Chemical Equilibrium Flashcards
When is a chemical reaction in equilibrium
A chemical reaction is in equilibrium when the composition of the reactants and products remains constant indefinitely.
Which constant characterises the equilibrium composition of the reaction mixture
The equilibrium constant ( K ) characterises the equilibrium composition of the reaction
mixture.
What does the value of an equilibrium constant indicate
The value of an equilibrium constant indicates the position of equilibrium.
What units do equilibrium constants have
Equilibrium constants have no units.
What causes a change in K for endothermic reactions
For endothermic reactions, a rise in temperature causes an increase in K and the yield of
the product is increased.
What causes a change in K for exothermic reactions
For exothermic reactions, a rise in temperature causes a decrease in K and the yield of
the product is decreased.
What does the presence of a catalyst not affect
The presence of a catalyst does not affect the value of the equilibrium constant.
Where is the equilibrium in water and aqueous solutions
In water and aqueous solutions there is an equilibrium between the water molecules and
hydronium (hydrogen) and hydroxide ions.
This ionisation of water can be represented by:
H20 + H20 ————> H30+ + OH-
What is the feature of water
Water is amphoteric (can react as an acid and a base)
What does the numerical value of the equilibrium constant depend on
The numerical value of the equilibrium constant depends on the reaction temperature and is independent of concentration and/or pressure.
What are the concentrations of pure solids and pure liquids at equilibrium taken as
The concentrations of pure solids and pure liquids at equilibrium are taken as constant and given a value of 1 in the equilibrium expression.
What is the disassociation constant for water known as and represented by
The dissociation constant for the ionisation of water is known as the ionic product and is
represented by Kw :
Kw
Kw = [H3O+] [OH-]
Where must the reaction take place fro equilibrium to be established
For an equilibrium to be established the reaction must take place in a closed system
A closed system is..
A closed system is one which allows energy to be transferred to or from surroundings but not the reactants or products. Reactants form products and products form reactants.
What is dynamic equilibrium
The rate of the forward reaction equals the rate of the reverse reaction
For the general reaction
aA +bB <———-> cC + dD
K is given by the equation
K = [products] /[reactants] = [C]^c. [D]^d / [A]^a. [B]b
What are a b c d
The stoichiometric coefficients in the balanced equation
If value of K is < 10^-3
Effectively no reaction
If value of K is 10^-3 to 10^3
Significant quantities of reactants and products at equilibrium
If value of K is >10^3
Reaction is effectively complete
When K = 1
[products] = [reactants]
What value are pure solids and solvents (liquids) given
It is given the concentration value of 1 in equilibrium equation
How is K calculated for reaction involving gases
K is calculated using the partial pressure of gases involved
For the general reaction involving gases
aA + bB <=> cC + dD
Kp = P[products] / P[reactants] = PC^c. PD^d/ PA^a. PB^b
What can be used in place of partial pressures
Concentrations of gases can be used in place of partial pressures
When is a equilibrium describes as homogeneous
An equilibrium can be describes as homogeneous I.e all the species are in one state
2NOCL(g) <——-> 2NO (g) + CL2 (g)
When is a catalyst describes as heterogenous
The species are in more than one state
CaCO3 (s) <———> CaO (s) + CO2(g)
La chatelier’s principle
La chateliers principle states that any change in a system at equilibrium results in a shift of the equilibrium in the direction which minimises the change
What does concentration change and not change
At a given temperature changes in concentration may alter the position of equilibrium by these changes do not alter the value of K
What effect does pressure (only applies to gases) have on the value of K
At a given temperature changes in pressure May later the position of equilibrium but these changes do not alter the value of K
What happens if there are equal numbers of moles of gas molecules on each side of a reaction
A change in pressure will have no effect on equilibrium position since there is equal no of moles or gas on each side
What happens when the temperature of a system is raised
When the temperature of a system is raised the system will move in a direction to reduce the temperature I.e the endothermic reaction will be favoured.
Is K affected by changes in temperature
K is affected by changes in temperature
Summary of factors affecting K
K is unaffected by changes in
- concentration
- pressure
K is affected by changes in temperature an increase in temperature will shift the equilibrium in the direction of the endothermic reaction.
What are acids historically defined as
Historically acids were defined as producing hydrogen ions in solution and bases as producing hydroxide ions in solution
What does the Brønsted-Lowry define acids and bases as
An acid is any substance capable of donating a proton.
A base is any substance capable of accepting a proton.
What species is left when an acid donates a proton
When an acid donates a proton the species left is called the conjugate base of that acid
When is a conjugate base formed for every acid
For every acid there is a conjugate base formed by the loss of a proton
What species is formed when a base accepts a proton
When a base accepts a proton the species formed is called the conjugate acid of that base
What is formed for every base by the gain of a proton
For every base there is a conjugate acid formed by the gain of a proton
What is the value of Kw at 25°C
The ionic product of water (Kw) for water has the value of 1.0x 10^-14 at 25°C since the reaction is endothermic
What happens to the ionic product of water as temperature increases
Ten ionic product of water increases as the temperature increases. Therefore the pH of the water decreases as the temperature increases it becomes more acidic.
The ph of any aqueous solutions of a strong acid or alkali can be calculated using:
pH = -log[H+]
What is the disassociation for strong acids and strong bases
Strong acids and strong bases are completely dissociated into ions in aqueous solution.
What is the disassociation for weak acids and weak bases
Weak acids and weak bases are only partially dissociated into ions in aqueous solution.
Examples of strong acids
Examples of strong acids include hydrochloric acid, sulfuric acid and nitric acid. Sodium hydroxide
Weak acid examples
Ethanoic acid, carbonic acid and sulfurous acid are examples of weak acids. Ammonia and ammines
Strong bases example
Solutions of metal hydroxides are strong bases.
Weak bases example
Ammonia and amines are examples of weak bases.
What is Ka
ka is known as the acid disassociation constant and is a measure of the strength of the acid
The smaller the value of Ka
The smaller the value of Ka the weaker the acid
How can the disassociation constant be expressed
pKa = -log Ka
The approximate pH of a weak acid can be calculated using
pH = 1/2 pka - 1/2 log10c
What does the name of the salt produced depend on
The name of the salt produced depends on the acid and base used.
What does a soluble salt of a strong acid and a base produce
A soluble salt of a strong acid and a strong base dissolves in water to produce a neutral
solution.
What does a soluble salt of a weak acid and a strong base dissolve to produce
A soluble salt of a weak acid and a strong base dissolves in water to produce an alkaline
solution.
What does a soluble salt of a strong acid and a weak base dissolve to produce
A soluble salt of a strong acid and a weak base dissolves in water to produce an acidic
solution.
What is a buffer solution
A buffer solution is one in which the pH remains approximately constant when small
amounts of acid, base or water are added.
What is an acid buffer
An acid buffer consists of a solution of a weak acid and one of its salts made from a strong base
How does an acid buffer solution work
In an acid buffer solution the weak acid provides hydrogen ions when these are removed
by the addition of a small amount of base. The salt of the weak acid provides the conjugate base, which can absorb excess hydrogen ions produced by the addition of a small amount of acid.
What is a basic buffer
A basic buffer consists of a solution of a weak base and one of its salts.
How does a basic buffer solution work
In a basic buffer solution the weak base removes excess hydrogen ions, and the conjugate acid provided by the salt supplies hydrogen ions when these are removed.
An approximate pH of an acid buffer solution can be calculated from its composition and
from the acid dissociation constant:
PH = pKa -log10 [acid] / [salt]
What is the relationship between pH and hydrogen ion concentration given by
PH = -log10 [H30+] and [H3O+] = 10 ^-PH
Which solutions have a weakly acidic nature
The weakly acidic nature of solutions of carboxylic acids, sulfur dioxide and carbon dioxide can be explained by reference to equations showing the equilibria.
Which solutions have a weakly alkaline nature
The weakly alkaline nature of a solution of ammonia or amines can be explained by
reference to an equation showing the equilibrium.
What are similarities and differences I’d equimolar solutions of weak and strong acids (or bases)
Equimolar solutions of weak and strong acids (or bases) have different pH values,
conductivity, and reaction rates, but the stoichiometry of reactions are the same.
The acid disassociation constant is represented by Ka:
Ka = [H3O+] [A-]/ [HA]
The acid indicator dissociation constant is represented as KIn and is given by the following expression
Kin = [H3O+] [In-] / [HIn]
What can suitable indicators be selected by
Suitable indicators can be selected from pH data, including titration curves.
What is the expression for the pH range over which a colour change occurs
The pH range over which a colour change occurs can be estimated by the expression:
pH = pKin +- 1
In aqueous solution the colour of an acid is….
In aqueous solution the colour of an acid indicator is distinctly different from that of its
What are the concentrations of H30+ and OH- in water and aqueous solutions with a pH of 7
In water and aqueous solutions with a pH value of 7 the concentrations of H3O+ (aq)
and OH- (aq) − are both 10^-7 at 25°C
What can be calculated if the concentration of H3O+ or OH- is known
If the concentration of H3O+ or the concentration of OH- is known the concentration of the other ion can be calculated using Kw or by using pH +pOH=14
What does the value of the ionic product vary with
The value of the ionic product varies with temperature
What are indicators and what is their dissociation
Indicators are weak acids for which the dissociation can be represented as:
HIn(aq) + H2O <=> H3O+ (aq) + In- (aq)
What is the colour of the indicator determined by
The colour of the indicator is determined by the ratio of [HIn] to [In-]
What is the theoretical point at which colour change occurs
The theoretical point at which a colour change occurs is when [H3O+] = Kin
When is the colour change assumed to be distinguishable
The colour change is assumed to be distinguishable when [Hin] and [In] differ by a factor of 10
