Download Enthalpy - Mr. Rowley

Unit 04 - Heat
Basic Info:
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
All chemical reactions involve energy changes, whether energy
is being absorbed or given off.
Where does this energy come from?
 All this energy is stored in chemical bonds. The bonds within a molecule
holding the atoms together are called intramolecular bonds.
 The bonds holding molecules to each other are called intermolecular
bonds.

When bonds are broken, energy is absorbed.
 This would be an “endothermic” process as energy needs to be put in to
break the bonds apart

When bonds are formed, energy is released.
 This would be an “exothermic” process as energy is given off when the
compound becomes more stable after a bond forms
Enthalpy
Thermodynamics is the study of heat. Specifically, we
are studying the energy changes during chemical
reactions.
 Enthalpy (H˚) is the heat content in a system, or, the
total amount of potential and kinetic energy within a
substance.
 This is energy, so it is measured in joules (J).
 Note: The ˚ symbol indicates a temperature of 25˚C
and 101 kPa, which is standard temperature and
pressure in thermodynamics (STP).

Heat of Reaction
The enthalpy of a system then comes from both the
bonds holding the substance together, and the motion
of the particles within the system.
 Although we cannot measure the enthalpy of a system,
we can calculate the enthalpy changes during a
reaction (ΔH˚).
 We also call this change in enthalpy the heat of
reaction.

Exothermic Reactions:
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Reactions in which there is a net release of enthalpy.
When enthalpy is released, an energy term will appear on the
product side of the equation.
i.e.: The thermite reaction is a highly exothermic reaction
between iron (III) oxide and aluminum, producing aluminum
oxide and molten iron:
Fe2O3 (s) + 2Al(s) → Al2O3(s)+ 2Fe(l)+ 847.6 kJ

If you had this reaction occurring in your hand (don't try this at
home!!!) your hand would feel warmer because the reaction is
releasing heat.
Enthalpy and Exothermic Reactions

Recall that an exothermic reaction releases more energy to
form the bonds in the products than it did to break the
bonds in the reactants.

Ex) Cu(s) + Cl2(g)  CuCl2(g) + 220.1 kJ
Exothermic reactions release heat because the reactants
have a higher heat content (enthalpy).
 The heat released is the ‘excess’ heat. Since CuCl2 has a
lower heat content, the extra heat is released to the
surroundings.

Enthalpy and Exothermic Reactions

Graphically, we can represent the potential energy for the
reactants and products like:
Enthalpy and Exothermic Reactions
The graph shows that the amount of potential energy in
the reactants is more than the potential energy in the
products.
 This is like rolling a boulder down a hill. As the boulder
goes down, it loses potential energy.
 The drop in potential energy is the heat released, or the
heat of reaction (ΔH). The value is negative since heat
is being released.

Enthalpy and Exothermic Reactions

We can then remove the energy from the equation and
record it as a change in enthalpy:

Cu(s) + Cl2(g)  CuCl2(g)

So, the heat of reaction above is -220.1 kJ. We
describe the reaction by saying that in the formation of
1 mole of CuCl2 from 1 mole of Cu and 1 mole of Cl2,
220.1 kJ was released.
ΔH˚ = -220.1 kJ
Endothermic Reactions:


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Reactions that require a net input of enthalpy.
This is indicated by writing the enthalpy term on the reactant
side of the equation.
i.e.: Consider the reaction when sulfur trioxide decomposes into
sulfur trioxide and oxygen. This reaction requires a net input of
198 kJ of energy in order to occur:
2 SO3(g) + 198 kJ → 2 SO2(g) + O2(g)

If you held this reaction in your hand, your hand would feel
colder because the energy required for the reaction to occur
would be taken from the surroundings, in this case your hand.
Enthalpy and Endothermic Reactions

Likewise, an endothermic reaction requires the addition
of energy. This ‘excess’ energy is stored as potential
energy in the bonds of the products.

Ex) H2O(g) + C(s) + 132 kJ  CO2(g) + 2H2(g)

Here, the potential energy is lower in the reactants.
Therefore, the enthalpy is increased during the
reaction.
Enthalpy and Endothermic Reactions
Enthalpy and Endothermic Reactions
This is like pushing a boulder up a hill… it takes
energy.
 We can now remove the energy from the equation and
record it as a change in enthalpy.


H2O(g) + C(s)  CO2(g) + 2H2(g)
ΔH˚ = +132 kJ