module 5 Flashcards
define rate of reaction
change in conc. of a product or reactant over a certain time
how can you calculate rate of reaction
change in conc/time (s)
what does it mean if a reactant is in zero order?
changing conc. had no effect on the rate
what does it mean if a reactant is in first order?
the rate is directly proportional to the concentration
what does it mean if a reactant is in second order?
change in rate = change in conc. squared
what is the generic rate equation
rate = k [X]^x [Y]^y
do zero order reactants appear on the rate equation? why?
no, as they do not affect rate
how is overall order determined
adding all of the orders
when the overall order is 3 what would the units of the rate be?
dm^6 mol^-2 s^-1
how can you measure rate experimentally?
use colorimeter at suitable intervals and see if colour changes
if gas involved use a gas syringe to collect or use a scale to measure mass lost
what is a half life
time taken for concentration of a reactant to reduce by half
what is the symbol for a half life
t1/2
what is the relationship between first order reactants and half life?
they have a constant half life
which equation is used to find rate constant using the half life in a first order reaction?
k = ln2/ t1/2
what is the relationship between rate and time
rate is directly proportional to 1/t
how to find rate constant from a rate conc. graph of first order
k = rate/conc.
what is the rate determining step?
the slowest step in a reaction with multiple steps
how does the rate determining step relate to the species involved in the rate equation?
any species involved in the RDS appear in the rate equation - species only involved after RDS do not appear in rate equation
for a reactant in the rate equation, what indicates how many molecules of that reactant are involved in the RDS?
the order of the reactant
what conditions affect the value of the rate constant
temperature
what is the effect of a 10 degree increase in temp on the rate of reaction
double
what is the Arrhenius equation? what does each term mean?
k = Ae^-Ea/RT k = rate constant A= pre-exponential factor e= mathematical quantity Ea = activation energy (j) R = gas constant T = temp (in kelvin)
how can you convert the Arrhenius equation into a useful form to plot a graph? how can you find each term?
Lnk = -Ea/RT - LnA
graph of Lnk against 1/T
gradient = -Ea/R and y Intercept is Ln A
what does it mean when a reaction is in equilibrium
the rate of the forward reaction is equal to the rate of the backward reaction
what are the methods to measure equilibrium?
- measure change in colour/colour intensity using colorimeter
- use pH probe
- measure electrical conductivity
- titration
what does mole fraction mean?
the amount of a given component in a reaction mixture
what equation is used to find the mole fraction?
mole fraction = no. moles of substance A/total mol.
what does partial pressure mean?
the pressure exerted by a single species in a reaction vessel
what equation is used to find partial pressure?
partial pressure = mol. fraction x total pressure
what is the relationship between conc. of a substance and its partial pressure?
conc. is proportional to partial pressure
in the reaction aA+bB=cC+dD what would the kp expression be?
kp = pC^c x pD^d / pA^a x pB^b
what does it mean if k (eq. constant) is greater than 1?
reaction favours product (right)
what does it mean if k (eq. constant)is less than 1?
reaction favours reactants (left)
what is the effect of increasing temp on k (eq. constant)
will shift in endothermic direction
what is the effect of decreasing temp on k (eq. constant)
will shift in exothermic direction
what is the only factor that effects k?
temperature
what is the effect of increasing temperature on k for a forward endothermic reaction?
k increases as temp increases
what is the effect of increasing temperature on k for a forward exothermic reaction?
k decreases as temp increases
what effect do catalysts have on k?
catalysts do no effect the equilibrium - only an effect on rate of reaction
what is a bronsted lowery acid?
proton donor
what is a bronsted lowery base?
proton acceptor
define Lewis acid
electron pair acceptor
define Lewis base
electron pair donor
what ion causes a solution to become acidic?
H+/ H3O+
what ion causes a solution to be alkaline?
OH-
what is the equation for the ionisation of water?
2H2O <=> H3O+ + OH-
or H2O <=> H+ + OH-
give an example of a mono basic acid
HCl
give an example of a dibasic acid
H2SO4
give an example of a tribasic acid
H3PO4
identify the acid base pairs for the following reaction:
CH3COOH + H20 <=> CH3COO- + H3O+
acid 1: CH3COOH
acid 2: H3O+
base 1: H2O
base 2: CH3COO-
define a strong acid
dissociate completely
give 3 examples of strong acids
sulphuric acid, hydrochloric acid, nitric acid
what is the difference between concentrated and strong?
conc = many mol per dm3 strong= amount of disassociation
define a weak acid
partially dissociates
give an example of any weak acid
methanol acid, (any organic acid)
what is the symbol for this acid dissociation constant
Ka
what does a larger Ka value mean?
Larger the Ka = greater extent of dissociation
what is the equation for pKa from Ka
pKa = -log10 Ka
what is the equation for Ka from pKa
Ka = 10^-pKa
what is the relationship between pKa and the strength of the acid
the smaller the pKa the stronger the acid
what is the equation to find pH from [H+]
pH = -log[H+]
what is the equation to find [H+] from pH
[H+] = 10^-pH
why is a pH scale useful rather than using H+ conc.
allows for a wide range of [H+] conc. to be expressed as simple positive values
what is the relationship between pH and [H+]
high pH value = small [H+]
if two solutions have a pH difference of 1 what is the difference of [H+]?
factor of 10
[H+] of strong acid is equal to what?
[HA]
what equation is used to find the [H+] of weak acids
[H+] = (sq. route)Ka x [HA]
what is the assumption made when calculating pH of weak acids
conc of acid at equilibrium = conc of acid after dissociation - because v little acid dissociates
what is the expression for ionic product of water - Kw
Kw = [H+][OH]-
what are the units for Kw
mol2dm-6
what is the value of Kw at 298k
1 x 10^-14
what physical values affect the values of Kw and how?
temp only - if temp is increased, equilibrium will move right, Kw will increase and pH of pure water will decrease
indices of [H+] and [OH]- should add up to what?
-14
define the term strong base
base that fully dissociates in water
give 3 examples of strong bases
NaOH
KOH
Ca(OH)2
give an example of a weak base
ammonia
what is the equation to find [H+] of strong bases?
[H+] = Kw/[OH}-
define a buffer solution
a mixture that minimises pH change on addition of small amounts of acid or base
what are the two ways a buffer can be made?
weak acid and conjugate base
weak acid and strong alkali
in which direction does the equilibrium shift when an acid is added to a buffer solution? why?
equilibrium shifts left because [H+] increases and conjugate base reacts with H+ to remove most of it
in which direction does the equilibrium shift when an alkali is added to a buffer solution? why?
equilibrium shifts right because [OH-] increases and the small conc of H+ reacts with it OH-. to restore H+ ions HA dissociates shifting the equilibrium
what equation is used to find [H+] of a buffer solution
[H+] = Ka x [HA]/[A-]
what buffer system maintains blood pH at 7.4? what happens when acid/alkali is added?
H+ + HCO3- <=> CO2 + H2O
add OH- = reacts with H+ to form H20, shifts equilibrium left to restore H+ lost
add H+ = equilibrium shifts right, removing excess H+
what is a titration?
the addition of an acid/base of known conc. to acid/base to determine conc. an indicator is used to show when neutralisation occurs, as is a pH meter
what is an equivalence point
the point at which the exact volume of base has been added to neutralise acid or vice versa
what is end point
the point at which pH changes rapidly
what are the properties of a good indicator of a reaction?
sharp colour change - no more than one drop of acid/alkali needed for the colour change
end point same as equivalence point (otherwise titration gives wrong answer)
distinct colour change - obvious when endpoint has been reached
what indicator would you use for a strong acid - strong base titration?
phenolphthalein or methyl orange
what indicator would you use for a strong acid - weak base titration?
methyl orange
what indicator would you use for a weak acid - strong base titration?
phenolphthalein
what colour is methyl orange in an acid, base and at end point
acid - red, base - yellow, end point -orange
what colour is phenolphthalein in acid and base
colourless in acid and red in base
what colour is bromothymol in acid and base
acid - yellow, base - blue
how do you use a pH meter?
remove pH probe from storage solution and rinse with distilled water
dry probe and place into solution with unknown pH
let probe stay in solution until it gives settled reading
define lattice enthalpy
the formation of one mole of an ionic lattice from gaseous ions under standard conditions
what does a more exothermic lattice enthalpy mean
more exothermic = stronger ionic bonds
why is it not possible to measure lattice enthalpy directly
it is not possible to form one mole of ionic solids from its gaseous ions
define enthalpy change of solution
enthalpy change that takes place when one mole of a solute is completely dissolved in water under standard conditions
define enthalpy change of hydration
the enthalpy change that takes place when dissolving one mole of gaseous ions in water
what are the factors that impact the size of lattice enthalpy
size of ions involved
charges on the ions
ionic bond strength
which ions have more negative lattice enthalpy values? why?
smaller ions because they can get closer, hence stronger attraction
describe hydration
when an ionic lattice is broken its ions become part of the solution
positive ions attracted to slightly negative oxygen and negative ions attracted to slightly positive hydorgeen
what are the factors that impact the magnitude of the enthalpy of hydration?
size of ion
charge on ions
define entropy
a measure of the dispersal of energy in a system which is greater when they system is more disordered
what is the symbol of entropy
s
which is more disordered, solid or gas?
gas
what is the unit of standard entropy
J K-1 mol-1
how does the temperature effect entropy?
greater temp = more energy = more movement
arrangement of particles will = more random
higher temp = higher entropy
when a solid ionic lattice is dissolved in solution, what happens to entropy?
entropy increases because ions are more disordered
how does a change in the no. of gas molecules in a reactions effect entropy
increase = increase in entropy decrease = decrease in entropy
what is the equation to calculate entropy
entropy = entropy (products) - entropy (reactants)
what is the gib’s free energy equation and what is each symbol for?
(delta)G = (delta)H - T(delta)S G = free energy H = enthalpy change T = temp in k s = entropy
when H is negative and S is positive what will G and the feasibility be
G will be negative and the reaction will be feasible
when H is positive and S is negative what will G and the feasibility be
G is positive and the reaction is not feasible
when H is positive and S is positive what will G and the feasibility be
G is negative (and therefore feasible) at high temperatures
when H is negative and S is negative what will G and the feasibility be
G is negative (and therefore feasible) at low temperatures
for a reaction to occur should (delta)G be positive or negative?
negative
what are the limitations of predictions of feasibility made by using (delta)G
reaction may have high activation energy
reaction may have low rate
define an oxidising agent
a species that is reduced in a reaction and causes another to be oxidised
define a reducing agent
a species that is oxidised in a reaction and causes another to be reduced
define oxidation
loss of electrons
define reduction
gain of electrons
what happens in a redox reaction
a reaction where electrons are transferred from one species to another
define standard electrode potential
the e.m.f of a half cell compared with a standard hydrogen half cell measured at 298k with 1 mol dm -3 and a gas pressure of 100kPa
what happens when a rod of metal is dipped in a solution of its own ions
an equilibrium is set up between the solid metal and aqueous metal ions
what is a standard hydrogen half cell made of
Hydrochloric acid 1moldm-3
Hydrogen gas at 100kPa
inert platinum electrode
why is a hydrogen half cell used as a standard half cell
easy to control purity and reproducibility
how do you make a simple salt bridge?
soak a piece of filter paper in KNO3, NH4NO3
why are salt bridges necessary
to complete the circuit by connecting two solutions - enables charge to be transferred between half cells and don’t react with electrodes
why may you use other standards electrodes occasionally
they are cheaper/easier/quicker and can be just as good
platinum is expensive
if an E value is more negative, what does it mean in terms of oxidising/reducing power?
Better reducing agent (easier to oxidise)
if an E value is more positive, what does it mean in terms of oxidising/reducing power?
better oxidising agent (easier to reduce)
how do you calculate emf from E values
Ecell = Epositive - Enegative
when would you use a platinum electrode?
when both oxidised and reduced form of the metal are in aqueous form
why is platinum chosen for electrodes
inert and good conductor to complete circuit
how can you predict if a reaction will occur according to Ecell
find E cell and if more than 0.4v reaction is feasible
what are the 3 main types of electrochemical cell
fuel
rechargeable
non-rechargeable
how do non-rechargeable cells work
provide electrical energy until all electrochemical cells have been reacted
how do rechargeable cells work?
chemicals In cell will provide electrical energy when recharging reactions of cells can be reversed
give an example of a rechargeable cell
lithium ion battery
lithium polymer battery
why is lithium used in laptop batteries
lithium has low density so electrode is light and very reactive
what are the drawbacks of lithium batteries
toxic if ingested
can cause fire/explosions due to rapid discharge of current
how do fuel cells work
cell uses external supplies of fuel and an oxidant, these external supplies need to be continuously supplied
modern fuel cells are based on what type of fuels?
hydrogen
hydrogen rich fuels (e.g. methanol)
what reactions take place at the two electrodes in an alkaline fuel cell
2H2 + 4OH- –> 4H2O + 4e-
O2 + 2H2O + 4e- –> 4OH-
what are the disadvantages of fuel cells?
hydrogen is a flammable gas with a low b.p. –> hard and dangerous to store and transport –> expensive to buy
fuel cells have a limited lifetime and use toxic chemicals in their manufacture
why can’t some cells be recharged
reaction of cell not reversible - product produced either dissipates or cannot be converted back into reactants
why may the e.m.f of a cell change?
conc. of ions change - reagents used up
how can e.m.f be kept constant?
reagents supplied constantly, conc. of ions constant, E remains constant
how do you write ionic equations
remove spectator ions from aqueous compounds
what is the rule regarding state of compounds in ionic equations
must be aqueous to be split into ions - remains in compound if sol, liquid or gas.
what is formed when an acid reacts with ammonia
ammonium salt
what are the units of Kw?
mol^2 dm^-6
what does a lower pKa suggest
stronger acid
what is a monoprotic acid
an acid which dissociates to produce one H+ ion for every acid molecule - conc. of acid = conc. of H+
how do you find the pH of a diprotic strong acid
-log(conc. x 2)
what are the assumptions made when using Ka
- only small amount of acid dissociates so can assume that [HA] at equilibrium is equal to [HA] at start of reaction
- dissociation of acid is greater than dissociation of water present in solution - assume all [H+] is from acid - so [H+] is equal to [A-]
how do you use a pH meter
- place in distilled water - should read 7.0
- repeat with standard solutions of pH 4.0 and 10
- rinse with distilled water between each solution
why can’t you use an indictor to measure the pH change with a weak acid and weak base
no sharp pH change
what is the name given to Kw?
ionic product
what factors determine the pH of a buffer solution
- Ka/pKa/acid strength / amount of dissociation
- temperature
- ratios of weak acid: conjugate base
which direction of reaction does a higher temperature favour
endothermic
what are two features in dynamic equilibrium
- conc. of reactants and products are constant but are constantly changing
- the rate of both forward and backward reaction are equal
if you need to write an equation which is not redox what should you make sure happens to the oxidation state of each element
remains the same
what are the acid equations for a fuel cell
H2 –> 2H+ + 2e-
4H+ + O2 + 4e- –> 2H2O
what are the alkali equations for a fuel cell
O2 + 2H2O + 4e- –> 4OH-
4OH- + H2 –> 4H2O + 4e-
what happens to ions in acid/alkali fuel cells?
alkali - OH- ions move right to left
acid - H+ ions move left to right
what is the equation for the MnO4 titration with iron
MnO4(-) + 8H(+) + 5Fe(2+) –> Mn(2+).+ 5Fe(3+) + 4H2O
what are the equations for the iodine - sodium thiosulphate titration
IO3(-) + 5I(-) + 6H(+) –> 3H2O + 3I2
I2 + 2S2O3(2-) –> S4O6 (2-) + 2I(-)
what is the colour change in the iron MnO4 titration?
purple –> colourless
define the ionic product (Kw)
conc. of H+ ions multiplied by conc. of OH- ions
why is it unlikely for a reaction to occur between HNO3 and H2SO4 if it is in water?
water is a stronger base (more likely to accept a proton) than HNO3
what will the pKa value of HNO3 be if the pKa value of H2SO4 is -3 (H2SO4 is stronger)
- any value less negative than -3
- HNO3 is weaker and will therefore have a larger pKa
what are two common assumptions when calculating pH of weak acids
- [H+] and [A-] are equal
- [HA] is the same at equilibrium and undissociated
what should be written in the question asking why a buffer solution is formed
- write equation
- some acid is in excess
how do you find A in an Arrhenius graph
- y intercept = lnA
- A = e^lnA
how do you find Ea from an Arrhenius graph
- find gradient
- multiply by negative gas constant (as -Ea/RT)
how do you rearrange the Arrhenius equation to find the activation energy
Ea = (lnA - lnK) RT
how do you rearrange the Arrhenius equation to find the temperature
T = Ea/R(lnA-lnK)
what does the gradient equal in a first order graph
k
if rate = k [A] ^2 [B] and A and B are both tripled what factor will the rate increase by
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