chemistry - unit 2.1 Flashcards
When is a chemical reaction 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 does an equilibrium constant have?
No units.
What are the concentration of pure solids and pure liquids at equilibrium 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 and is independent of concentration and/ or pressure.
For endothermic reactions, what does a rise in temperature cause?
An increase in K and the yield of the product is increased.
For exothermic reactions, what does a rise in temperature cause?
A decrease in K and the yield of the product is decreased.
What does the presence of a catalyst do?
It does not affect the value of the equilibrium constant.
In water and aqueous solutions, where is there an equilibrium?
Between the water molecules and hydronium and hydroxide ions.
How can this ionisation of water be represented by?
H2O(l) + H2O(l ) -> H3O+ (aq) + OH− (aq)
What does H3O+(aq) represent?
A hydronium ion, a hydrated proton. A shorthand representation of H3O+ (aq) is H+ (aq).
What is water?
Amphoteric (can react as an acid and a base).
What is the dissociation constant for the ionisation of water known as the ionic product, Kw?
Kw = [H3O+][OH-]
What does the value of the ionic product vary with?
Temperature.
At 25°C, what is the value of Kw?
approximately 1 × 10-14.
What is the relationship between pH and the hydrogen ion concentration given by?
pH=−log[H3O+] and [H3O+] = 10−pH
In water and aqueous solutions with a pH value of 7, what are the concentrations of H3O+ (aq) and OH− (aq)?
They are both 10-7 mol l-1 at 25°C.
If the concentration of H3O+ (aq) or the concentration of OH− (aq) is known, what can be calculated?
The concentration of the other ion using
Kw or by using pH + pOH = 14 .
What is the Brønsted-Lowry definition of an acid?
A proton donor.
What is the Brønsted-Lowry definition of a base?
A proton acceptor.
For every acid, what is there?
A conjugate base, formed by the loss of a proton.
For every base, what is there?
A conjugate acid, formed by the gain of a proton.
What are strong acids and strong bases?
Completely dissociated into ions in aqueous solution.
What are weak acids and weak bases?
Only partially dissociated into ions in aqueous solution.
What are examples of strong acids?
Hydrochloric acid, sulfuric acid and nitric acid.
What are examples of weak acids?
Ethanoic acid, carbonic acid and sulphurous acid.
What are strong bases?
Solutions of metal hydroxides.
What are examples of weak bases?
Ammonia and amines.
What can the weakly acidic nature of solutions of carboxylic acids, sulfur dioxide and carbon dioxide be explained by?
Reference to equations showing the equilibria.
What can the weakly alkaline nature of a solution of ammonia or amines be explained by?
Reference to an equation showing the equilibrium.
What do 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.
What is the acid dissociation constant represented by Ka?
[H3O+][A-] / [HA]
or
pKa where pKa =−log10(Ka)
How can the approximate pH of a weak acid be calculated?
Using pH=1/2pKa −1/2log10(c)
What does a soluble salt of a strong acid and a strong base dissolve in water to produce?
A neutral solution.
What does a soluble salt of a weak acid and a strong base dissolve in water to produce?
An alkaline solution.
What does a soluble salt of a strong acid and a weak base dissolve in water to produce?
An acidic solution.
What does the name of the salt produced depend on?
The acid and base used.
How can the changes in concentrations of
H O+ and OH− ions of salt solutions be explained?
Using the appropriate equilibria.
What is a buffer solution?
One in which the pH remains approximately constant when small amounts of acid, base or water are added.
What does an acid buffer consist of?
A solution of a weak acid and one of its salts made from a strong base.
In an acid buffer solution, what does the weak acid provide?
Hydrogen ions when these are removed by the addition of a small amount of base.
What does the salt of the weak acid provide, in an acid buffer?
The conjugate bae, which can absorb excess hydrogen ions produced by the addition of a small amount of acid.
What does a basic buffer consist of?
A solution of a weak base and one of its salts.
In a basic buffer solution, what does the weak base do?
It removes excess hydrogen ions.
In a basic buffer solution, what does the conjugate acid provided by the salt do?
it supplies hydrogen ions when these are removed.
How can an approximate pH of an acid buffer solution be calculated?
From its composition and from the acid dissociation constant:
pH = pKa − log[acid]/[salt]
What are indicators?
Weak acids for which the dissociation can be represented as:
HIn(aq)+H2O(l) -> H3O(aq) + In-(aq)
In aqueous solution, what is the colour of an acid indicator?
Distinctly different from that of its conjugate base.
What is the colour of the indicator determined by?
The ratio of [HIn] to [In−]
What is the theoretical point at which colour change occurs?
When [H3O+]=Kln.
When is the colour change assumed to be distinguishable?
When [HIn] and [In−] differ by a factor of 10.
What is the pH range over which a colour change occurs estimated by?
The expression:
pH = pKIn +/-1
Where can suitable indicators be selected from?
pH data, including titration curves.
