5.1 Flashcards
what’s the unit of rate
mol dm^-3 s^-1
what’s the relationship between rate of reaction and the concentration of a reactant raise to its order power
proportional
what is rate of reaction
the change in concentration of a reactant or a product per unit of time
what is 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 constant k
the constant that links the rate of reaction with the concentration of the reactants raised to the powers of their orders in the rate equation
what is the rate equation of A+B—>C if m is the order of A and n is the order of B
rate=k[A]^m[B]^n
how do you work out the overall order of a reaction
add together all the individual orders
what is the effect of a zero order reactant on the rate of a reaction
no effect, any concentration keeps same rate
what is the effect of a first order reactant on the rate of a reaction
linear effect - double conc means double rate
what is the effect of a second order reactant on the rate of a reaction
exponential effect - double conc means quadruple rate
how do you work out rate of reaction in an experience t
conc of product/time
what is half life (t1/2)
the time taken for the reactant concentration to decrease by half
how can you calculate the rate constant for first order reactions using half life
k=ln2/t1/2
how will a zero order concentration time graph look
linear, straight line decreasing steadily, half life decreases
what does the gradient in a concentration time graph represent
rate
how will a first order concentration time graph look
smooth curve with equal half lives
how will a second order concentration time graph look
steeper curve with an increasing half life
what should you be especially careful with when reading concentration time graphs
watching your axis: units and scale
what is the unit of k when you use a first order reaction and half life to calculate it
s^-1
what does a zero order rate concentration graph look like
a flat line parallel to the x axis
what does the gradient on a rate concentration graph represent
k
how does a first order rate concentration graph look
directly proportional
how does a second order rate concentration graph look
exponential graph
what’s the effect of a higher temperature on the rate constant k
increases value of k
what rate is used in rate equations
initial rate of reaction (tangent of the start of a concentration time graph)
why may one use a large excess of a reactant that one isn’t investigating the effect of on rate
to keep the concentration effectively constant, to make it not effect the reaction, act as zero order
what is the rate determining step
the slowest step in the reaction mechanism of a multi step reaction
how could you get values for a concentration time graph
continuous monitoring:
- monitoring by gas collection
- monitoring by mass loss
- monitoring using a colourimeter
what does a colourimeter measure
the intensity of light passing through a sample, the wavelength of light passing through a coloured solution is controlled by a filter, the amount of light absorbed by a solution is measured
what is the initial rate of reaction
the instantaneous rate at the start of a reaction when the time t=0
what is a clock reaction
a more convenient way of obtaining the initial rate of a reaction by taking a single measurement - the time t from the start of an experiment is measured for a visual change to be observed, the initial rate is then proportional to 1/t, repeat experiment with different concentrations
what’s an 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
how does order of reactants impact the rate determining step
order = number of moles of that reactant used in rate determining step
how must you approach a rate determining step question where you outline the steps in the reaction
make rds according to rate equation, MAKE SURE TO LABEL, try to make any products of overall equation, doesn’t have to be normal things, then use what’s left to make the rest
what’s a brønsted-lowry acid
a proton donor
what’s a brønsted-lowry base
a proton acceptor
what is a buffer
a mixture that minimises pH changes on additions of small amounts of acid or base
what type of acids do you use in buffers and why
weak acids because they don’t fully dissociate - reversible reaction
what are acid buffers made of
a weak acid and it’s sodium or potassium salt
what’s the general relationship between temp and rate
10°C increase means k roughly doubled, so rate doubled
what does k stand for in the arrhenius equation and units
k= rate constant (varied units)
what does A stand for in the arrhenius equation and units
A=frequency factor of collisions (pre-exponential factor) (units are always the same as k because the e^(-Ea/RT) part cancels out)
what does Ea stand for in the arrhenius equation and units
activation energy (Jmol^-1)
what does R stand for in the arrhenius equation and units
gas constant (8.314 J mol^-1 K^-1)
what does T stand for in the arrhenius equation and units
temperature (K)
how do you write out the arrhenius equation in the form y=mx+c
lnk = -Ea/R (1/T) + lnA
what are the axis for an arrhenius graph
lnk and 1/T
what’s a mole fraction
shows the proportion that a molecule accounts for of the total moles present
how do you calculate mole fractions
moles of substance/total moles present
how do you figure out total system pressure from partial pressures
add them all up
how do you work out partial pressure
mole fraction x total pressure:
moles/total moles x total pressure
how would you write the partial pressure of A
(P small A)
not square brackets becuase it’s not concentration
how do you calculate Kc
the concentration of the products divided by the concentration of the reactants raised to the power of the number of moles of that substance
what would be the Kc expression of
aA+bB <==>cC+dD
Kc= [C]^c [D]^d/[A]^a [B]^b
what are the units of Kc
varied, use the expression to work out units
how can Kc be found for a homogenous reaction
as normal
how can Kc be found for a heterogenous reaction
do as normal but do NOT include any solids present in reaction
what is Kp
the equilibrium constant used for gaseous equilibria
how do you calculate Kp
the product of partial pressures of the products over the partial products of the reactants with each partial pressure raised to the power of the moles of the substance present
what are Kp’s units
varies depending on the Kp expression
how would you calculate Kp for a homogenous reaction
as normal
how can Kp be found for a heterogenous reaction
as normal just excluding any solids
what factor affects the values of Kp and Kc
temperature because it shifts the position of equilibrium
what happens to the equilibrium constant if equilibrium shifts to the left to favour reactants
decreases
what happens to the equilibrium constant if equilibrium shifts to the right to favour products
increases
what does acid-base equilibria involve
the transfer of protons between substances
what is a conjugate acid
the species formed when a base accepts a proton
what is a conjugate base
the species formed when an acid donates a proton
what type of acid is suphuric acid
strong diprotic acid
what is a conjugate acid base pair
a set of two species that transform into eachother by gain or loss of a proton
how should you label a conjugate acid base pair equation
acid 1 and base 1 should be labelled as the pair not the first two you come across, draw square bracket links between the conjugate acid base pairs
if a conjugate acid base pair had a strong acid, what would the other component of the pair be
a weak base
what happens if you have a diprotic acid undergo a conjugate acid base pair reaction
it only loses one H+ at a time because conjugate acid base pairs only differ by one proton
is H2O an acid or a base
it’s amphoteric so can act as both
what does a bigger Ka mean with a weak acid
a strongER weak acid
what does a smaller Ka mean with a weak acid
a weakER weak acid
how is a weak weak acid represented in equilibrium
the less an acid dissociates the more the equilibrium moves to the left towards the whole acid
what assumptions do you make in weak acid Ka calculations and when does the assumption get worse
that the concentration of the acid stays the same, this gets less accurate the stronger the weak acid is because more of the acid dissociates
and that the concentration of water is constant, ignoring H3O+ on the top and H2O on the bottom, this also becomes more inaccurate the stronger the weak acid is
how should you treat the conjugate acid H3O+ in a Ka calculation
we ignore water so use [H+] instead
how do you work out percentage dissociation
[H+]/[HA] x100
how do you work out the [A-] if it’s not given but it’s a 1:1 molar ratio
use [H+]^2
what is a strong acid
an acid that fully dissociates into its ions when in solution
what is a weak acid
an acid that only partially dissociates into its ions when in solution
whats the pH of strong acids
0-2
what’s the pH of weak acids
3-7
what’s the pH of weak bases
7-11
what’s the pH of strong bases
12-14
what is Ka
the acid dissociation constant
what calculation does p mean as a prefix
-log10
how to get pKa from Ka
-logKa
how to get Ka from pKa
10^-pKa
what does a low pKa mean about the Ka and acid strength
large Ka and a strong acid
how to calculate pH from [H+]
-log[H+]
how to calculate [H+] from pH
10^-pH
how can you work out [H+] of a strong acid if you’re only given the concentration of the acid
it’s the same because they fully dissociate
what happens if you dilute a strong acid 10x
pH will increase by 1 unit because of the logarithmic scale
what happens if you dilute a weak acid by 10x
increase pH by less than one unit because weak acids aren’t fully dissociated in solution, so are a reversible reaction, which means that equilibrium shifts to counter the change
what does water do
slightly dissociates to form hydroxide and hydrogen ions in an equilibrium (constant Kw)
what’s the expression for Kw
Kw=[H+][OH-]
what’s the relationship between [OH-] and [H+] in the dissociation of water
they are equal to eachother
what’s the value of Kw at 25°C (room temp)
1.0x10^-14
Kw unit
mol^2 dm^-6
what happens to water as temperature increases (forward reaction is endo)
equilibrium shifts to right and forward reaction is favoured so it becomes more acidic because more H+ ions
what is a buffer solution
a system that minimises pH changes on additions of small amounts of acid or base
how are buffers formed
from a weak acid and it’s sodium or potassium salt
or
an excess of a weak acid and a strong alkali
what type of acids can be used as buffers
weak acids because they only partially disscoiate
when are buffers most effective
when pH=pKa +/- 1 (max tenfold difference between [HA] and [A-]
what quantity of the OH- ions is needed and why
large amount to react with excess H+ from an acid imbalance and to keep the ratio of acid to base in mixture almost constant
what happens if extra alkali is added to a solution with a buffer
the OH- reacts with H+ to make water
what happens if extra acid is added to a solution with a buffer
the A- ion reacts with it to make the original product, equilibrium shifts to the left
why must we use a salt with the acid in a buffer
to ensure large concentrations of both the acid and the A- ion
what’s the buffer calculation
Ka=[H+][A-]/[HA]
what’s the relationship between [H+] and [A-] in buffer calculations
they aren’t the same becuase there’s excess [A-] in buffers
what must happen when making a buffer out to a weak acid and a strong alkali
the weak acid MUST have more moles than the strong alkali, otherwise it will fully neutralise and the buffer will stop working
how do you find [HA] in a weak acid strong base buffer reaction
acid and alkali will neutralise eachother so work out how much acid is left in moles, convert moles into conc (use new acid volume)
how do you find [A-] in a weak acid strong base buffer reaction
moles of alkali reacted = moles of salt formed, work out salt conc = [A-]
how does a strong acid/ base look on a neutralisation titration curve
starts or ends horizontal at very high or low becuase theyre low or high pH and dissociate fully
and
sharp curve to vertical becuase fully dissociates quickly
how does weak acid/base look in a neutralisation titration curve
starts high/ low becuase weak and never is horizontal because it doesn’t fully dissociate
and never vertical because doesn’t fully dissociate
how can you know if an indicator is suitable for a neutralisation titration curve
if the pH range of the indicator is fully in the vertical stage
what happens when small volume of acids or bases are added to buffers
the pH can change 0.1 or 0.01 units of pH
uses of buffers
biological systems to regulate pH
shampoo
how to do obtain a pH titration curve
alkali is slowly added to acid or vice versa and the pH is regularly measured with a pH probe
what’s the neutralisation point of a strong acid base reaction
around 7
what’s the effect on neutralisation point when a strong acid weak base react
lower less than 7
what’s the affect on neutralisation point when a weak acid and strong base react
higher and bigger pH
what’s the neutralisation point of a wells acid and base reaction
normally 7
what do you call the vertical region of a titration curve
the equivalence point
when is pH equal to pKa according to the equivalence point
half the equivalence point
what indicator would you use for a more acidic neutralisation point
methyl orange
red in acid yellow at neutralisation point
what indicator would you use for a more basic neutralisation point
phenolphthalein
pink in alkali to colourless at neutralisation point
why should you only use a few drops of indicators
because they are weak acids and may affect the pH
why does the colour change occur in an indicator
due to an equilibrium shift between the HA and A- forms of the indicator
