Ch5.1 Rates, equilibrium and pH Flashcards
What does a rate equation allow you to do?
- to calculate exactly how fast a chemical reaction will proceed
What is the general form of the rate equation?
rate = k [A]ᵐ [B]ⁿ
k = rate constant
A and B = concs of reactants a and b
m and n = the orders of reaction with respect to each reactant
the orders can be 0, 1 or 2
What must be considered before constructing a rate equation?
- think carefully about how to design our experiment
- decide what we’re going to record as a measure of reaction rate eg. vol of gas
- a trial experiment will establish a suitable range of concs to use
- we decide on actual values for variables such as concs
Describe the method for determining the orders of reaction
- keep conc of A the same and systematically change B
- draw a reaction curve for each experiment
- draw a tangent on each curve at the initial rate
- plot the gradients against the concs on a new graph
- flat line = 0, straight diagonal = 1, curve upwards = 2
- repeat for any other reactants
When would the half-lives method be used to work out the orders of reaction in a rate equation?
if nothing in a reaction is visibly measurable eg. esterification, 2 clear colourless liquids react to produce 2 clear colourless liquids
Describe the half-lives method
- mix a large volume of the reaction mixture in a big beaker
- at fixed intervals (usually 1 min) withdraw small samples of the mixture with a volumetric pipette and quench each to stop the reaction
- when sufficient samples have been collected/quenched, carry out titrations on each
- plot graph of conc falling against time
- plot 3 successive half-lives
What will the 3 successive hallf-lives tell you at the end of the half-lives method?
- if half-lives decrease, the reaction is 0 order
- if they remain constant, the reaction is 1 order
- if they increase, the reaction is 2 order
What 2 things can a rate equation tell you?
- how quickly the reaction will proceed for any given concs
- how many steps there are likely to be in the reaction which will help us to work out the mechanism
What is the rate-determining step?
- the slowest step in a reaction
- it determines the overall speed
How can the rate equation tell you the rate-determining step?
- if the conc of a reactant is in the rate equation, then that reactant or something derived from it takes part in the slow step of the reaction
- if the conc of a reactant doesn’t appear in the rate equation then neither the reactant nor anything derived from it participates in the slow step
If the equation for the reaction of CO and NO2 is this:
CO+NO2 -> NO+CO2
and the rate equation for this is:
rate = k[NO2]^2,
what does this tell us?
- the rate determining step (usually the first step) involves two molecules of NO2 and nothing else
- there must be a separate and faster step involving the products of the first step and CO
- if 2 molecules of NO2 are used in the first steps, we must produce one in the second step to be consistent with the overall equation
What should be the only thing changing when we work out the rate equation for a particular chemical reaction?
the only variable that changes should be conc
What is the Arrhenius equation?
k=Ae^(-Ea / RT)
k= rate constant
A = pre-exponential factor
T = temp in K
R = gas constant
Ea = activation energy
If concentrations are kept the same, what will happen to the value of k as the temp increases?
it will increase
What is the shape and bond angles of an oxonium ion?
pyramidal 107’
Where do acidic properties come from?
the reaction of H+ ions with water to make oxonium ions
word equation for metal and acid
metal + acid -> salt + hydrogen
word equation for metal oxide and acid
metal oxide + acid -> salt + water
word equation for metal hydroxide and acid
metal hydroxide + acid -> salt + water
word equation for metal carbonate and acid
metal carbonate + acid -> salt + water
What happens when an acid is mixed with water?
when an acid donates a proton in the presence of water, the water behaves as a base however the water has become H3O+ which is now an acid
acid in reactants becomes conjugate base and base in reactants becomes conjugate acid in products
What happens to bases and acids during neutralisation?
- bases become acids
- acids become bases
What are all acid-base reactions?
REVERSIBLE
they are EQUILIBRIUM reactions
What is Ka?
- acid dissociation constant
if equation is
HA ⇌ H+ + A-
where A is an acid
Ka = ([H+][A-]) / [HA]
How are the dissociations of strong and weak acids different?
strong= complete dissociation
weak= partial dissociation
What is the general rule for which acids are strong and weak?
strong acids are inorganic and weak acids are organic (contain carbon)
Describe the Ka values of strong and weak acids
strong = high Ka value (always >1)
weak= small (between 0 and 1)
What is the formula for calculating pH?
pH= -log10[H+]
Describe how monobasic, dibasic and tribasic acids behave in dissociation
- monobasic= split apart completely to produce 1 H+ ion
- dibasic = split apart completely to produce 2 H+ ions
- tribasic = split apart completely to produce 3 H+ ions
What is Kw?
- the ionic product of water
Kw = [H+][OH-] = 1x10^-14
What assumptions are made for weak acid calculations?
2 CONTRADICTORY ASSUMPTIONS
- none of the weak acid dissociates (this isn’t as strange an assumption as it seems, for ethanoic acid, 1/1000 molecules dissociate)
- obviously some molecules have to dissociate and when they do, they give equal numbers of [H+] and [A-]
After assumptions have been made about weak acid calculations, what conclusion can be made?
Ka= ([H+][A-])/[HA]
= [H+]^2/[HA]
What is a buffer?
- a solution which is capable of neutralising small additions of H+ and OH-
- this maintains the pH of the solution
Which 2 chemical processes are buffers important for and why?
- most dyes are indicators, if pH isn’t kept constant while they’re being used then they change colour
- many drugs exist as optical isomers (they have right and left handed forms). right-handed forms can be harmful so they need to be screened out during manufacture. using an enzyme in one of the reaction steps ensures that only left-handed isomers are produced. buffer needed so enzyme doesn’t denature
