Understanding Redox Reactions

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Questions and Answers

In a redox reaction, if a species is reduced, what occurs in terms of electron transfer and what happens to its oxidation number?

  • It loses electrons, and the oxidation number decreases.
  • It gains electrons, and the oxidation number decreases. (correct)
  • It loses electrons, and the oxidation number increases.
  • It gains electrons, and the oxidation number increases.

Consider the polyatomic ion $SO_4^{2-}$. What is the overall oxidation number of this ion, and what principle dictates this value?

  • -8, which is the total oxidation number of the four oxygen atoms.
  • -2, because the overall oxidation number of a polyatomic ion equals its charge. (correct)
  • +2, because the sum of oxidation numbers of oxygen must be positive.
  • 0, because the sum of oxidation numbers in a polyatomic ion is always zero.

During a redox reaction, a substance undergoes oxidation. Which of the following statements accurately describes what happens to this substance?

  • It gains electrons and its oxidation number decreases.
  • It loses electrons and its oxidation number increases. (correct)
  • It gains protons and its oxidation number remains constant.
  • It donates protons and its oxidation number decreases.

When balancing redox half-equations, why is it crucial to balance the oxygen atoms by adding $H_2O$ to the appropriate side of the equation?

<p>To maintain mass balance by accounting for oxygen atoms present in the reactants or products. (D)</p> Signup and view all the answers

In the compound $OF_2$, what is the oxidation number of oxygen, and why does it differ from its usual oxidation number?

<p>+2, because fluorine is more electronegative than oxygen. (B)</p> Signup and view all the answers

What is the primary reason for balancing redox equations, beyond simply ensuring the same number of atoms of each element on both sides?

<p>To ensure that the total charge is balanced, reflecting the conservation of charge. (A)</p> Signup and view all the answers

Consider a redox reaction where a metal atom, M, is oxidized to $M^{2+}$. What specific changes occur at the atomic level during this oxidation process?

<p>The metal atom loses two electrons, increasing its oxidation number. (B)</p> Signup and view all the answers

If a species is described as being reduced in a redox reaction, what directly measurable effect does it have on another species involved in the same reaction?

<p>It causes the other species to undergo oxidation, thus increasing its oxidation number. (D)</p> Signup and view all the answers

For a compound such as $H_2O$, how do you confirm that the sum of the oxidation numbers of all atoms equals zero, and what does this principle indicate about charge distribution?

<p>By assigning hydrogen a +1 and oxygen a -2, their sum confirms the molecule's neutrality, indicating balanced charge distribution. (D)</p> Signup and view all the answers

What is the critical role of balancing 'charge with e-' in the final step of balancing redox half-equations, and what chemical reality does it ensure?

<p>It mandates that the total number of electrons lost in oxidation equals the total number gained in reduction, upholding electron conservation. (A)</p> Signup and view all the answers

In balancing redox reactions, why is it necessary to balance hydrogen atoms ($H$) by adding $H^+$ ions, and what specific conditions does this assumption typically imply about the reaction environment?

<p>To balance the atoms assuming an acidic medium. (A)</p> Signup and view all the answers

In an electrolytic process, if a metal ion $M^{3+}$ gains three electrons to deposit as a solid metal on the cathode, how does this reduction affect the local electrochemical environment?

<p>It promotes a local reduction, decreasing the concentration of positively charged ions near the cathode. (C)</p> Signup and view all the answers

Consider a scenario where hydrogen is bonded to a metal, such as in sodium hydride ($NaH$). How does this affect the oxidation number of hydrogen, and why?

<p>Hydrogen's oxidation number becomes -1 because it is more electronegative than sodium. (A)</p> Signup and view all the answers

Given the unbalanced redox reaction: $MnO_4^- + Fe^{2+} ightarrow Mn^{2+} + Fe^{3+}$ in an acidic solution, what is the stoichiometric coefficient for $Fe^{2+}$ once the equation is fully balanced?

<p>5 (C)</p> Signup and view all the answers

If chlorine gas ($Cl_2$) is bubbled into a solution containing bromide ions ($Br^−$), what observable change would indicate a redox reaction has occurred, and what is the underlying chemical process?

<p>The solution darkens as bromine ($Br_2$) is formed, indicating chlorine has oxidized bromide ions. (C)</p> Signup and view all the answers

In the context of oxidation numbers, how does the concept of electronegativity govern the assignment of oxidation states in covalent compounds?

<p>The more electronegative atom is assigned a negative oxidation state, assuming it gains electrons. (A)</p> Signup and view all the answers

What is the implication of the statement 'the species with the e- is getting oxidized' in terms of electron availability during a chemical reaction?

<p>The species donates electrons, enabling it to reduce other species in close proximity. (D)</p> Signup and view all the answers

Given a voltaic cell constructed with zinc and copper electrodes, predict what occurs at the zinc electrode as the voltaic cell discharges, and justify your prediction based on redox principles.

<p>Zinc is oxidized, releasing electrons and dissolving into the solution as $Zn^{2+}$ ions, causing the electrode to corrode. (C)</p> Signup and view all the answers

Consider a galvanic cell where the half-reactions involve complex ion formation. How does the formation of a stable complex ion affect the reduction potential of a metal ion in the half-cell?

<p>It decreases the reduction potential, as the concentration of free metal ions is lowered. (D)</p> Signup and view all the answers

Flashcards

Oxidation

Loss of electrons during a reaction.

Reduction

Gain of electrons during a reaction.

Oxidation Number

The charge an atom would have if all bonds were ionic.

Oxidation number of elements

Zero.

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Oxidation number of monatomic ions

Equals the charge of the ion.

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Oxidation number of Hydrogen

+1 for hydrogen when bonded to non-metals, -1 when bonded to metals.

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Oxidation number of Oxygen

-2 unless bonded to Fluorine or itself.

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Oxidation number for polyatomic molecules

Zero.

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Oxidation number in a polyatomic ion

Charge on the ion.

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Balancing Redox Half Equations

Balance the number of atoms and charges in a redox reaction.

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Identifying oxidized species

The one gaining a negative charge.

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Identifying reduced species

The one losing a negative charge.

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Study Notes

Redox Reactions

  • Oxidation involves gaining oxygen, losing hydrogen ions, losing electrons, and an increasing oxidation number.
  • Reduction involves losing oxygen, gaining hydrogen ions, gaining electrons, and a decreasing oxidation number.
  • "OIL RIG" is a mnemonic: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).

Oxidation Number

  • All elements have an oxidation number of 0 (e.g., Na, H2, N2).
  • The oxidation number of monoatomic ions equals the charge of the ion (e.g., Li+, Mg2+, O2-).
  • Hydrogen has an oxidation number of +1, but is -1 when joined to a metal.
  • Oxygen has an oxidation number of -2, unless attached to a more electronegative atom.
  • The oxidation number for a polyatomic molecule is 0 (e.g., SO3, H2SO4, C2H5OH).
  • The oxidation number in a polyatomic ion equals the charge on the ion.

Balancing Redox Half Equations

  • Determine formulas of all species.
  • Balance all species.
  • Balance oxygen atoms by adding Hâ‚‚O.
  • Balance hydrogen atoms by adding H+.
  • Balance charge by adding e-.

Oxidation Number Examples

  • For MnO4-1:
    • Oxidation number of O4 = (-2 x 4) = -8
    • Mn + (-8) = -1
    • Mn = +7
  • For Cr2O72-:
    • Oxidation number of O7 = (-2 x 7) = -14
    • Cr2 + (-14) = -2
    • Cr2 = +12
    • Cr = +6

Electron Transfer Rule

  • The species gaining electrons is getting oxidized.
  • The species losing electrons is getting reduced.

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