Chemistry Tips (Finals) (16 Nov)

Galvanic cell uses (2)

torches and car batteries

Electrolytic cells uses (2)

electro-plating and production of Cl₂

which redox reaction is spontaneous?

galvanic

where is Eᶿ negative

electrolytic cell

what energy conversion takes place in each cell

Electrolytic: electrical energy → chemical energy

Galvanic: chemical energy → electrical energy

which terminal is which in electrolytic

anode = positive terminal

cathode = negtive terminal

which terminal is which in galvanic

anode = negative terminal

cathode = postive terminal

what’s nearest negative terminal of the battery?

• physical terminal that supplies electrons to the cathode

• giving it a negative charge so that reduction can take place.

in which direction does current flow?

anode to cathode

which mass increases and decreases?

anode decreases in mass

cathode increases in mass

functions of salt bridge (3)

• completes the circuit

• maintains electrical neutrality in the half cells

• separates the half cells

writing cell notation

how to determine if reaction is spontanrous

if ‘Eᶿ cell = Eᶿ cathode - Eᶿ anode’ is positive

when does the current stop being delivered

until the reaction reaaches chrmical equilibrum then the voltage = 0

inc surface area of terminals =

inc RoR = inc maximum current

• DOES NOT affect emf

what lowers internal resistance? (affect on current)

• wider, shorter salt bridge

• more conductive salt brige

• inc current and deoesn’t affect emf

acronym for anode and cathode

AN OX

RED CAT

uses of graphite electrodes

• inert (chemically unreactive)

• conducts electricity

what does putting the electrolyte in a molten or aqueous solution afford you?

• ions are free (mobile)

• it allows the electrolyte to be conductive

electrolytic cell diagram

industrial uses of electrolytic cells (3)

1. electroplating

2. refining copper

3. production of chlorine

what is the aim of electroplating and which is the cathode?

• aim is to coat metals with useful or preciouis metals

• object that is coated is cathode

when refining copper which is anode, cathode, and electrolytes?

anode = impure copper

cathode = thin sheet of pure copper

electrolyte = Cu ions

why is the production of chlorine needed? what should you use and why?

• important in the production of bleach, plastics and insectides

• use a diaphragm or membrane cell to keep the products seperate

what is the electrolyte in the production of chlorine

concentrated NaCl

what is diaphragm made of?

porous asbestos and polymers

why can the solution form diaphragm seep through and where to?

• diaphragm’s pores let the liquid seep from the positive side (anode) to negative side (cathode)

which side is more liquid kept on and why

• higher liquid level on anode side to ensure liquid flows from left to right

  • prevents any NaOH that is formed from moving back to where Cl₂ is produced which would cause unwanted side reactions

acronym for oxidation and reduction

OIL RIG

oxidising agent ~

substance that accepts electrons

reducing agent ~

substance that donates electrons

what is required for a spontaneous redox reaction

• substance that is more easily oxidised must react with substance that is more easily reduce

• (substance on right of table must be higher up than the substance on left)

electrode potential of half cell meaning

the tendency fir a half cell to attract electron

what are the standard cinditons for eletrode potentials

25℃ and 1 mol.dm⁻³

facts abt standard hydrogen electrode (SHE)

• consists of hydrogen gas at 101,3kPa and 25℃ bubbling around a platinum electrode in 1 mol.dm⁻³ H⁺ ion solution

• writeen as “Pt(s) I H_{2 (g) II other half cel’}

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annoted table of potentials

basic rule for naming:

1. identify longest chain

2. name longest chain

3. identify and name branches

4. write in a way that functional then branches get lowest number

suffix for CA, ester, and ketone

CA - anoic acid

Ester - oate

Ketone - one

how to write ester?

alcohol then CA

format for naming

prefix + stem + suffix

(suffix shows homologous series and prefix is position and branches)

types of structural isomers

1. Chain - straight/branched

2. Positional - functional group on diffrent carbon numbers

3. Functional - aldehyde & ketone, ester & CAformat for naming

prefixes for 1 to 7

meth-

eth-

prop-

but-

pent-

hex-

hep-

effects on IMF (3)

1. type of IMF (hydrogen/lond/etc)

2. number and type functional group(homologous series)

3. structure (no. carbons <chain length. and straight/branched chain <straight is strong>)

structure for stonger IMF’s

1, state homologous series and IMF on both *MENTION BOTH HAVE LONDON

2. state which compound has stronger IMF

3. state that it means more energy needed to overcome IMF

4. state which has higher BP/VP/MP

a bit about elimination (2)

specific, reaction

specific: cracking

long alkane → broken into smaller alkanes and alkenees

a bit about combustion (3)

specific: heat and excess oxygen

alcohol AND alkene AND alkane

CO₂ + H₂O

a bit about substituon (X₂) (3)

specif, condition, change

specific: halogenation

condition: ultraviolet light or heat

Alkane → Haloalkane

a bit about addition (of H₂) (4)

specific, catalyst, condition, reaction

specific: hydrogenation

catalyst: Pt, Pd or Ni

condition: alkene dissolved in organic solvent

Alkene → alkane

a bit about esterification (3)

condition, elements, products

conditions: heat and a catalyst of conc. sulfuric acid

• with CA and alcohol

Ester + H₂O

a bit about addition of H-X (3)

specific, condition, reaction

specific: hydrohalogenation

condition: no water

Alkene → Haloalkane

a bit about elimination of H₂O (2)

specific, condition, reaction

specific: dehydration

condition: heat with excess conc. sulfuric acid or H₃PO₄

Alcohol → Alkene

a bit about substitution (w/ dilute base) (3)

specific, condition, reaction

specific: hydrolysis

condition: heat under reflux (dilute base)

Haloalkane → Alcohol

a bit about addition of H₂O (4)

specific, condition, catalyst, reaction

specific: hydration

condition: steam

catalyst: H₃PO₄ (many others)

Alkene → Alcohol

a bit about addition of X₂ (2)

specific: halogenation

Alkene → Haloalkane

types of cracking and conditions

1. thermal - high press and temp

2. catalytic - lower press and press than thermal

a bit about elimination of H-X (3)

specific, condition, reaction

specific: dehydrohalogenation

condition: hot conc. NaOH or KOH dissolved in ethanol AND water cannot be present

'

Haloalkane → Alkene

types if organic reactions

1. addition

2. elimination

3. combustion

4. esterification

5. elimination of H₂O

6. Addition (of H₂)

7. substitution (with X₂)

8. elimination of H-X

9. Addition of X₂

10. Addition of H₂O'

11. Substitution (with dilute base)

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nitrate

ammonium

nitrite

thiosulfate

chlorate

sulfate

NO₃⁻

NH₄⁺

NO₂⁻

S₂O₃²⁻

ClO₃⁻

SO₄²⁻

permanganate

ethanoate/ acetane

dichromate

phosphate

hydronuim

MnO₄⁻

CH₃COO⁻

Cr₂O₇⁻

PO₄³⁻

H₃O⁺

hydrogen carbonate

hydrogen sulfate

sulfite

hydroxide

carbonate

HCO₃⁻

HSO₄⁻

SO₃²⁻

OH⁻

CO₃²⁻

name a condition for hydrolysis (2)

• strong dilute base

• mild heat

catalyst for esterifiation

sulfuric acid

what to remember about naming esters?

-yl goes on alcohol (eg. propyl ethanoate*

% purity =

(m _pure) / (m _total) x 100

format for investigative question

“what is the relationship between…”

what to do if highly flammable (2)

• use waterbath to heat

• keep away from open flames

what type of hydrocarbons decolourise in water

unsaturated

why do alkanes only react in sunlight/ with heat

• they need energy added to it

• sunlight supplies activation energy that is needed

molecular formula after in bromine water?

gets two Br and no extra H₂O

-substituted

galvanic diagram

• no external power source needed

• used in batteries

electrolytic diagram

• battery or power cell needed

• electroplating, electro refining, water electrolysis

explain change in direction of electron flow when half cellls are changed and refer to reducing strength

X is a better reducing agent than Y therefore X is reduced

n = (for charges and stuff)

Q/q_e

during electroysis why are only A and B oxidised and not C

• the potential difference is not strong enough to oxidize C

% yield =

(actual yield) / (theo. yield) x 100

how to solve for limiting (5)

- balance equation
- moles for reactant
- pick a reactant and work out others
- determine what's lmitng

table for empirical

element I mass (g) I n (n=m/M)(mol) I ratio (divide by smallest) I simplest ratio

ions on table

+1 +2 +3 ± -3 -2 -1 0

what do you round to with empirical

to biggest half then need to multiply out fraction (2 base)

how to find mass reacted

1. find mass of product (n=m/M)

2. use mol ratio to get mass of reactant that reacted

3. get mass of reactant

4. subtract as needed

What are the standard conditions for electrochemical cells?

• Temperature: 25 degrees C (298 K)

• Pressure: 1 atm (101.3 kPa) for gases

• Concentration: 1 mol/dm3 for aqueous solutions (electrolytes)

What must always be included in standard cell notation regarding platinum (Pt) and cell orientation?

• Include platinum (Pt) as an inert electrode when a half-cell contains only aqueous ions or gases (no solid metal conductor).

• By convention, the anode (oxidation) is written on the left and the cathode (reduction) on the right (Oxidation || Reduction). Temperature is implied and not written in the notation unless conditions are asked.

What should you remember when a disturbance is applied to an equilibrium system or electrochemical cell?

• Apply Le Chatelier's principle: the equilibrium shifts in the direction (forward or reverse) that counteracts or relieves the imposed stress.

• Focus strictly on how the cell's equilibrium responds to the change made, rather than focusing only on the change itself.

• always state the disturbance, the direction favored, and the net effect on yields/concentrations.

What are the physical and chemical benefits of aluminium?

• Physical: Lightweight and durable.

• Chemical: Forms a thin, adherent, passive protective oxide layer on its surface that prevents underlying metal from further corrosion.

What are the economic and environmental benefits of using cryolite in aluminium extraction?

• Economic: Lowers the melting point of alumina (Al2O3) from over 2000 degrees C down to 950 to 1000 degrees C, drastically reducing electrical energy costs.

• Environmental: Lower energy consumption means fewer fossil fuels burned at power stations, thereby reducing greenhouse gas emissions (like CO2).

What are the types of intramolecular bonds and their constituent bonding conditions?

• Covalent: Non-metal + Non-metal (sharing of electron pairs).

• Ionic: Metal + Non-metal (transfer of valence electrons forming oppositely charged ions).

• Metallic: Metal + Metal (lattice of positive metal ions in a sea of delocalized valence electrons).

What are the types of intermolecular forces and the conditions for each?

• Ion-dipole forces: Between an ion and a polar molecule.

• Hydrogen bonding: A strong dipole-dipole force where H is covalently bonded to highly electronegative atoms (F, O, or N).

• Dipole-dipole forces: Between two polar molecules.

• London dispersion forces: Present in all molecules (especially non-polar molecules) due to temporary instantaneous dipoles.

Why do ionic compounds have high melting and boiling points?

• They consist of a giant ionic crystal lattice

• held together by strong electrostatic forces of attraction between oppositely charged ions.

• Because there are many of these strong bonds throughout the lattice, a large amount of thermal energy is required to overcome them.

What must electrodes be made of, and how do inert electrodes function?

Electrodes must be solid and electrically conductive. If an inert electrode (such as platinum or graphite) is used, it does not participate chemically in the redox reaction itself; instead, it provides a solid conductive surface for electron transfer while the ions or gases in the electrolyte undergo oxidation or reduction.

Why isn't water oxidized in preference to chloride ions during brine electrolysis, despite water having a more favorable standard reduction potential?

Although thermodynamic standard potentials favor water oxidation, the extremely high concentration of Cl- ions in concentrated brine shifts the reaction rate and kinetics, making the oxidation of Cl- ions predominant.

What must you remember when using half-reactions with 2e- (or multiple electrons) when calculating moles of electrons?

Always ensure that half-reactions are balanced and multiplied by the appropriate stoichiometric factor so that the total moles of electrons lost in oxidation equal the moles gained in reduction when combining them.v

What can the cathode compartment be contaminated with in certain industrial electrolysis cells (e.g., chlor-alkali cells), and why?

In membrane or diaphragm chlor-alkali cells, the cathode compartment (where reduction of water produces H2 gas and OH- ions) can experience trace contamination or reduced current efficiency if oxygen gas (O2) or chlorine gas leaks across the separator/membrane from the anode side.

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What does a non-polar molecule indicate regarding its intermolecular forces?

A non-polar molecule indicates a symmetrical charge distribution (zero net dipole moment). Consequently, the only intermolecular forces operating between them are weak London dispersion forces.

What does having more electrons in a molecule mean for its intermolecular forces?

More electrons mean a larger, more polarizable electron cloud. This leads to larger temporary (instantaneous) dipoles forming, resulting in stronger London dispersion forces, which require more thermal energy to overcome (hence higher melting/boiling points).

Fully explain why chlorine gas (Cl2) is produced predominantly at the anode during the electrolysis of concentrated sodium chloride (brine) instead of water.

• Under standard conditions, water (H2O) has a more negative standard reduction potential than chloride ions (Cl-), meaning water is thermodynamically more easily oxidized.

• However, in concentrated brine, the very high concentration of Cl- ions drastically increases the rate of chloride oxidation.

• Coupled with kinetic overpotential for oxygen evolution at the electrode, the oxidation of Cl- to Cl2 becomes the predominant reaction.

pH =

-log[H₃O]

what do diff pH’s mean?

pH = 7 means neutral

pH < 7 means acidic (acid left over; reaction will ionise)

pH > 7 means basic (alkaline; base left over; reaction will dissociate)