Balancing Redox Reactions Using the Half-Reaction Method (11.1.3) Flashcards
• In the half-reaction method of balancing redox reactions, the half-reactions are balanced separately for redox atoms, electrons, oxygen atoms and hydrogen atoms.
• In the half-reaction method of balancing redox reactions, the half-reactions are balanced separately for redox atoms, electrons, oxygen atoms and hydrogen atoms.
• The half-reactions are combined when the number of electrons transferred are made equal.
• The half-reactions are combined when the number of electrons transferred are made equal.
In the half-reaction method of balancing redox
reactions, the half-reactions are balanced
separately for redox atoms, electrons, oxygen
atoms and hydrogen atoms.
The example begins with identifying the halfreactions
and assigning oxidation numbers to the
redox atoms. The values shown are per atom and
the values for oxygen atoms are not shown. Next,
the redox atoms are balanced and each of the halfreactions
are balanced for charge by adding
electrons. Finally, oxygen atoms are balanced for a
reaction in acid by adding H2O molecules and
hydrogen atoms are balanced by adding sufficient
H+
ions. To obtain the half-equations for a reaction
in a base, oxygen atoms are balanced by adding
OH–
ions and hydrogen atoms by adding H2O
molecules.
The half-reactions are combined when the number
of electrons transferred are made equal.
In the example, the number of electrons in each of
the half-reactions are made equal by multiplying the
entire half-equation by the lowest common factors
(4 and 3, in this case). The half-reactions are now
added together and species present on both sides
of the resulting chemical equation are cancelled to
yield the simplest expression of the chemical
equation.
In the half-reaction method of balancing redox
reactions, the half-reactions are balanced
separately for redox atoms, electrons, oxygen
atoms and hydrogen atoms.
The example begins with identifying the halfreactions
and assigning oxidation numbers to the
redox atoms. The values shown are per atom and
the values for oxygen atoms are not shown. Next,
the redox atoms are balanced and each of the halfreactions
are balanced for charge by adding
electrons. Finally, oxygen atoms are balanced for a
reaction in acid by adding H2O molecules and
hydrogen atoms are balanced by adding sufficient
H+
ions. To obtain the half-equations for a reaction
in a base, oxygen atoms are balanced by adding
OH–
ions and hydrogen atoms by adding H2O
molecules.
The half-reactions are combined when the number
of electrons transferred are made equal.
In the example, the number of electrons in each of
the half-reactions are made equal by multiplying the
entire half-equation by the lowest common factors
(4 and 3, in this case). The half-reactions are now
added together and species present on both sides
of the resulting chemical equation are cancelled to
yield the simplest expression of the chemical
equation.
How does the half-reaction (or half-cell) balancing method differ from the oxidation number method?
The half-reaction (or half-cell) method isolates the two half-reactions (i.e., the oxidation step and the reduction step) that, together, comprise a redox reaction. (C)
It is simply a different way of doing the same thing as the oxidation number method. You approach it by concentrating on the two half-reactions: the reduction step and the oxidation step.
Which of the following correctly describes the two half-reactions in the redox reaction below?
CuS(s) + NO3− (aq) → Cu2+ (aq) + SO42− (aq) + NO(g)
CuS(s)→ Cu2+ (aq) + SO42− + 8e − is the oxidation step.
NO3− (aq) + 3e −→ NO(g) is the reduction step. (C)
Which statement about the following redox reaction in acidic solution is not correct?
MnO4− (aq) + I − (aq) → MnO2(s) + I2(aq)
The redox atoms are already balanced in this reaction. (D)
Which statement about balancing this simple redox reaction using the half-reaction method is correct?
Cu(s) + Ag+1 (aq) → Cu+2 (aq) + Ag(s)
At the end of the balancing method, the total number of electron transfers for each of the two half-reactions is the same. (D)
How does assigning oxidation states in compounds help you balance redox reactions?
The oxidation state tells you about an atom or molecule’s potential to accept or donate electrons. (B)
The six steps for balancing a redox reaction by the half-reaction method are listed in random order below:
Balance the oxygen atoms with H2O and H + or H2O and OH − (depending on whether the solution is acidic or basic).
Assign oxidation numbers to the redox atoms.
Add the half-reactions.
Balance the electrons for each of the half-reactions.
Balance the redox atoms.
Write the half-reactions.
Which of the following shows the correct order for the half-reaction method of balancing redox reactions?
f, b, e, d, a, c (D)
Look at the unbalanced redox reaction (in an acidic solution).
S2O32− (aq) + I2(aq) → SO42− (aq) + I − (aq)
Which of the following is not part of the final balanced reaction?
10H2O(l) (A)
You need 5 H2O(l) molecules, not 10.
The correct balanced equation is:
5H2O(l) + S2O32− (aq) + 4I2(aq) → 2SO42− (aq) + 8I − (aq) + 10H + (aq)
Suppose that you have to balance the following reaction in basic solution.
NH3(aq) + ClO− (aq) → N2H4(aq) + Cl − (aq)
Which of the following is not a step you would take to balance this reaction?
After balancing the oxygen atoms, multiply one of the half-reactions by 2 to balance electrons. (D)
Suppose that you have to balance the following reaction in basic solution.
S2− (aq) + CrO42− (aq) → S(s) + Cr(OH)3(s)
Which of the following is not a step you would take to balance this reaction?
Add 10OH − (aq) to one side of a half-reaction. (C)
