a level physical

what is pressure?

the force exerted by gas molecules in a given volume

what is partial pressure?

the portion of the total pressure exerted by the presence of a single type of gas molecule

give the eqn for the mole fraction:

no. of moles of a gas / total no. of moles of gas

what does the sum of the mole fractions always = ?

1

what is K_p? what is it deduced from?

• the equilibrium constant calculated from partial pressures for a system at constant temperature

• deduced from the eqn for a reversible reaction occurring in the gas phase

give the eqn for partial pressure:

partial pressure = mole fraction x total pressure

how do you write a K_p expression and deduce its units?

• same as K_c - instead of using [], use () and write pp before each thing (to show it’s a partial pressure)

• deduce units by cancelling (same as K_c)

how does temperature affect K_p?

• if the temperature change causes the equilibrium to shift right, K_p increases

• if the temperature change causes the equilibrium to shift left, K_p decreases

how does a catalyst affect K_p?

no effect!

why might K_p have no units?

moles of products = moles of reactants (so units cancel)

what is an acid?

acid = H⁺ donor

what is a base?

base = H⁺ acceptor

what does acid-base equilibria involve?

transfer of protons

what denotes a strong acid?

acid which fully dissociates in soln (e.g. HCl → H⁺ + Cl^-)

what is a concentrated acid?

acid w/ a larger amount of acid particles in a given volume

what is the formula used to calculate pH?

pH = -log₁₀ [H⁺]

what is the formula used to calculate [H⁺] ions?

[H⁺] = 10^-pH

give the general eqn for acids in aqueous soln:

• HA + H₂O → H₃O⁺

•  OR HA → H⁺ + A^-

what is a monoprotic acid? give 2 examples: 

• acid which donates one proton for every acid molecule

• so [H⁺] = 1 mol dm^-3

• e.g. HCl, HNO₃

what is a diprotic acid? give an example:

• acid which donates 2 protons for every acid molecule 

• so [H⁺] = 2 mol dm^-3

• e.g. H₂SO₄

what is a triprotic acid? give an example:

• acid which donates 3 protons for every acid molecule

• so [H⁺] = 3 mol dm^-3

• e.g. H₃PO₄

give the expression for the ionic product of water, K_w:

K_w = [H⁺][OH^-]

explain why [H₂O] is not shown in the K_w expression:

[H₂O] is (almost) constant

explain why the value of K_w increases as the temperature increases (2)

• equilibrium is endothermic

• equilibrium shifts to the RHS to minimise/oppose temp increase (so more H⁺ and OH^- formed)

explain why water is neutral:

[H⁺] = [OH^-] - dissociation of each water molecule gives 1 H⁺ and 1 OH^-

a pH meter is calibrated using a calibration graph - to create the calibration, the pH meter is used to measure the pH of separate sons, each w/ a known, accurate pH.

suggest why the pH probe is washed w/ distilled water between each of the calibration measurements (1)

either:

• different solns must not contaminate each other

• to wash off any residual soln/substance (which could interfere w/ the reading)

a pH meter is calibrated using a calibration graph - to create the calibration, the pH meter is used to measure the pH of separate sons, each w/ a known, accurate pH.

the calibrated pH meter is used to monitor the pH during a titration of HCl w/ NaOH - explain why the vol of NaOH soln added between each pH measurement is smaller as the end point of the titration is approached (1)

to avoid missing the endpoint

state why all 3 of the indicators in Table 2 are suitable for this titration (1)

all have a colour change/pH range w/in the stepp/vertical part of the titration curve

for pure water at 40^oC, pH = 6.67.

a student thought that the water was acidic - explain why the student was incorrect (1)

[H⁺] = [OH^-]

suggest briefly a practical procedure that a student could use to obtain data from which the curve in the graph could be plotted (3)

• place a fixed vol of alkali in a flask or beaker

• add acid in small portions from a burette

• stir and use a pH meter to record the pH after each addition of acid

the student was provided w/ samples of 3 different indicators - suggest how the practical procedure could be refined by the student to identify the most suitable indicator (2)

• repeat the experiment w/ each indicator

• select the indicator that changes colour rapidly when the pH changes from about 7 to 4

explain briefly why a pH meter should be calibrated before use (1)

over time/after storage meter does not give accurate readings

what denotes a weak acid? give the eqn for this:

• only slightly dissociate in aq soln

• HA ⇌ H⁺ + A^-

what denotes a weak base?

dissociates only slightly in aq soln

give the expression for K_a, the acid dissociation constant:

Ka = [H⁺][A^-] / [HA]

give the expression for pKa:

pK_a = -log₁₀ (K_a)

give the expression for calculating Ka from pKa:

Ka = 10^-pK_a

how does pK_a vary w/ the strength of the acid?

• the larger the pK_a, the weaker the acid

• the smaller the pK_a, the stronger the acid

explain how K_a varies w/ the strength of an acid according to the PoE:

• the bigger the K_a, the further the PoE lies to the RHS and the stronger the acid

• the smaller the K_a, the further the PoE lies to the LHS and the weaker the acid

give the assumption we make for weak acids and give the expression derived from this:

• [H⁺] = [A^-]

• so K_a = [H⁺] / [HA]

give the shape of the titration curve for a strong base added to a strong acid:

give the shape of the titration curve for a weak base added to a strong acid:

give the shape of the titration curve for a strong base added to a weak acid:

give the shape of the titration curve for a weak base added to a weak acid:

what is the equivalence point?

point at which sufficient base has been added to just neutralise the acid/base

how can we calculate pK_a from a titration curve?

pH at half neutralisation = pK_a

explain the general shape of a pH curve:

1. pH changes little as acid is in XS

2. equivalence point - point at which sufficient base has been added to just neutralise the acid

3. pH changes little as base in XS

give the general shape of a pH curve if an acid is added to a base:

what do you need to take into account when choosing a suitable indicator?

should change colour over a narrow range and at the vertical section of the curve (at equivalence)

give the method for calibrating the pH probe:

give the method for measuring the pH of the mixture of acid and alkali:

give the general eqn for the reaction of a weak acid w/ a strong base - give the assumption we cannot make:

• HA + OH^- → A^- + H₂O

• we cannot make the assumption that [H⁺] = [A^-]

what is a buffer soln?

soln which maintains an approximately constant pH, despite dilution or addition of small amounts of acid or base

what is an acidic buffer - what do they contain and why do they work?

• maintain pH below 7

• made from a weak acid (HA) and a soluble salt of the acid (A^-)

• dissociation of acid is at equilibrium (HA ⇌ H⁺ + A^-)

describe what happens when a small amount of base is added to an acidic buffer:

• OH^- react w/ H⁺ in buffer, removing some H⁺

• the PoE in the equilibrium HA ⇌ H⁺ + A^- moves to the right to replace the H⁺ which reacted w/ the OH^-

  • OH^-+ HA → H₂O + A^-

• this keeps the pH almost constant

describe what happens when a small amount of acid is added to an acidic buffer?

• H⁺ conc increases so PoE of HA ⇌ H⁺ + A^- shifts left

• soluble salt of the acid provides A^- to remove the added H⁺ so pH remains roughly unchanged

  • A^- + H⁺ → HA

give the 3 methods of making acidic buffers:

• mixture of weak acid and soluble salt of acid

• reaction of an XS of weak acid w/ strong alkali

• ½ neutralise acid to make buffer w/ pH = to pK_a

what is a basic buffer? what do they contain?

• maintains pH above 7

• mixture of weak base and soluble salt of that base

• dissociation of base at equilibrium

  • B + H₂O ⇌ BH⁺ + OH^-

give 2 methods of making basic buffers:

• mixture of a weak base and soluble salt of that base

• reaction of an XS of a weak base w/ a strong acid

what happens when a small amount of acid/base is added to the basic buffer?

what is the standard electrode potential (E^θ)?

• p.d. measured under standard conditions when the ½ cell is connected to a standard hydrogen electrode

• a measure of where the PoE lies for each ½ cell

how can we relate electrode potentials to PoE?

the more +ve the electrode potential, the further right the PoE

how are ½ equations always written for standard electrode potentials?

always written as reduction (see image for e.g.) 

what can we use to measure the E^θ for a single ½ cell?

standard hydrogen electrode = electrode consisting of H₂ gas in contact w/ H⁺ ions on a platinum surface

describe how we can use the standard hydrogen electrode to measure the E^θ for a single ½ cell: 

• H₂ gas flows over the inert Pt electrode to establish the equilibrium 2H⁺ _(aq) + 2e^- ⇌ H_2 (g)

• the standard hydrogen electrode is connected to the ½ cell under investigation under standard conditions

• the E^θ of the standard hydrogen electrode is 0.00 V by definition

• so we can determine the E^θ by using the formula E_cell = E_right - E_left

state the standard conditions necessary when using the standard hydrogen electrode:

• 298 K

• H₂ gas at 100 kPa

• all solns have a conc of 1 mol dm^-3

which side is the standard hydrogen electrode always positioned on?

LHS - regardless of E^θ values of other cells

give the formula used to calculate the EMF (E_cell) of a cell:

E_cell = E_right - E_left

give the symbols used in cell notation and state what they represent:

which species is placed closest to the salt bridge || when writing cell notation?

species w/ the highest oxidation state (i.e. oxidised species)

on which side of the salt bridge || do you write each species in cell notation?

as seen/given in a diagram:

• -ve ½ cell (oxidised) on LHS

• +ve ½ cell (reduced) on RHS

why is Pt used as the contact electrode in the standard hydrogen electrode?

inert

give and explain the cell notation for the standard hydrogen electrode being used to measure the EMF of a Cu/Cu²⁺ ½ cell:

Pt _(s) | H_{2 (g)} | H⁺ _(aq) || Cu²⁺ _(aq) | Cu _{(s) :}

• species w/ highest oxidation state closest so salt bridge

• | separate species w/ diff states

• Pt _(s) included - used as a contact: solid contact must be used if there are only aq/gaseous species

you are provided with the Daniell cell referred to in part (b), including a Zn electrode of known mass - briefly outline how you could carry out an experiment to confirm your answer to part (b) (3)

• allow cell to discharge until [CuSO₄] = 0.5 mol dm^-3

• confirm using colorimetry

• weigh mass of Zn electrode before and after experiment

use data from the table above to explain why Au⁺ ions are not normally found in aq soln (2)

• E^θ of Au⁺ bigger than E^θ of H₂O

• so Au⁺ ions oxidise H₂O

use data from the table to predict and explain the redox reactions that occur when iron powder is added to an XS of aq silver nitrate (3)

• E^θ of Ag⁺ bigger than E^θ of Fe²⁺

• E^θ of Ag⁺ bigger than E^θ of Fe³⁺

• so Ag⁺ oxidise Fe → Fe²⁺ and → Fe³⁺

give the features of non rechargeable cells and explain why they are necessary:

• porous separator - allows ions to move/flow/transfer

• carbon rod - made from graphite to allow e^- to flow

• zinc - container for cell (gets used up)

give an example of a non rechargeable cell:

either:

• alkaline battery

• dry cell

suggest why a non rechargeable cell often leaks after being used for a long time (1)

Zn oxidised

give an example of a rechargeable cell and give the 2 ½ eqns associated w/ it:

lithium ion:

• Li⁺ + CoO₂ + e^- ⇌ LiCoO₂ (+ve electrode)

• Li⁺ + e^- ⇌ Li (-ve electrode) 

give the overall eqn for the reaction that occurs in a lithium-ion cell:

Li + CoO₂ ⇌ LiCoO₂

for a rechargeable reaction, what do the different directions of the reaction apply to?

• forward reaction = discharge (being used)

• backward reaction = recharge (being charged)

give the 3 types of cell:

• non rechargeable (irreversible)

• rechargeable (reversible)

• fuel cells

describe how an alkaline hydrogen-oxygen fuel cell works:

• H₂ and O₂ gases are supplied to separate platinum-containing electrodes

• an anion-exchange membrane separates the electrodes, allowing passage of anions (OH^-) and water but not the gaseous reactants

• the electrolyte is an aq KOH soln

• e^- flow from the -ve electrode through an external circuit to the +ve electrode, while OH^- ions migrate through the anion-exchange membrane towards the -ve electrode

give the eqn for the reaction at the -ve electrode of an alkaline hydrogen-oxygen fuel cell:

H_{2 (g)} + 2OH^- _(aq) → 2H₂O _(l) + 2e^-

give the eqn for the reaction at the +ve electrode of an alkaline hydrogen-oxygen fuel cell:

O_{2 (g)} + 2H₂O _(l) + 4e^- → 4OH^- _(aq)

give the overall eqn for the reaction that occurs in an alkaline hydrogen-oxygen fuel cell:

2H₂ + O₂ → H₂O

give a benefit of using non-rechargeable cells:

cheap

give a drawback of using non-rechargeable cells:

difficult to dispose of/waste issues

give 3 benefits of using rechargeable cells:

• less waste as metal compounds reused

• cheaper in the long run

• lower environmental impact

give 2 drawbacks of using rechargeable cells:

• need E source to recharge

• some waste at end of useful life

give 3 benefits of using hydrogen fuel cells:

• converts more of the available E from combustion of H₂ into KE of car

• no pollutants (CO₂, SO₂ etc.) as only waste product is H₂O

• efficient

• no need to recharge

give 4 drawbacks of using hydrogen fuel cells:

• need constant fuel supply

• H₂ is flammable and explosive

• high cost

• difficult to source H₂ (usually made using fossil fuels)

suggest why the recharging of a lithium cell may lead to the release of CO₂ into the atmosphere (1)

electricity from recharging the cell may come from power stations burning fossil fuel

describe an electrochemical cell:

• 2 half cells - electrodes submerged in solns of own ions

• half cells connected by high resistance voltmeter and salt bridge

what is the purpose of the high resistance voltmeter?

• measures p.d. between electrodes - this is the EMF/E_cell

• typically e^- would flow from more reactive to less reactive metal but high resistance inhibits e^- flow to maintain conc of ions in soln

what is the salt bridge?

• filter paper soaked in saturated soluble salt soln e.g. potassium nitrate

• OR a tube containing unreactive ions in an agar gel

what is the purpose of a salt bridge?

• helps to maintain electrical neutrality w/in internal circuit

• allows movement of ions to complete the circuit and so prevent charge buildup ∴ prevents cell from taking its reaction to equilibrium

• connects 2 ½ cells/electrodes but keeps solns separate

how does a salt bridge work?

to maintain electrical neutrality:

• -vely charged ion moves to +ve ½ cell via salt bridge

• +vely charged ion move to -ve ½ cell via salt bridge 

what happens when a piece of metal is submerged into a soln of its own ions?

• an equilibrium is set up

• there is a tendency for the metal to form +ve ions and go into soln