Chemical Reactivity (Casey)

what is collision theory

substances must collide with the correct orientation with sufficient energy to break the original bonds

what factors affect reaction rate

• concentration

• surface area

• temperature

• catalyst

how does concentration affect reaction rate

increasing the concentration increases the number of particles available to react in a given volume, so there will be more effective collisions per second resulting in an increased reaction rate

how does surface area affect reaction rate

increasing the surface area increases the number of particles available to collide, so there will be more effective collisions per second resulting in an increased reaction rate

how does temperature affect reaction rate

increasing the temperature increases the kinetic energy of the particles, this means the particles move faster, increasing the frequency of collisions. In addition, the collisions are more likely to be effective because the kinetic energy of the particles has increased, so a greater proportion of particles have sufficient energy to overcome the activation energy, resulting in an increase in reaction rate

how does a catalyst affect reaction rate

a catalyst provides an alternative pathway for the reaction with a lower activation energy, therefore, more reacting particles will collide with sufficient energy to overcome the activation energy, so there will be more successful collisions per second so an increase in the rate of reaction

what happens at equilibrium

• the rate of the forward reaction equals the rate of the reverse reaction

• the concentrations of the reactants and products are constant

what is Kc

the equilibrium constant, Kc = products divided by reactants

what do we get from the Kc value

the magnitude of Kc gives information about the relative reactant and product concentrations at equilibrium, the bigger the Kc the more products, the smaller the Kc the more reactants

what is the Kc value affected by

a change in temperature

what is Q

the reaction quotient, Q is calculated by substituting given concentrations into the expression, this is then compared to the Kc value

if Q < Kc

the product concentration is too low, the forward reaction will be favoured to each equilibrium

if Q > Kc

the product concentration is too high, the reverse reaction will be favoured to each equilibrium

what is the equilibrium position affected by

• a change in concentration

• a change in pressure

• a change in temperature

how does concentration affect equilibrium

if a reactant or product is added, the system will oppose the change by favouring the reaction (forward or reverse) that uses up the added chemical, and re-establishes equilibrium

how does pressure affect equilibrium

• increasing the pressure results in the system opposing the change by shifting in the direction of the lowest number of gas moles, this will decrease the pressure and re-establish equilibrium

• decreasing the pressure results in the system opposing the change by shifting in the direction of the greatest number of gas moles, this will increase the pressure and re-establish equilibrium

how does temperature affect equilibrium

• an increase in temperature causes the system to oppose the change, by shifting in the endothermic direction to absorb the added heat energy

• a decrease in temperature causes the system to oppose the change, by shifting in the exothermic direction to replace the lost heat energy

how does temperature affect the Kc value

• a change in temperature changes the ratio of products and reactants, and therefore the value of Kc

• if the temperature change shifts the equilibrium towards the products, the value of Kc is increased

• if the temperature change shifts the equilibrium towards the reactants, the value of Kc is decreased

what is an acid

a proton donor

what is a base

a proton acceptor

what is the equilibrium expression for the ionisation of water

Kw = [H3O+][OH-]

what is the value of Kw

1.00 × 10^-14

H3O+ vs OH- acidic

[H3O+] > [OH-]

H3O+ vs OH- basic

[H3O+] < [OH-]

H3O+ vs OH- neutral

[H3O+] = [OH-]

what is pH

the concentration of hydronium ions in a solution

what is pH used for

to express the acidity or basicity of an aqueous solution

calculate pH

pH = -log[H3O+]

calculate [H3O+] with pH

[H3O+] = 10^-pH

what is a strong acid

an acid that completely ionises

what is a weak acid

an acid that partially ionises

what is a strong base

a base that completely dissociates

what is a weak base

a base that partially ionises

what are the strong acids

• HCl

• HNO3

• H2SO4

• H2CO3

• H3PO4

• HBr

what are the weak acids

• carboxylic acids

• HF

• conjugates of weak bases

what are the strong bases

• metal OH (NaOH)

• metal O (MgO)

what are the weak bases

• NH3 (ammonia)

• NH2 (amines)

• conjugates of weak acids

what are neutral solutions

if a salt dissolves in water and neither of the dissociated ions react with water, the resulting solution is neutral

what is an example of a neutral solution

• NaCl → Na+ + Cl-

• neither the Na+ ions or the Cl- ions react with water, therefore no H3O+ or OH- are produced

• [H3O+] = [OH-], pH = 7

what is hydrolysis

when a salt dissolves in water and one of the dissociated ions reacts with water, the resulting solution is either acidic or basic

what are acidic solutions

if a salt dissolves in water and one of the dissociated ions reacts with water to produce H3O+, the resulting solution is acidic

what is an example of an acidic solution

• NH4Cl → NH4+ + Cl-

• NH4+ + H2O → NH3- + H3O+

what are basic solutions

if a salt dissolves in water and one of the dissociated ions react with water to produce OH-, the resulting solution is basic

what is an example of a basic solution

• NaCH3COO → Na+ + CH3COO-

• CH3COO- + H2O → CH3COOH+ + OH-