HS Chemistry - Heat in Reactions

Enthalpy

Overview of The Page

This page will cover:

• What is enthalpy? What is the enthalpy change of a chemical reaction?
• How is the enthalpy change of a chemical reaction calculated?

Let's take a reaction:

H + Cl → HCl

In this reaction, one Hydrogen atom bonds with one Chlorine atom to form one molecule of Hydrochloric acid $HCl$.

When the Hydrogen atom bonds with the Chlorine atom, both atoms achieve a full valence shell. After that, they don't bond with other atoms, because their valence shells are full.

Hydrogen needs one more electron to fill its valence shell. Chlorine needs one more electron to fill its valence shell. Both atoms will react with other atoms to fill their valence shell, as atoms with a full valence shell are more stable.

The Hydrogen and Chlorine atoms will react with one another to form a HCl molecule. Both atoms now have full valence shells, so they won't react with other atoms. They're more stable this way.

The Hydrogen and Chlorine atom are less reactive when bonded together than when they're separate. They're more stable, and therefore have less energy, when they're bonded together than when they're separate atoms.

This holds true for any group of atoms that form a molecule. They have less energy when bonded together as a molecule than when they're separate atoms.

As shown above, in this reaction, any Hydrogen atom and any Chlorine atom must be either separate or bonded to each other; the same atom can't be both separate and bonded to another atom. We are observing changes to a system.

Before the system changes, the atoms are separate. Each atom has a certain amount of energy. The energies of all the separate parts of the system combined will give the system's total energy, or the system's enthalpy. The enthalpy of a system is the total energy of the system.

So the system has a certain enthalpy before it changes $let's call it Enthalpy 1$, and a certain, different enthalpy after it changes $let's call it Enthalpy 2$.

The system has a greater enthalpy before the reaction than it does afterwards, because atoms have less energy when bonded together in a molecule than when they're separate. So there is a decrease in enthalpy down the reaction pathway, from the reactants to the products. This change in enthalpy between the reactants and products is called the enthalpy change, represented by ∆H, and its unit is Joules $J$ or kiloJoules $kJ$. We experience it as heat energy.

A decrease in enthalpy in a system is shown as a negative ∆H value, while an increase in enthalpy in a system is shown as a positive ∆H value. A quick way to calculate enthalpy change would be to subtract the enthalpy of the system before the change from the enthalpy of the system after the change $Enthalpy 2 - Enthalpy 1 = Enthalpy Change$.

The energy isn't "destroyed" in this reaction, where there is a decrease in enthalpy down the reaction pathway. According to the 1^st law of thermodynamics, energy can't be created or destroyed - it can only be converted from one form to another. The energy is converted into heat energy and released to the surroundings. Similarly, in reactions where there is an increase in enthalpy down the reaction pathway, the energy isn't "created", it's just absorbed from the surroundings as heat energy.

We could write all this information down. Or we could show it in a two-dimensional graph $**a potential energy diagram**$.

And this graph shows just what we need it to show - it shows the enthalpy of the reactants, the enthalpy of the products, the enthalpy change in the system, the direction of the reaction pathway, and it tells us whether there was an increase or a decrease in enthalpy $in this case, a decrease$.

If we want, we can add some more information to the graph.

But the first one works fine as well.

Calculating Enthalpy Change

How do we calculate the enthalpy change in a system, though? To do that, we need to know the bond energies of the reactants and products $the energy stored in the bonds between the atoms of the molecules$. This is the amount of energy that the molecule needs to absorb in order to be broken into separate atoms, and it is also the amount of energy that is released when the atoms form that molecule. Those two energies are the exact same, and that is the bond energy of the molecule.

To find the enthalpy change of the system, we subtract the final enthalpy from the initial enthalpy. That comes out to:

Enthalpy Change (∆H) = New Enthalpy - Original Enthalpy

OR

Enthalpy Change = Enthalpy of Products - Enthalpy of Reactants

In our example reaction, H + Cl → HCl, the enthalpy change is:

Enthalpy of Products: HCl

• 1 × -431 = -431

Enthalpy of Reactants: H + Cl

• 0 + 0 = 0

Enthalpy Change: Enthalpy of Products - Enthalpy of Reactants = -431 - 0 = -431 kJ

431 kJ of heat is released, or evolved, in this reaction.