Equilibrium in a closed vessel and the rate of reaction
الأسبوع 10 · الباب الثالث «الاتزان الكيميائي» (١): مقدمة الباب والاتزان الفيزيائي (التبخر والتكثف) والتفاعلات التامة والعكسية والاتزان الكيميائي حالة ديناميكية ومعدل التفاعل وما يتوقف عليه وطبيعة المواد المتفاعلة · 3 دروس
2
Complete and reversible reactions: a dynamic balance
Chemistry Sec 3 Term 1, Chapter 3: pp. 53–58
GoalTell complete and reversible reactions apart, and describe chemical equilibrium as a dynamic balance with equal rates but not equal amounts.
New words
complete reaction · a reaction that goes on until a reactant is used up, because a product leaves as a gas or a precipitatereversible reaction · a reaction that can go both ways, written with ⇌chemical equilibrium · the state of a reversible reaction in which forward and backward rates are equal and concentrations stay constant
Explanation
Some reactions are complete. A precipitate or a gas leaves the reacting mixture, so the reverse cannot happen: BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl (the solid BaSO₄ settles) and Zn + 2HCl → ZnCl₂ + H₂ (the gas escapes from an open vessel). Other reactions are reversible, such as acetic acid with ethanol: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O. The products can react to give the reactants again, and the mixture still contains acid, so a blue litmus test would still turn red. At chemical equilibrium the forward rate equals the backward rate, the concentrations stay constant, and all reactants and products are present, as long as the temperature and pressure do not change. Two traps: at equilibrium things are not “still”, because both reactions go on, and the amounts are not necessarily equal; they are only constant. Equations here are for reading only, and esters are met in Term 2.
Worked examples
Sort the reactions. BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl: complete, because the solid BaSO₄ leaves the solution. CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂ in an open vessel: complete, because CO₂ escapes. CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O: reversible, because nothing leaves the mixture and the arrow points both ways.
Tray
Tokens
Chance to move
Moves per round
A
9
1/3
3
B
6
1/2
3
Equal rates, unequal amounts. In the dice game, 15 tokens move between tray A and tray B. A token in A moves to B on a roll of 1 or 2; a token in B moves to A on 1, 2 or 3. Then A → B is 9 × 1/3 = 3 tokens a round and B → A is 6 × 1/2 = 3 tokens a round when A has 9 and B has 6. The rates are equal but the amounts are not: 9 and 6.
Count the atoms in the boxes. N₂ + 3H₂ ⇌ 2NH₃ is shown as three boxes. At the start there are 2 N₂ and 6 H₂, which is 4 N atoms and 12 H atoms. Later there are 1 N₂, 3 H₂ and 2 NH₃: N = 2 + 2 = 4 and H = 6 + 6 = 12. The counts are the same in the last two boxes: the system has reached equilibrium, but both reactions still go on. The numbers are invented for the picture.
Class activity
Token and dice game, paper and dice only. Put 15 tokens (buttons or coins) in tray A and none in tray B. Each round, roll a die for every token in A: on 1 or 2 move it to B. Roll for every token in B: on 1, 2 or 3 move it to A. Roll everything before moving. Play 8 rounds, write the counts in A and B after each round, and draw them as two lines. Predict first: the counts should settle near 9 in A and 6 in B, with small ups and downs, while tokens keep moving. Your dice results will differ, and that is fine.
