6Half ReactionsShow what happens to electrons in the two parts of a redox reaction: the reduction and the oxidationSteel hulls in ships have zinc blocks attached because they oxidize and release electrons. These electrons get consumed by the steel and prevent corrosion.
7Rules for Writing Half-Reactions Equations The number of electrons gained must equalthe number of electrons lost.The equation must balance with respect toboth atoms and charge.(Charge and mass are conserved in redox equations.)
100 0 2+ 1- Mg (s) + Br2 (l) → MgBr2 (s) Write the half-reaction equations for the redox reaction Mg (s) + Br2 (l) → MgBr2 (s)Step 1: Use oxidation numbers to determine what is oxidized and what is reducedMg (s) + Br2 (l) → MgBr2 (s)What happened to Mg?Mg gained a positive charge so it lost electrons.It is oxidized.
110 0 2+ 1- Mg (s) + Br2 (l) → MgBr2 (s) Mg → Mg2+ Br2 → 2Br- Mg (s) + Br2 (l) → MgBr2 (s)What happened to Br2?Br2 got a negative charge so it gained electrons.It is reduced.Step 2 : Write two separate equations: one showing oxidation and another showing reduction. Use coefficients to balance the number of atoms.Mg → Mg2+Br2 → 2Br-
12Step 3: Make the charge balance in each equation by adding electrons. Mg → Mg2+ + 2e-Br2 + 2e- → 2Br-Check:The number of electrons gained must equal the number of electrons lost.The equation must balance with respect to both atoms and charge.
13Redox Reactions and Covalent Bonds When hydrogen burns in chlorine, a covalent bond forms from the sharing of two electrons.The pair of electrons is more strongly attracted to the chlorine atom because of its higher electronegativity.Neither atom has totally lost or totally gained any electrons.Hydrogen has donated a share of its bonding electron to thechlorine but has not completely transferred that electron.
14Objectives Explain what must be conserved in redox equations. Balance redox equations by using the half- reaction method.Relate chemical activity to oxidizing and reducing strength.Explain the concept of disproportionation.