Chem110 – Class Guide: Redox, Concentrations (Chapter 4, Sections 4 and 5)
Chapter 4, Sections 4 & 5 Learning Goals:
Upon completion of Chapter 4, Sections 4 and 5, you should be able to:
Given a balanced equation, identify if it is an exchange, acid/base, or redox reaction.
Given a balanced equation, write the oxidation state for each element.
Given a balanced equation, determine which substance is oxidized, which is reduced, which
substance is the oxidizing agent, and which is the reducing agent.
Define the concentration unit of molarity, and be able to describe the preparation of solutions
of specified molarity. These solutions may involve the concentration of ions in aqueous
solutions.
Given a concentrated solution, calculate either the amount of solution necessary to make a
dilute solution or the molarity of the dilute solution.
Chapter Reading Guide: Chapter 4, Section 4 and 5
Section 4: OXIDATION-REDUCTION (REDOX) REACTIONS:
You can skip “Oxidation of Metals by Acids and Salts” and “The Activity Series” in this
section.
Oxidation Numbers – every element in a compound can be assigned an oxidation number.
Apply the rules found on page 132 in your textbook for assigning oxidation numbers.
When you think you have a good understanding of oxidation numbers, try this interactive
exercise on oxidation numbers.
(http://www.berks.psu.edu/clt/OxidationNumbers2.swf)
Try Practice Exercise 4.8.
Look carefully at Reaction 4.25 below (I know it’s in a section I told you to skip. I just needed
an example of a redox reaction from your text.)
Mg(s) + 2 HCl (aq)  MgCl2 (aq) + H2 (g)
0
+1 -1
+2 -1
0
Once you have the correct oxidation numbers for each element, you cab determine which element
is oxidized (the one whose number increased), and which element is reduced (the one whose
number decreased).
In this example, Mg is oxidized (went from 0 to +2, it lost 2 electrons) and H is reduced (went from
+1 to 0, it gained 1 electron). Now try Practice Exercise 4.9.
CONCENTRATIONS OF SOLUTIONS
Watch this movie: Solution Formation from Solid
(http://www.berks.psu.edu/clt/SolFormationFromSolid.mov) for making 250mL of a 1.0M
Copper Sulfate solution. Pay close attention to the words the narrator uses.
Listen to how she pronounces “250mL of a 1.0M Copper Sulfate Solution”. By learning to
talk like a chemist, you’re learning to think like a chemist.
Molarity is the favorite concentration unit of chemists!
molarity (M) =
moles of solute
liters of solution
For now, we can assume liters of solve = liters of solution. This will change when we get to
Chapter 13 though!
Try practice exercises 4.11 and 4.12 to calculate molarity. Remember that the molarity of
ions can be different than the molarity of the solution!
When using molarity in dimensional analysis (for example, to convert between molarity,
moles, and volume), it’s better to write mol/L, rather than M. That way, you can still cancel
your units! Look at Sample Exercise 4.13 and then try Practice Exercise 4.13.
Sometimes, we need to dilute solutions – start with a higher concentration, and add solvent
to get to a weaker concentration. The moles of solute won’t change, but the volume of
solution will, so the new concentration will be lower.
To calculate either a new concentration or volume after dilution, use: M1V1 = M2V2; where
M = molarity and V = volume. Try practice exercise 4.14.
Learning Resources
Chapter Learning Goals
Chapter 4, Sections 4 and 5 Learning Goals
Interactive Exercises
Oxidation Numbers
(http://www.berks.psu.edu/clt/OxidationNumbers2.swf)
Additional Chapter Resources
Movie: Solution Formation from Solids
(http://www.berks.psu.edu/clt/SolFormationFromSolid.mov)
Pre Class Assignment: This assignment must be completed prior to the next class. Check your syllabus
for the exact due date and time.
Connect to the pre class assignment found here: (berks.psu.edu/clt/chem110/Redox_HW.doc)
Submit a copy to the dropbox located in ANGEL called “Pre Class Assignment Submission: Redox
and Concentrations”
End of Chapter Problems:
Practice with these problems if you are having difficulty with any of the concepts covered in this
class guide AFTER we have met in class. If you cannot easily complete these problems, seek
help from your instructor, your mentor, or the learning center.
Chapter 4: 49, 51, 61, 63, 69, 73, 75, 96