adding the extra reactions with water do not seem directly part of the goal equation.

Evis said:
Adding the extra reactions with water do not seem directly part of the goal equation.

But is necessary to cancel out hydrogen and water. Your solution is incorrect as your equation is not identical with the goal equation, and it must be.

## 1. What is the definition of enthalpy?

Enthalpy is a thermodynamic property that represents the total energy of a system, including both its internal energy and the work required to create or maintain its structure.

## 2. How is enthalpy calculated?

Enthalpy is calculated by taking into account the internal energy, pressure, and volume of a system using the equation H = U + PV, where H is enthalpy, U is internal energy, P is pressure, and V is volume.

## 3. Why is enthalpy important in chemical reactions?

Enthalpy is important in chemical reactions because it helps us understand the changes in energy that occur during a reaction. It also allows us to predict whether a reaction will release or absorb heat.

## 4. What is the enthalpy change for the reaction 4B + 3O2-->2B2O3?

The enthalpy change for this reaction is the difference in enthalpy between the reactants (4B + 3O2) and the products (2B2O3). This can be determined experimentally by measuring the heat released or absorbed during the reaction.

## 5. How does the enthalpy of a reaction affect its spontaneity?

The enthalpy of a reaction is a factor in determining its spontaneity. A negative enthalpy change (exothermic reaction) will make the reaction more likely to occur spontaneously, while a positive enthalpy change (endothermic reaction) will make the reaction less likely to occur spontaneously.

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