Practice questions Flashcards
A sample of Gallium consists of two isotopes, Ga-69 and Ga-71. The RAM of the sample was found to be 69.7. Calculate the percentage abundance of each isotope.
65% Ga-69 and 35% Ga-71
working: 69.7 = ((69x + 71(100-x))/100
x = 65
How many neutrons does the isotope boron-11 have?
6
working: 11 - 5
Which of the molecules have the strongest intermolecular forces? BF3, H2S, NH3 or H2
NH3
reason: hydrogen bonding
Which structure would sulfur, S8, have?
simple covalent lattice
What is used as the standard measurement of relative isotopic mass?
carbon-12
Give the meaning of ‘isotopes’
atoms with the same number of protons but different numbers of neutrons
Explain how a student could analyse a TLC chromatogram to identify alpha-amino acids present.
calculate Rf values and compare to a database of known values
Suggest why alpha-amino acids with similar structures could cause problems when using TLC to analyse mixtures of alpha-amino acids.
may have similar Rf values
spots may merge together
Explain how GC-MS can be used to identify compounds.
separates compounds to produce fragmentation patterns which can be compared to a database
How does a liquid stationary phase separate organic compounds in a mixture in gas chromatography?
solubility
Which would have the longest retention time when using an alkane stationary phase in GC?
A) alkane
B) alcohol
C) ester
A) alkane
Explain what is meant by the term condensation polymerisation.
the joining of monomers to form a polymer chain with the loss of a small molecule such as H20 or HCl
Polyester x is a degradable polymer prepared by bacterial fermentation of sugars. State two ways polyester x can be degraded.
hydrolysis
photodegradable with sunlight/UV
State and the shape and bond angle around a carbon atom in the alkyl group of propanoic acid. Explain the shape.
shape = tetrahedral
bond angle = 109.5 degrees
explanation = 4 bonding pairs of electrons repelling as far apart as possible
When carrying out a titration, which piece of apparatus should be used to measure the 25cm3 portion of acid solution that has been made up in a volumetric flask?
graduated pipette
How can you reduce the effect of any random error in a titration?
repeat the titration until two concordant results are achieved
State 3 reasons why use of catalysts in industrial processes can be beneficial to the environment.
less energy required
lower carbon dioxide emissions
greater atom economy / less waste
Describe the bonding in silicon.
giant covalent lattice with strong covalent bonds
NO radicals catalyse the breakdown of ozone in the stratosphere. What two equations show how NO radicals catalyse this breakdown?
O3 + .NO -> O2 + NO2.
NO2. + O -> NO. + O2
What should there not be when heating an alcohol?
A naked flame (alcohols are flammable so a sand bath could be used instead of a Bunsen burner)
Explain, in terms of subatomic particles, the meaning of the term isotopes.
atoms of the same element with the same number of protons but with different numbers of neutrons, so with different atomic masses
The two isotopes of potassium have the same chemical properties. Why?
same number of electrons in the outer shell
A mass spectrometer measures the relative abundance of ions with different m/z values. Explain the meaning of ‘m’.
relative mass
What is the chemical name for bicarbonate of soda, NaHCO3?
sodium hydrogen carbonate
Suggest a reason for using an oil bath rather than a water bath.
water could not reach a temperature above 100 degrees c (would evaporate)
Suggest a safe way to extinguish a magnesium fire.
cover with sand / carbon dioxide
Car manufacturers are increasingly using magnesium to make car components rather than iron. Suggest why.
Mg has a lower RFM value and mass than Fe, improving fuel efficiency
When reacting magnesium with hydrochloric acid and measuring the volume of gas evolved, what is the purpose of having the magnesium inside a test tube within a conical flask of the acid?
ensure no gas is lost between time of adding the magnesium and replacing the bung
What does the (III) in phosphorus (III) oxide signify?
phosphorus has a +3 charge
Dilute cobalt chloride solution can be used as invisible ink. The ink is made visible by holding the paper under a hot light bulb. Suggest the chemistry that makes the ink visible again.
heating causes water in the solution to evaporate so the ink is then visible as anhydrous cobalt (II) chloride which is blue
Explain what effect, if any, using a conical flask wet with distilled water instead of a dry conical flask will have on a titre value.
no effect - distilled water will not alter the moles of substance e.g. ethanoic acid, added to the conical flask
Define what is meant by a weak acid?
weak acids only partially dissociate in water
Define orbital
a region around the nucleus that can hold up to 2 electrons with opposite spins
Describe the shape of an s-orbital
spherical
Krypton is found in period 4. Explain how its electron configuration confirms this.
highest shell number is 4
What property of electrons is represented by the arrows?
spin
State another name for a dative covalent bond.
co-ordinate bond
The structure and bonding of CO2 and SO2 both differ from each other. State the following:
structures of CO2 and SO2
bonding within CO2 and SO2
CO2 structure = simple molecular
SO2 structure = giant
bonding within CO2 = double covalent
bonding within SO2 = single covalent
Predict the structure and bonding in lead oxide, PbO
giant structure
ionic bonding
Each C-Cl bond within the molecule tetrachloromethane, CCl4 is polar. Why is this?
bond between two atoms with different electronegativity values
more electronegative Cl atom will attract bonding electrons more, leading to a partial negative charge
Predict, with a reason, whether you would expect sodium chloride, NaCl, to be soluble in tetrachloromethane, CCl4
NaCl is ionic so not soluble is a non-polar solvent
unlikely attraction between ions and non-polar solvent molecules will be strong enough to overcome strong ionic bonds
Predict, with reasons, whether ammonia, NH3, will have a higher or lower boiling point than phosphine, PH3 (they are both group 5 hydrides)
higher melting point
hydrogen bonding is stronger than permanent dipole-dipole interactions between phosphine molecules
Would you expect astatine, another group 7 element, to have a higher or lower boiling point than iodine? Explain your answer.
higher - more electrons so stronger induced dipole-dipole interactions
Rank the 3 molecules (propanone, butane and prop-2-en-1-ol) in order of increasing boiling point. Explain in terms of intermolecular forces why.
propan-2-en-1-ol - hydrogen bonding
propanone - permanent dipole-dipole interactions
butane - induced dipole-dipole interactions
hydrogen bonding > permanent > induced
Give 2 similarities and 2 differences between graphene and graphite.
similarities: both contain single C-C covalent bonds
layers of hexagons with delocalised electrons
differences: graphene = 2D; 1 layer
graphite = 3D; series of parallel layers with intermolecular forces between
Explain whether you would expect Gallium to be soluble or insoluble in water.
insoluble - very strong metallic bonds
Explain why the first ionisation energy of oxygen is lower than the first ionisation energy of nitrogen (both period 2 elements)
oxygen has two paired electrons in the 2p subshell (nitrogen has none)
paired electrons repel one another so are easier to remove
Explain why the first ionisation energy of nitrogen is higher than the first ionisation energy of carbon (both period 2 elements)
nitrogen has a greater nuclear charge due to more protons, increasing attraction between the nucleus and outer electrons, making them harder to remove
electron shielding is similar across a period so has no effect
The variation in first ionisation energies across a period provided evidence for what structure within an atom?
sub-shells of electrons
From the following elements: lithium, beryllium, and fluorine, predict which one will have the largest second ionisation energy. Explain your answer.
lithium - 2nd electron is 1st shell is closer to the nucleus
attraction to nucleus is stronger so more energy is needed to remove
Describe and explain the trend in first ionisation energy values as you move down group 2.
first ionisation energy decreases - increase in shell number, increased electron shielding and increased atomic radius, allowing outer electrons to be more easily removed as weaker attraction to nucleus
Define the term activation energy
the minimum amount of energy required for a reaction to take place
Suggest a reason why an enthalpy change calculated from average bond enthalpies might not be accurate.
average bond strength in lots of different molecules taken into account
Suggest, with a reason, an appropriate material that a calorimeter should be made from.
metal - good conductor of heat
Catalytic converters on cars are less effective on short journeys. They only become effective after the first 5 miles, suggest why.
temperature is too low at the start of the journey
only a small proportion of reactant molecules have energy exceeding the activation energy
Give the effect, if any, and reason, on the position of equilibrium when adding a catalyst.
no effect - forward and backward reaction rates are increased by the same amount
Suggest one environmental disadvantage linked with the use of catalysts.
some catalysts are toxic
Suggest a reason, other than cost, why a perfume manufacturer would choose to use a chemically manufactured fragrance rather than the natural product.
consistent concentration/quality/fragrance
not dependent on pineapple growing/season/weather
purer product
less waste products
Define the term structural isomer.
a molecule with the same molecular formula but with a different structural formula
Explain, in terms of bond enthalpies and bond dipoles, why alkanes are generally unreactive.
high bond enthalpies
non-polar
no dipoles as atoms have the same electronegativity values
State the shape around each carbon atom in an aliphatic alkane.
tetrahedral
Suggest how the C-C-C bond angle in cyclopropane would differ from the C-C-C bond angle in propane.
smaller
How is a sigma bond created?
overlap of orbitals directly between bonding atoms
Define the term radical
a species with an unpaired electron
When cyclohexane reacts with chlorine under UV light, how many different di-substituted halogencyclohexane isomers could be made?
4
If you wanted to synthesise the mono-substituted molecule bromocyclopentane, C5H9Br, why would you avoid using a radical substitution process to synthesise it?
mixture of products formed which are costly/time-consuming to separate
Give 2 possible advantages and disadvantages of disposing of waste polymers by combustion, which generates large amounts of heat.
advantages: heat generated could be converted to electricity
avoids disposal in landfill
disadvantages: toxic gases produced
uses finite resources of oil to produce new polymers
PVC can produce toxic gases if combusted. Why waste gas can be removed by a reaction with sodium hydrogencarbonate?
hydrogen chloride
Give 2 examples of degradable polymers.
biodegradable
photodegradable
Explain how manufacturers of biodegradable polymers from plant material may claim they are ‘carbon neutral’ and suggest why this is unlikely overall.
amount of carbon dioxide released when the polymer degrades is equal to the carbon dioxide taken in by plants
not carbon neutral as harvesting/manufacture/transport of crop is likely to use fossil fuels
Explain the potential danger of incomplete combustion.
produces carbon monoxide which is toxic to humans
Which substance contains hydrogen bonding in the liquid state? A) CH3(CH2)4CH3 B) CH3(CH2)3CHFCH3 C) CH3(CH2)3COCH3 D) CH3(CH2)3CH(OH)CH3
D) CH3(CH2)3CH(OH)CH3
reason: the F of B) and O of C) are not bonded to a H
Which of the following errors could have contributed to an enthalpy change being less exothermic than expected?
1) After removing the spirit burner from under the beaker, allowing the flame to burn for a further 5 mins before weighing it.
2) Recording the final temperature 5 mins after removing the burner
3) Spilling some water from the measuring cylinder that should have gone into the beaker
only 1 and 2
reason: 3 would result in a more exothermic enthalpy
When preparing a standard solution for titration, which of the following would result in a larger titre than it should be?
1) Water added to completely fill the volumetric flask, rather than to the graduation line
2) Washing out the conical flask with water before each titration
3) Washing out the pipette with water before each titration
only 1
Why do Br2 and I2 not exist in the gaseous state under standard conditions?
fairly large molecules so have relatively strong induced dipole-dipole interactions
Suggest the formula of the electrophile formed by the heterolytic fission of an I-Br bond.
I+
reason: Br is more electronegative
What is the formula of ammonium sulphide?
(NH4)2S
reason: -ate contains oxygen
S is found in group 6 so is a 2- ion
How many orbitals are occupied in a silicon atom?
8
Which electron configuration represents the element with the largest first ionisation energy? A) 1s2 2s2 B) 1s2 2s2 2p4 C) 1s2 2s2 2p6 D) 1s2 2s2 2p6 3s2
C) 1s2 2s2 2p6
reason: across a period the first ionisation energy increases as there are more protons increasing nuclear charge, with a smaller atomic radius meaning electrons are harder to release
D) 1s2 2s2 2p6 3s2 is in the next shell out (higher energy level), so outer electrons are further from the nucleus and can reach the ionisation energy easier
When heated with NaOH(aq), 1-iodobutane is hydrolysed at a much faster rate than 1-chlorobutane, why?
The C-I bond requires less energy to break than the C-Cl bond
State the similarity between the atomic structure of isotopes of the same element.
same number of protons and electrons
Write the electron configuration of a bromide ion, in terms of sub-shells.
1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6
reason: electron configuration of the closest noble gas
Which statement is not correct for group 2 metals?
A) an unpaired electron is present in an s-orbital
B) chemical reactivity increases with increasing atomic number
C) the first ionisation energy decreases with increasing atomic number
D) atomic radius increases with increasing atomic number
A) an unpaired electron is present in an s-orbital
HBr(aq) forms two ions in solution. What is the correct observation for reactions of HBr(aq)?
A) effervesces addition of sodium carbonate solution
B) forms a white ppt on addition of silver nitrate solution
C) turns orange on addition of silver nitrate solution
D) turns brown on addition of potassium chloride solution
A) effervesces addition of sodium carbonate solution
reason: HBr + NaCO3 -> NaBr + H20 + CO2
What is the general formula of alkyl groups?
CnH2n+1
Suggest the formula of the acid and base used to prepare the salt H4O5N2
acid = HNO3 base = NH3
Which molecule is non-polar? A) SF6 B) H2S C) PF3 D) NH3
A) SF6
Which equation represents a redox reaction? A) Mg + 2HCl -> MgCl2 + H2 B) MgO + 2HCl -> H2O + MgCl2 C) MgCO3 + 2HCl -> CO2 + H2O + MgCl2 D) Mg(OH)2 + 2HCl -> MgCl2 + 2H2O
A) Mg + 2HCl -> MgCl2 + H2
A student mixes 100cm3 of 0.2mol dm-3 NaCl(aq) wutg 100cm3 of 0.2mol dm-3 Na2CO3(aq)
What is the total concentration of Na+ ions in the mixture formed?
0.3mol dm-3
What is the formula of sodium sulfate?
Na2SO4
Which of the following is the Avogadro constant equal to? A) molecules in 1g of hydrogen gas B) atoms in 23g of sodium-23 C) electrons in 32g of oxygen molecules D) moles in 12g of carbon-12
B) atoms in 23g of sodium-23
Give the name of the intermolecular force acting between molecules of chlorine and describe how this intermolecular force arises?
induced dipole-dipole interactions arise when random movement of electrons causes negative electrons to be spread unevenly, causing an instantaneous dipole which induces a dipole in a neighbouring molecule
Suggest how a chemical process could have a high percentage yield, but a low atom economy.
good conversion of reactants into products but lots of waste products produced
Suggest why highly concentrated solutions of hydrogen peroxide (which easily decomposes into water and oxygen) are kept in dark brown bottles, rather than the standard colourless bottles.
Identify a potential safety issue of storing hydrogen peroxide incorrectly in standard colourless bottles.
stop light reaching the chemical
oxygen gas made by decomposition would increase pressure inside the bottle and risk explosion
When testing an unknown solution to identify anions, why is it necessary to add acid before adding barium chloride solution?
removes carbonate ions which would otherwise form barium carbonate, a white ppt and give a false positive result for sulfate ions
What causes the trend in first ionisation energy from left to right across the periodic table? A) greater number of electrons B) greater shielding C) increased nuclear charge D) large atomic radii
C) increased nuclear charge
reason: more protons = greater attraction to nucleus
Which is the most soluble in water?
A) Mg(OH)2
B) Ba(OH)2
B) Ba(OH)2
reason: solubility increases down the group
Which will not result in a reaction?
A) potassium bromide with aqueous iodine
B) potassium iodide with chlorine water
A) potassium bromide with aqueous iodine
reason: I is less reactive than Br and cannot displace it
Which is the most common product of the reaction between HBr and 3-methylpent-2-ene?
A) 2-bromo-3-methylpentane
B) 3-bromo-3-methylpentane
B) 3-bromo-3-methylpentane
reason: H will attach to the carbon with the most hydrogens on
Which of the following statement(s) is/are true?
1) photodegradable polymers form CO2 and H2O when they break down
2) plastics can be formed from renewable and non-renewable sources
3) halogenated polymers can produce toxic chemicals when burned
only 2 and 3
reason: it is biodegradable polymers which break down into CO2 and H2O
Which type of bond is not linked to high levels of IR absorption and the greenhouse effect? A) C-H B) C=O C) C-O D) O-H
C) C-O
Which redox reaction contains the largest change in oxidation state for sulphur? A) H2SO4 + 8HI -> H2S + 4I2 + 4H2O B) S + O2 -> SO2 C) S2O3 2- + 2H+ -> SO2 + S + H2O D) S + 6HNO3 -> H2SO4 + 6NO2 + 2H2O
A) H2SO4 + 8HI -> H2S + 4I2 + 4H2O
Determine the formula of ‘methane hydrate’, CH4.xH2O (methane makes up about 13.4% of its mass)
In the formula, show the value of x to 2 decimal places
CH4. 5.74H2O
What is the systematic name for NaNO2?
sodium nitrate (III)
What are the units of the rate constant for a reaction with an overall order of 3?
dm6 mol-2 s-1
As the end point of an iodine-thiosulfate titration is approached, a solution is added to detect the end point. State the solution and explain the colour change observed at the end point.
add starch
turns from blue-black to colourless when all the I2 has been used up / reacted
Write the electron configuration, in terms of sub-shells, for a copper atom.
1s2 2s2 2p6 3s2 3p6 3d10 4s1
How can pH be measured continuously?
pH meter or pH probe connected to data logger
‘Soluble aspirin’ is usually sold as the sodium or calcium salt of aspirin. Suggest why salts of aspirin are more soluble than aspirin in water.
salts are ionic and attracted to polar H2O
14.4g of an element X reacts with oxygen to form 24.0g of a compound XO2 Which element is X? Fluorine Magnesium Titanium Molybdenum
Titanium
Why property is not correct for calcium?
A) acts as an oxidising agent
B) it forms a basic oxide
C) it reacts with water to form hydrogen gas
D) its hydroxide is more alkaline than magnesium hydroxide
A) acts as an oxidising agent
Why do successive ionisation energies increase with ionisation number?
radius decreases AND attraction between (the remaining) electrons and nucleus increases
The chemist adds more nitrogen to the equilibrium mixture in (b)
N2(g) + 3H2(g) <=> 2NH3(g) ΔH = -92kJ mol-1
The temperature is kept at 300K and the volume at 5dm^3
The chemist predicts that the addition of nitrogen will increase the proportion of H2(g) that reacts.
Explain whether the chemist’s prediction is correct.
Kc does not change (with pressure/concentration)
[N2] increases / denominator increases
Chemist is correct
denominator decreases OR numerator increases to restore equilibrium Kc
The chemist adds more nitrogen to the equilibrium mixture in (b)
N2(g) + 3H2(g) <=> 2NH3(g) ΔH = -92kJ mol-1
The temperature is kept at 300K and the volume at 5dm^3
The chemist predicts that the addition of nitrogen will increase the proportion of H2(g) that reacts.
Suggest why the chemist is more concerned with increasing the proportion of H2 that reacts rather than the proportion of N2 that reacts.
N2 obtained from the air AND H2 must be manufactured/does not occur naturally
A student measures the pH of 0.045mol dm-3 solutions of HNO3 and HNO2 and found that the acids had different pH values. Why?
HNO3 is a strong acid AND HNO2 is a weak acid
Silicon can be made by heating silicon tetrachloride, SiCl4, with zinc. SiCl4 + 2Zn -> Si + 2ZnCl2
8.50g of SiCl4 is reacted with an excess of zinc. The percentage yield of silicon is 90%. What is the mass of silicon made?
1.26g
Aqueous Cr3+ ions are reacted with an excess pf aqueous sodium hydroxide. Which product is formed?
A) Cr(OH)6 3-
B) Cr(OH)3
A) Cr(OH)6 3-
A cell is constructed from the two redox systems below.
Cu2+(aq) + 2e- <=> Cu(s) E⦵ = +0.34V
Ag+(aq) + e- <=> Ag(s) E⦵ = +0.80V
Which statement(s) is/are correct for the cell?
1. The cell potential is 1.14V
2. The reaction at the copper electrode is Cu(s) -> Cu2+(aq) + 2e-
3. The silver electrode increases in mass
Only 2 and 3
Why would the boiling point of PH3 be lower than the boiling point of NH3?
NH3 has hydrogen bonding which needs more energy to overcome
Why is the boiling point of PH3 lower than the boiling point of AsH3?
AsH3 has more electrons (than PH3)
in AsH3, stronger/more induced dipole–dipole interactions, more energy required to overcome
The reactivity of the group 2 elements Mg-Ba increases down the group, why?
Atomic radius increases and more (electron) shielding Nuclear attraction decreases / (outer) electron(s) experience less attraction
Ionisation energy decreases / less energy needed to remove electron(s)
For the change that accompanies the standard enthalpy change of atomisation of iodine give the following:
equation with state symbols
explanation of entropy change
½ I2(s) -> I(g)
entropy increases
gas has more disorder
Which equation matches the enthalpy change of atomisation of iodine? A) I2 (g) -> 2I(g) B) 1/2 I(g) -> I(g) C) I2 (s) -> 2I(g) D) 1/2 I2(g) -> I(g)
D) 1/2 I2(g) -> I(g)
one mole of gaseous atoms forms from the element in its standard state
What is the reason that zinc is not classified as a transition element?
A) zinc atoms contain a full d-sub-shell
B) there are no zinc ions with an incomplete d-sub-shell
B) there are no zinc ions with an incomplete d-sub-shell
What is the benefit of using a catalyst for energy demand?
reactions can take place at a lower temperature
What are two benefits of using a catalyst for the environment?
reduced CO2 emissions / burning fossil fuels
different reactions possible with better atom economy / less waste / less hazardous chemicals
What is the specific heat capacity of water, c?
4.18 J g-1 K-1
When heated with dilute acid, MnO4 2-(aq) ions disproportionate into MnO4 - and MnO2
Although MnO4 2-(aq) ions disproportionate in acidic conditions, MnO4 2-(aq) ions are stable under alkaline conditions. Explain this difference in stability, in terms of equilibrium.
In acidic conditions: (Concentration of) H+ increases AND equilibrium (position) shifts to the right to reduce concentration of H+/remove H+
In alkaline conditions: OH– reacts with H+ AND equilibrium (position) shifts to the left to increase concentration of H+/add H+
