40 short questions of 9th Class Chemistry Chapter 6, Equilibria, with exam-ready answers in the book's words: the textbook exercise questions plus SLO-based questions asked in board papers. Punjab Board new syllabus.
Exam-ready answers. Learn the meaning, then write them in your own words.
Set A: Textbook exercise short questions 5 questions
The short questions at the end of the chapter in your book.
Short Q 6.1
How is dynamic equilibrium different from static equilibrium?
Answer
Dynamic equilibrium
Static equilibrium
The forward and backward processes continue at equal rates.
Nothing is moving or changing at all.
Changes are going on at the particle level but there is no visible change.
There is no change at any level.
Example: N₂ + 3H₂ ⇌ 2NH₃ in a closed vessel; water and its vapour in a closed bottle.
Example: a book lying on a table; a balanced see-saw at rest.
Since the reaction does not cease at equilibrium, rather it keeps on going in both directions, it is called dynamic equilibrium.
Short Q 6.2
How will the reversible reaction 2H₂O(l) ⇌ 2H₂(g) + O₂(g) be affected if its temperature is increased?
Answer
The decomposition of water is endothermic (it absorbs the 571.6 kJ that is released when 2 mol of water form). Heating favours the endothermic direction, so increasing the temperature drives the reaction forward: more water decomposes into hydrogen and oxygen. (Water decomposes noticeably only at very high temperatures, above about 2000 °C.)
Check your book
The book writes this equilibrium with "electric current". Electrolysis decomposes water because the current drives it; it is not an equilibrium, as H₂ and O₂ do not recombine at room temperature.
Short Q 6.3
How can you get the maximum yield in a reversible reaction?
Answer
Remove a product as soon as it is formed (liquefy NH₃; let CO₂ escape; remove water when making ethyl acetate).
Add more of a reactant (usually the cheaper one).
Choose the temperature: low for an exothermic forward reaction, high for an endothermic one.
Choose the pressure for gas reactions: high if the products have fewer gas moles.
A reversible reaction may be forced to go to completion if one or all of the products are withdrawn from the reaction mixture as soon as they are formed.
Short Q 6.4
How can you decrease the time to attain the position of equilibrium in a reversible reaction?
Answer
Use a catalyst: a catalyst increases both the rates of forward and backward reactions, so it decreases the time taken by the reaction to attain the state of equilibrium. Raising the temperature also speeds up both reactions (though it may shift the equilibrium position).
Short Q 6.5
What is the effect of increasing pressure on the reaction N₂(g) + O₂(g) ⇌ 2NO(g) at equilibrium?
Answer
No effect. There are 2 moles of gas on the left (1 N₂ + 1 O₂) and 2 moles on the right (2 NO). Change of pressure disturbs only those gaseous reactions in which the number of moles of the reacting gases is different from the number of moles of the gases produced.
Set B: More short questions 35 questions
The book's in-text exercises first, then short questions on every topic of the chapter (SLO-based), as asked in board papers.
Practice exercise questions (in text)
SQ 6.1
Activity: heat blue copper sulphate gently and then strongly in a dry test tube, let it cool, and note your observations.
Answer
On heating, the blue crystals turn white and water droplets collect on the cool upper part of the tube.
On strong heating the solid becomes a white powder: anhydrous CuSO₄.
On cooling it stays white; if the water droplets run back, or a few drops are added, it turns blue again and gets warm.
CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(g)
SQ 6.2
Elaborate an example of dynamic equilibrium which exists in this world between the three physical states of water.
Answer
In the water cycle (and in any closed container), water evaporates from seas, lakes and ice, and vapour condenses back as clouds, rain and dew; ice melts and water freezes. In a closed bottle the rates of evaporation and condensation become equal: H₂O(l) ⇌ H₂O(g). In a glass of ice-water at 0 °C, melting and freezing go on at equal rates: H₂O(s) ⇌ H₂O(l). The amounts stay the same although molecules keep changing state.
Liquid and vapour in a closed bottle.
SQ 6.3
N₂O₄ is a colourless gas that slowly changes to brown NO₂ at 100 °C. Predict how the colour of the mixture will change if N₂O₄ is kept in a sealed flask at 100 °C.
Answer
At first the gas is colourless. As some N₂O₄ changes to NO₂, a brown colour appears and deepens. NO₂ also combines back into N₂O₄; once the two rates become equal, the amount of NO₂ stops changing, and the colour stays a constant light brown. It never becomes dark brown, because N₂O₄ is never used up.
The colour settles once equilibrium is reached.
SQ 6.4
PCl₅(g) ⇌ PCl₃(g) + Cl₂(g) is endothermic. What will happen if the gas mixture is compressed?
Answer
Compressing raises the pressure. The equilibrium moves to the side with fewer moles of gas: 1 mole (PCl₅) against 2 moles (PCl₃ + Cl₂). So the backward reaction is favoured and more PCl₅ forms.
SQ 6.5
What will happen if we add Cl₂ gas to the PCl₅ equilibrium mixture?
Answer
Adding a product increases its concentration. To restore equilibrium, Cl₂ reacts with PCl₃ to form more PCl₅: the backward reaction is favoured, until a new equilibrium is reached.
SQ 6.6
What will happen if the temperature of the PCl₅ reaction is increased?
Answer
The forward reaction is endothermic, so heating favours it: more PCl₅ decomposes into PCl₃ and Cl₂.
SQ 6.7
Suggest a suitable method to withdraw water from the reaction mixture in the preparation of ethyl acetate.
Answer
Add concentrated sulphuric acid (instead of dilute): it is a strong dehydrating agent and absorbs the water as it forms, while still acting as the catalyst.
Or add a drying agent such as anhydrous calcium chloride or anhydrous copper sulphate to take up the water.
Or distil off ethyl acetate (b.p. 77 °C) as it forms; removing either product drives the reaction forward.
Reversible and irreversible reactions
SQ 6.8
Define irreversible reaction with an example.
Answer
A reaction which goes to completion and moves in the forward direction only is called an irreversible reaction. If stoichiometric amounts are used, no reactant is left. Example: NaCl(aq) + AgNO₃(aq) → AgCl(s) + NaNO₃(aq).
SQ 6.9
Define reversible reaction with an example.
Answer
A reaction which moves in both the forward and backward directions under the same conditions is called a reversible reaction. It is shown by ⇌. Example: N₂ + 3H₂ ⇌ 2NH₃.
SQ 6.10
Why does a reversible reaction never go to completion?
Answer
As soon as products form, they react to give back the reactants. At any time all the species are simultaneously present in the reaction mixture, so the reactants are never used up completely (in a closed system).
SQ 6.11
How can a reversible reaction be made to go to completion?
Answer
If either one of the products is withdrawn from the reaction mixture or, being a gas, it escapes into the atmosphere. CaCO₃ heated in an open vessel decomposes completely, because CO₂ escapes.
SQ 6.12
What happens when CaCO₃ is heated in an open and in a closed container?
Answer
Open and closed vessels.
Open: CO₂ escapes as soon as it forms, and the reaction goes to completion. Closed: CO₂ reacts with CaO to give back CaCO₃, and equilibrium is set up.
SQ 6.13
Are physical changes also reversible? Give an example.
Answer
Yes, physical changes may also be reversible in nature. Water ⇌ ice and water ⇌ steam; the book gives the heating of blue copper sulphate (CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O).
Hydrated salts
SQ 6.14
What happens when blue copper sulphate is heated? Write the equation.
Answer
CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(g)
blue → white on heating
It loses its five water molecules and becomes white anhydrous copper sulphate. When white anhydrous copper sulphate absorbs moisture from the atmosphere, it turns blue again.
SQ 6.15
What are the colours of cobalt chloride with different amounts of water?
Answer
Form
Formula
Colour
Hexahydrate
CoCl₂·6H₂O
pink
Dihydrate
CoCl₂·2H₂O
purple
Anhydrous
CoCl₂
blue
Pink, purple, blue.
SQ 6.16
What happens when blue anhydrous cobalt chloride absorbs moisture?
Answer
With limited moisture it is first converted into a dihydrate which is purple. The dihydrate then absorbs four more water molecules to become the hexahydrate, which is pink.
SQ 6.17
How can copper sulphate and cobalt chloride be used to test for water?
Answer
White anhydrous copper sulphate turns blue, and blue cobalt chloride (paper) turns pink, when water is added. These changes show that a liquid contains water.
SQ 6.18
What is water of crystallisation?
Answer
The fixed number of water molecules that are part of the crystal of a hydrated salt: five in CuSO₄·5H₂O, six in CoCl₂·6H₂O. It gives the crystals their shape and colour and is driven off by heating.
Dynamic equilibrium
SQ 6.19
What is chemical equilibrium?
Answer
The state of a reversible reaction at which the rate of the forward reaction becomes equal to the rate of the backward reaction. The number of reactant molecules disappearing in the forward reaction equals the number formed in the reverse reaction.
SQ 6.20
Why is chemical equilibrium called dynamic?
Answer
Because the reaction does not cease at the state of equilibrium; rather it keeps on going in both directions. It only appears as if nothing is happening.
SQ 6.21
What happens to the concentrations at equilibrium?
Answer
The concentrations of the reacting species (reactants and products) remain constant at equilibrium. They are not necessarily equal.
Concentrations level off at equilibrium.
SQ 6.22
How do the forward and backward rates change as a reaction approaches equilibrium?
Answer
Rates against time.
The forward rate is greatest at the start and decreases as reactants are used up; the backward rate is zero at the start and increases as products build up. At equilibrium they are equal.
SQ 6.23
On what factors does the time to reach equilibrium depend?
Answer
The time a reaction takes to attain dynamic equilibrium depends upon the nature of the reaction and the conditions at which it is performed. For very slow reactions it may take years; ammonia formation at 400 °C with a catalyst takes minutes.
SQ 6.24
Can equilibrium be reached in an open container?
Answer
Not if a product is a gas: it escapes, so the backward reaction cannot occur and the reaction goes to completion. A dynamic equilibrium needs a closed system, where nothing enters or leaves.
Disturbing an equilibrium
SQ 6.25
In what ways can a reversible reaction at equilibrium be disturbed?
Answer
Adding or withdrawing one or more of the reacting species.
Adding or withdrawing one or more of the products.
Changing the temperature.
Adding a catalyst (changes only how fast equilibrium is reached).
Changing the pressure, if gases are involved.
SQ 6.26
What is the effect of adding N₂ to the ammonia equilibrium?
Answer
The concentration of N₂ increases and the reaction no longer maintains equilibrium. Nitrogen reacts with hydrogen to produce more ammonia until a new equilibrium is reached, with concentrations different from before.
SQ 6.27
What is the effect of withdrawing ammonia from the equilibrium mixture?
Answer
Its concentration decreases. To restore equilibrium, more nitrogen and hydrogen react to produce ammonia.
SQ 6.28
Why does increasing temperature decrease the yield of ammonia?
Answer
The formation of ammonia is exothermic (ΔH negative); the reverse reaction is endothermic. Increasing the temperature pushes the reaction in the backward direction. Decreasing the temperature drives it forward.
SQ 6.29
Why is ammonia formation favoured at high pressure?
Answer
4 moles of reacting gases produce 2 moles of product gas. When the pressure is increased, the reaction moves to the side with fewer moles, i.e. forward.
SQ 6.30
Which reactions are not affected by pressure?
Answer
Gas reactions with the same number of moles of gas on both sides, such as N₂ + O₂ ⇌ 2NO and H₂ + I₂(g) ⇌ 2HI(g), and reactions with no gases at all.
SQ 6.31
Why does a fizzy drink lose its gas when the can is opened?
Answer
CO₂ is dissolved under pressure: CO₂(g) ⇌ CO₂(aq). Opening the can lowers the pressure, the equilibrium moves towards the gas, and bubbles of CO₂ come out.
Catalyst and industry
SQ 6.32
What is the effect of a catalyst on a reversible reaction?
Answer
A catalyst increases both the rates of forward and backward reactions. So it decreases the time taken by the reaction to attain the state of equilibrium. It does not change the amounts present at equilibrium.
SQ 6.33
How is practically 100 % conversion achieved in the Haber process?
Answer
The ability of ammonia gas to be converted into its liquid form easily is used to drive the reaction to completion. Ammonia is liquefied and removed, and the unreacted N₂ and H₂ are recycled through the catalyst.
SQ 6.34
Why is ethyl acetate important? How is its yield improved?
Answer
It is used as a thinner in the paint industry. C₂H₅OH + CH₃COOH ⇌ CH₃COOC₂H₅ + H₂O; the yield is improved by removing water from the reaction mixture as soon as it is formed.
SQ 6.35
What is catalytic methanation?
Answer
Coal reacted with steam gives water gas: C + H₂O → CO + H₂. These products then react by a reversible reaction called catalytic methanation to yield methane: CO + 3H₂ ⇌ CH₄ + H₂O. Pakistan's Thar coal could be used this way.
