Edexcel A T10 and T11 - Equilibrium

A sample of N₂O₄(g) is placed into an evacuated container at 373K and allowed to undergo the reversible reaction N₂O₄(g) ⇄ 2 NO₂(g). The concentration of each species is measured over time, and the data are used to make the graph shown above. Which of the following identifies when equilibrium is first reached and provides a correct explanation?

• At 14 seconds, because [N₂O₄] is twice [NO₂], which implies that the forward and reverse reaction rates are equal.

• At 23 seconds, because [NO₂] equals [N₂O₄], which shows that equal concentrations are present at equilibrium.

• At 40 seconds, because [NO₂] is twice [N₂O₄], which matches the stoichiometry of the balanced chemical equation.

• At 60 seconds, because [NO₂] and [N₂O₄] remain constant, indicating that the forward and reverse reaction rates are equal.

If the equilibrium constant is much less than one (K << 1) then

• Equilibrium is not established.
• Equilibrium lies to the right (products are favored)
• Equilibrium lies to the left (reactants are favored)
• Neither products or reactants are favored.

N₂O₄(g) ↔ 2 NO₂(g)
What is the concentration equilibrium constant expression?

• K_c = [NO₂]²/[N₂O₄]
• K_c = [N₂O₄]/[NO₂]²
• K_c = [N₂O₄]²/[NO₂]
• K_c = [NO₂]/[N₂O₄]²

N₂O₄(g) ↔ 2 NO₂(g)
What is the equilibrium constant expression?

• K_p = (P_NO2)²/ (P_N2O4)
• K_p = (P_N2O4) / (P_NO2)^₂
• K_p = (P_N2O4)² / (P_NO2)
• K_p = (P_NO2) / (P_N2O4)²

What is the equilibrium-constant expression for 
CO₂(g) + H₂(g) ↔ CO(g) + H₂O(l)

• K_c= [CO][H₂O] / [CO₂][H₂]  
• K_c= [CO₂][H₂] / [CO]
• K_c= [CO₂][H₂] / [CO][H₂O]
• K_c= [CO] / [CO₂][H₂]  

What is the equilibrium expression for:
Fe₃O₄(s) + 4H₂(g) <--> 3Fe(s) + 4H₂O(g)

• ([Fe]³[H₂O]⁴ ) / ([Fe₃O₄][H₂]⁴)
• ([Fe₃O₄][H₂]⁴)/ ([Fe]³[H₂O]⁴)
• [H₂O]⁴ / [H₂]⁴
• ([Fe][H₂O]) / ([Fe₃O₄][H₂])

Keq < 1

• There are more products than reactants when the reaction reached equilibrium.
• There are more reactant than products when the reaction reached equilibrium.
  
• The amount of reactants is equal to the amount of products.

What kind of equilibrium does the reaction below show?
CaCO_{3 (s)} ⇌  CaO _(s) + CO_{2 (g)}

• Heterogeneous equilibrium, all the reactants and products are in the same physical state.
• Homogeneous equilibrium, all the reactants and products are in the same physical state.
• Heterogeneous equilibrium, the reactants and products are present in more than one physical state
• Homogeneous equilibrium, the reactants and products are present in more than one physical state

An equilibrium constant with a large magnitude indicates…

• A very fast reaction
• More products at equilibrium
• More reactants at equilibrium
• nothing, without considering the stoichiometry of the reaction

Define chemical equilibrium.

• A reaction is reversible.
• The concentration of the reactants is equal to the concentration of the products.
• The rate of a forward reaction is equal to the rate of the reverse reaction.
• The reaction stops and no further change in concentration occurs.