2a- Chemical equilibrium Flashcards
A chemical reaction is said to be in equilibrium when…?
The composition of the reactants and products remains constant indefinitely
What does the equilibrium constant (K) characterise?
The equilibrium composition of the reaction mixture
What does the value of an equilibrium constant indicate?
The position of equilibrium
What units do equilibrium constants have?
They have no units
How can the concentrations of pure solids and pure liquids at equilibrium be taken?
As constant and given a value of 1 in the equilibrium expression
What does the numerical value of the equilibrium constant depend on?
The reaction temperature (concentration and pressure have no effect)
In an exothermic reaction an increase in temperature causes what to happen to the K value and the yield of product?
In an exothermic reaction an increase in temperature causes an increase of the K value and an increase in the yield of product
In an endothermic reaction an increase in temperature causes what to happen to the K value and the yield of product?
In an endothermic reaction an increase in temperature causes a decrease in the K value and a decrease in the yield of product
What affect does the presence of a catalyst have on the value of the equilibrium constant?
A catalyst has no affect
Water is said to be amphoteric, what does this mean?
It can react as both an acid and a base
What does the value of the ionic product of water vary with?
Temperature
At 25*c what is the approximate value of Kw?
1 x 10^-14
In water and aqueous solutions with a pH value of 7, what are the concentrations of both hydronium and hydroxide ions at 25*c?
Concentrations of H3O+ and OH- are both 10^-7 mol/l
What do the Bronsted-Lowry definitions of acids and bases state?
An acid is a proton donor and a base is a proton acceptor
For every acid there is a…?
Conjugate base which is formed by the loss of a proton
For every base there is a…?
Conjugate acid which is formed by the gain of a proton
What is a characteristic of strong acids and bases?
Strong acids and bases completely dissociate into ions in aqueous solution
What is a characteristic of weak acids and bases?
Weak acids and bases only partially dissociate into their ions in aqueous solution
What are three examples of strong acids?
- hydrochloric acid (HCl)
- sulfuric acid (H2SO4)
- nitric acid (HNO3)
What are three examples of weak acids?
- carboxylic acids (e.g. ethanoic acid)
- carbonic acid (H2CO3)
- sulfurous acid(H2SO3)
What is an example of a strong base?
- solutions of metal hydroxides (except Mg)
What are two examples of weak bases?
- ammonia (NH3)
- amines
What is the relationship between the Ka value and the strength of the acid?
The smaller the Ka value, the weaker the acid is
A soluble salt of a strong acid and a strong base dissolves in water to produce what?
A neutral solution
A soluble salt of a strong acid and a weak base dissolves in water to produce what?
An acidic solution
A soluble salt of a weak acid and a strong base dissolves in water to produce what?
An alkaline solution
What does the name of a salt depend on?
The acid and base used
What is a buffer solution?
One in which the pH remains approximately constant when small amounts of acid, base or water are added
What is an acidic buffer made with?
A weak acid and the one of its salts made from a strong base
What is a basic buffer solution made with?
A weak base and one of its salts
What are indicators?
Weak acids which change colour depending on the pH of the solution
How is the colour of the indicator determined?
By the ratio of the concentrations of HIn ions to In- ions
The colour change is for an indicator is assumed to be distinguishable at what point?
When the concentrations of HIn ions and In- ions differ by a factor of 10
How can suitable indicators be selected?
From pH data, including titration curves
