1.7 Simple equilibria and acid-base reactions Flashcards
What is meant by a ‘reversible reaction’?
A reversible reaction is a reaction in which the products can react together to form the original reactants.
What is meant by ‘dynamic equilibrium’?
Dynamic equilibrium occurs in a reversible reaction when the rate of the forward reaction equals the rate of the backwards reaction.
The concentration of the reactants and products are constant.
What is Le Chatelier’s principle?
If a change is made to the conditions of a reversible reaction, the position of equilibrium will move to counteract the change.
How does increasing the temperature affect the equilibrium position?
Increasing the temperature will favour the endothermic reaction, so the equilibrium will shift towards the products of the endothermic reaction.
How does decreasing the temperature affect the equilibrium position?
Decreasing the temperature will favour the exothermic reaction, so the equilibrium will shift towards the products of the exothermic reaction.
How does increasing the pressure affect the equilibrium position?
Increasing the pressure favours the side of the reaction with fewer moles of gas, so the position of the equilibrium will move to that side.
How does increasing the concentration of the reactants affect the position of equilibrium?
Equilibrium will shift to the right to increase the concentration of the products.
What is Kc?
The equilibrium constant.
The magnitude indicates whether there are more reactants or products in an equilibrium system.
How do you calculate Kc?
aA + bB ⇌ cC + dD
Kc = [C]ᶜ[D]ᵈ/ [A]ª[B]ᵇ
Deduce an expression for Kc for the equation below:
3H₂(g) + N₂(g) ⇌ 2NH₃(g)
Kc =[NH₃]²/[N₂][H₂]³
How do you work out the units for Kc from the expression?
Kc =[NH₃]²/[N₂][H₂]³
Substitute in the concentration units and cancel down:
Kc =[NH₃]²/[N₂][H₂]³
Kc = (mol dm⁻³)² / mol dm⁻³ x (mol dm⁻³)³
= mol² dm⁻⁶/ mol⁴ dm⁻¹² = dm⁶ mol⁻²
What is the effect on Kc if the concentration of the reactants are increased?
Concentration has no effect on Kc
What is the effect on Kc if the pressure of the system is increased?
Pressure has no effect on Kc.
Why does the addition of a catalyst not affect the value of Kc?
A catalyst does not move the position of equilibrium, it only increases the rate at which equilibrium is reached.
If the forward reaction of a reversible reaction is endothermic, what effect will increasing the temperature have on the position of equilibrium?
Increasing the temperature will favour the forward reaction so the position of equilibrium will shift to the right in order to oppose the change.
How does increasing the pressure affect the position of equilibrium of the following reaction?
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
When there is an increase in pressure, the position of equilibrium shifts to the side with fewer moles of gas. Therefore the position of equilibrium will shift to the right.
How does decreasing the pressure affect the position of equilibrium of the following reaction?
H₂(g)+ I₂(g) ⇌ 2HI(g)
There is an equal number of molecules of gas on either side of the equation so the position of equilibrium is not affected by a change in pressure.
If the forward reaction is exothermic, what effect will increasing the temperature have on the value of Kc?
If the temperature is increased then the backwards endothermic reaction will be favoured.
This means the position of equilibrium will move to the left and the concentration of reactants compared to products will increase.
Therefore the value of Kc will decrease as the denominator of the Kc.
expression is increasing.
What is a Brønsted-Lowry acid?
A proton donor.
What is a Brønsted-Lowry base?
A proton acceptor.
What do acid-base reactions involve the transfer of?
Protons (H⁺ ions).
Acids donate H⁺(aq) ions to the bases, which accept the H⁺(aq) ions.
What is the proton donor and proton acceptor in this reaction:
HNO₂(s) + H₂O(l) → H₃O⁺ (aq) + NO₂⁻(aq)
Proton donor: HNO₂(aq)
Proton acceptor: H₂O(l)
What is pH?
A value which expresses the acidity or alkalinity of a substance, on a scale where 7 is neutral.
It is a simple way of measuring hydrogen ion concentration.
How do you calculate the pH of a strong acid?
HA → H⁺ + A⁻
For a strong acid the concentration of
acid = concentration of H⁺ ions.
pH = -log[H⁺]