5.1 Rates, equilibrium and pH Flashcards
Define homogeneous equilibrium
An equilibrium in which all the specified making up the reactants and products are in the same physical state
Define heterogenous equilibrium
An equilibrium in which species making up the reactants and products are in different physical states
Most common ways of determining concentrations of reactants and products of equilibrium
Titration
Colorimeter
Drawbacks to using titration to determine concentration in equilibrium
Reactants or product must be removed and this will alter the position of equilibrium
Other substances present may affect the results of titration
Define mole fraction
Measure of how much of a given substance is present in a reaction mixture
Define partial pressure
The pressure an individual gaseous substance would exert if it occupied the whole reaction vessel
What happens when K»1 ?
Reaction is product favoured
The products on the right-hand side predominate at equilibrium
What happens when K«1?
Reaction is reactant favoured
Reactants on the left-hand side predominate at equilibrium
What is rate of reaction?
the change in concentration of a reactant or a product per unit time
What is the rate constant, k?
the constant that links the rate of reaction with the concentrations of reactants raised to the powers of their orders in the rate equation
What is the rate equation?
for a reaction A+B=C, rate = k[A]^m[B}n where m and n are orders of reaction with respect to A and B
What is the overall order?
The sum of individual orders, m+n
Units of k for zero order?
moldm-3s-1
Units of k for first order?
s-1
Units of k for second order?
dm3mol-1s-1
Units of k for third order?
dm6mol-2s-1
What is a half life?
the time taken for the concentration of the reaction to reduce by half
Techniques used to measure concentrations of reactants or products in a reaction?
- pH changes by carrying out titrations
- pH changes using a pH meter
- change in volume or pressure
- the loss of mass in reactants
- formation of a precipitate
- a colour change
Symbol for half life
t1/2
How can you find the order on a concentration-time graph?
By looking at the half life
Shape of a zero order reaction concentration-time graph?
Straight line graph, half life decreases with time
Shape of a first order reaction concentration-time graph?
Curved graph going downwards with a constant half-life
Which type of reaction has a constant half life?
First order reaction
How can you use half lives to find the rate constant?
By applying the formula:
k= (ln2)/(t1/2) to a first order reaction
How can you calculate the initial rate by looking at a concentration-time graph?
Tangent line drawn at t=0 and gradient calculated
Shape of a zero order rate-concentration graph?
horizontal line
changes in concentration have no effect on rate
Shape of a first order rate-concentration graph
straight line normal positive gradient
What is the gradient of a concentration-time graph?
Rate of reaction
What is the gradient of a rate-concentration graph?
k
What does a second order rate-concentration graph look like?
curvy and going up
Define reaction mechanism
a series of steps that, together, make up the overall reaction
Define rate-determining step
the slowest step in the reaction mechanism of a multi-step reaction
Define intermediate
A species formed in one 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
What are clock reactions?
Where you measure the time taken for a certain amount of product to form as you vary to concentration of one of the reactants
Assumptions made in clock reactions
- concentration of the reactant does not change significantly over given timescale
- constant temp
- when we observe the end point the reaction has not proceeded too far
How to find k from a rate-concentration graph
Gradient of FIRST order reactions
Which rate-concentration graphs pass through the origin?
All with orders greater than 0
What does the order of the reactants of the rate equations tell us?
How many molecules of the reactant are involved in the rate determining step
What is an Arrhenius plot?
a graph of lnk = lnA - (Ea/R) x (1/T), whee lnk is plotted against 1/T
What happens to k when temperature increases?
k increases
What is Ea in Arrhenius equation?
Activation energy
What is A in the Arrhenius equation?
pre-exponential factor
What is R in the Arrhenius equation?
gas constant
For every 10 degree increase in temperature, what is the change in rate?
Rate doubles
What does the Arrhenius equation tell us?
- temperature and the rate constant are related exponentially
- as temperature increases, rate constant increases
What is y in an Arrhenius plot?
ln k
What is in an Arrhenius plot?
ln A
What is m in an Arrhenius plot?
Ea/R
What is x in an Arrhenius plot?
1/T
What can an Arrhenius plot be used to identify?
- A, the y intercept is lnA
- Ea, gradient is -Ea/R
What does the equilibrium law tell us?
for the equilibrium aA + bB = cC + dD
Kc = [C]^c[D]^d/[A]^a[B]^b
What is a homogeneous equilibrium?
equilibrium in which all species making up the reactants and products are in the same physical state
What is a heterogeneous equilibrium?
equilibrium in which species making up the reactants and products are in different physical states
What colour is dinitrogen tetraoxide and what is its formula? (and state)
colourless gas
N2O4
What colour is nitrogen oxide, NO2?
Brown gas
Describe the dinitrogen tetraoxide decomposition equilibrium?
N2O4 (g) = 2NO2 (g)
- At first, conc of N2O2 is high and NO2 is zero but then NO2 rises and N2O2 falls
- colourless to brown
When does Kc change?
When temperature of the reaction changes
When do we get a dynamic equilibrium?
-rate of the forward reaction = rate of the backward reaction
Concentration of the products and reactants is constant
Examples of homogeneous equilibria
- decomposition of dinitrogen tetraoxide
- reaction between iodine gas and hydrogen
Examples of heterogenous equilibria
H2O (g) = H2O (l)
What do we not include in Kc expressions?
pure liquids or solids
Why do we not include pure liquids or solids in Kc expressions?
Their concentrations do not change
Most common ways of measuring concentraions?
- titration
- colorimeter
Problems with titration to find concentration
-either the product or reactant is being used up
this will move the equilibrium
-other chemicals, eg catalysts, may be present and change position of equilibrium
Equation for mole fraction
number of moles in substance A/ total number of moles of all substances
Equation for partial pressure
partial pressure = mole fraction x total pressure
What is Kp?
Equilibrium constant for gasses using partial pressures instead of concentrations
`What is total pressure?
The sum of the partial pressures
What does an equilibrium constant value of 1 indicate?
Position of equilibrium is halfway between the reactants and the products
What does K>1 indicate?
-reaction is product favoured
What does K<1 indicate?
-reaction is reactant-favoured
What does an increase in temperature do to equilibrium?
shifts in endothermic direction
delta H = positive
What does a decrease in temperature do to equilibrium?
shifts in the exothermic direction
delta H = negative
What is Le Chatelier’s principle?
change in the temperature, pressure or concentration of a system will result in the position of eqilibrium moving to counteract the change
What happens to K if the forward reaction is endothermic and the temperature increases?
K increases
What happens to K if the forward reaction is endothermic and the temperature decreases?
K decreases
What happens to K if the forward reaction is exothermic and the temperature increases?
K decreases
What happens to K if the forward reaction is endothermic and the temperature decreases?
K increases
What can change the value of K?
Temperature ONLY
What is a Brønsted-Lowry acid?
Proton, H+, donor
What is a Brønsted-Lowry base?
A proton acceptor
What is a conjugate acid-base pair?
A set of two species that transform into each other by gain or loss of proton
What is an alkali?
Base that dissolved in water forming OH- ions
Define neutralisation
A chemical reaction in which an acid and a base react together to produce salt and water
What is a strong acid?
An acid that completely dissociates in solution
What is a weak acid?
An acid that partially dissociates in solution
What is the acid dissociation constant and what is the symbol?
For an acid, HA, it is define as
Ka = [H+][A-]/[HA]
What is pKa?
-log Ka
What is Ka?
10^-pKa
Difference between strength and concentration
strength is the extent an acid dissociates into H+ and A-
conc is how may moles of acid are present in a volume
List of strong acids
HCl HNO3 H2SO4 HBr (Hydrobromic acid) HI HClO4 (chloric acid)
What measures the extent of acid dissociation?
Acid dissociation constant, Ka
What are the units of Ka always?
moldm-3
What does a large value of Ka indicate?
Large extent of dissociation
Strong acid
What does a small value of Ka indicate?
Small extent of dissociation
Weak acid
What is pH?
-log([H+(aq)])
What is [H+] equal to?
10^-pH
What is the relationship between pH and [H+] often described as?
see-saw, when one is up, the other is down
Ka equation for weak acids
Ka = [H+]^2/[HA]
Limitations of calculating pH of weak acids
- need to assume that so little of the original acid has dissociated that [HA] undissociated = [HA]dissociated
- If <5% of [HA] dissociates, it is deemed safe to use approximation
What do you need to know to calculate the Ka for weak acids?
- measure the pH of a solution of the weak acid using a pH meter
- concentration of weak acid
What is the ionic product of water defined as?
Kw = [H+][OH-]
At 25 degrees, Kw = 1.00 x 10^-14 mol2dm-6
Two features of a system in dynamic equilibrium
rate of forward reaction = rate of reverse reaction
concentrations of reactants and products are constant but they are constantly interchanging
What is the strength of a base judged on?
Their ability to dissociate and form OH- ions
Examples of strong bases
Hydroxides of group 1 and 2 metals
NaOH KOH and Ca(OH)2
Example of a weak base
Ammonia
What is a buffer solution?
a mixture that minimises pH changes on addition of small amounts of acid or base
What is a buffer solution a mixture of?
A weak acid (HA) and its conjugate base (A-)
can be weak acid and its salt which fully dissociates to form base
Why is the equilibrium of a buffer system pushed to the left?
The salt dissolves to form the conjugate base and so we have a lot of the conjugate base
The system counteracts this by pushing equilibrium towards the reactants
What pushes the equilibrium of a buffer system even further to the left?
The conjugate base and acid reacting to form the weak acid
What are there large reservoirs of in a buffer system?
Weak acid, HA
Conjugate base, A-
Alternative method of setting up a buffer
excess weak acid and strong alkali
forms the salt and water
What happens to a buffer system when we add a small amount of acid?
[H+] increases
H+ used up to keep Ka constant
What happens to a buffer system when we add a small amount of alkali?
Decreases [H+]
This is replaced by more of the acid dissociating to make sure Ka stays constant
What pH does blood plasma need to be?
Between 7.35 and 7.45
Which buffer system controls pH of blood plasma?
carbonic acid-hydrogencarbonate ion buffer
Describe the carbonic acid - hydrogencarbonate buffer system
carbonic acid, H2CO3 acts as the weak acid
hydrogencarbonate ion HCO3- acts as conjugate base
H2CO3 = HCO3- + H+
What is the equivalence point?
the point in a titration at which the volume of one solution has reacted exactly with the volume of the second solution.
This matches the stoichiometry of the reaction taking place.
What is the end point?
the point in a titration at which there are equal concentrations of the weak acid and conjugate base forms of the indicator. The colour at the end point is midway between the colours of the acid and conjugate base forms
What can be used to measure the pH of the reaction mixture as solution from a burette is added over time?
-pH meters or data loggers
What are on the axis of a typical acid-base titration pH curve?
pH on the y-axis
volume of base added on the x-axis
How is an indicator chosen in titrations?
so that the pH of the end point is equal to the pH of the equivalence point
pH range of methyl orange
3-4
pH range of phenolphthalein
8.3-10
pH range of bromothymol blue
6-7.6
What is an acid base indicator and what can we use to represent it?
A weak acid
We use the formula HIn with the equilibrium:
HIn = In- + H+
What is the HIn colour of bromothymol blue
+ the In- colour?
HIn = yellow (in acid) In- = blue (in base)
Things to consider when selecting a suitable indicator
- sharp colour change (if gradual we will not find equivalence point)
- end point of indicator must be near equivalence point
- clear colour change
What is the HIn colour of methyl orange
+ the In- colour?
HIn =red (acid)
In- = yellow (base)
What is the HIn colour of phenolphthalein
+ the In- colour?
HIn = colourless (in acid) In- = pinky purple (alkali)
What indicator do we use for strong acid - strong base titrations?
- methyl orange
- phenolphthalein
- 3-11
What indicator do we use for strong acid - weak base titrations?
- methyl orange
- 3-7.5 pH
- the salts formed between weak base and strong acid are acidic
What indicator do we use for weak acid - strong base titrations?
-phenophthalein
because of the weak acid, the pH is much higher before the neutralisation, alkaline salt forms when you mix a strong base and weak acid
6.5-11.5
Things to note about weak acid - weak base titrations?
- no vertical section
- neither indicator has an end point near equivalence point
- indicator would change gradually
Why do we get the flat sections of titration curves?
- When we add a strong base to a weak acid we get a salt, -is what we need for buffer solution
- pH doesn’t change much
How can we work out Ka from titration curve (combined equation)?
pH = pKa + log([salt]/[acid])
What is half-eqivalence pH the same as?
pKa
What factors determine the pH of a buffer solution?
-Ka /pKa /acid strength/amount of dissociation
-temperature
-ratio/amounts/concentrations of weak acid and
conjugate base/salt
Chemical formula of limestone
CaCO3
What is meant by a conjugate acid- base pair?
two species differing by H+
Equation for pH using OH in it
pH = 14 - pOH
