WJEC Chemistry for AS: Student Bk

Redox Many chemical reactions involve the loss or gain of electrons, a species being oxidised if it loses electrons and reduced if it gains them. Since electrons do not vanish or appear from nowhere, all these reactions involve a transfer of electrons from the species being oxidised to the one being reduced. E.g., in Na + 1 2 Cl 2 → Na + + Cl − , the Na is being oxidised and loses an electron and the Cl gains an electron and is reduced. The popular nemonic OILRIG is helpful if used carefully with the atom saying ‘ O xidised I L ose electrons, R educed I G ain electrons. Confusion is very common. Oxidation numbers (states) This is a useful accounting system for redox with simple rules: 1. All elements have an oxidation state of zero. 2. Hydrogen in compounds is usually 1 (or +1). 3. Oxygen is usually −2 or −II. 4. Group 1 and 2 elements in compounds are 1 and 2 respectively. 5. Group 6 and 7 elements in compounds are usually −2 and −1 respectively. 6. An element bonded to itself is still 0. 7. The oxidation numbers of the elements in a compound or ion must add up to zero or the charge on the ion. Important The oxidation number does not imply a charge, e.g. in MnO 4 − the oxidation numbers are Mn(7) and four O(4 × −2) giving an overall charge of minus 1. The Mn is not 7+. The S-block elements The elements are all reactive, electropositive (low electronegativity) metals forming cations with oxidation numbers 1 or 2 respectively. Oxides are formed with oxygen/air as in Ca + 1 2 O 2 → CaO. Hydrogen is liberated with water and an oxide or hydroxide formed; Na + H 2 O → NaOH + 1 2 H 2 The reaction of Group 1 and Group 2 elements with acids is similar to this except that a salt is formed as in Mg + 2HCl → MgCl 2 + H 2 and the elements in both groups show their typical reaction as reducing agents, donating electron(s) to reduce the acid or water to hydrogen and being themselves oxidised. Mg + 2HCl → Mg 2+ + H 2 + 2Cl − Oxidation number O 2(1)( – 1) 2 O 2( – 1) In all these cases reactivity increases down the group and Group 1 elements are more reactive than Group 2. Lithium reacts slowly with water while potassium reacts violently; magnesium react slowly but barium is faster. All the s-block metals react vigorously with acids. Knowledge check (i) Give the oxidation states of all the atoms in the following compounds: FeCl 3 H 2 O 2 K 2 Cr 2 O 7 Br 2 H 2 CO 3 (ii) Write down the oxidation states of all the atoms on both sides of the following unbalanced reaction: NaI + H 2 SO 4 → Na 2 SO 4 + I 2 + H 2 S + H 2 O 39 Knowledge check Redox (oxidation/reduction) reactions involve the transfer of from one agent to another. A substance that gives one to another substance is called a agent and is thus by this process while the receiving agent is said to be . 38 C1.1 see page 12 Link ▲ Sodium metal on water 58 WJEC AS Level Chemistry: Unit 1