Communicating Changes in Enthalpy
Objective
lesson 3
chapter 9
You will be able to
· identify the type of reaction based on the enthalpy change.
· Correctly indicate the change in enthalpy in 4 ways.
For a balanced equation you can include the change in
enthalpy as a separate value.
For example,
CO (g) + 2H (g) ➔ CH OH (l) ∆H=-128kJ
2
3
Notice this is an exothermic reaction, indicated by the
negative sign.
Example: Write the change in enthalpy if the following
reaction absorbs 136 kJ of energy.
C H (g) ➔ C H (g) + H (g)
2 6
2 4
2
Molar Enthalpy
Example: given the reaction, ∆H=-128 kJ
CO (g) + 2H2 (g) ➔CH3OH (l)
What is the molar enthalpy for hydrogen gas?
∆H=n∆rH
Example
What is the molar enthalpy for oxygen, given the
following reaction?
Al2O3(s) ➔2Al(s) + (3/2)O2 (g)
∆H=+1676 kJ
Exothermic reactions with enthalpy change in the equation
Reactants ➔Products + energy
CO (g) + 2H2 (g)➔CH3OH (l)+128 kJ
Endothermic reactions
Reactants + energy➔Products
Al2O3(s) +1676 kJ➔2Al(s) + (3/2)O2 (g)
Example
Write the thermochemical reactions including the energy in
the equation for the following.
1. The combustion of methane releases 802.5 kJ
2. The reaction of water with carbon dioxide produces
oxygen and glucose in photosynthesis. This process requires
2802.5 kJ of energy from the sun.
Potential Energy Diagrams
Exothermic
Reactants ➔Products + energy
How would endothermic be different?
Summary
Enthalpy change
· ∆H
· Units = kJ
· Used in balanced equations
Molar Enthalpy
· ∆rH
· Units = kJ/mol
· Used for a specific reactant or product
Change included in the balanced equation
· ∆H (kJ) included as either a reactant or product.
· Absolute value used, so no negative values!
Potential Energy diagrams
· Reactants on one line, products on another.
· ∆H is the difference between energy levels.
Assignment
Read p.332-350
Do p. 337 #1-4, p. 350 #1-7