5.1 rates, equilibrium and pH Flashcards
define rate of reaction
change of concentration of reactant or product per unit of time
give the equation for overall rate of reaction
change in conc of reactant or product ÷ time
what is order of a chemical?
order with respect to a chemical shows how changing concentration of the chemical affects rate of reaction
what is overall order?
the orders of each reactant added up
what is the rate determining step?
the slowest step in a chemical reaction
zero order:
- describe the conc-time graph a chemical with this order
- how does changing the conc of a chemical of this order affect rate?
- how does the half life on a conc-time graph change over time?
- linear graph, negative gradient
- changing conc does not affect rate
- half life decreases over time
first order:
- describe the conc-time graph a chemical with this order
- how does changing the conc of a chemical of this order affect rate?
- how does the half life on a conc-time graph change over time?
- curved, negative gradient, starts steep and gets shallower
- changing conc changed rate by the same factor
- half life is constant
second order:
- describe the conc-time graph a chemical with this order
- how does changing the conc of a chemical of this order affect rate?
- how does the half life on a conc-time graph change over time?
- just like first order graph but starts VERY steep
- changing conc changes rate by the same factor squared
- half life increases with time
what would be the rate equation of
A + B → C
rate = k[A]ˣ[B]ʸ
where x and y are the orders of reactants A and B
how do you find the rate constant and its units?
- work out order of all reactants
- substitute in known values of the rate and concentrations of reactants
- rearrange to find k
- units are derived from units of rate and concentrations to the power of their orders
describe the rate-concentration graph of a zero order chemical
horizontal line
describe the rate-concentration graph of a first order chemical
linear
starting at origin
positive gradient
describe the rate-concentration graph of a second order chemical
exponential
starting at origin
positive gradient
how do you find the rate constant from a first order rate-concentration graph?
k = gradient
ONLY for first order
how do you find the rate constant from a first order concentration-time graph?
k = ln2 ÷ half life
ONLY for first order
what are two rules regarding order, rate equations and the rate determining step?
- if a chemical is NOT in the rate equation it is NOT in the RDS.
- the order of a chemical = the number of molecules in the RDS.
in reaction mechanisms, what are intermediates?
species formed that do not appear in final equation, they are catalysts
give the 2 equations of the arrhenius equation
name each variable and give the units
k = Ae^-Ea/RT lnk = -Ea/RT + lnA
k = rate constant (units to be determined) A = pre-exponential factor (same units as k) Ea = activation energy (Jmol-1) R = gas constant 8.314 (JK-1mol-1) T = temp in KELVIN
what can the arrhenius equation be approximated to? what do each part of the arrhenius equation represent in the approximation?
equation of straight line - y = mx + c y = lnk m = -Ea/R x = 1/T c = lnA
plotting arrhenius equation:
what goes on the x and y axis?
y axis = lnk
x axis = 1/T
plotting arrhenius equation
how do you find Ea from the graph?
- find gradient of the line of best fit using a triangle
- multiply gradient by gas constant 8.314 to find Ea in Jmol-1
- divide by 1000 to find it in kJmol-1
plotting arrhenius equation
how do you find A or the pre-exponential factor from the graph?
- extrapolate backwards to the y axis and find the y intercept
- lnA = y intercept so put this number to the power of e
OR - once you have found the Ea, substitute data from the table into either of the equations
- rearrange to find A
what is a homogenous system?
a reaction where all reactants and products are the same state
what is a heterogenous system?
a reaction where more than one state of matter is present in reactants and products
in the equation:
aA + bB ⇋ cC + dD
what is the Kc equation?
Kc =[C]^c[D]^d / [A]^a[B]^b
product conc / reactants conc
what states of matter are included in the Kc equation?
only aqueous and gases concentrations are included
how do you find the moles/concentration of substances in equilibrium from a reaction?
using RICE tables:
R - reaction and molar ratios go here
I - initial moles of reactants and initial mol of products = 0
C - change of moles: changes with molar ratios: opposite operation happens to products and reactions (eg. if change is +ve for reactants its -ve for products)
E - equilibrium moles in this row
what does the equilibrium constant, Kc, indicate?
the position of equilibrium
where is the position of equilibrium when:
- Kc = 1 ?
- Kc < 1 ?
- Kc > 1 ?
- Kc «_space;1 ?
- Kc»_space; 1 ?
- in the middle of products and reactants
- favours reactants
- favours products
- lies far to the left
- lies far to the right
what is the only factor that affects the equilibrium constant?
temperature
what effect does increasing/decreasing temperature do to position of equilibrium?
increasing - to endothermic direction
decreasing - to exothermic direction
what happens to Kc when you change concentration or pressure? why?
Kc doesn’t change because the system works to restore Kc to it’s original value
what is Kp
same constant as Kc but instead of concentration, partial pressure of gases are used
define mole fraction and give the equation
proportion of given substance present in the reaction mixture
mole fraction = moles of compound/total moles of products and reactants
define partial pressure and give the equation
the pressure exerted by an individual gas if it occupied the whole vessel on its own
partial pressure = mole fraction x total pressure
give the equation of Kp given the reaction
aA + bB ⇋ cC + dD
what must you use in the equation?
partial pressure of each gas raised to the power of their stoichiometry
Kp = (Pc)^c (PD)^d / (PA)^a (PB)^b
must use ROUND brackets
how do you find the value of Kp?
use a rice table but add rows M (mole fraction) and P (partial pressure)
give the bronsted lowry (BL) definitions of an acid and base
acid - proton donor (loses H+)
base - proton acceptor (gains H+)
what are three names given to acids? what do these names refer to?
monobasic/protic
dibasic/protic
tribasic/protic
names refer to how many H+ ions the acid loses
what is a conjugated acid and base pair?
two species that can transform into each other by loss or gain of H+ ions
in this reaction
NH₃ + HCl ⇋ NH₄⁺ + Cl⁻
what are the conjugated acid-base pairs?
NH₃ is the BL base and NH₄⁺ is the conjugate acid because it can LOSE a H+ ion to become NH₃
HCl is the BL acid and Cl⁻ is the conjugate base because it can GAIN a H+ ion to become HCl
how do you find the concentration of H+ ions when given the pH?
[H⁺] = 10 ^-pH
10 to the power of -pH
how do you find pH of a solution when given the concentration of H⁺ ions?
pH = -log[H⁺]
negative log of the conc of H+ ions
pH increases = [H⁺] ions _______
decreases
pH decreases = [H⁺] ions _______
increases
what is assumed about the concentration of a strong acid?
in this strong acid HA —> H⁺ + A-
[HA] = [H⁺] is assumed because we assume all H+ ions completely disassociate
derive the equation involving the equilibrium constant for acids
HA —> H⁺ + A-
Ka = [H⁺][A-] / [HA]
[H⁺] = [A-] so numerator can be written as [H⁺]²
multiply by [HA] and square root to get
[H⁺] = √(Ka[HA])
how do you find the pKa value?
pKa = -log(Ka) Ka = 10^-pKa
what does Kw symbolise?
- Kw is the equilibrium constant for the dissociation water
- the ionic product of water
- Kw = [H⁺][OH-]]
- Kw = 1x10^-14
