Chapter 9.4 - Bond Energy & Chemical Change

What does the heat emitted or absorbed from a chemical reaction depend on?

The differences between the reactant and product bond energies

What are the most important factors with kinetic energy?

A molecule’s movements through space and their rotations and vibrations

How does kinetic energy relate to temperature?

It’s proportional to temperature

What are the most important factors with potential energy?

Phase changes, changes in the attraction between vibrating atoms, between the nucleus and electrons in each atom, between the protons and neutrons in each nucleus, and between nuclei and the shared electron pair in each bond

What is the two step process for forming a chemical compound, as is allowed by Hess’s law?

(1) an amount of heat is absorbed (positive) to break the reactant bonds and form separate atoms (2) an amount of heat is emitted (negative) when the atoms form product bonds

How do you get the enthalpy of reaction from the two step process permitted by Hess’s law?

The enthalpy of reaction is the sum of the two steps in the process

Which is greater in an exothermic reaction: the reactant bonds breaking or the product bonds forming?

The product bonds forming

Which is greater in an endothermic reaction, the reactant bonds breaking or the product bonds forming?

The reactant bonds breaking

What is the method for calculating the enthalpy change of the reaction?

(1) break all the bonds on the reactant side (2) use all atoms to form product bonds (3) add the bond energies with appropriate signs to get the final enthalpy change of the reaction

In reality, are some or all of the bonds broken in a chemical reaction?

Some

Will there be any discrepancies between the enthalpy change of the reaction and the enthalpy change of formation?

Yes; slightly

Why are there discrepancies between the enthalpy change of the reaction and the enthalpy change of formation?

Because the enthalpy change of formation is an average of the bond energies for the compound

What is a fuel?

A material that reacts with O₂ to release energy

What are the most common fuels?

Hydrocarbons and coal

Is a combustion reaction exothermic or endothermic?

Exothermic

Why is a combustion reaction exothermic?

Because it takes less energy to break the weaker bonds of the reactants than energy is released when the strong product bonds form

Do fuels with more weak reactant bonds yield more or less energy than fuels with less weak reactant bonds?

More

If there are more bonds to oxygen in a fuel, will it yield more or less energy?

Less

What element to several common explosives contain?

Nitrogen

Why is an explosive highly exothermic?

Because of the strong product bonds that form, which are N₂ and CO