11.1.2 Balancing Redox Reactions by the Oxidation Number Method Flashcards Preview

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Flashcards in 11.1.2 Balancing Redox Reactions by the Oxidation Number Method Deck (11)
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1
Q

Balancing Redox Reactions by the Oxidation Number Method

A
  • Formal charge and oxidation state (or oxidation number) are both models of what is occurring with electron distribution in a molecule or ion.
  • A reaction is balanced by assigning oxidation states, balancing the transferred electrons, and balancing the redox atoms.
  • Balancing atoms of oxygen and hydrogen depends on the solution.
2
Q

note

A
  • Formal charge and oxidation state (or oxidation number) are both models of what is occurring with electron distribution in a molecule or ion.
  • Formal charge reflects the average of the charge on an atom in a molecule. For oxidation number, the electrons are assigned entirely to the more electronegative atom. Oxidation number is a formalism that facilitates the balancing of oxidation-reduction reactions.
  • A reaction is balanced by assigning oxidation states, balancing the transferred electrons, and balancing the redox atoms.
  • In the example, first oxidation numbers are assigned to each redox atom. The values shown are per atom and the values for oxygen atoms are not shown. Next, the redox atoms (Cr and Cl) are balanced, and then the electrons transferred are identified based on the change in oxidation number of each atom.
  • Electrons are balanced by multiplying each half reaction by the lowest common factor (4 and 3), and these coefficients are applied to the redox molecules (by multiplication) to complete the balancing as shown in the box. Only the oxygen atoms remain unbalanced.
  • Balancing atoms of oxygen and hydrogen depends on the solution.
  • In an acidic solution, the oxygen atoms are balanced with H2O molecules and the hydrogen atoms with
    H+ ions. In the example, the left side of the equation has 40 oxygen atoms and the right side has 21 oxygen atoms. Adding 19 H2O molecules (making up the difference) to the right side to balance oxygen atoms. Adding 38 H+ ions to the left side accounts for the added H+ ions.
  • In a basic solution, the oxygen atoms are balanced with 2OH– ions and the hydrogen atoms with H2O molecules. In the example, add 38 OH– ions to the right side to balance the oxygen atoms and 19 H2O molecules to the left side.
3
Q

Which of the following correctly shows the oxidation numbers for each of the elements in H2PO4−, I2, and Al(OH)4−?

A

H2PO4−: H = +1, P = +5, O = −2; I2 : I = 0; Al(OH)4− : Al = +3; O = −2; H = +1

4
Q

Which of the following best describes what a redox reaction is?

A

It is a reaction in which the oxidation state of one element increases and the oxidation state of another element decreases.

5
Q

Which of the following correctly identifies the oxidizing agent and the reducing agent?

3H2S(aq) + 2NO3−(aq) + 2H +(aq) → 3S(s) + 2NO(g) + 4H2O(l)

A

H2S is the reducing agent. NO3− is the oxidizing agent.

6
Q

Which of the following shows the correct oxidation states for iodine, sulfur, and oxygen in the products of this reaction?

10H +(aq) + 8I −(aq) + SO42−(aq) → H2S(aq) + 4I2(aq) + 4H2O(l)

A

I is 0; S is −2; O is −2

7
Q

Which of the following is not a correct rule for determining oxidation states?

A

Any compound’s component species have oxidation states that add up to zero.

8
Q

Look at the reaction between oxalate and iodate ions
5C2O42-(aq) + 2IOe-(aq) + 12H+(aq) -> 10CO2(g) + I2(aq) + 6H2O(l)
Which of the following correctly identifies the oxidizing agent, the reducing agent, the oxidized compound, and the reduced compound

A

IO3− is the oxidizing agent and the reduced compound.

C2O42− is the reducing agent and the oxidized compound

9
Q

Which statement about the following redox reaction in acidic solution is not correct?

H2S(aq) + Cr2O72− (aq) → S(s) + Cr3+ (aq)

A

The final step involves adding oxygen atoms to the left side of the reaction and hydrogen atoms to the right side of the reaction.

10
Q

Which of the following correctly identifies the oxidizing agent, the reducing agent, the oxidized substance, and the reduced substance in the reaction?

6Fe2+(aq) + Cr2O72−(aq) + 14H +(aq) → 6Fe3+(aq) + 2Cr3+(aq) + 7H2O(l)

A

Cr2O72− is the oxidizing agent. Fe2+ is the reducing agent. Fe2+ is the oxidized substance. Cr2O72− is the reduced substance.

11
Q

Which of the following correctly summarizes the electron transfer in the given reaction?

10H +(aq) + 8I −(aq) + SO42−(aq) → H2S(aq) + 4I2(aq) + 4H2O(l)

A

I − → I 0; S6+ → S2−

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