الكيمياء (بالإنجليزية) الصف الثالث الثانوي، الأسبوع 7: Thiosulphate and nitrite

The rest of the anions and a decision chart

الأسبوع 7 · الباب الثاني (٢): بقية الأنيونات: الثيوكبريتات والنيتريت ومجموعة حمض الكبريتيك المركز (الكلوريد والبروميد واليوديد والنيترات واختبار الحلقة البنية) ومجموعة كلوريد الباريوم (الفوسفات والكبريتات) وتدريب من إعداد Darslab على تصنيف الأنيونات · 3 دروس

Thiosulphate and nitrite

Chemistry Sec 3 Term 1, Chapter 2: pp. 35–39

GoalRecognise thiosulphate and nitrite by what dilute HCl releases, and confirm each with a second test (equations on paper).

New words

thiosulphate · the ion S₂O₃²⁻; with dilute HCl it gives SO₂ gas and a yellow solid, sulphurnitrite · the ion NO₂⁻; with dilute HCl it gives colourless NO that turns brown in airdecolourise · to remove a colour, such as the brown of iodine or the violet of permanganate

Explanation

Thiosulphate and nitrite finish the dilute HCl group. With thiosulphate the acid gives sulphur dioxide and also a yellow solid: Na₂S₂O₃ + 2HCl → 2NaCl + H₂O + SO₂ + S. The liquid turns cloudy yellow, which a sulphite does not do. The confirmation is that thiosulphate removes the brown colour of iodine and forms the tetrathionate ion: 2Na₂S₂O₃ + I₂ → Na₂S₄O₆ + 2NaI. Nitrite is different, because the acid it gives, HNO₂, is so unstable that it breaks down at once: NaNO₂ + HCl → NaCl + HNO₂, then 3HNO₂ → HNO₃ + 2NO + H₂O. Colourless NO meets oxygen in the air and turns reddish-brown just where it leaves the tube: 2NO + O₂ → 2NO₂. Its confirmation is that acidified permanganate loses its violet colour, because nitrite is oxidised to nitrate. Everything here is a paper equation. The gases and the permanganate are for trained people in a laboratory, and no smell test is ever used.

Anion	What dilute HCl gives	Confirmation
thiosulphate S₂O₃²⁻	SO₂ and a yellow solid, S	brown iodine colour is removed
nitrite NO₂⁻	NO, brown NO₂ at the tube mouth	violet permanganate colour is removed

Worked examples

	Left	Right
Na	2	2
S	2	1 + 1 = 2
O	3	1 + 2 = 3
H	2	2
Cl	2	2

Count the atoms. Na₂S₂O₃ + 2HCl → 2NaCl + H₂O + SO₂ + S. Left: Na 2, S 2, O 3, H 2, Cl 2. Right: Na 2, Cl 2, H 2, O 1 + 2 = 3, S 1 + 1 = 2. The equation is balanced, and one of the two sulphur atoms leaves as a solid.

ƒThree steps

NaNO₂ + HCl → NaCl + HNO₂ ; 3HNO₂ → HNO₃ + 2NO + H₂O ; 2NO + O₂ → 2NO₂

Nitrite in three steps. (1) NaNO₂ + HCl → NaCl + HNO₂. (2) 3HNO₂ → HNO₃ + 2NO + H₂O: N 3 and O 6 on both sides, H 3 on both sides. (3) 2NO + O₂ → 2NO₂. The colourless gas of step 2 becomes brown in step 3 when it meets air, which is why the colour appears where the gas leaves the tube and not inside the liquid.

Anion	Gas	Liquid after the acid
sulphite	SO₂	clear
thiosulphate	SO₂	cloudy yellow (sulphur)

Spot the difference. Sulphite and thiosulphate both give SO₂ with dilute HCl. A solution that stays clear points to sulphite; a solution that turns cloudy yellow points to thiosulphate, because the extra solid is sulphur. The brown iodine test then confirms thiosulphate.

Class activity

Atom tokens, paper only. Cut small paper squares and colour them: one colour each for N, O and H. Lay out 3HNO₂ with tokens (3 N, 6 O, 3 H), then regroup the same tokens into HNO₃, two NO and one H₂O. Check that no token is left over or missing. Then make tokens for the first step, Na₂S₂O₃ + 2HCl, and move them into the products. Nothing is mixed or heated; these acids and gases are handled only by trained people in a laboratory.

Practice

Complete and balance: K₂S₂O₃ + HCl → ?

In the step 2NO + O₂ → 2NO₂, 0.30 mol of NO reacts with enough oxygen. How many moles of O₂ are needed, and what volume is that at STP (22.4 L per mole)?

7.9 g of sodium thiosulphate, Na₂S₂O₃ (Na = 23, S = 32, O = 16), reacts with excess dilute HCl. What mass of sulphur forms, using the equation Na₂S₂O₃ + 2HCl → 2NaCl + H₂O + SO₂ + S?

A brown gas seen where the gas leaves the tube does not alone prove nitrite. Why?