Question

2. 2Cr02 (aq, yellow)+ 2H,0-(aq) Cr20 (aq, orange) +3H20(D (AH = -ve) a. What would you observe if you add concentrated acid to the system? Describe the appearance and how it changes. Briefly explain your answer in terms of Le Châtelier's Principle. b. What would you observe if you add some K2Cr20, crystals to the system? Describe the appearance and how it changes. Briefly explain your answer in terms of Le Châtelier's Principle.

Answer 1

The basic idea of Le Chateliers principle is that, if a system at equilibrium is disturbed (due to addition of reactant/product from outside , or by changing pressure, temperature etc) , then the system will react in such a way so as to counter the disturbance.

For eg, if we add more products to the system, then the system will shift its equilibrium towards the reactant.

2)

2CrO₄^2- +2H₃O⁺ ↔ Cr₂O₇^2- + 3H₂O

a)

Acid is present as a reactant. So, if we add concentrated acid, the equilibrium will shift in the forward direction (leading to formation of more product) to counter the effect of having an increased amount of reactant.

So, the colour will become like Cr₂O₇^2- (orange) faster.

b)

K₂Cr₂O₇ crystals when added to the system will give Cr₂O₇^2- , which is present on the product side. So, according to Le Chateliers principle, the equilibrium will shift in the reverse direction, and the colour of the system will be yellow.

c)

When base (NaOH) is added, it will neutralise the acid present in the system, and so, the amount of H₃O⁺ (reactant) will decrease. So, according to Le Chateliers principle, the equilibrium will shift towards the reactant so that the amount of reactant formed is more (or,the amount of product forming is lower), i.e , the equilibrium will lie towards the left.

So, the colour of the system will be yellow.

d)

The enthalpy change for the forward reaction is negative (ΔH =-ve). This means, in the forward reaction, heat is evolved to the surroundings(or, the forward reaction is exothermic). This can be thought of as: heat is a product (since it is released). Thus, heat is on the product side.

(For an endothermic reaction, similarly, heat will be on the reactant side)

If the temperature of the system is increased, then it means that the product increases. So, the equilibrium will shift towards the reactants. Colour of the system will be yellow.