module 5.1 - rates, equilibrium and pH Flashcards
what is a brønsted-lowry acid
- species that donates a proton
what is a brønsted-lowry base
- species that accepts a proton
what is different in an equation of a weak acid compared to a strong acid
- weak acid equation = reversible arrow
what is a conjugate acid-base pair
- pairs of molecules that are interchangeable by H+
what are monobasic, dibasic and tribasic acids
acids that can be classified by the number of bases that they can donate protons to in a reaction
what is the acid dissociation constant/ equilibrium constant of acids
Ka
difference between strong and weak acids
strong = fully ionise
weak = partially ionise
what is the extent of dissociation of weak acid determined by
Ka
rate equation for Ka
[products] / [reactants]
low Ka =
high pKa (weak acid)
high Ka =
low pKa (strong acid)
pKa =
- log (Ka)
Ka =
10 -pKa
what does pH depend on
- concentration of acid [HA]
- Ka - constant
approximations
- HA dissociates to produce equal concentrations of H+ and A-
- assume that weak acids don’t dissociate to a great extent so the overall concentration of HA doesn’t change much
generic equation of a weak acid
HA -><- H+ + A-
Ka equation using concentrations
[H+]2 / [HA]
calculating pH of a weak acid
[H+] = √ Ka x [HA]
pH = -log [H+]
pH =
- log [H+]
[H+] =
10 -pH
Kw valuw
1 x10 -14
Kw equation
Kw = [H+][OH-]
using Kw to find pH steps
- find conc of OH-
- use Kw = [H+][OH-] to find [H+]
- calculate pH
pOH method to find pH
- log [OH-]
- 14 - ans = pH
what is a buffer
system that minimises pH changes on addition of small amounts of an acid or base
what does a buffer contain
- a weak acid
- the conjugate base of the acid
what are the ways of preparing a buffer
route 1; mixture of weak acid and one of its salts. e.g ethanoic acid and sodium ethanoate
route 2; mixture of an excess of weak acid and a strong base (partial neutralisation) e.g excess ethanoic acid and sodium hydroxide
what experiment is used to find initial rate of a reaction
disappearing cross method
effect of zero order reactants
- changing concentration of reactant has no effect on rate
effect of first order reactants
changing concentration of reactant has a directly proportional effect on rate
effect of second order reactants
changing concentration of reactant changes rate by the square of the change
rate equation
rate = k[A]^a[B]^b
rules for rate equation
- only reactants appear in rate equation; not products
- rate equation does not have to match overall equation
what is on the x axis of a concentration time graph
time/s
what is on the y axis of a concentration time graph
[A] mol/dm^3
zero order concentration time graph
straight line down
first order concentration time graph
slight curve down
second order concentration time graph
steeper curve down
half life definition
time taken for half a reactant to be consumed
equation for half life
t1/2 = ln(2) / k
equation for rate constant using half life
K = ln(2) / t 1/2
zero order concentration rate graph
straight line across
- y intercept = k
first order concentration rate graph
straight line up
- gradient = k
second order concentration rate graph
curve upwards
what is the rate determining step
slowest step in a reaction
steps to writing rate determining step
- write rate equation
- use rate equation to write out another equation and create products that are found in equation
e.g rate = k[H2O2][I-] so H2O2 + I- -> H2O + IO- - find intermediates (if any) and use immediately in next reaction
e.g IO- = intermediate as it is not in the products of original equation
so IO- + I- + 2H+ -> I2 + H2O - find overall equation by crossing out intermediates/ substances that appear on both side of equation
arrhenius equation
K = Ae ^-Ea/RT
what is A in the arrhenius equation
- frequency factor
- how frequently molecules collide in the correct orientation
how does increasing temperature increase rate in arrhenius equation
- if T increases, Ea/RT decreases and e ^-Ea/RT increases so k increases and so does rate
how does adding a catalyst increase rate in arrhenius equation
- if Ea decreases (as a catalyst allows a reaction to occur by finding a lower activation energy), then Ea/RT decreases so e ^-EA/RT increases. and rate and k also increase
how to determine equation for a straight line using arrhenius equation
ln(k) = -EA/R x 1/T + ln(A)
y = m x + c
what is ln(a) in the arrhenius equation
y-intercept = ln(a)
what is -EA/R in the arrhenius equation
gradient
how to calculate Ea in graph version of arrhenius equation
Ea = gradient x R
Ea = kJmol so divide by 1000
