الأسبوع 5 · الباب الأول (٥): الحديد النقي وحالتا التأكسد (+2) و(+3) وتفاعلات الحديد مع الهواء وبخار الماء والكلور والكبريت والأحماض وأكاسيد الحديد الثلاثة وتحضيرها وخواصها (معادلات وحسابات على الورق فقط) · 3 دروس
3
The black magnetic oxide and the three oxides compared
Chemistry Sec 3 Term 1, Chapter 1: pp. 20–24
GoalShow that Fe₃O₄ is a mixed oxide FeO·Fe₂O₃, write its preparation and reactions, and compare the three oxides of iron.
New words
mixed oxide · an oxide that behaves as two oxides together: Fe₃O₄ = FeO·Fe₂O₃, with iron in both the +2 and +3 statesmagnetite · the ore Fe₃O₄, black and strongly attracted by a magnetoxidation of an oxide · taking up oxygen so that the metal reaches a higher state, as in Fe₃O₄ to Fe₂O₃
Explanation
The black oxide Fe₃O₄ occurs as the ore magnetite and is strongly magnetic. It is best seen as a mixed oxide, FeO·Fe₂O₃: in each formula unit one iron atom is in the +2 state and two are in the +3 state, since 2 + 2 × 3 = +8 balances 4 × (−2). Do not write iron in Fe₃O₄ as a single ion with the charge 8/3: no iron atom has that state. On paper it is prepared from red-hot iron with air or steam (lesson 1), or by partial reduction of Fe₂O₃: 3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂. With hot concentrated sulphuric acid it gives both salts together, which proves that it is mixed: Fe₃O₄ + 4H₂SO₄ → FeSO₄ + Fe₂(SO₄)₃ + 4H₂O. Heated in air it oxidises to Fe₂O₃: 4Fe₃O₄ + O₂ → 6Fe₂O₃ (the book’s half-integer form, 2Fe₃O₄ + ½O₂ → 3Fe₂O₃, is the same equation halved). Of the three oxides only Fe₃O₄ is strongly attracted by a magnet.
Oxide
Colour
Iron, % by mass
Iron states
Strongly magnetic?
FeO
black
77.8
+2
no
Fe₂O₃
reddish-brown
70.0
+3
no
Fe₃O₄
black
72.4
+2 and +3
yes
Worked examples
ƒFormula
Fe₃O₄ = FeO · Fe₂O₃
1 iron(II) and 2 iron(III) per formula unit
Checking that Fe₃O₄ is FeO + Fe₂O₃. Atoms: Fe 1 + 2 = 3 and O 1 + 3 = 4, as in Fe₃O₄. States: one iron(II) and two iron(III) per formula unit, so one third of the iron atoms are iron(II) (33.3 %).
Fe₃O₄ with hot concentrated sulphuric acid on paper (Fe = 56, S = 32, O = 16, H = 1): Fe₃O₄ + 4H₂SO₄ → FeSO₄ + Fe₂(SO₄)₃ + 4H₂O. For 23.2 g of Fe₃O₄, n = 23.2 ÷ 232 = 0.1 mol. Acid: 0.4 mol × 98 = 39.2 g. FeSO₄: 0.1 × 152 = 15.2 g. Fe₂(SO₄)₃: 0.1 × 400 = 40.0 g. Water: 0.4 × 18 = 7.2 g. Check: 23.2 + 39.2 = 62.4 and 15.2 + 40 + 7.2 = 62.4.
Step
Equation
1
3Fe₂O₃ + CO → 2Fe₃O₄ + CO₂
2
Fe₃O₄ + CO → 3FeO + CO₂
3
FeO + CO → Fe + CO₂
back up
4FeO + O₂ → 2Fe₂O₃
A ladder of reductions on paper. Carbon monoxide takes Fe₂O₃ down step by step, and oxygen takes the lower oxides back up. Each line is balanced.
Class activity
A decision chart on paper. Draw a “which oxide?” chart with three yes–no questions that separate FeO, Fe₂O₃ and Fe₃O₄: for example “Is it reddish-brown?” and “Is it strongly attracted by a magnet?”. Test your chart on three descriptions that a friend writes on cards (without naming the oxide). No powders are handled: the chart works from the descriptions.
Practice
1
4Fe₃O₄ + O₂ → 6Fe₂O₃. For 46.4 g of Fe₃O₄ (Fe = 56, O = 16) find the mass of oxygen used and of Fe₂O₃ formed.
