F331 common questions

A value for the enthalpy change of combustion of octane can be obtained from a simple experiment using the following apparatus.

(i) Describe how you could use this apparatus to obtain data from which you could calculate a value for the enthalpy change of combustion of octane. You should also explain how you would use your experimental results to calculate this value

burn measured mass / amount of fuel / octane (1)

allow measure starting and finishing
temperature / mass of octane / fuel
do not allow just ‘final’ temp. recorded

measure temp rise (1)

of a fixed volume / mass / amount of water (1)

ignore reference to solution

use – energy transferred = mass of water × specific heat
capacity (of water) × temp rise (1)

allow q / energy = mcΔT or mcθ allow ‘m’ as mass of water unless conned
eg allow answer divided by moles burnt

scale up to one mole of fuel / octane used / AW (1)

Results obtained using this apparatus give a much less negative value for the enthalpy change of combustion of octane than the data book value.

Suggest two limitations in the practical procedure which would result in a less negative value for the enthalpy change of combustion of octane than the data book value.

any two from 4:

heat loss to surroundings / air / effect of draughts; etc (1)

do not allow ‘not standard conditions’ / reference to data book values / AW

heat losses to calorimeter / apparatus; (1)

do not allow ‘enthalpy may escape’

incomplete combustion of fuel / lack of (enough) oxygen; (1)

evaporation of fuel (from wick); (1)

ignore evaporation of water / measurement error / human error

The following reaction occurs when butane burns in an engine.

C₄H₁₀(g) + 6.5O₂ (g) → 4CO₂(g) + 5H₂O(g)

This reaction is accompanied by an increase in entropy.

Explain why the entropy increases.

more molecules in products(1);

more disorder/ways of arranging them(1);

allow ‘increase in number of moles’
allow exothermic reaction giving more kinetic energy for 1^st mark

not more ways of arranging atoms

In the 1970s the UK produced around 100 000 tons of a compound called lead tetraethyl, Pb(C₂H₅)₄. This compound was used in petrol to stop auto-ignition.

Explain what is meant by auto-ignition and why it is undesirable in an engine.

fuel igniting too early / before or without spark/on
compression(1);

can damage engine / loss of power / efficiency(1);

do not allow tendency to auto ignite

do not allow causes knocking

The data book value for the enthalpy change of combustion of liquid limonene is slightly different from the value calculated above.

Suggest two reasons why this is the case.

bond enthalpies are for the gaseous state / not in
standard state (1);

bond enthalpy values are averages(1);

no other reference to standard states / conditions

Solid calcium oxide reacts with an excess of water to give calcium hydroxide solution. A group of students set out to determine the enthalpy change of this reaction.

Suggest an experimental procedure they might use and list the measurements they would make.

experimental procedure Stir (1);

Insulated beaker / polystyrene cup / water jacket (1);

measurements Measure temperature rise / change / initial & final temperatures (1) ;

Measure mass of solid / (known) amount / quantities (1)

Measure volume / mass of water / (known) amount / quantities

Toxic oxides of nitrogen (NO_x) are produced when coal burns at high temperatures. Explain how this NO_x is produced.

Nitrogen (& O₂) from air/fuel/coal(1); react with O₂ 2

/combust/oxidized/bond with O₂ (1)

(ignore refs. to incomplete combustion)

Describe a possible hazard from using a medical tracer and explain how it might arise.

Two from:
leukaemia / cancer / radiation / cell / tissue / blood vessel damage / rapid
division of cells (1);
cells / tissue / molecules being ionised / radical formation (1);
mutations / DNA damage (1)

The half-life of the sodium-24 tracer is 15 hours. Explain why this half-life is suitable, rather than a much longer one or a much shorter one.

Longer – little radiation to measure / lasts longer than needed
(for investigation) / longer exposure (to radiation) than necessary /
prevent repeat investigation(AW)

Shorter – short burst of radiation (AW) / decays before measurement
made / isotope detected / does not pass round body in time

What causes ‘knock’ in a car engine and what term is used to indicate the tendency of a fuel to cause knocking?

(Fuel) pre-igniting(AW) (1); Octane number/rating (1) ;

Complete and label the enthalpy level diagram below to represent a reaction that causes a temperature decrease in the surroundings.

Use the following labels: reactants; products; enthalpy change of reaction

Reactants label to left of products (1); Reactants label below products (1); 3

single headed ’vertical’ arrow going up, labelled in words or using H (1)

ecf for products below reactants

Give one reason why it is desirable to reduce carbon monoxide emissions.

CO  toxic/poisonous/photochemical smog (1); NOT harmful/health 1
hazard/acid rain/greenhouse

Explain, in terms of bond breaking and bond making, why the combustion of a fuel gives out heat.

Bonds broken, energy in/endothermic (1); formed, energy out/exo (1);

more out than in/more exo- than endo-thermic (1) AW (independent)

Autogas has a higher octane number than ordinary petrol.

Why is a fuel with a high octane number an advantage?

less/reduces/ tendency to autoignite/knock/pre-ignite (1);NOT stops

avoids damage to engine (1);

higher compression ratio/power/efficiency possible (1)

Explain the meaning of the term half-life.

Time taken(qualified) to decay(1) but NOT half the time taken(CON) 2

for amount/ count rate to drop by half (from starting point)/

half radioactivity decays/half the atoms decay/half radiation emitted/

half radioactive nuclei decay/half mass of original isotope (1) (AW)

hat is meant by the term isotopes?

Same atomic no./protons (1); different mass no./ neutrons (1)

Complete this table:

property

α-particle

β-particle

relative charge

relative mass

4

negligible

stopped by

paper

deflection by electric field

large

+2; low/small (1); –1; aluminium/lead (1) (one mark for each column)

The enthalpy change for this reaction could be measured directly from experiment by adding solid ammonium bicarbonate to dilute hydrochloric acid in an appropriate container.

Describe the essential measurements that would need to be taken in order to determine this enthalpy change.

Mass/weight of ammonium bicarbonate (1); Ignore references to mass after
reaction

Vol./mass of HCl (1) NOT volume of reactants; Concentration of HCl (1);

Starting temperature (1); finishing temps (1); NB (temp change/T scores two)

mass of reactants on its own = 1 mark; temp increase  1 mark only

What happens in a nuclear fusion reaction?

(Light) nuclei (1) (fuse/join/come together) (1); NOT atoms

to form a heavier nucleus/atom/element or larger nucleus (1)

any reference to bonding is a CON

What conditions in the Sun make fusion possible?

High/extreme pressure/gravity/density (1); Not  lots of pressure 2

high/extreme temp/extreme heat/energy/KE (1) allow high temp and
pressure

The change in melting point across the period can be explained in terms of the structure and bonding of the elements.

Describe the changes, both in type of bonding and in structure, as the period is crossed from left to right.

bonding: (one mark available)

metallic on left of period changing to covalent going
right(1); may be combined with later marks

structure: (three marks available)

(giant) metallic at start(1);

giant / large molecular / covalent or network / lattice in
middle(1);

(simple) molecular on right(1);

allow description using quoted elements eg Li, Be, (B) metallic. etc

allow ‘carbon / C’ for middle marking point

Explain:

 why excited atoms emit light

 why the emission spectrum consists of discrete lines

 why there are sets of lines.

emit light:- electrons raised to higher electronic levels (1);

electrons drop back to lower levels losing energy (as ‘light’)1

discrete lines:- energy levels ‘quantized’ (AW)/drops give out a specific

amount of energy/drops between levels (1

relates to specific frequencies/ ()E=hu (1)

sets of lines:- each set represent drops to a different lower level/

mention of specific example eg Lyman (1)

Excited ATOMS/no mention of electrons max four marks

NB these points could be gained from an annotated diagram.

Strontium also produces an absorption spectrum.

Describe two similarities and one difference between the appearance of the emission and absorption spectra of strontium.

similarities

lines (spectrum) (1);lines in same place/same 3

spacing/lines converge (1)

difference

– black lines (on a bright background)

compared to coloured lines (on a black background) (1);

Explain the term standard enthalpy change of formation.

Enthalpy change when 1 mole of compound (1); formed from elements

in their standard states NOT conditions (1)

Why did Mendeleev leave gaps in his Periodic Table?

How did later evidence from the scientific community support Mendeleev’s decision to leave gaps in his Periodic Table?

i)

for elements yet to be discovered(1)

ii)

new elements(1);

showed properties fitting in with group(1);

Showed properties / characteristics / chemistry predicted / fitted – for second mark

allow specific reference to a “new” element

Which group in the modern Periodic Table is missing from Mendeleev’s table?

noble(inert) gases / group 0 / group 8 / VIII (1);

wrongly named group cons correct group number

Newlands and Mendeleev both put the elements in order of atomic mass.

What property is used to order the elements in the modern Periodic Table?

Atomic / proton number(1);

Suggest why Newlands’ arrangement was less useful than Mendeleev’s arrangement.

properties of some of the elements did not match up in
Newlands(1);

allow “he didn’t leave gaps”

Catalytic converters could not be used with ‘leaded’ petrol because the lead acted as a catalyst poison.

(i) Describe the type of catalysis used in catalytic converters and describe the mechanism of this type of catalysis.

In your answer, you should use appropriate technical terms, spelt correctly.

QWC – heterogeneous (1); spelling of word must be correct

adsorption of reactants(1);

bonds (in reactants) weaken and break(1);

new bonds (in products) form(1);

products diffuse off / desorbed / released from catalyst(1);

do not allow absorption but allow anything else which suggests “on the surface”

bonds between reactants break cons 3^rd mark

do not allow forms an intermediate

do not allow petrol as a named reactant

Explain how a catalyst poison works.

Poison irreversibly adsorbed blocking surface of catalyst (AW) (1);

reactants / other molecules cannot bond to surface(1);

allow poison binds irreversibly for 1st mark

Other radioactive isotopes are used as medical tracers.

Suggest why it is not advisable to use an isotope with either a very short or a very long half-life as a medical tracer.

very short half-life

not around in sufficient levels long enough to detect(AW) /
burst of harmful radiation (1);

very long half-life

possible harm to patient/too little radiation to detect(1);

do not allow isotope finished / stopped; must be about diagnosis / detection / tracing
allow ‘not around long enough to be useful as a tracer’ / decays too fast / quickly for use(AW)

Complete this table:

property                       type of emission

                                     α             β            γ

relative
charge                                 –1           0

relative mass              4                          0

nature                              (nuclear) 

                                         electron /

                                          correct
symbol

range in air

(few) cms /
mm

stopped by

Lead /
aluminium /
thick metal
(sheet) /
concrete

deflection by
an electric
field

High / big /
large

none

property

type of emission

α

β

γ

relative
charge

+2

–1

0

relative mass

4

0.00055

0

nature

helium
nucleus

(nuclear)
electron /
correct
symbol

very high
frequency
electromagnetic
radiation

range in air

(few) cms /
mm

few metres

very long

stopped by

tissue paper

metal foil

Lead /
aluminium /
thick metal
(sheet) /
concrete

deflection by
an electric
field

low

High / big /
large

none

A data book shows that when one mole of ethanol burns under standard conditions, 1370 kJ are produced.

Suggest two reasons why this value is very different from the student’s experimental result.

heat losses (to surroundings) (1);

plus one more from:

incomplete combustion;

heat loss to calorimeter;

evaporation of fuel from wick;

not standard conditions;

do not allow answers in terms of operator error unless describing heat loss

Use the ideas of bond making and bond breaking to explain why the combustion of ethanol gives out energy.

energy in / endothermic to break reactant bonds (1);

more energy given out / exothermic to form product bonds (1);

ignore references to number of double bonds formed

more bonds formed cons second mark

note: second marking point now runs two marking points from previous mark schemes into one

Write a balanced equation for the reaction between nitrogen and oxygen to produce nitrogen monoxide. Include state symbols.

Suggest why high temperatures are needed for this reaction to take place.

N₂(g) + O₂(g) → 2NO(g)

need to break NN bond / NN bond / stable N₂ molecule

very high bond enthalpy / high E_A / very strong(1);

must refer to nitrogen bond (or imply) ignore references to oxygen but con reference to other bond types

Explain why:

• mass numbers of atoms are always whole numbers;

• the relative atomic mass of an element may not be a whole number.

mass number is protons + neutrons / nucleons (1);

Ar is average / mean (1);

of (naturally occurring) isotopes (1);

allow cannot have fractional protons and neutrons

do not allow sub-atomic particles

The search for alternatives to fuels made from crude oil is becoming more important. ‘Biofuels’ such as ethanol and biodiesel are now readily available.

Suggest two benefits associated with using ‘biofuels’.

any two from:

sustainable/renewable;

biodegradable;

avoids wasting/using up fossil fuels;

Less CO;

carbon neutral/no net emission of CO₂ / AW;

do not allow any answers in terms of expense

do not allow produces less pollution / no SO₂ / high octane number / less CO₂ / less global warming

The half-life of potassium-40 is approximately 12,000 million years.

Suggest and explain why this long half-life makes potassium-argon dating unreliable for determining the age of lavas that are only thousands of years old.

difficult to detect very small amounts of Ar-40 formed K-40
decayed / dating errors very large when so little decay has
taken place / AW (1)

do not allow answers that talk only in terms of ‘not even finished one half life’

Some rocks contain radioactive isotopes that can be used to date the rocks.

Dating of rocks requires being able to accurately measure the amount of both original (parent) radioisotope and finishing (daughter) stable isotope.

Suggest two assumptions that must be made if a radioisotope is to be used for dating a rock. any two from:

any two from:

half life / decay constant has remained unchanged;

no loss / gain of parent isotope / original radioisotope same age as
rock;

no loss / gain of daughter products/no daughter product at start;

‘original rock contained the parent isotope but not the daughter isotope’ …..for this answer,

allow one mark for idea of assuming no presence of daughter product from an alternative source

allow no loss of gas

do not allow half life/mass must be known

Explain the term aliphatic.

(hydrocarbon) contains no benzene rings / not an arene (1)

do not allow contains no rings

Suggest a value for the bond angle shown in the structure below and explain your value.

120

3 areas of electron density (1)

do not allow 3 ‘atoms’ or ‘electron pairs’
allow names or descriptions of electron groups
eg double bond

around central C (1)

allow clear diagram or description

areas of electron density / pairs repel as far apart as possible /
minimize energy (1)

do not allow repel as much as possible

TAKE CARE repel and ‘as far apart’ run together for only one mark

allow bonds (but not atoms) repel

The strength of metals and metal alloys is due to their strong metallic bonding.

Draw a labelled diagram to show a simple model of metallic bonding.

Delocalised electrons (1)

First two points can be on diagram or labels

minimum of five cations shown (can touch)

Regular array of cations / positive ions / residues (1)

allow positive atoms

do not allow positive nucleus or positive metal

Labels but any used must be correct (1)

Explain the origin of carbon monoxide emissions.

Incomplete combustion (1)

allow not enough oxygen or air linked to the idea of combustion / uncomplete combustion

of hydrocarbons (1)

Second mark depends on the first.
allow fossil fuel or named fossil fuel / carbon in the fuel /
organic fuel
do not allow just ‘fuel’ or carbon as the fuel

Give two reasons why carbon monoxide is classed as a polluting gas.

Toxic / poisonous / reduces the capacity of blood to carry
oxygen around the body / AW (1)

allow respiratory problems, but not breathing problems.
ignore harmful / dangerous AND

Any one from:

causes (photochemical) smog (1)

oxidised to CO₂ which is a greenhouse gas / reacts with O₂ to form
CO₂ which is a greenhouse gas (1)

Answer must have the CO₂ AND the greenhouse gas for this alternative.
Allow global warming instead of greenhouse gas.

Describe the main stages of the time-of-flight mass spectrometer [5]

sample ionised / X+ produced 

(ions) are accelerated / move into acceleration area OR increased velocity (or speed) OR given KE (in electric field/plates) 

to the same kinetic energy 

drift region 

heavier ions move across to detector more slowly (ora) /

time taken to reach detector is a measure of its mass / different mass take different times to reach detector 

pollutant

how the pollutants in the exhaust gases are formed

oxides of nitrogen

From the reaction of nitrogen and oxygen gas in the air, at the high temperatures of the combustion chamber.

carbon monoxide

CO – partial/incomplete combustion

oxides of sulfur

SO_x – combustion of sulphur
impurities/atoms in fuel (1);

hydrocarbons

hydrocarbons – unburnt fuel (1);