Question

An imaginary planet was just discovered that has a similar environment to our planet Earth. All the chemistry is similar except for the values of bond energies. Use the planet's given bond energies to calculate the enthalpy of reaction for the combustion of 1 mole of pentane.
Enter a number to O decimal places in kJ/mol
Bond Energy (kJ/mol)
H-H 585
C-H 257
H-O 478
C-C 370
C C 779
CC 992
0-0 490
C-0 251
C-0 740

Answer 1

Answer: $\Delta H_{rxn}=-4652$ kJ/mol

Explanation: In a reaction, a bond of a molecule can break or rearrange itself. When it happens, it absorb or release energy.

Bond Energy is the energy necessary to break or to make a particular bond, producing gaseous fragments at atmospheric temperature. This type of energy can be used to show how stable a compound is or how easy a bond can break.

Enthalpy of Reaction is the energy absorption or release at a constant temperature resulting from a chemical reaction.

To calculate the Enthalpy using bond energy, you have:

1) Write the balanced equation with all the all the reactants and products in gaseous form:

$C_{5}H_{12}_{(g)}+8O_{2}_{(g)}$ ⇒ $5CO_{2}_{(g)}+6H_{2}O_{(g)}$

2) Count each bond of each molecule from both sides, including double or triple bonding, if they exists:

Reactants:

$C_{5}H_{12}$ : 12 C-H

            4 C-C

O₂ : 1 O=O

Products:

CO₂ : 2 C=O

H₂O : 2 H-O

3) From the balanced reaction, we there are 8 moles of oxygen gas, 5 moles of carbon dioxide and 6 moles of water, which has to be included when adding each side.

$\Sigma H_{reactants}=$ 12.257 + 4.370 + 8.490 = 8484

$\Sigma H_{products}=$ 5.2.740 + 6.2.478 = 13136

4) Calculate Enthalpy of Reaction by:

$\Delta H_{rxn}= \Sigma H_{reactants}-\Sigma H_{products}$

$\Delta H_{rxn}=8484-13136$

$\Delta H_{rxn}=-4652$

The enthalpy of reaction for the combustion of 1 mole of pentane on a just discovered imaginary planet is an exothermic process of magnitude $\Delta H_{rxn}=$ - 4652kJ/mol