F332 Repeated Questions

Front 1

How do CFC's damage the ozone layer

Back 1

 C-Cl bonds strong so CFC’s not broken down in troposphere.

 High energy (high frequency) UV radiation in stratosphere breaks C-Cl bonds.

 Creates Cl radicals which catalyse break down of ozone molecules into oxygen.

 Ozone absorbs high energy UV that causes skin cancer, damage to eyes (cataracts), DNA damage.

Front 2

What type of bond breaking/ what mechanism of bond breaking is involved in initation?

Back 2

Homolytic fission

Front 3

Ozone is not formed by sunlight in the troposphere because

Back 3

High energy UV needed to break bonds in oxygen (O2) molecules is not found in the troposphere OR UV in troposphere does not have enough energy to break bonds in O2.

Front 4

If bromoalkanes (e.g bromomethane) mentioned in a CFC question include

Back 4

 C-Br bond weaker than C-Cl
 Low energy UV found in troposphere can break C-Br bonds (not C-Cl).
 Bromoalkanes react in troposphere and do not reach the stratosphere.
 C-Cl bonds too strong to break in troposphere and need high energy UV only found in stratosphere to break and create Cl radicals.

Front 5

What is the advantages of HFCs

Back 5

 C-F bonds so strong they do not break down to form radicals under action of UV in stratosphere.
 C-H bonds mean they can be broken down by low energy UV in troposphere and do not reach stratosphere.
 Have same essential properties as the CFC’s they replace.
 Disadvantages : cause global warming, presence of halogens makes them expensive to make.

Front 6

What reacts faster, chloroalkanes or bromoalkanes with a nucleophile ? (e.g NaOH) –mention polarity and bond strength.

Back 6

 C-Cl more polar as Cl more electronegative than Br. (bigger electronegativity difference in C-Cl than C-Br)
 C-Br has lower bond strength (bond enthalpy) than C-Cl
 Br is a larger atom than Cl so C-Br bond is longer than C-Cl.
 Longer bonds are weaker/take less energy to break.
 Bromoalkanes react faster proving lower bond strength is more important than higher polarity.

Front 7

Describe the appearance of halogens

Back 7

Halogens – appearance (asked recently).
 Iodine is a black or grey solid, chlorine is a green gas, liquid bromine is red/brown (jun12).
 Bromine water is brown / orange / yellow (jan2012) not “red” and colourless when decolourised (not clear).
 Iodine dissolved in water is brown or yellow.
 Iodine dissolved in organic solvents (hexane, cyclohexane etc) is violet/purple.

Front 8

Explain how rising levels of carbon dioxide may increase global warming.

Back 8

 UV radiation from the sun is absorbed by the Earth’s surface.
 Earth’s surface re-emits this energy as infra-red radiation.
 Carbon dioxide molecules absorb infra-red radiation.
 Absorbing infra-red makes bonds in CO2 vibrate more.
 More vibration means kinetic energy of CO2 rises causing a rise in temperature (excited CO2 may re-emit some absorbed IR radiation in all directions including some back to the surface) Not “reflect” IR
 More CO2 means more energy absorbed so larger temperature rise.

Front 9

Compare the structure and properties of CO2 and SiO2.

Back 9

 SiO2 is a solid at room temperature while CO2 is a gas.
 CO2 is simple molecular (avoid simple covalent) while SiO2 is a giant covalent network.
 Strong covalent bonds between atoms in SiO2 must be broken to melt/boil, requiring much energy. NOT “intermolecular” bonds in SiO2 (as no discrete molecules in a network)!
 Only weak intermolecular bonds (instantaneous dipole-induced dipole ) have to be broken between CO2 molecules needing little energy-to separate molecules, so CO2 easily boiled.
(Idea of weak imb compared to strong covalent bonds is important)
 CO2 slightly soluble as water molecules can hydrogen bond to oxygen atoms in CO2.
 SiO2 totally insoluble as water molecules can’t replace strong covalent bonds in SiO2 with stronger intermolecular bonds.

Front 10

Why does a reaction go faster when heated (temperature and rates)?

Back 10

 Raising temperature increases kinetic energy of molecules.
 More molecules will exceed the activation enthalpy (minimum energy for a collision to lead to a reaction).
 More collisions will be successful per unit time = higher reaction rate.

Front 11

Why does raising pressure increase rate?

Back 11

 Higher pressure increases the number of particles per unit volume of gas.
 More collisions will occur per unit time = higher/faster rate.

Front 12

How does adding a catalyst increase rate of reaction?

Back 12

 Catalyst provides alternative pathway for the reaction
 With lower activation enthalpy.
 Activation enthalpy = minimum energy needed for colliding particles to react.
 More particles will now exceed activation enthalpy (does NOT increase energy of particles)
 More successful collisions per unit time = higher (faster) rate.

Front 13

What is dynamic equilibrium?

Back 13

 The rate of the forward and reverse reactions are equal in a closed system.
 The concentrations of reactants and products no longer change

Front 14

What affect does increasing pressure has on the equilibrium poistion?

Back 14

Increasing the pressure pushes the equilibrium position to the side with fewer moles of gas which is the left, hence less product is made

Front 15

What affect does rising temperature has on the equilibrium poistion?

Back 15

Raising the temperature pushes the equilibrium position in the endothermic direction (the reverse reaction is endothermic) which is the left. Less product is made

Front 16

“The best conditions for making “x” are very high pressures, suggest why only medium pressure is used in reality.”

Back 16

 Thick walled apparatus to cope with pressure very costly
 Large energy costs in creating/keeping high pressure using heavy duty pumps.

Front 17

Describe the ways to reduce CO2 levels

Back 17

 Burn less fossil fuels/ used a named non-carbon based alternative such as geothermal, tidal, nuclear etc. Not “use hydrogen” as is often made from fossil fuels.
 Plant more trees to increase photosynthesis
 Pumping it under pressure to the deep ocean floor. (not just into the sea)
 Pumping it under pressure into disused oil /gas wells. (not just underground or into rocks)

Front 18

Why is carbon capture and storage not widely used?

Back 18

 Apparatus and energy costs of pumping CO2 at pressure to bottom of deep sea make it very expensive.
 Concern that CO2 may harm ecosystems/marine life in deep oceans by increasing acidity of water.

Front 19

What industrial processes produce carbon dioxide?

Back 19

 Refining oil
 Generating electricity
 Making iron/steel
 Fermentation

Front 20

Explain in terms of intermolecular bonds why bromine Br2 has a higher boiling point than chlorine Cl2. Explain how the intermolecular bond occurs.

Back 20

 Non-polar molecules only have instantaneous dipole-induced dipole intermolecular bonds.
 Random movement of electrons in molecule creates uneven distribution of charge (=instantaneous dipole).
 This induces a dipole in a neighbouring molecule leading to an attraction.
 Molecules with more electrons (Br2) will form stronger intermolecular bonds.
 More energy will needed to overcome/break the stronger intermolecular bonds so boiling point is higher.

Front 21

What is E/Z isomerism

Back 21

-the C=C double bond prevents rotation
-there must be two different groups on each carbon of the Carbon –carbon double bond.

Front 22

What is a radical?

Back 22

Particle (species) with an unpaired electron

Front 23

Why is the molecule CF3Cl polar?

Back 23

 C-Cl and C-F bonds have different polarities
 The dipoles/charges do not cancel out OR centre of positive and negative charges don’t coincide
 OR there is a greater (partial) negative charge on the F side of the molecule.

Front 24

Explain the shape of the SO2 molecule and give the bond angle.

Back 24

 There are 3 regions of electron density around the (central atom) S (2 double bonds and a lone pair around central atom which is S in this case).
 (Regions of electron density) repel (not just bonds/ atoms/lone pairs of electrons repel)
 As far apart as possible to minimise repulsion.
 The shape is bent/ V-shaped with a bond angle of 120o.

Front 25

What is meant by heating under reflux?

Back 25

 Boiling (a liquid reaction mixture)
 With a vertical condenser – preventing products and reactants from boiling away (not preventing evaporation, if drawn, condenser must be open at top)
What is

Front 26

What is meant by a (homogeneous) catalyst?

Back 26

 Catalyst provides alternative pathway for reaction with lower activation enthalpy.
 Is chemically unchanged at the end of the reaction.
 (Homogeneous means) catalyst is in same state as reactants.

Front 27

What is meant by “addition polymerisation”?

Back 27

 (polymer) Many monomers /molecules bonded/joined to form a long chain.
 There is only one product / no molecules eliminated

Front 28

How are ID-ID formed?

Back 28

-Electron movements causes unequal distribution of charge leading to temporary dipole, this induces a temporary dipole of a neighbourig molecule and attracts it.

Front 29

Describe one use, apart from refrigerants, to which CFCs were put.

Back 29

- Blowing agents for expanded plastics
- propellants for aerosols
- solvents for degreasing

Front 30

Explain the effect of CFCs on the stratosphere which led to them being banned

Back 30

Breaks down to give chlorine atoms; which causes ozone depletion

Front 31

What safety precautions are needed when using/transporting halogens- especially chlorine

Back 31

Chlorine is volatile/ a gas , toxic and causes respiratory diseases.

Front 32

Why is electrolysis particularly useful industrial process

Back 32

-100% atom economy
- no waste products

Front 33

Chloride ions to chlorine molecules - Write a half equation for this reaction.

Back 33

2Cl- (aq) --> Cl2 (aq) + 2e-

Or..... ( replace X with Chlorine, Bromine or iodine)

2X- (aq) --> X2(aq) + 2 e-

Front 34

Explain, in terms of intermolecular bonds, why something is a gas at room temperature and
Pressure but something is a liquid under the same conditions. As part of your answer, you should
Explain how the intermolecular bonds arise.

Back 34

- Instantaneous dipole - Induced dipole , electron movements in the molecules create an uneven distribution of charge, a dipole is induced in a neighbouring molecule leading to attraction , Intermolecular bonds are stronger in something, something has more electrons ( if molecule or element).

Front 35

Give the general equation for the reaction between a halide and sliver ions.

Back 35

X- (aq) + Ag+ (aq)  AgX (s) ( X being the halide)

Front 36

Explain what is meant by dynamic equilibrium.

Back 36

Rate of forward reactions = rete of back reactions/ reactants and products are formed at the same rate.
Concentration of reactants and products remain constant/ closed system.

Front 37

Carbon dioxide is described as a ‘Greenhouse gas’. Explain how increased concentrations of
Carbon dioxide in the troposphere could be linked to global warming, saying where the energy
comes from originally.
In your answer, you should make it clear how the steps you describe are linked to one
another.
(Answer in logical order )!

Back 37

- The sun emits energy, in the form of UV light, onto the Earth

- The Earth absorbs this energy and re-emits IR

- CO2, in the troposphere, absorbs some IR in the 'IR window'

- making the bonds vibrate more

- this turns into kinetic energy, raising the temp.
- some O2 molecules radiate IR which warms the Earth

- more CO2 means more radiation and hence a warmer Earth

Front 38

Explain what is meant by the term electrophile

Back 38

Partially positively charged / electron deficient reagent bonds by accepting a pair of electrons.

Front 39

What is meant by 'heating under reflux'?

Back 39

-Boiling a liquid in a closed system
- So that no product is lost.

Front 40

How do modern catalysts work?

Back 40

-adsorb the molecule
- Break it's bonds
- Make new bonds
-Release it

Front 41

What is the main intermolecular bond that arises between polyethene chains and how does it occur?

Back 41

-Instantaneous dipole-induced dipole
-At a certain moment in time the electrons may be closer to one side of the molecule creating an instantaneous dipole
-This can induce a dipole in an unchanged molecule then repel it

Front 42

How can compounds exist as a pair of E/Z isomers?

Back 42

-It has a double bond to prevent it from rotating.
-it has two different groups on each of the C=C

Front 43

Why does formation of ozone not take place in the troposphere?

Back 43

-High frequency radiation only present in the stratosphere
-O radicals are formed by Photodissociation of O2

Front 44

How do you make a halogenoalkane

Back 44

- Shake methylpropan-2-ol with conc. HCl (keep releasing the pressure)
- Allow the mixture to settle and run off the lower aqueous layer, leaving the impure halogenoalkane
- Neutralise the excess acid by adding sodium hydrogen carbonate solution and run the lower layer off
- Add distilled water and run off the lower layer (this gets rid of any remaining inorganic impurities)
- Remove any remaining water by adding anhydrous sodium sulfate (a drying agent)
- Remove remaining organic impurities by distilling the mixture

Front 45

What happens to the temperature in the atmosphere as you move away from the Earths surface to the top of the stratosphere?

Back 45

- the temperature decreases in the troposphere and you move away from the Earth
- because hot gases near the Earths surface rise and cool
- within the stratosphere
- the temperature of the atmosphere rises as you move away from the Earth
- due to exothermic reactions

Front 46

How do we know that halogenoalkanes deplete ozone?

Back 46

- They break down in the presence of UV light to form radicals
- the radicals catalyse the breakdown
- low concentrations of ozone were found above the antarctic

Front 47

Why do reactions between atoms occur more quickly at the top of the troposphere?

Back 47

-Conc of gases is higher at the top of the troposphere
- So more collisions occur between the molecules per second.

Front 48

Define initiation , propagation and termination

Back 48

-Initiation - radical produced from a non-radical
-Propagation- radical replaced by another with a radical on both sides of equation
-Termination - radical used up to produce non-radicals

Front 49

Explain the meaning of the term thermoplastic.

Back 49

is deformed when warmed / heated

Front 50

Give one use bromine

Back 50

- making medicines
- making flame retardants
- water purification

Front 51

Give one problem associated with tropospheric ozone.

Back 51

- breathing diffculties
- photochemical smog

Front 52

Explain what is meant by the fingerprint region in an infrared spectrum

Back 52

-Region below 1500 (cm-1)
-Unique (part of the spectrum) for molecule

Front 53

Explain why the presence of ozone in the stratosphere is important for humans.

Back 53

-it absorbs / removes any type of high frequnecy uv radiation
- which could cause skin cancer / damage DNA / affect crops

Front 54

Suggest why OH radicals are not produced in troposhere by action of sunglight on water molecules

Back 54

-Frequency of uv is not high enough to break the bond

Front 55

Explain what is ment by the term activation enthalpy

Back 55

- minimum energy need for a collision to cause a reaction

Front 56

Describe and explain the effect, if any, of an increase in pressure on the rate of reaction

Back 56

- reactions rate increase
- particles are close together
- so colide more frequently

Front 57

Describe and explain the effect, if any, of an increase in pressure on the equilibrium yield of the reaction

Back 57

- equilibrium yield decreases
- equilibrium position moves to oppose the change towards the reactants
- because fewer molecules on th reactant side

Front 58

Describe and explain the effect, if any, of an increase in temperature on the equilibrium yield of the reaction

Back 58

-yield would increase
-equilibrium position moves to oppose the change towards products
-in the endrothermic direction

Front 59

Explain how increased concentration of methane in the troposphere could be linked to global warming

Back 59

-More methane means more radiation is absorbed
-molecules radiate air and that warms Earth/atmosphere