5.1 Flashcards
What is the rate of reaction?
The rate at which products are formed.
Rate of reaction = Δ [products or reactants] / time
What is the order and overall order of a reaction?
The order defines the power of the relationship to which the concentration of a reactant affects the rate of the reaction.
The overall order is the sum of all the orders.
What is the half-life and how can it be used to identify the order of a reaction?
The time it takes for the concentration of a reactant to halve.
The half-life is constant for a first order reaction. This can be used to differentiate between first and second order reactants’ concentration-time graphs. Zero order reactants have a constant gradient.
What is the rate determining step?
The slowest step in a multistage reaction.
What is the rate equation?
rate = k [A]^m [B]^n, where solids are not considered
How is the half-life calculated?
k = ln2 / half-life
What conclusions can be drawn from rate-concentration graphs?
If the graph shows a completely horizontal line (so one where the rate is unaffected by the concentration) the reactant is zeroth order.
If the graph has a constant gradient going through (0,0) (so the rate is directly proportional to the concentration) the reactant is first order.
If the graph shows a curve going through (0,0) (so the rate is disproportionally affected by the concentration) the reactant is second order.
How can k be found from a rate-concentration graph?
k is the gradient of rate-concentration graphs but only for first order reactions.
What are the techniques and procedures used to investigate reaction rates?
Initial rates method and continuous monitoring (colour, mass loss ect.)
change the initial conditions and see how this affects the initial rate.
What does the rate equation show?
The stoicheometrty lof the rate determining step.
When suggesting possible mechanisms, always have the first step producing one of the overall products.
What factors affect the rate constant?
Temperature. Higher temperatures increase the rate constant.
What is the mole fraction?
moles of gas A / total moles of all gases
What is partial pressure?
Partial Pressure is the pressure each gas would exert if it occupied the entire vessel on its own at the same temperature.
Partial pressure of A = mole fraction x total pressure
What are the expressions for Kc and Kp?
Kc = [A]^m / [B]^n
Kc does not consider solids and is only affected by temperature.
Kp = p^m (A) / p^n (B)
Kp only considers gases and is only affected by temperature.
What conditions affect the equilibrium constants?
Temperature. Higher temperatures increase the rate constant. For all other conditions the equilibrium position will shift in the direction which opposes the change and restore the value of Kc/Kp
What are acids and bases?
Acids are proton donors .
Bases are proton acceptors.
What are monobasic, dibasic, and tribasic acids?
Monobasic: HCl, HNO3
Dibasic: H2SO4
Tribasic: H3PO4
What is a conjugate acid-base pair?
HCl + H2O —-> H3O+ + Cl-
acid 1 base 1
base 2 acid 2
What is there to know about Ka?
HA <-> H+ + A-
Ka = [H+][A-] / [HA]
Since Ka can be so large we use the logarithmic scale given by: pKa =-logKa.
The smaller the value of pKa, the stronger the acid.
What is the relationship between [H+] and pH?
pH = - Log([H+])
For strong mono basic acids which completely deionise in water [H+] = [Acid]
For weak acids which partially dissociate, Ka may need to be used: Ka = [H+]^2 / [HA]
For these approximations: the extent of disassociation is so small that [HA]»_space; [H+], any water present will have disassociated by such a small amount that it will not affect the concentration of [H+] so [H+] = [A-], and [HA] - [H+] = [HA].
What is Kw?
Water can act as both an acid (-> H+ + OH-) and a base (H+ -> H3O+).
Water exists in equilibrium: H2O <-> H+ + OH-
This is known as the ionisation of water. The equilibrium lies well to the left.
Kw = Kc x [H2O] = [H+][OH-] where [H+]=[OH-]
Kw is called the ionic product of water.
At 25°C Kw = 1 x 10^-14 mol^2dm^-6.
The pH of a strong base can be calculated by knowing that [H+] = Kw / [OH-]
where [OH-] = [strong monobasic alkali].
What are buffer solutions?
A buffer solution minimises pH changes on addition of small amounts of an acid or base.
A buffer solution is made from:
(1) (CH3COOH/NaOH)
A weak acid and a strong alkali.
(2) (CH3COOH/CH3COONa)
A weak acid and a salt of a weak acid. The weak acid partially dissociates whilst the salt of the weak acid salt completely dissociates so: HA <-> H+ + A-
On addition of an acid: [H+] is increased, the conjugate base, A-, reaches with the H+ ions and the equilibrium shifts to the left, removing most of the added H+ ions.
On addition of an alkali, [OH-] is increased, the small concentration of H+ ions react with the OH- ions producing water, so HA dissociates, shifting the equilibrium to the right to restore the reacted H+ ions.
For a type (2) buffer: Ka = [H+][A-] / [[HA] where [HA] equilibrium = [HA] undissociated and [A-] = [salt].
What is an example of a buffer system?
The carbonic acid–hydrogencarbonate buffer system which keeps the blood between a pH of 7.35-7.45.
The weak acid is carbonic acid, H2CO3.
The conjugate base is HCO3-
SO, H2CO3 <-> H+ + HCO3-
Most materials released into the blood are acidic so are converted into H2CO3 which can be turned into aqueous CO2 with an enzyme which can then be converted into a gas and exhaled in the lungs.
What is the equivalence point?
The point at which the acid has exactly reacted with the alkali.
The centre of the vertical section of the titration curve.
How do indicators work?
Indicators are often represented as HIn where they undergo: HIn <-> H+ + I-
When there are equal amounts of weak acid and conjugate base, the indicator has reached it’s end point.
How is a pH probe calibrated?
The probe is rinsed with water.
It is then blotted dry.
It is then placed in solutions of known pH to ensure that it is accurate.
[titration curve diagrams needed]
