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ACHEM - REVISION > Stuff to know > Flashcards

Stuff to know Flashcards

1
Q

Explain how a sigma bond is formed

A

End-on overlap with electron density between the two atoms

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2
Q

Explain the creation of a Pi bond

A

Sideways overlap with electron density above and below sigma bond

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3
Q

How to determine if reaction is spontaneous

A

If delta G is negative

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4
Q

Outline two differences between an electrolytic cell and a voltaic cell

A

Electrolytic cell converts electrical energy to chemical energy

Voltaic cell converts chemical energy to electrical energy

Electrolytic cells use electricity to carry out a redox chemical reaction and a voltaic cell uses a redox reaction to create electricity

Electrolytic cells require a power supply and voltaic cells do not

In an electrolytic cell, cathode is negative and anode is positive, this is vice versa for voltaic cell

Voltaic cell has two separate solutions and electrolytic cell has one solution

Voltaic cell has a salt bridge and electrolytic cell has no salt bridge

In electrolytic cell, oxidation occurs at positive electrode/anode and voltaic cell, oxidation occurs at the negative electrode/anode

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5
Q

State one reason why aluminum is preferred to iron in many uses regarding electrolysis

A

Al does not corrode/rust
Al is less dense
Better conductor
More malleable

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6
Q

Distinguish in terms of electronic structure, between the terms “group” and “period”

A

period has same number of valence shells

group has same number of electrons in valence shell

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7
Q

Electroplating is an important application of electrolysis. State the composition of the electrodes and the electrolyte used in the silver electroplating process.

A

Cathode: object to be plated: spoon
Anode: Silver/ Ag

Electrolyte: [AgNo3] or [Ag(CN)2]-

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8
Q

Define average bond enthalpy

A

minimum energy required to break 1 mole of a bond in a gaseous molecule/state

average values obtained from a number of similar bonds/compounds

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9
Q

Describe, using equations, the complete and incomplete combustion of alkanes.

A

C8H18 + 12.5 O2 -> 8CO2 + 9H2O

Gasoline and many other fuels rarely burn this way
If supply of air/oxygen is limited, then incomplete combustion occurs
Carbon monoxide may be formed
Hydrogen still converted to water

Highly toxic and a major source of air pollution

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10
Q

How can Alkanes react with chlorine or bromine

A

Through a substitution reaction to give an initial product in which one hydrogen atom is replaced by the halogen

Usually brought about by exposure to ultraviolet light or sunlight, though they will also occur without light at very high temperatures

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11
Q

Free radical chain reaction

A

Reaction with an alkane, such as methane, with a halogen
Split up into three steps: Initiation, propagation, and termination

Initiation

Cl-Cl -> Cl’ + ‘Cl

Propagation

Cl’ + H-CH3 -> Cl-H + ‘CH3

‘CH3 + Cl-Cl -> Cl-CH3 + ‘Cl

Termination

Cl’ + ‘Cl -> Cl-Cl

Cl’ + ‘CH3 -> CH3-Cl

Ch3’ + ‘CH3 -> Ch3-Ch3

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12
Q

Addition reaction

A

A reaction in which the double bond of an alkene is converted to a single bond and two new bonds are formed to the species it reacts with

typical of alkenes and alkynes

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13
Q

Hydrogenation

A

Addition reaction between alkenes and hydrogen halides such as Hydrogen chloride

Activation energy is slightly higher, but if a gaseous mixture of an alkene and hydrogen is passed over a heated nickel catalyst, an addition reaction to form an alkane occurs

Basis of conversion of vegetable oils, which contain a number of C=C double bonds, into margarine, which has fewer double bonds and hence a higher melting point

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14
Q

Define a polymer

A

long chain molecules that are formed by the joining together of a large number of repeating units, called monomers, by a process of polymerization

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15
Q

Define monomers

A

repeating units that join together in large quantities through the process of polymerization in order to create a polymer

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16
Q

Describe the Addition Polymerization Reaction

A

Monomers contain double bonds and in the addition reaction new bonds form between these monomer units

Simplest polymerization reaction of this type is that of ethene when heated with a catalyst to form polyethene

n CH2=CH2 -> [-CH2-CH2-]n

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17
Q

Name three common polymers

A

polythene
poly(chloroethene)
polypropene

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18
Q

Describe, using equations, the complete combustion of alcohols

A

Alcohols burn readily in air to form CO2 and H2O

As with all such combustion reactions, carbon monoxide or even carbon can be produced if the air supply is restricted
Hydrogen is never a product of combustion

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19
Q

Describe the oxidation reactions of alcohols

A

Hydrogen atoms attached to the same carbon as the -OH group are readily oxidized and so these three classes of alcohols behave in rather different ways when they react with oxidizing agents such as acidified potassium dichromate

primary (One C atom on -OH carbon)
have two readily oxidized H and so undergo 2 stages of oxidation to yield aldehydes and then carboxylic acids

secondary (2 C atoms on -OH C)
Have one reactive H and so undergo one stage of oxidation to yield ketones

tertiary (3 C atoms on -OH C)
Do not have any reactive hydrogen atoms and are not readily oxidized

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20
Q

How to obtain an aldehyde from the oxidation of a primary alcohol

A

Both aldehydes and alcohols are polar
alcohols have higher boiling points as these experience hydrogen bonding in addition to dipole-dipole forces
In practice to obtain an aldehyde, the alcohol is added to the boiling oxidizing agent so that as soon as the more volatile aldehyde is formed, it distils off before it can be further oxidized.

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21
Q

How to obtain a carboxylic acid from the oxidation of a primary alcohol

A

A more concentrated solution of the oxidizing agent is used and the mixture is refluxed so that the aldehyde cannot escape further oxidation

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22
Q

What does heating under reflux allow?

A

Allows us to carry out a reaction at the boiling point of the solvent without any loss of the solvent.
The vapor from the boiling solvent turns back to a liquid in the vertical condenser and drips back into the flask.

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23
Q

Homologous series

A

Series of compounds that have the same functional group (hence similar chemical properties) and only differ from each other by the presence of an additional carbon atom and it’s two associated hydrogen atoms in the molecule

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24
Q

Explain why the hydroxide ion is a better nucleophile than water

A

It is negatively charged and hence attracted to the partial positive charge on the carbon attached to the halogen

As a result SN2 hydrolysis reactions occur more rapidly in aqueous alkali than in neutral solution

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25
Q

What is a nucleophilic substitution reaction?

A

The reaction of halogenoalkanes with the hydroxide ion

Occurs in 2 different mechanisms
SN1
SN2

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26
Q

Describe SN1 mechanism

A

Slow heterolytic fission of the carbon-halogen bond to form a carbocation intermediate is the rate determining step

Intermediate reacts rapidly with the nucleophile to form the final product.

Intermediate has a finite existence and occurs at potential energy minimum

27
Q

Describe SN2

A

Breaking down of carbon-halogen bond occurs simultaneously with the formation of the new bond to the nucleophile. The point at which both are half completed is known as the transition state and it occurs at a potential energy maximum in the energy level diagram

28
Q

What does the rate of a nucleophilic reaction depend on?

A

Nature of nucleophile and halogenoalkanes

Stronger nucleophiles more readily attack a carbon with a partial positive charge
2factors of halogen:
Polarity of bond- the less polar the slower the rate of reaction; partial charge on carbon becomes smaller

Bond strength- the weaker the bond, the quicker the rate

Bond strength is dominant factor

29
Q

Steps to a redox equation

A

1) balance number of atoms of the element being oxidized of reduced
2) add water molecules to balance the number of oxygen atoms on both sides
3) add hydrogen ions to balance number of hydrogen atoms on both sides
4) add electrons so that the electrical charges on both sides balance

30
Q

Define a voltaic cell

A

Device for converting chemical energy into electrical energy through a spontaneous redox reaction as a source of electrical energy

Basis of electrochemical cells

More reactive metal will always be anode

Salt bridge allows movement of ions between solutions, thus completing the circuit and maintaining electrical neutrality

31
Q

Define standard hydrogen electrode

A

Electrode potentials measures to SHE

SHE has potential of zero

Needs 1M HCl, hydrogen gas at 1 ATM, wire to very high resistance volt meter, and platinum coated with platinum black

32
Q

Why is platinum black necessary in a SHE

A

Very finely divided platinum which catalyze a the reaction:

2H+ + 2e -> H2 (g)

33
Q

Explain the electrolysis of a molten salt

A

Cations attracted to cathode where they undergo reduction

Anions attracted to anode where they undergo oxidation

Water can also be oxidized or reduced

34
Q

What type of conductors are the electrodes made of in electrolysis

A

Inert (graphite or platinum)

If the anode is made from a metal that is not inert, like copper, then the anode may be oxidized and dissolve into the solution

35
Q

Produces of the electrolysis of an aqueous solution

A

Cathode- formation of the metal or formation of hydrogen

Anode- formation of the non-metal formation of oxygen or oxidation of electrode

Most energetically favorable will take place: for the cathode the most positive electrode potential; the the anode the most negative

36
Q

The amount of product that results from electrolysis will depend upon:

A

Number of electrons required to produce one mole of the product

The magnitude if the current (rate of flow of electrons)

Time for which the current is passed

37
Q

Bronstead Lowry definition of an acid

A

Substance that acts as a donor if hydrogen

38
Q

What is a conjugate base

A

When an acid loses one hydrogen ion

39
Q

Define amphiprotic

A

Species that can act as both acids and bases

40
Q

Properties of acids

A

pH<7

React with active metals (above hydrogen in the reactivity series) to give a salt and hydrogen gas

React with bases to form a salt and water

React with metal and hydrogen carbonates to give a salt, water, and carbon dioxide, which appears as effervescence

41
Q

What concentration of hydrofluoric acid is required to give a solution of pH 2 and what percentage of the hydrofluoric acid is dissociated at this pH, if the dissociation constant of the acid is 6.76x10^-4 mol dm^-3?

A

Ka= x^2/a

Set Ka= 6.76x10^-4, and x to 10^-2

Find a, and then divided 10^-2 by a

42
Q

How are Ka, Kb, and Kw related?

A

Ka x Kb= Kw

43
Q

How are pKa, pKb, and pKw related?

A

pKa + pKb = 14.00

44
Q

Describe the composition of a buffer solution and explain its action

A

If a small volume of strong acid or base is added to water, then pH of water changes significantly

If acid or base is added to a mixture of a weak acid and it’s conjugate based then change in pH would be much less

45
Q

What does the pH of a buffer solution depend on?

A

Ka of the weak acid
Ratio of the concentrations of the acid and it’s conjugate base

A buffer solution of any desired pH can be prepared

46
Q

What happens when a buffer is diluted?

A

pH does not change, but buffer will be less effective as the amount of strong acid/ base required to completely react with all of one of the buffer components decreases

47
Q

When is a buffer most effective?

A

When concentration of acid and base are equal

pH is equal to the pKa

48
Q

Solid sodium ehtanoate is added to 0.2 mol dm^-3 ethanol acid until the concentration is 0.05 mol dm^-3.
Given that Ka for ethanoic acid is 1.74x 10^-5 mol dm^-3, and assuming no volume change on dissolving the solid, calculate the pH of the buffer solution formed

A

[H+]= Ka x [HA]/[A-]
= 1.74x 10^-5 x 0.2/0.05
=6.96 x 10^-5 mol dm^-3

pH=-log[H+]=4.16

49
Q

Define a salt

A

An ionic compound comprised of cations from a base and anions from an acid which are completely dissociated into ions in aqueous solution.

50
Q

Cations can act as ___

A

Acids

51
Q

The stronger the conjugate acid a salt is derived from,

A

The weaker the acid-base activity of the ion

52
Q

What is the equivalence point?

A

When amount of acid = amount if base

53
Q

What is an indicator?

A

Substance (often an organic dye) that has a different color in acidic and alkaline solutions and hence can be used to detect the end point of a titration

Occurs because an indicator is a weak acid/base in which the two forms have different colors and are in equilibrium with each other

54
Q

What change in conditions affects the Kc in Le Chatlier’s principle?

A

Temperature

Whether it increases or decreases depends on to which side it shifts

55
Q

Define the Haber Process

A

Direct combination of N and H to produce ammonia

Mixture of both in a 1:3 ratio by volume is compressed and passed over a heated iron catalyst

56
Q

Uses of ammonia

A

Manufacture of fertilizers bc N is vital for plant growth

Manufacture of polymers containing nitrogen, such as nylon

57
Q

Explain Contact process

A

The production of sulfuric acid by the oxidation of sulfur

Pure sulfite is burnt in air to form sulfur dioxide, then mixed with air and passed over Vanadium (V) oxide catalyst to produce sulfur trioxide

Later reacted with water to produce sulfuric acid

58
Q

Use of sulfuric acid

A

Manufacture of fertilizers, polymers, detergents, paints and pigments

59
Q

Describe the equilibrium established between a liquid and it’s own vapor and how it is affected by temperature

A

Molecules of the liquid will escape from the surface and enter the vapor phase. These molecules in the vapor phase will collide with the walls of the container and exert a pressure. Some of the molecules will also strike the surface of the liquid and condense back into the liquid phase. Initially the rate of return will be low, but as more and more molecules escape into the vapor phase and the pressure increases, the rate of return also increases until it becomes equal to the rate at which the particles vaporize so that

Rate of vaporization= rate of condensation

At this point the system is in a state of dynamic equilibrium, similar to a chemical equilibrium, and the pressure exerted by the particles in the vapor phase is known as the vapor pressure of the liquid. Altering the surface are at the liquid affects both the rates equally, so that it has no overall effect on vapor pressure, though it will affect the time taken to reach equilibrium - the greater the surface area, the more rapidly equilibrium is achieved

60
Q

Define enthalpy of vaporization

A

The amount of energy required to convert one mole of the substance from the liquid to the gaseous state

Mainly required to overcome intermolecular forces, though some is required to do work against the atmosphere

The stronger the forces between the particles, the greater the enthalpy of vaporization, the lower the vapor pressure at a given temperature, and the higher the boiling point.

61
Q

Define boiling point

A

The temperature at which the vapor pressure is equal to STP

62
Q

What is the Qc

A

Found the same way as an equilibrium constant except the reaction is not in equilibrium

63
Q

Define rate of a chemical reaction

A

A measure of the speed at which products are formed, measured as the change in the concentration divided by the change in time

DeltaP/deltaT=-deltaR/deltaT

64
Q

Explain the term hybridization

A

Merging of atomic orbitals to form orbitals of equal energy

ACHEM - REVISION (4 decks)

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