Chapters 1-25 OCR A-Level Chem: ⚗️

Bronsted-Lowry acid: 

proton donor

Bronsted-Lowry base:

proton acceptor

what is a hydronium ion?

H₃O⁺ → often just written as H⁺ instead

what does monobasic, dibasic, and tribasic acid mean?

the number of hydrogen ions in the acid that will be replaced per molecule

organic acids do not replace any hydrogen atoms from…

the carbon chain

acid + base → 🫵

salt + water 🫵

an alkali is…

a basic solution

a low value of [H⁺_(aq)] is a … pH

high

a high value of [H⁺_(aq)] is a … pH

low

relationship between pH and  [H⁺_(aq)]

pH = -log [H⁺_(aq)]

reverse relationship between [H⁺_(aq)] and pH

[H⁺_(aq)] = 10^-pH

because it is logarithmic, a change of 1 pH number is equal to

10 times different in [H⁺_(aq)]

for a strong acid (that completely dissociates into ions), [H⁺_(aq)] is equal to…

the concentration of the acid

general formula for the dissociation of weak acids: 

HA_(aq) ⇌ H⁺_(aq) + A^-_(aq) 

what K_a?

the acid dissociation constant

how to calculate the acid dissociation constant?

[H⁺_(aq)] [A^-_(aq)] / [HA_(aq)]

units of the acid dissociation constant?

mol dm^-3

the larger the value of K_a, the further the equilibrium is…

to the right 

K_a can be converted into a logarithm (pK_a) as to avoid negative indices. What is the relationship between them?

pK_a = -logK_a

reverse relationship between K_a and pK_a

K_a = 10^-pKa

for a weak acid, there is …. in a dissociation:

an equilibrium

When HA molecules dissociate, H⁺_(aq) and A^-_(aq) ions are formed…

in equal quantities 

what 2 approximations are made to simplify the Ka expression?

1. HA dissociates to product concentrations of H+ and A- that are equal

2. the equilibrium concentration of HA is smaller than the undissociated concentration

K_a expression:

working out [H⁺_(aq)] from the Ka expression:

2 approximations in calculations involving weak acids: 

1. dissociation of water is negligible 

2. concentration of acid is much greater than the H⁺ concentration at equilibrium

water ionises very slightly, acting as an…

acid and base:

dissociation of water equation:

H₂O (l) ⇌ H⁺(aq) + OH^-(aq)

what is K_w?

the ionic product of water → [H⁺(aq)] x [OH^-(aq)]

Ka x [H2O (l)] is a constant: 

Kw 

Kw at 25 degrees Celsius?

1.00 × 10^-14 mol² dm^-6

for a dibasic acid/acid, multiply the concentration of OH or H by [what] in pH calculations

2

molecular formula of benzene:

C6H6

2 ways of showing benzene:

3 pieces of evidence that disprove Kekulé model of benzene:

1. lack of reactivity:

   1. does not undergo electrophilic additions

   2. does not decolourise bromine under normal conditions

   3. therefore no C=C bonds 

2. length of carbon-carbon bonds:

   1. bond length found to be between the length of a single + a double bond 

3. hydrogenation enthalpies:

   1. expected to have an enthalpy change of hydrogenation that is 3x that of cyclohexane 

enthalpy change of hydrogenation of cyclohexane, benzene, and predicted of benzene 

cyclohexane = -120 kJ mol-1

benzene = -208 kJ mol -1

predicted of benzene = -360 kJ mol -1

delocalised model of benzene:

1. planar

2. each carbon uses 3 of its available 4 electrons in bond to 2 other carbon atoms + 1 atom

3. each carbon has 1 electron in a p-orbital at right angles to the plane of the bonded carbon + h atoms

4. adjacent p-orbitals overlap sideways, in both directions above + below plane of carbons → forms a ring of electron density

5. overlapping of p-orbitals creates a system of pi bonds, spread over all 6 of the carbon atoms

6. the six electrons occupying the system of pi bonds is said to be delocalised

benzene nomenclature rules:

1. one substituent group = prefix + benzene

2. alkyl group + functional group, alkyl group with seven or more carbon atoms = phenyl + suffix

3 exceptions to benzene nomenclature:

1. benzoic acid

2. phenylamine

3. benzaldehyde

substituent groups are listed in…

alphabetical order

typical equation representing electrophilic substitution of benzene:

a hydrogen is substituted out of benzene:

Nitration of benzene:

• slow!!

• H2SO4 catalyst

• heated to 50 degrees to maintain a good rate of reaction → water bath

• a hydrogen is replaced by a -NO₂ group

• benzene + HNO₃ → nitrobenzene + H₂O

nitration of benzene equation:

if the temperature of the nitration of benzene rises about 50 degrees C, what could/will happen?

further substitution leads to the production of dinitrobenzene

in the nitration of benzene, nitric acid is NOT the electrophile. What is?

the nitronium ion, NO₂⁺ → produced in the reaction of concentrated nitirc acid with concentrated sulphuric acid

mechanism for the nitration of benzene: 

halogenation of benzene:

halogens do NOT react with benzene unless a catalyst called a halogen carrier is present.

common halogen carriers:

AlCl₃, FeCl₃, AlBr₃, FeBr₃

bromination of benzene:

electrophile = bromonium ion, Br_⁺ - generated from the halogen carrier catalyst

bromonium ion accepts a pair of electrons from benzene to form a dative covalent bond 

mechanism of the bromination of benzene:

alkylation of benzene requirements:

• benzene + haloalkane, in the presence of AlCl₃

• AlCl₃ acts as a halogen catalyst, generating the electrophile

acylation reactions: 

benzene + acyl chloride in the presence of an AlCl₃ catalyst → forms an aromatic ketones 

ethanoyl chloride + benzene →

phenylethanone + HCl

why are alkenes more reactive than arenes? (with bromine)

1. the pi bond in an alkene contains localised electrons, above and below the plan of the C=C double bond.

2. this is an area of high electron density.

3. localised electrons in the ⫪bond induce a dipole in the non-polar bromine molecule → δ⁺Br——Brδ^-

mechanism for the electrophilic addition of Br₂ with cyclohexene:

why does benzene only react with a halogen carrier present? 

1. benzene has delocalised pi electrons spread above and below the plane of the carbon atoms in the ring structure 

2. the electron density around any 2 carbon atoms in the benzene ring is less than that in a C=C double bond in an alkene

3. when a non-polar molecule approaches, there is insufficient pi electron density around any 2 carbon atoms to polarise the bromine molecule 

alkenes react with bromine via…

benzene reacts with bromine via…

electrophilic addition

electrophilic substitution

what is a phenol?

an organic chemical containing a hydroxyl, -OH, functional group directed directly to an aromatic ring

difference between a phenol and an aromatic alcohol:

phenol is a … acid?

weak → presence of a non-polar benzene ring

when dissolved in water, phenol partially dissociates to form:

phenoxide ion and a proton

comparable acidity of phenols, alcohols, and carboxylic acids:

Carboxylic acid = most acidic

phenol

alcohol = least acidic

ethanol, phenol, and ethanoic acid reacting with sodium hydroxide + sodium carbonate:

ethanol = will NOT reach with either

phenol = react with sodium hydroxide (strong base)

ethanoic acid = will react with sodium hydroxide AND sodium carbonate (weak base)

reaction with sodium carbonate can be used to distinguish between a … and a …

phenol (will not react)

carboxylic acid (will react)

electrophilic substitution reaction of phenol + sodium hydroxide:

takes place under mild conditions

bromination of phenol: 

phenol reacts with an aqueous solution of bromine to form 2,4,6-tribromophenol

decolourises the bromine water (orange to colourless) 

a halogen carrier is not required! 

nitration of phenol:

reacts readily at room temperature

forms a mixture of 2-nitrophenol and 4-nitrophenol

dilute nitric acid

addition of bromine water to phenol results in 2 observations:

1. bromine decolourises

   1. white precipitate forms

why is phenol more reactive than benzene? 

1. lone pair of electrons from the oxygen p-orbital of the -OH group being donated into the pi system of phenol

2. electron density of benzene ring increases 

3. increased electron density attracts electrophiles more strongly than benzene

4. aromatic ring in phenol is therefore more susceptible to attack from electrophiles than in benzene

the electron density in the phenol ring is sufficient to …

polarise bromine molecules → no halogen carrier catalyst required

nitrobenzene reacts … with bromine, requiring:

slowly → halogen carrier catalyst and a high temperature

NH₂ group [ what ] a benzene ring?

activates

what is an activating group? 

helps the aromatic ring react more readily with electrophiles → e.g. NH₂wh

what is a deactivating group?

makes the aromatic ring react less readily with electrophiles

-NH₂ is said to be…

2- and 4- directing

-NO₂ is said to be …

3- directing

directing effect: 

a substituent group on benzene will have a directing effect on any second substituent group. 

all 2- and 4- directing groups are…

activating groups

all 3- directing groups are …

deactivating groups

2- and 4- directing groups:

1. amine, NH₂ or NHR

2. hydroxyl, -OH

3. ketone, -OR

4. -R or -C₆H₅

5. -halogen

a h k r h

rate of reaction definition:

the change in concentration of a reactant / product in a given time

what shape does a typical concentration/time graph have?

curve:

• steepest at start

• becomes less steep

• flattens off

4 factors that can change the rate of a reaction:

1. concentration (or pressure when reactants are gases)

2. temperature

3. use of a catalyst

4. surface area of solid contacts

what does collision theory state?

2 particles must collide to react

why are some collisions effective, and some ineffective?

1. particles must collide with correct orientation.

2. particles must have sufficient energy to overcome the activation energy barrier of the reaction

how does increasing concentration affect the rate of reaction?

• rate of reaction generally increases

• increases the number of particles in the same volume

• particles are closer together + collide more frequently

• therefore → more frequency + successful collisions (correct orientation + sufficient energy) → increased rate of reaction

how does increasing the pressure of a gas affect the rate of reaction?

• gas = compressed in a smaller volume → pressure of gas is increased → rate of reaction increases

• concentration of gas molecules increases as the same number of gas molecules occupy a smaller volume

• gas molecules = closer together + collide more frequently → leads to more effective collision in the same time

2 methods for following the progress of a reaction:

• monitor removal (decrease in conc.) of reactant

• monitor formation (increase in conc.) of product

what 2 methods can be used for monitoring reactions that produce gas?

1. monitor volume of gas produced at regular time intervals → gas collection

2. monitor loss of mass of reactants using a balance → mass removal

draw the equipment for gas collection:

on a concentration-time graph, what represents the rate? 🫵

the gradient 🫵

on a (gas collection or mass lost) / time graph, what represents the rate?

the gradient

3 rules for catalysts:

1. catalyst not used up in the reaction

2. may form an intermediate / provide a surface on which the reaction can take place

3. catalyst is regenerated at the end of reaction

Exo + Endothermic reaction enthalpy profile diagrams (with catalyst!)

3 things to remember when drawing enthalpy profile diagrams?

• reactants + products written out

• ∆H shown with an arrow pointing in correct direction

• activation energy shown with upwards arrow

homogeneous catalysts:

• same physical state as the reactants

• forms an intermediate → breaks down to give catalyst + product

2 reactions that use homogeneous catalysis:

1. making esters (with sulphuric acid as a catalyst)

2. ozone depletion (Cl• catalyst)

heterogeneous catalyst:

• different physical state from the reactants

• usually solid → in contact with gaseous reactants/reactants in solution

• reactant molecules are adsorbed (weakly bonded) → onto surface of catalyst → where the reaction takes place

• product leaves surface of catalyst by desorption → at the end of the reaction

lower activation energy often means a lower…

temperature → this makes using catalysts more sustainable (less electricity etc.)

what does a Boltzmann distribution graph look like?