Unit 6: Oxidation & Reduction
Defining the fundamental concepts of electron transfer and oxidation states.
6.1 Definitions (Electron Transfer, O/H Gain/Loss)
Redox is a shorthand term for reduction-oxidation. These two processes always occur simultaneously in a chemical reaction. There are several ways to define them.
In Terms of Electron Transfer (Most General):
- Oxidation Is Loss of electrons. A species that loses electrons is said to be oxidised. (Mnemonic: OIL)
- Reduction Is Gain of electrons. A species that gains electrons is said to be reduced. (Mnemonic: RIG)
Example: In the reaction $2Na + Cl_2 \rightarrow 2NaCl$,
- Sodium is oxidised: $Na \rightarrow Na^+ + e^-$
- Chlorine is reduced: $Cl_2 + 2e^- \rightarrow 2Cl^-$
In Terms of Oxygen and Hydrogen (Common in Organic Chemistry):
- Oxidation is the gain of oxygen or the loss of hydrogen.
- Reduction is the loss of oxygen or the gain of hydrogen.
Example: The oxidation of ethanol to ethanoic acid:
$CH_3CH_2OH \xrightarrow{[O]} CH_3COOH$
Here, ethanol gains an oxygen atom and loses hydrogen atoms.
Solved Examples:
- In the reaction $Zn + Cu^{2+} \rightarrow Zn^{2+} + Cu$, which species is oxidised?
Solution: Zinc ($Zn$) is oxidised because it loses two electrons to become $Zn^{2+}$. - Define reduction in terms of electron transfer.
Solution: Reduction is the gain of electrons. - When iron rusts, $Fe \rightarrow Fe^{3+}$, is this oxidation or reduction?
Solution: Oxidation, because the iron atom loses three electrons. - The conversion of an aldehyde ($R-CHO$) to an alcohol ($R-CH_2OH$) involves the gain of hydrogen. Is this oxidation or reduction?
Solution: Reduction. - Identify the process: $2Br^- \rightarrow Br_2 + 2e^-$.
Solution: Oxidation, as electrons are lost. - When magnesium burns in air ($2Mg + O_2 \rightarrow 2MgO$), what happens to the oxygen in terms of electrons?
Solution: The oxygen molecule gains electrons to form oxide ions ($O^{2-}$), so it is reduced. - Use the OIL RIG mnemonic to define oxidation.
Solution: OIL stands for Oxidation Is Loss (of electrons). - Is the reaction $H_2 + F_2 \rightarrow 2HF$ a redox reaction? Explain.
Solution: Yes. Hydrogen is oxidised (loses an electron to fluorine) and fluorine is reduced (gains an electron from hydrogen). - When copper oxide ($CuO$) is converted to copper ($Cu$), has it been oxidised or reduced?
Solution: Reduced, because it has lost oxygen. - In the half-reaction $Fe^{3+} + e^- \rightarrow Fe^{2+}$, what process is occurring?
Solution: Reduction, as the iron(III) ion is gaining an electron.
6.2 Oxidation Numbers
To track electron transfer in all compounds (including covalent ones), we use the concept of oxidation numbers (or oxidation states). The oxidation number is the hypothetical charge an atom would have if all its bonds to different elements were 100% ionic.
Rules for Assigning Oxidation Numbers:
- The oxidation number of an atom in its elemental form (e.g., $Na$, $O_2$, $S_8$) is 0.
- The oxidation number of a simple monatomic ion (e.g., $Na^+$, $Cl^-$) is equal to its charge.
- In compounds, the oxidation number of fluorine is always -1.
- In most compounds, the oxidation number of oxygen is -2 (exceptions: peroxides like $H_2O_2$ where it is -1, and in $OF_2$ where it is +2).
- In most compounds, the oxidation number of hydrogen is +1 (exception: metal hydrides like $NaH$ where it is -1).
- The sum of the oxidation numbers in a neutral compound is 0.
- The sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion.
In a redox reaction:
- Oxidation is an increase in oxidation number.
- Reduction is a decrease in oxidation number.
Solved Examples:
- What is the oxidation number of sulphur in sulphuric acid, $H_2SO_4$?
Solution: H is +1, O is -2. Let S be x. 2(+1) + x + 4(-2) = 0. 2 + x - 8 = 0. So, x = +6. - What is the oxidation number of manganese in the permanganate ion, $MnO_4^-$?
Solution: O is -2. Let Mn be x. x + 4(-2) = -1. x - 8 = -1. So, x = +7. - What is the oxidation number of chlorine in $Cl_2$?
Solution: 0, because it is an element. - Find the oxidation number of chromium in the dichromate ion, $Cr_2O_7^{2-}$.
Solution: O is -2. Let Cr be x. 2x + 7(-2) = -2. 2x - 14 = -2. 2x = 12. So, x = +6. - In the reaction $2FeCl_2 + Cl_2 \rightarrow 2FeCl_3$, what happens to the oxidation number of iron?
Solution: In $FeCl_2$, Fe is +2. In $FeCl_3$, Fe is +3. The oxidation number increases from +2 to +3, so iron is oxidised. - What is the oxidation number of oxygen in hydrogen peroxide, $H_2O_2$?
Solution: -1. This is an exception to the usual -2 rule for oxygen. - What is the oxidation number of hydrogen in sodium hydride, $NaH$?
Solution: -1. This is an exception as it is bonded to a metal. - Determine the oxidation number of nitrogen in the nitrate ion, $NO_3^-$.
Solution: O is -2. Let N be x. x + 3(-2) = -1. x - 6 = -1. So, x = +5. - Has sulphur been oxidised or reduced in the change from $SO_2$ to $SO_3$?
Solution: In $SO_2$, S is +4. In $SO_3$, S is +6. The oxidation number has increased, so it has been oxidised. - What is the oxidation number of carbon in the carbonate ion, $CO_3^{2-}$?
Solution: O is -2. Let C be x. x + 3(-2) = -2. x - 6 = -2. So, x = +4.
Knowledge Check (20 Questions)
Answer: Electrons.
Answer: 0.
Answer: +6.
Answer: Oxidation.
Answer: -3.
Answer: Electrons.
Answer: Reduced.
Answer: 0.
Answer: +7.
Answer: Gain.
Answer: +1.
Answer: A decrease.
Answer: +2.
Answer: Oxidised (its oxidation number changes from -4 in $CH_4$ to +4 in $CO_2$).
Answer: -2.
Answer: +3.
Answer: Gain.
Answer: True.
Answer: 0.
Answer: Oxidised.