Day # 20 march 18
CONCEPT: Reaction types: Redox
TOOLS:
1.
2.
3.
4.
5.
6.
key definitions – already covered in CH 3, but review quickly
assigning oxidation numbers – already covered in CH 3, refresh your memory
balancing equations
calculating free energy from cell potential
calculating free energy from cell potentials under non-ideal conditions
periodic trends and periodic relations – extend your chemical knowledge
Go back to the beginning of the semester. Do you remember the different types of
chemical reactions?
Acid/base – Lewis and Bronsted
Solid dissolving
Salt solutions precipitating
Redox reactions
Radical reactions
Take a moment to think about whether you can recall what each type of reaction
represents and how to recognize each type of reaction.
Basic tools for reactivity:
Balancing reactions
Stoichiometry
Unit conversions
Free energy – tells you driving force for reaction (under standard and non-standard
conditions)
Enthalpy – tells you heat released during a reaction
Entropy – helps understand why reactions have the free energy that they have
Equilibrium – enables you to predict the concentrations of reactants and products once
the reaction has reached its lowest energy state and all the free energy from the reaction
has been exhausted.
We have not talked about how to measure free energy, with good reason. For many types
of reactions, it is quite hard. We’re now going to focus on a class of reactions where
measuring the free energy change as reactants are converted to products is relatively easy.
This class of reactions is redox reactions and since these reactions involve the transfer of
electrons, we can often take advantage of the tools of electromagnetism to measure free
energies.
Redox reactions are also the defining class of reactions of biogeochemical cycles.
Put up the sulfur cycle and define key terms
Example of important sulfur reaction:
Metals too tend to undergo spontaneous reactions with acids:
Bring in some Ca, Zn, Mg, Cu and acid.
Begin by reviewing how to balance redox reactions
H+ + Zn  Zn2+ + H2
1.
2.
3.
4.
5.
6.
7.
identify each half reaction
balance all atoms except O and H
balance Os with water
balance H with H+
if in base, add OH- to both sides to neutralize H+
id number of electrons involved in each half reaction
multiply by appropriate coefficients so that both half reactions have same number
of electrons
8. combine half reactions back together
9. check work.
A few more examples.
Work problems 17: 3, 4, 5, 6, 7 in text