Equilibria A2
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Define Kc.
Kc is the equilibrium constant for concentrations. It measures the ratio of product concentrations to reactant concentrations at equilibrium.
How is Kc calculated?
Molar concentration of the products/reactants, with powers based on the number of moles of each present in the balanced equation
How are expressions for Kc constructed for homogeneous equilibria?
For homogeneous equilibria (same phase), the Kc expression includes concentrations of substances in that phase.
What is omitted in the Kc expression for heterogeneous equilibria?
For heterogeneous equilibria (different phases), solids and liquids are not included in the Kc expression.
What happens to Kc when the temperature increases in an endothermic reaction?
In an endothermic reaction, an increase in temperature shifts the equilibrium position to favor the products, increasing Kc.
What happens to Kc when the temperature increases in an exothermic reaction?
In an exothermic reaction, an increase in temperature shifts the equilibrium position to favor the reactants, decreasing Kc.
How does an increase in reactant concentration affect Kc?
An increase in reactant concentration shifts the equilibrium towards the product side, increasing Kc.
How does an increase in pressure (for gases) affect Kc?
An increase in pressure shifts the equilibrium toward the side with fewer moles of gas, changing Kc accordingly (for gas-phase reactions).
What is reaction rate?
The change in concentration of a reactant or product in a given time.
What makes an effective collision?
Particles must have sufficient energy Particles must collide in correct orientation
Which factors can change the rate of a chemical reaction?
Concentration Temperature Use of a catalyst Surface area of solid reactants
What effect does increasing temperature have on reaction rate?
Reaction rate increases
Explain why increasing temperature has its effect on reaction rate
Particles have more energy Higher speed Collide more frequently More successful collisions Rate increases
What is the rule of thumb for reaction rate and temperature?
A 10C increase in temperature doubles the rate.
What effect does increasing concentration/pressure have on reaction rate?
Reaction rate increases
Explain why increasing concentration/pressure has its effect on reaction rate
Concentration increases More particles per unit volume More frequent collisions More successful collisions Rate increases
How can progress of a chemical reaction be followed?
Monitoring the removal of a reactant. Following the formation of a product.
What are the two methods of determining reaction rate, when a gas is produced?
Monitoring the volume of gas produced Monitoring the loss of mass of reactants
What does a catalyst do?
A substance that changes the rate of a chemical reaction without undergoing any permanent change itself.
What are the main properties of a catalyst?
Not used up in the reaction. May react with a reactant to form an intermediate or provide a surface for the reaction to take place. The catalyst is regenerated at the end of the reaction.
What is a homogeneous catalyst?
Has the same physical state as the reactants.
How does a homogeneous catalyst interact with the reactants?
Forms an intermediate This intermediate breaks down to give the product and regenerates the catalyst
Give two examples of homogeneous catalysts
Sulphuric acid in the production of esters Chlorine radicals in ozone depletion
How are the units of Kc worked out?
Substitute units into the expression Cancel common units