2.1 Rates, Equilibrium And pH Flashcards
What units is the rate of reaction measured in?
Mol dm^-3 s^-1
How do you work out the rate of a reaction?
(Change in conc of reactant or product)/(time for the change to take place)
Usually plot a graph of concentration against time, then work out the gradient of a tangent to the curve
What is a shorthand way of writing ‘concentration of product in mol dm^-3)?
[product]
What happens to the conc of the reactants as a reaction proceeds? Why?
The concs decrease:
- Fewer collisions take place per second between reactant particles
- The rate slows down
What happens to the concentration of a product as a reaction proceeds?
The concentration increases
When is the rate of reaction fastest?
At the beginning of the reaction
For reactions involving acids or bases, what can you measure in order to monitor the rate of reaction?
- pH changes by carrying out titrations
- pH changes by using a pH meter
For reactions that produce gases, what can you measure in order to monitor the rate of reaction?
- The change in volume or pressure
- The loss of mass of reactants
For reactions that produce visual changes, what can you observe in order to monitor the rate of reaction?
- The formation of a precipitate
- A colour change (by using a colorimeter)
What is the order with respect to a reactant?
The power to which the concentration of the reactant is raised in the rate equation
What is the rate equation?
Take the reaction A + B + C —> products
Let the orders with respect to each reactant be A=0, B=1, C=2
Rate α [A]^0, rate α [B]^1, rate α [C]^2
So rate α [A]^0[B]^1[C]^2
So the rate equation is: rate = k [A]^0[B]^1[C]^2
Anything to the power of 0 is 1, so rate = k [B]^1[C]^2
What is the overall order of a reaction?
The sum of the individual orders
Definition of half-life of a reactant
The time taken for the concentration of the reactant to reduce by half
What happens to the concentration and half-life of a reactant with zero order?
Concentration decreases at a constant rate
Half-life decreases with time
What happens to the concentration and half-life of a reactant with first order?
Concentration halves in equal time intervals
Half-life is constant
What happens to the concentration and half-life of a reactant with second order?
Concentration decreases rapidly, but the rate of decrease then slows down
Half-life increases with time
What does the reaction rate depend on?
The rate constant and the concs of the reactants present in the rate equation
What is the key factor affecting the reaction rate?
The number of collisions that exceed the activation energy
What happens if the temp of a reaction is increased but the concs of the reactants stay the same?
The rate of reaction increases
The rate constant increases
What is the rate-determining step?
The slowest step in the reaction mechanism of a multi-step reaction
- The overall reaction can be no faster than the slowest step
How can you determine which reactants, and how many particles of each reactant, are involved in the rate-determining step?
- If a reactant appears in the rate equation, that reactant is involved in the rate-determining step
- The order with respect to the reactant tells you how many particles of the reactant are involved in the rate-determining step
What is an intermediate?
A species formed in 1 step of a multi-step reaction that is used up in a subsequent step, and is not seen as either a reactant or a product of the overall equation
How can you work out the equilibrium constant, (Kc), for the equation aA + bB cC + dD
Kc = ([C]^c x [D]^d)/([A]^a x [B]^b)
What are the units for Kc?
Depends on the Kc expression:
Unit for concs is mol dm^-3
These will cancel
If the value of Kc is 1, what does that mean?
That the position of equilibrium is halfway between reactants and products
If the value of Kc is greater than 1, what does that mean?
The reaction is product-favoured
If the value of Kc is less than 1, what does that mean?
The reaction is reactant-favoured
How does an increase in temperature affect the position of equilibrium?
It shifts it in the endothermic direction
If the forward reaction is endothermic: Kc increases
If the forward reaction is exothermic: Kc decreases
How does an decrease in temperature affect the position of equilibrium?
It shifts it in the exothermic direction
How does an increase in reactant concentration affect the position of equilibrium?
The equilibrium position shifts from left to right
How does an increase in pressure affect the position of equilibrium?
The equilibrium position shifts towards the side with fewer gaseous moles
How does the presence of a catalyst affect Kc?
It doesn’t: catalysts speed up both the forward and reverse reactions by the same factor
If a reaction is at a low temperature, how big are Kc and k (rate constant)?
Assume forward reaction is exothermic
Kc is large, but k is small
So high yield but slow reaction
If a reaction is at a high temperature, how big are Kc and k (rate constant)?
Assume forward reaction is exothermic
Kc is small but k is large
Low yield but quick reaction
What is oxidation?
- The loss of electrons
- An increase in oxidation number
What is reduction?
- The gain of electrons
- A decrease in oxidation number
What happens when an acid is added to water?
The acid dissociates, releasing protons into solution
What is a monobasic acid?
An acid that releases 1 proton per molecule
What is a dibasic acid?
An acid that releases 2 protons per molecule
- Done in 2 stages
What is a tribasic acid?
An acid that releases 3 protons per molecule
- Done in 3 stages
What is neutralisation?
A chemical reaction in which an acid and a base react together to produce a salt and water
What are hydronium ions?
H3O^+
- A hydrogen ion that is dative covalently bonded to a water molecule
- Represents protons in aqueous solution
What is an acid-base pair?
A set of 2 species that transform into each other by gain or loss of a proton
E.g. HNO2 + H2O >< H3O^+ NO2^-
What relationship is there between pH and [H^+] (hydrogen ion concentration)?
pH = -log[H^+] [H^+] = 10^(-pH)
What does a low pH value mean!
A large [H^+]
If the acid is strong, how much of it will dissociate in aqueous solution?
100%
What is a weak acid?
An acid that only partially dissociates in solution
What is the acid dissociation constant?
- An equilibrium constant that measures the actual extent of acid dissociation
- Ka = ([H^+][A^-])/[HA] for HA >< H^+ +A^-
- Units = mol dm^(-3)
What does a large Ka value mean?
There is a large extent of dissociation, so the acid is strong
What does a small Ka value indicate?
There is a small extent of dissociation, so the acid is weak
What are the pKa and Ka conversions?
pKa = -log(Ka) Ka = 10^(-pKa)
Which acids completely dissociate in water?
HCl
HNO3
H2SO4
How can you work out [H^+] for weak acids?
[H^+] = square root(Ka x [HA])
In water and neutral solutions, will the conc of H+ ions be greater than, less than or equal to the conc of OH- ions?
Equal to
In acidic solutions, will the conc of H+ ions be greater than, less than or equal to the conc of OH- ions?
Greater than
In alkaline solutions, will the conc of H+ ions be greater than, less than or equal to the conc of OH- ions?
Less than
If you know the conc of H+ ions in a solution at 25°C, how can you work out the conc of OH- ions?
Kw = [H+] x [OH-]
But we also know that Kw always = 1.00 x 10^-14 mol^2 dm^-6
How is the strength of a base measured?
By its dissociation in solution to generate OH- ions
- Strong bases (alkalis) dissociate more than weak bases
What are alkalis?
Bases that dissociate in water to release hydroxide ions
What is a buffer solution?
A mixture that minimises pH changes in addition of small amounts of acid or base
What is a buffer solution a mixture of?
- A weak acid, HA
- Its conjugate base, A^-
How can buffer solutions be made?
- From a weak acid and a salt of the weak acid
- From partial neutralisation (using an aqueous alkali) of a weak acid to give a solution containing a mixture of the salt and the excess of weak acid
How does a buffer solution work?
- The weak acid partially dissociates, forming its conjugate base and H^+ ions (H+ and A-)
- The salt of the weak acid completely dissociates, forming more if the conjugate base
- This means that there is a large reservoir of weak acid, HA, and a large reservoir of conjugate base, A-. The concentration of H+ ions is very small
How does a buffer work?
It minimises the pH by using the equilibrium:
HA H+ + A-
- The weak acid, HA, removes added alkali
- The conjugate base, A-, removes added acid
What happens when an acid, H+, is added to a buffer solution?
- [H+] is increased
- The conjugate base, A-, reacts with H+ ions
- The equilibrium shifts to the left, removing most of the added H+ ions
What happens when an alkali, OH-, is added to a buffer solution?
- [OH-] is increased
- The small conc of H+ ions react with the OH- ions, producing H2O
- HA dissociates, shifting the equilibrium to the right to restore most of the H+ ions that have reacted
How can you calculate the pH of a buffer solution?
Given conc. of HA, conc of salt of HA and Ka
What is the most important buffer system for controlling the pH of blood?
The carbonic acid-hydrogencarbonate ion buffer
- Carbonic acid (H2CO3) acts as the weak acid
- HCO3- acts as the conjugate base
What is the equivalence point?
The point in a titration at which the volume of 1 solution has reacted exactly with the volume of the 2nd solution
Describe the key features of an acid-base titration curve
- When the base is first added, the pH increases very slightly - the acid is in great excess
- Within 1-2 cm^3 of the equivalence point, the pH starts to increase more quickly. There is now only a small excess of acid present
- Eventually, there is a very sharp increase in pH brought about by a very small addition of base (usually just 1 drop)
- This shows on the titration curve as a vertical section
- The equivalence point is at the centre of this vertical section
- As further base is added, there is little additional change in pH - the base is now in great excess
How can a titration curve be made?
- The pH of an acid-base titration can be data logged continuously
- A titration curve can be plotted by a computer
Which indicators are suitable for a strong acid-strong base titration?
Phenolphthalein
Methyl orange
Which indicators are suitable for a strong acid-weak base titration?
Methyl orange
Which indicators are suitable for a weak acid-strong base titration?
Phenolphthalein
How is an indicator chosen for a titration?
- A weak acid is used
- Has 1 colour in its acid form and a different colour in its conjugate base form (the colour at the end point is midway between)
- The pH value of the end point of the indicator is as close as possible to the pH value of the titration’s equivalence
- A suitable indicator changes colour within the pH range in the vital section of the titration curve
Define ‘end point’
The point in a titration at which there are equal concentrations of the weak acid and conjugate base forms of the indicator
How can an acid-base indicator be represented?
HIn
- Acid form = HIn
- Conjugate base form = In-
Which indicators are suitable for a weak acid-weak base titration?
No indicators are really suitable
- As the base is added, the pH changes slowly through the equivalence point and there is no vertical section to the titration curve
- An indicator would change colour gradually over a few cm^3
What is the standard enthalpy change of neutralisation?
The energy change that accompanies the neutralisation of an aqueous acid by an aqueous base to form 1 mole of H2O under standard conditions
How can you determine the enthalpy change of neutralisation by an experiment?
- Measure the temperature change
- Work out the heat gained by surroundings, using Q = mcΔT
- Hence find the heat lost by the chemical system (change sign)
- Work out the amount, in mol, that reacted (n = c x v)
- Then scale the quantities to match the molar quantities needed to form 1 mole of H2O
Why do we get the same value for enthalpy change of neutralisation for different strong acids?
- Strong acids are completely dissociated
- The actual reaction taking place has the same ionic equation:
H^+ + OH^- —-> H2O - The other ions are spectator ions that do not take part in the reaction
Why do weak acids give a less exothermic value for the enthalpy change of neutralisation using strong acids?
- Weak acids are only partially dissociated
- Although the same neutralisation reaction takes place, most of the weak acid molecules must first dissociate to release H^+
- This requires energy, so the enthalpy change is slightly less exothermic
