extra flashcards unit 1 + unit 2
1
Q
in the emission/absorption spectras, an (atom/electron) absorbs energy??
A
electron
2
Q
in an absorption/emission spectra, when a white light no longer shines on a sample, so the electron drops/falls to the lower energy levels, what does it emit?
A
electron falls down, emitting a photon of light
3
Q
why are there several series of lines on an emission spectra?
A
because there are different energy levels that electrons can fall back down to
(be promoted to if it was an absorption spectra)
4
Q
what does the convergence limit represent?
A
the ionisation of the atom
5
Q
units of E=hf
A
E = joules
f = Hz
6
Q
units of f = c / λ
A
f = Hz
λ = Meters!!!
7
Q
how to get from kJ to kJmol^-1?
A
kJ x avogadros number
8
Q
find the frequency of the convergence limit? from an enthalpy change? of 376kJmol^-1
A
376kJmol^-1 x1000 = 376000 Jmol^-1
376000Jmol^-1 / avogadros no. = 6.25x10^-19J
E = hf
6.25x10^-19 / h = f
f = 9.42 x10^14 Hz
9
Q
for group 2 solubility, what are the elements we should look at?
A
magnesium, calcium, barium
10
Q
why do we use gravimetric analysis?
A
to calculate the mass of an impure substance
11
Q
what colour solution is iodine?
A
yellow/brown solution
12
Q
what colour solution is bromine?
A
orange/brown solution
13
Q
in an endothermic reaction, why would you add a lid to the calorimeter?
A
to prevent heat gain
14
Q
why do you never get a real value when doing an experiment of e.g combustion of alcohol?
A
- its not 100% pure
- incomplete combustion - we assume all of it goes to complete combustion
15
Q
when a product is reacted with bromine water, what is the formula of bromine water?
A
Br-Br
16
Q
suggest why the overall yield of a two-step synthesis is likely to be lower than the yield of a direct reaction? [1]
A
- product is lost at the end of each step
- so two steps will mean a greater loss
17
Q
explain why the enthalpy change of (e.g combustion) for isomers should be similar [2]
A
- the number and type of bonds are the same
- the energy absorbed by breaking bonds and the energy released by forming bonds will be similar
18
Q
2-2-dimethylbutane is the isomer of C6H14 which ignites most readily. suggest a reason for this.
A
- it has the lowest boiling point/is the most volatile/vaporises most quickly
19
Q
what types of bonding is present in iodine?
A
van der waals AND covalent
20
Q
what arrow represents the transition corresponding to the first line in the visible region in the atomic spectrum?
A
the arrow from n=3 to n=2
21
Q
POSITION of equilibrium
A
22
Q
a student said that the bonds in an ammonia molecule are not purely covalent. explain why she is correct [2]
A
- nitrogen more electronegative than hydrogen / nitrogen and hydrogen have different electronegativities
- this results in polarity / unequal electron distribution in the bond
23
Q
at 25°C and a pressure of 1 atm, 225^3 of gas is produced.
the volume of a gas is directly proportional to its temperature at constant pressure.
a student said that if the gas formed in this experiment were collected at a temperature of 50°C and at 1 atm pressure, the volume formed would he double.
do you agree? justify your answer [1]
A
student incorrect
- this relationship is true when temperature is in K
24
Q
in a volumetric analysis experiment, the students diluted the seawater before reacting it with silver nitrate.
suggest why? [1]
A
- undiluted seawater would give too high a titre
- reduces Cl- ion concentration (so less AgNO3 needed)
25
describe how the students should dilue the (seawater) by a factor of five [3]
- use a pipette/burette to measure (seawater)
- into a volumetric flask
- add 50cm^3 of seawater to a 250cm^3 volumetric flask (and make up to mark with deionised water) / 20cm^3 of seawater to a 100cm^3 volumetric flask
26
describe and explain ONE action the student might take just before the endpoint of the titration, to ensure that the volume of silver nitrate added at the endpoint is accurate [2]
- add drop by drop … to avoid ‘overshooting’ the endpoint
- shake the flask/rinse the side of the flask … to ensure that all the reactants react
- place flask on a white tile … to clearly see colour change
27
experiment to measure enthalpy change in reaction —> (neutralisation)
explain why the maximum temperature recorded decreases when more than 30^3 of sodium hydroxide is added [2]
- all the (H+) is neutralised
- excess OH- added cools the solution
28
Y is a halogenocompound in which each molecule contains one atom of chlorine, bromine or iodine.
describe a chemical test to determine which halogen is present in Y [3]
- add aqueous NaOH and the (neutralise with) HNO3
- add aqueous AgNO3
- yellow ppt for iodine, cream for bromine and white for chlorine
29
in cold weather the wings of an aeroplane can become covered in ice. for safety reasons the wings must be de-iced.
the liquid ethane-1,2-diol, CH2OHCH2OH is used, mixed with water, since this lowers the freezing temperature of water and causes ice to melt.
suggest a reason why the addition of ethane-1,2-diol lowers the freezing temperature of water [1]
any from:
- hydrogen bonding interferes with the formation of the ice lattice
- hydrogen binding stops the ice lattice from forming
- disrupts the hydrogen bonding between water molecules
- water molecules hydrogen bond more strongly with diol molecules than each other
- water prefers to form hydrogen bonds with the diol than with itself
30
describe the nature of the bonding in simple alkenes and explain how this governs their chemical behaviour. [4]
- σ-bonds formed between C and H atoms and σ- and 𝛑- bonds formed between two C atoms
- 𝛑-bonds formed by sideways overlap of p-orbitals
- 𝛑-bonds gives region of high electron density (and weaker)
- react by electrophilic addition
31
the hydrochloric acid used in an experiment could be measured using either a volumetric pipette or a measuring cylinder.
give one practical advantage of using a volumetric pipette:
give one practical advantage of using a measuring cylinder:
- more accurate/more precise/lower percentage error
- faster/easier/more convenient
32
write the equation that corresponds to the molar first ionisation energy of an element. use X to represent the element [1]
X (g) —> X+ (g) + e
33
how to find the concentration of [H+] from the pH?
[H+] = 10^-pH
34
why dont tertiary alcohols get oxidised?
bc oxidation product would otherwise contain too many bonds on carbon atom
35
how do anti-bumping granules work?
take KE away from boiling liquid
36
why is the top of a reflux left open?
- bc otherwise pressure builds up so its to stop it exploding
37
IUPAC/systematic name :
normal name
38
what does the numerical value of Kc tell is about the position of equilibrium?
if Kc > 1 RHS
if Kc < 1 LHS
39
how does chlorine have a higher boiling point that hydrogen chloride?
melting temp chlorine = 172
melting temp hydrogen chloride = 158
both simple covalent
- but chlorine has big mass number!(35.5x2), so more VdW
although HCl has H bonding, lot of VdW outweighs it
40
solubility = mass / volume
41
is CBr4 polar or non-polar? why?
not polar
- electron density being pulled in completely different directions - no net dipole
42
what does the propagation stage of free radical substitution always contain?
____ + X• —> ____ + HX
____ + X2 —> ____ + X•
43
what would you see if magnesium ions were present and you added sodium sulfate solution?
- colourless solution/no observable change
44
explain the origins of emission spectra [2]
- ELECTRONS in an atom are excited and the additional energy promotes them to HIGHER ENERGY LEVELS
- (when the source is removed) the electrons FALL BACK DOWN) to a lower energy level EMITTING ENERGY/LIGHT (in form of a photon)
45
bromine reacts with methanoic acid according to the equation below:
Br2 (aq) + HCOOH (aq) —> 2Br- (aq) + 2H+ (aq) + CO2 (g)
a student wanted to follow the rate of the reaction and mixed solutions of known concentration of bromine and methanoic acid.
suggest two methods which the student could use to follow the rate of this reaction as it proceeds [2]
- use a colorimeter to measure colour change WITH TIME
- use a pH meter to measure pH changes WITH TIME
- use a gas syringe/collect gas over water to measure changes in volume WITH TIME
- use a balance to measure mass changes WITH TIME
46
suggest a reason why bond enthalpies are described as being average bond enthalpies [1]
average used since each individual bond will be in a different environment and therefore have a different strength
47
all hydrocarbons can be burned but, apart from in combustion reactions, alkenes are more reactive than alkanes.
describe the bonding in propene and use this to explain its reactivity [5]
- alkenes contain σ and 𝛑 bonds
- σ bonds are formed from s-s orbital overlap / end-on orbital overlap
- 𝛑 bonds are formed from sideways overlap of p orbitals / overlap above and below plane
- the 𝛑 bond gives a region of high electron density
- this is susceptible to electrophilic attack / attack by an electron deficient species
- (this attack) leads to addition reactions
48
when is it necessary to use a reflux technique in a reaction? [1]
- when reaction is slow
- allows time for equilibrium to be established
- stops reactants / products boiling away
49
gareth was asked to analyse a mixture of potassium methanoate and potassium oxalate. he weighed out 4.69g of the mixture and carried out the method below.
- the mixture was completely dissolved in distilled water
- calcium chloride solution was added to the solution (only calcium oxalate was precipitated)
- the mixture was filtered and the precipitate calcium oxalate washed and dried.
suggest how gareth would know when enough calcium chloride solution had been added to react with all of the potassium oxalate present [2]
- allow to settle / test the filtrate
- add a few drops of calcium chloride solution and see if a ppt forms
50
describe a test, apart from the use of chlorine, to show that a solution contains bromide ions.
reagent(s):
observation(s):
- silver nitrate solution
- cream ppt
51
explain why sodium has a lower melting temperature than aluminium [1]
- aluminium has more valence electrons than sodium therefore stronger metallic bonds
52
explain why silicon has a higher melting temperature than phosphorus [1]
silicon has a giant molecular structure, phosphorus only has weak forces between the molecules
53
explain why hydrogen atoms emit only certain definite frequencies of visible light [2]
- electron falls from higher energy levels to lower energy levels / to n=2
- the difference between any two energy levels is fixed/ energy levels are quantised
54
N2 + 3H2 ⇌ 2NH3
when the temperature is increased the equilibrium yield of NB3 decreases. the student said that the reaction is endothermic. is he correct? justify your answer by using le chatelier’s principle [2]
- student is incorrect since equilibrium shifts to left
- system opposes change by taking in heat, favouring endothermic direction
55
suggest why it is difficult to identify a material as a metal when it is in powdered form? [1]
- can be difficult to get powdered metal to conduct electricity / powdered metal not shiny / not malleable
56
relative atomic mass:
relative isotopic mass:
relative formula mass :
- relative atomic mass: average mass of one atom of the element relative to 1/12th the mass of one atom of carbon-12
- relative isotopic mass : mass of an atom of an isotope relative to 1/12th mass of an atom of carbon 12
- relative formula mass : average mass of a molecule relative to 1/12th the mass of an atom of carbon-12
57
intermolecular forces are those that occur between molecules
intramolecular forces are those that occur within molecules
58
intermolecular forces determine physical properties of compounds and the intramolecular forces determine chemical properties, or how the compound reacts
59
changing pressure only has an effect when it is gases that are reacting together
(in equilibrium for le chats principle)
60
the spectrum is seen as a series of lines because electron energy levels are quantised
energy levels are quantised
61
ionisation:
attraction BETWEEN negative OUTER electron and positive nucleus
62
B
Be
- further
- more shielding
63
N
O
- half filled shell - more stable
- O has repulsion
64
IE of F bigger than O bc repulsion between electrons outweighed by nuclear charge
(nuclear charge outweighs extra repulsion)
65
cobalt-60 in radiotherapy
66
give a reason why it is necessary to ionise the magnesium atoms in a sample in order for it to work in the mass spectrometer? [1]
- to be able to accelerate the ions (to high speeds) / so that they can be deflected by a magnetic field
67
why is it difficult to measure the standard enthalpy change of formation of ethanol directly? [1]
- if carbon, hydrogen and oxygen are reacted, other products (as well as ethanol) would form
- the activation energy for the reaction is too high
- carbon, hydrogen and oxygen do not easily react with each other under these conditions
68
two students carried out an experiment using the apparatus of a spirit burner heating a beaker of water.
neither followed the given method correctly. euan extinguished the flame after only 10 seconds. carys continued to heat for two minutes after the water had boiled.
explain the effect that each of the students’ errors had on the values they obtained in this experiment [2]
euan: temperature rise/loss in mass very small therefore greater percentage error / less accurate
carys : mass of fuel used after boiling didnt result in temperature rise therefore calculated value of enthalpy change too small
69
a student said that she had seen cobalt (II) chloride speed up the rate of production of oxygen very significantly in a different reaction.
describe how the students could determine is cobalt (II) chloride is a better catalyst than manganese (IV) oxide in the decomposition of hydrogen peroxide [3]
- measure volume of gas at a particular time / time how long it takes to produce a fixed volume
- use the same amount of catalyst
- use the same volume of the same concentration of hydrogen peroxide
- better catalyst increases rate by more than
70
until recently chlorofluorocarbons, CFCs were used in many commercial products, but nowadays hydrofluorocarbons, HFCs are preferred
explain why HFCs are preferred to CFCs [5]
- CFCs destroy ozone
- we are then more exposed to UV radiation / greater risk of skin cancer
- (CFCs) produce Cl• / chlorine radical / HFCs do not produce Cl• / chlorine radical starts chain reaction
- HFCs contain C-H / C-F bonds and these are too strong to be broken by UV light
71
kyran states that because diamond is an element, its enthalpy of formation under standard conditions must be 0. state whether kyran is correct and give a reason to support your answer [1]
- he is incorrect
- as diamond is not the standard state of carbon
72
in an enthalpy change question, give a reason why the sign of the enthalpy value calculated is different from the sign of the temperature change measured? [1]
enthalpy measures chemical energy, and as heat energy increases, chemical energy must decrease
73
callum prepares copper (II) sulfate solution from hydrated copper (II) sulfate, CuSO4•5H2O
describe, giving full practical details, how callum should prepare 250cm^3 of copper(II) sulfate solution [6 QER]
- weighing method
- dissolve copper sulfate in a smaller volume of distilled water
- transfer to 250cm^3 volumetric / standard flask
- use of funnel
- wash funnel / glass rod / beaker with distilled water into volumetric flask
- add distilled water up to mark
- shake solution/ mix thoroughly
74
the process to make phenol is carried out in the gas phase and uses a solid zeolite catalyst. the operating temperature is around 400°
C6H6 + N2O —> C6H5OH + N2
∆H = -286kJmol^-1
the reactants are the hydrocarbon benzene and nitrogen oxide, which is a potent greenhouse gas. the nitrogen oxide is obtained from another process, where it is produced as an undesirable side product.
use the account and the equation to comment on the environmental and green chemistry advantages of this process [5 QER]
- process uses a (heterogenous) catalyst, which can easily be separated from the gaseous products (thus saving energy)
- the only other product of the reaction is gaseous nitrogen, which is non-toxic/safe/not a harmful product
- the process uses nitrogen oxide which is used up, rather than being released into the atmosphere from the other process (and causing global warming)
- the process is exothermic and the heat produced can be used elsewhere
- a relatively moderate operating temperature reduces overall costs
- high atom economy
75
state what is meant by molar mass [1]
the mass of one mole of compound
76
suggest why it’s important that the hydrochloric acid and the sodium hydroxide are at the same temperature in an enthalpy change reaction [1]
otherwise a temperature change would occur on adding the acid which had nothing to do with the reaction
77
in terms of bond strength, explain why enthalpy changes of combustion are negative [1]
negative enthalpy change means energy in bonds broken is less than that in bonds made
78
explain why the enthalpy change of combustion of propanol is more negative than that of ethanol [1]
more bonds broken and made in propanol and therefore more energy released
79
biogas refers to a mixture of different gases produced by the breakdown of organic matter in the absence of oxygen. its main component is methane.
suggest why burning biogas is considered to be more environmentally friendly than burning natural gas [2]
- (when biogas burns) no net change in atmospheric CO2 levels
- CO2 released is equivalent to that absorbed in the growth of the bio-resource
dont accept biogas is renewable/carbon neutral unless qualified
80
anti-bumping granules are placed in a flask.
suggest why these granules prevent bumping [1]
- small(er) bubbles form
- prevent large bubbles
- provide surface for bubbles to form
81
why should water go in at the bottom of the condenser and out at the top?
keeps condenser full of water / prevents ‘air-lock’ / allows better cooling
82
explain why something exhibits E-Z isomerism
- has two DIFFERENT groups attached to BOTH carbons in the double bond
83
give an example of an electrophile [1]
- H+
- Na+
84
halogenalkanes can also be formed from alkenes.
when 3-methylbut-1-ene is reacted with hydrogen bromide, a mixture of two different products is formed : 1-bromo-3-methylbutane and 2-bromo-3-methylbutane
state and explain which of the two products is more likely to be formed [1]
- 2-bromo-3-methylbutane
- since intermediate carbocation formed is more stable
85
a student carried out an experiment to determine the enthalpy change of a decomposition reaction. since the reaction is very slow, she was told to use a catalyst. state how the value of the enthalpy change would be affected if the experiment were repeated using a different catalyst [1]
no change since catalysts do not effect enthalpy change
86
a student is asked to prepare a sample of ethanal by oxidising ethanol. describe, giving brief experimental details, how he can carry out the reaction [4]
- place acid in flask and add sodium dichromate (VI) until it has dissolved
- (cool mixture) and add ethanol dropwise (shaking between additions)
- set up distillation apparatus
- heat gently until liquid boils over
87
give a disadvantage of biofuels compared with fossil-based fuels [1]
- large quantities of land needed to grow crops for biofuels
- growing crops for biofuels needs large quantities of water (and fertilisers)
88
what are the axis on a boltzmann energy distribution curve?
• x-axis = energy
• y-axis = no. of particles
89
explain what is meant by a carbon-neutral fuel? [3]
- when fuel from renewable / biological source (burns)
- no net change in atmospheric carbon dioxide levels
- carbon dioxide released is that absorbed by photosynthesis as the plant grew
90
explain what is meant by bond enthalpy? [2]
- (average) energy required to break
- 1 mol of bonds
91
what are some problems with enzymes as catalysts?
- enzyme activity is affected by temperature (increases until the protein denatures)
- enzyme activity is affected by pH (diff enzymes have diff optimum pH levels)
- enzymes can be difficult to be removed from liquid products
92
state why the hydrogenation of alkenes to alkanes is so commercially important? [1]
changes liquid vegetable oils into solid edible fats (e.g margarines)
93
halogenoalkanes are compounds in which one or more hydrogen atoms in an alkane have been replaced by halogen atoms. today they are widely used commercially; however many have also been shown to be serious pollutants.
halogenoalkanes can be formed directly from alkanes and alkenes but the ease of formation differs greatly. briefly outline and explain this difference by considering the types of reactions involved and the bonding in the hydrocarbons [6QER]
- alkenes react more readily than alkanes
- alkanes react by radical substitution
- alkanes are unreactive since they contain strong σ-bonds only
- alkenes react by electrophilic addition
- alkenes contain σ-bonds and 𝛑-bonds
- 𝛑 bond is weaker than σ bond so is easily broken
- 𝛑 bond gives region of high electron density
94
explain why 1-chlorobutane has a higher boiling point than chloroethane [2]
- has stronger / more VdW forces
- because it had a larger surface area/hydrocarbon chain
95
CFCs have been shown to be serious pollutants due to their contribution to ozone depletion. chemists are now replacing them with HFCs. suggest two properties which HFCs should have [2]
- non-flammable
- non-toxic
- suitable volatility
96
how to increase accuracy in enthalpy change reaction? (calorimeter)
- record steady temperature of the water before adding (e.g ammonium nitrate)
- ∆T can be calculated accurately
- record the temperature at timed intervals after adding (ammonium nitrate)
- graph can be extrapolated
- place lid on cup / insulate cup
- prevent heat loss
- crush the ammonium nitrate
- solid dissolves more quickly
97
what density does water have?
1 gcm^-1
98
explain why but-2-ene can form E- and Z- isomers but but-1-ene cannot [2]
- E-Z isomerism occurs due to restricted rotation about the double bond
- but-2-energy has two different groups attached to the carbons in the double bond while but-1-energy has two hydrogens attached to one carbon in the double bond
99
alkanes are derived from petroleum and many are used as fuels. some of the compounds found in petroleum contain sulfur.
state and explain one reason why sulfur compounds should be removed from fuels before they are used [2]
- sulfur combines with oxygen to form sulfur dioxide
- reacts with rainwater to form acid rain
100
how to reduce error due to heat loss in the spirit burner experiment?
- reduce distance between flame and beaker
- protect flame from draughts
101
suggest one reason, other than heat loss, why the value obtained for the enthalpy change of combustion is smaller than the theoretical value [1]
- incomplete combustion
102
in the iodine clock reaction, give a reason why the peroxide is measured into a boiling tube first and now added directly to the flask from a burette [1]
- so that the peroxide can be added quickly to the mixture rather than over a few seconds since the reaction starts when peroxide is added / to ensure that the correct volumes of peroxide and water are added
103
what are some changes which could be made to improve the results in the iodine clock experiment
- ensure that temperature is constant / use a water bath
- because all rates of reactions are temperature dependent
- repeat each experiment and calculate a mean of the concordant results
- ensures that the mean value is more accurate
- using colorimeter - (and valid reason)
104
in the iodine clock experiment, suggest another method, not using sodium thiosulfate, by which the rate of the oxidation reaction could be measured [2]
- the intensity of the colour of the iodine can be monitored over time
- using a colorimeter
105
what are the control variables for the iodine clock experiment?
- temperature of reactants / surroundings
- volume of all reactants (except H2O2 and water)
- concentrations of all reactants (except H2O2)
106
why might the iodine clock method be carried out on a white tile?
- it allows the colour change (from colourless to blue-black to be easily and quickly identified
107
hydrogen peroxide reacts with acidified potassium iodide according to the equation below:
2H+ + 2I- + H2O2 —> I2 + 2H2O.
this reaction was studied using an iodine clock reaction. describe the principles of how the rate of a clock reaction is determined. experimental details are not required [2]
- measure time taken for sudden colour change
- rate = 1/time
108
suggest a method that could be used to follow changes over time as the reaction of nitrogen monoxide, carbon monoxide and oxygen proceeds.
NO(g) + CO(g) + O2(g) —> NO2(g) + CO2(g). [2]
- monitor changes in volume of gas / use a gas syringe / monitor changes in pressure / use a manometer
- reagents have more moles of gas
109
describe the environmental implications on the atmosphere if the reaction:
NO(g) + CO(g) + O2(g) —> NO2(g) + CO2(g)
occurs on a large scale [2]
- NO/NO2/CO are toxic
- CO2 contributes to greenhouse effect/global warming
- NO2 - contribute to acid rain
110
briefly describe how fractional distillation can be carried out [2]
- heated / evaporated
- fractions condense at different temperatures / separated into fractional with different boiling temperatures
111
why are the halogens oxidising agents?
- gain an electron to form a complete shell
