Reaction types and reaction energy
Today‘s topics
Some basic types of chemical reactions
Energy diagrams for chemical reactions
Catalysts
Before we get started!
To repeat some of the most
important terms for this lesson, please complete the intro
questions!
And one more thing I‘ve forgotten
We had talked about the differences between giant covalent structures (such as diamond), that are completely connected through covalent bonds, and molecular solids that contain small covalent molecules that are interconnected
through weak forces of attraction.
Name differences in properties between the two types
Well, let’s have a look at iodine
Let‘s get started!
Please fill in the gaps on the worksheet as we go!
Please fill out the back of the worksheet with your neighbor!
3.1 Decomposition and synthesis
Worksheet 12
Two common types of chemical reactions are decomposition (dt. Analyse) and synthesis (dt.
Synthese) reactions.
Decomposition reactions are defined by one pure substance reacting to form at least two pure
substances. By definition the substance decomposing has to be a compound!
General equation: A B + C
Experiment: Synthesis of iron sulfide
Iron powder and sulfur powder are mixed in a test tube in a 7:4 mass ratio
A magnet is held against the test tube wall A Bunsen burner is used to heat the
contents of the test tube
After cooling a magnet is held against the test tube wall again
Note your observations!
M
O
Synthesis reactions are defined by at least two pure substance reacting to form one pure
substance. By definition the synthesized substance has to be a compound!
General equation: A + B C
3.2 Energy in chemical reactions
Brainstorm with your neighbor (2 mins): Were the two reactions we observed exothermic or
endothermic??
Brainstorm pt. 2 (4 mins): Try to draw an energy diagram for both reactions with the energy
contained in the compounds as the y-Axis that
shows the energy change during the reaction (use scrap paper!)
Brainstorm pt 3. (2 mins): Even the exothermic synthesis did not start immediately, we needed to add energy for it to start. Try to show this in your energy diagram!
3.2 Energy in chemical reactions
Catalysts
Catalysts are substances that lower the activation energy of a reaction. This is done by enabling an
alternate transition state (the state where the reactants are just reacting) with a lower energy.
A catalyst itself is not changed in the reaction, thus it is not a reactant.
The result of catalysts being added to a reaction is the reaction speeding up.
Different reactions need different catalysts.
Living cells contain a wide range of catalysts for
biochemical reactions. These biocatalysts are called enzymes. They are heat sensitive though.
