# What Is the Approximate Heat of Reaction for 2H2 + O2→2H2O?

• brake4country
In summary, the heat of reaction for 2H2 + O2→2H2O is approximately -488 KJ/mol, which indicates an exothermic reaction. This can be calculated by subtracting the total bond energies of the reactants (2H2 and O2) from the total bond energies of the products (2H2O). The correct answer is -488 KJ/mol, as one molecule of water has two hydrogen-oxygen bonds.
brake4country

## Homework Statement

Given that the bond energy of hydrogen-hydrogen bonds is 436 KJ/mol, that of hydrogen-oxygen bonds is 464 KJ/mol, and those in oxygen molecules 496 KJ/mol, what is the approximate heat of reaction for 2H2 + O2→2H2O

## Homework Equations

Hproducts-Hreactants

## The Attempt at a Solution

So, I understand that heats of reaction involving hydrogen gas and oxygen gas to liquid water is exothermic, so my answer must be negative. My book gives and answer of -488 KJ/mol but I keep ending up with -440 KJ/mol:

2(464) - [2(436) + 496] = -440 KJ/mol.

Calculation error? I went over the numbers twice. Thanks!

How many molecules of water does the reaction yield, and how many hydrogen-oxygen bonds are in each molecule? I hope this helps.

Actually, I looked up a video to see where my reasoning went astray. With the original equation, we have:

2H2+ O2→ 2H2O

Bonds broken:
2H-H = 436 KJ/mol
02 = 496 KJ/mol

Bonds formed:
2H2O

Using Hp-Hr, we get:
4(464)-[2(436) + 496)] = 488 KJ/mol but must be negative because g-->l is exothermic.

Since this is the right answer, I feel better now, but I prefer the experts here to make sure my work is valid. Thanks in advance!

Calaver
Your answer is correct, and your original answer had the right concept, it just looked like to me that you forgot to count one of the water molecules produced (or one of the hydrogen-oxygen bonds within each molecule).

You are right. I had to draw out the bonds in the products to see what I did wrong. Thanks again!

## 1. What is a bond energy?

Bond energy is the amount of energy required to break a chemical bond between two atoms in a molecule.

## 2. How is bond energy related to heat of reaction?

Bond energy is directly related to heat of reaction. When a chemical reaction occurs, bonds are broken and formed, and the difference in bond energies between the reactants and products determines the amount of heat released or absorbed.

## 3. Can bond energies be measured experimentally?

Yes, bond energies can be measured experimentally using techniques such as calorimetry or spectroscopy.

## 4. What factors affect bond energies?

The type of atoms involved, the type of bond (ionic, covalent, etc.), and the length and strength of the bond all affect bond energies.

## 5. How are bond energies used in chemical reactions?

Bond energies are used to calculate the overall energy change in a chemical reaction. This information can then be used to predict the feasibility and direction of a reaction.

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