Redox Reactions
Easy Overview
Every time you charge your phone, start a fire, or watch iron rust, you're looking at redox reactions. Redox is short for reduction-oxidation. One species loses electrons (oxidation), another gains them (reduction). It's the ultimate give-and-take of chemistry. This chapter teaches you how to track those electrons and balance even the messiest reactions.
Oxidation and Reduction — The Classic Dance
Oxidation is loss of electrons. Reduction is gain of electrons. A mnemonic: OIL RIG (Oxidation Is Loss, Reduction Is Gain). But there's also the older definition: oxidation adds oxygen or removes hydrogen, reduction removes oxygen or adds hydrogen. These always happen together — you can't have one without the other.
Oxidation Number — The Electron Scorecard
Oxidation number is a pretend charge an atom would have if all bonds were ionic. It helps you track electrons. Rules: elements in free state = 0; oxygen is almost always -2; hydrogen is +1; sum of oxidation numbers in a compound = 0; in an ion = charge on the ion. Simple rules, but they make everything else work.
Balancing Redox Reactions — The Half-Reaction Method
Split the reaction into two halves — one for oxidation, one for reduction. Balance atoms other than H and O first. Add H₂O to balance oxygen, then H⁺ to balance hydrogen. Add electrons to balance charge. Then multiply each half so electrons cancel out, and add them back together. In basic medium, add OH⁻ to neutralize H⁺.
Electrochemical Cells — Batteries and Beyond
A redox reaction can be used to generate electricity. Put oxidation in one container and reduction in another, connect them with a wire and a salt bridge, and electrons flow through the wire. That's an electrochemical cell. Daniel cell uses Zn (oxidation) and Cu (reduction). The voltage depends on the tendency of metals to lose electrons.
Electrochemical Series — The Popularity Chart
Metals arranged by their tendency to lose electrons form the electrochemical series. Li is at the top (most likely to get oxidized) and Au is at the bottom (least likely). Any metal above can displace any metal below from its salt solution. It also tells you which metal works best in a battery.
Key Points
- •OIL RIG: Oxidation Is Loss, Reduction Is Gain of electrons
- •Oxidation number rules: free element = 0, O = -2, H = +1, sum = 0 for compounds
- •Oxidizing agent: gets reduced (gains electrons); Reducing agent: gets oxidized (loses electrons)
- •Half-reaction method: split, balance atoms, add electrons, combine
- •In acidic medium: use H⁺ and H₂O; in basic medium: use OH⁻ and H₂O
- •Daniel cell: Zn (anode, oxidation) and Cu (cathode, reduction), voltage ≈ 1.1 V
- •Electrochemical series: stronger reducing agents at the top (Li), stronger oxidizing agents at the bottom (F₂)
Practice Questions
- Calculate the oxidation number of Cr in K₂Cr₂O₇ and of Mn in KMnO₄.
- Balance the following redox reaction in acidic medium: Cr₂O₇²⁻ + Fe²⁺ → Cr³⁺ + Fe³⁺
- What is an oxidizing agent? Identify the oxidizing and reducing agents in the reaction: Zn + CuSO₄ → ZnSO₄ + Cu
- Explain the construction and working of a Daniel cell with a neat diagram.
- Why is fluorine the strongest oxidizing agent in the periodic table?