chem paper 1 Flashcards
(25 cards)
2020
There is a general trend for an increase in ionisation energy across Period 3. Give one example of an element that deviates from this trend. Explain why this deviation occurs. (3 marker)
M1- Aluminium
M2- Outer electron in 3p orbital / sub-shell
M3- 3p higher in energy / slightly more shielded (than 3s) / slightly
further away (than 3s)
2020
Define the term relative atomic mass. (2 marker)
M1-The average mass of an atom of an element
M2-Compared to 1/12th the mass of an atom of carbon-12
2020
In a TOF mass spectrometer, ions are accelerated to the same kinetic energy (KE).
The kinetic energy of an ion is given by the equation KE =1/2mv2
Where:
KE = kinetic energy / J
m = mass / kg
v = speed / m s–1
In a TOF mass spectrometer, each 84Kr+ ion is accelerated to a
kinetic energy of 4.83 × 10−16 J and the time of flight is 1.72 × 10−5 s
Calculate the length, in metres, of the TOF flight tube.
The Avogadro constant, L = 6.022 × 1023 mol−1 (4 marker)
M1 m = (84/1000)/6.02 x 1023
(= 1.395 x 10-25 kg)
M2 v2 = 2ke/m
= 2 x (4.83 × 10−16) / (1.395 x 10-25)
M3 v = √(6924731183)
= 83214.97
M4 d = v x t
= 83214.97 x 1.72 × 10−5
= 1.43 (m)
2020
A student determines the enthalpy change for the reaction between
calcium carbonate and hydrochloric acid.
CaCO3(s) + 2 HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)
The student follows this method:
* measure out 50 cm3 of 1.00 mol dm–3 aqueous hydrochloric acid using a measuring
cylinder and pour the acid into a 100 cm3 glass beaker
* weigh out 2.50 g of solid calcium carbonate on a watch glass and tip the solid into
the acid
* stir the mixture with a thermometer
* record the maximum temperature reached.
The student uses the data to determine a value for the enthalpy change.
Explain how the experimental method and use of apparatus can be improved to
provide more accurate data.
Describe how this data from the improved method can be used to determine an
accurate value for the temperature change. (6 marker)
Stage 1: Apparatus
1a. Use a burette/pipette (instead of a measuring
cylinder)
1b. Use a polystyrene cup (instead of a beaker) /
insulate beaker
1c. Reweigh the watchglass after adding the solid
1d: Use powdered solid
Stage 2: Temperature Measurements
2a. Measure and record the initial temperature of
the solution for a few minutes before addition
2b. Measure and record the temperature after the
addition at regular intervals (eg each minute) for 8+ minutes/until a trend is observed
Stage 3: Temperature Determination
3a. Plot a graph of temperature against time
3b. Extrapolate to the point of addition
3c. Determine ΔT at the point of addition
2020
in a different experiment 50.0cm3 of 0.500 mol dm-3 aqueous hydrochloric acid are reacted with 50.0 cm3 of 0.500 mol dm–3 aqueous sodium hydroxide.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) ΔH = –57.1 kJ mol–1
The initial temperature of each solution is 18.5 °C
Calculate the maximum final temperature of the reaction mixture.
Assume that the specific heat capacity of the reaction mixture, c = 4.18 J K–1 g–1
Assume that the density of the reaction mixture = 1.00 g cm–3
[5 marks]
M1- n(HCl) or n(NaOH) = 50 x 0.500 / 1000 = 0.025 moles
M2- q = –ΔH x n = 57.1 x 0.025 = 1.4275 kJ
M3- ΔT = q/mc
M4- ΔT = (1.4275 x 1000) / (100 x 4.18) = 3.4(2) °C
M5- Final Temperature = 18.5 + 3.4 = 21.9 °C
2020
Explain why the melting point of magnesium is higher than the melting point of sodium.
(2 marker)
M1- Mg2+ has a higher charge than Na+
M2- Stronger attraction to delocalised sea of electrons / stronger metallic
bonding
2020
This question is about shapes of molecules and ions.
Draw the shape of NCl3 and of NCl4+
Include any lone pairs of electrons that influence the shape.
Name the shape of NCl3
State and explain the bond angle in NCl4
+
[5 marks]
check mark scheme for shapes
M3- Name of shape of NCl3 = Pyramidal
M4- Bond Angle = 109.5°
M5- (4 bp and 0 lp) electron pairs repel equally / electron pairs repel to be
as far apart as possible
2020
Solid sodium iodide reacts with concentrated sulfuric acid to form iodine and sulfur in
a redox reaction.
1.Give a half-equation to show the conversion of iodide ions to iodine.
2.Give a half-equation to show the conversion of sulfuric acid to sulfur.
3.Give an overall equation for this redox reaction.
4.Identify one other sulfur-containing reduction product formed when solid sodium iodide
reacts with concentrated sulfuric acid.
(4 marker)
M1-2I- → I2 + 2e
M2-H2SO4 + 6H+ + 6e → S + 4H2O
M3-6H+ + 6I- + H2SO4 → 3I2 + S + 4H2O
M4-SO2 or H2S
2022
Explain why the first ionisation energy of the Group 2 elements decreases down the group.
(2 marker)
M1-the outer electron is in a higher (energy) level / there is an increase in
shielding / the atoms get larger / more shells
M2-There is a weaker attraction between the nucleus and the outer
electron.
2022
a student makes a solution of citric acid by dissolving some solid citric acid in water.Describe a method to add an accurately known mass of solid to a beaker to make
a solution. (2 marker)
M1-measure the mass of the weighing boat (or similar) and solid
M2- Add the solid to a beaker (or other suitable container) and
then reweigh the weighing boat (and subtract to find the mass
of solid added.)
2022
The student uses this method to complete a titration.
* Rinse a burette with distilled water.
* Fill the burette with sodium hydroxide solution.
* Use a measuring cylinder to transfer 25 cm3 of the citric acid solution into a
conical flask.
* Add 5 cm3 of indicator.
* Slowly add the sodium hydroxide solution from the burette into the conical flask.
* Add the sodium hydroxide solution dropwise near the end point until the indicator
just changes colour.
* Repeat the titration to get concordant results.
The method used by the student includes three practical steps that will lead to an
inaccurate final result.
For each of these three steps
* identify the mistake
* explain why it is a mistake
* suggest how the mistake can be overcome.
(6 marker)
stage 1
a. Problem – using a measuring cylinder
b. Explanation – large uncertainty / not
accurate enough
c. Improvement – use a (volumetric)
pipette (Not dropping pipette)
Stage 2
a. Problem – too much indicator
b. Explanation – may react and affect the endpoint reading
c. Improvement – use a smaller volume (2-6 drops)
Stage 3
a. Problem – rinsing the burette with
distilled or deionised water
b. Explanation – will slightly dilute the
alkali solution
c. Improvement – rinse the burette with alkali solution
2022
Suggest, in terms of the intermolecular forces for each compound, why CBr4 has a
higher boiling point than CHBr3
(3 marker)
M1 CBr4 has van der Waals’ forces between molecules
M2 CHBr3 has van der Waals’ forces and dipole-dipole intermolecular
forces
M3 The van der Waals’ between CBr4 molecules are stronger than the
dipole-dipole and van der Waals’ forces between CHBr3
2022
When iodide ions are oxidised using concentrated sulfuric acid, sulfur dioxide, a
yellow solid and a foul-smelling gas are all formed.
Give an equation to show the reaction between iodide ions and
concentrated sulfuric acid to form the yellow solid.
Identify the foul-smelling gas.
(2 marker)
M1-6 I– + 6 H+ + H2SO4 → S + 3 I2 + 4 H2O
M2- H2S
2022
Hydrogen and nitrogen react to form ammonia.
3 H2(g) + N2(g) ⇌ 2 NH3(g)
At 745 K, the equilibrium constant, Kc = 0.118 mol–2 dm6
At 745 K, 0.150 dm3 of an equilibrium mixture contains 0.0285 mol of hydrogen and
0.0870 mol of nitrogen.
Calculate the amount, in moles, of ammonia present in this equilibrium mixture.
[5 marks]
mark sheme
answer= 3.25 x 10-3 mols
2022
Test 1
Add Na2CO3(s)
A Effervescence
B Effervescence
C No visible change
D No visible change
Test 2
Add acidified AgNO3(aq)
A no visible change
B white ppt
C no visible change
D very pale yellow ppt
Q1.Identify the gas formed in Test 1.
Describe a further test to confirm the identity of this gas. (2 marker)
Q2.Explain how the observations from Test 1 and Test 2 can be used to show that solution B contains hydrochloric acid. (2 marker)
Q1
Identity of gas: Carbon dioxide / CO2
Test: When gas bubbled through limewater it turns cloudy
Q2
Effervescence (with Na2CO3,) so contains H+ ions
White ppt (with AgNO3,) so contains chloride ions
2022
Describe a series of tests that the student can use to show that solution C contains
ammonium sulfate. (4 marker)
M1-(Warm with some) NaOH,
M2-Damp red litmus at the mouth of the tube turns blue
M3-Add (acidified) BaCl 2 / Ba(NO3)2
M4-White ppt formed
2018
This question is about sodium fluoride (NaF).
Some toothpastes contain sodium fluoride.
The concentration of sodium fluoride can be expressed in parts per million (ppm).
1 ppm represents a concentration of 1 mg in every 1 kg of toothpaste.
A 1.00 g sample of toothpaste was found to contain 2.88 x 10–5 mol of
sodium fluoride.
Calculate the concentration of sodium fluoride, in ppm, for the sample of toothpaste.
Give your answer to 3 significant figures.
4 marker
Mr NaF = 42(.0) Mass NaF in 1 g = 2.88 × 10-5 × 42.0 (= 1.210 (1.2096) × 10-3 g)
Mass NaF in 1 kg = 1.210 (1.2096) g
(Mass in mg = 1210 (1209.6) mg)
Concentration of NaF = 1210 (ppm)
2018
Separate unlabelled solid samples of three anhydrous sodium compounds are
provided for a student to identify.
These compounds are known to be sodium carbonate, sodium fluoride and
sodium chloride but it is not known which sample is which.
Outline a logical sequence of test-tube reactions that the student could carry out to
identify each of these compounds.
Include the observations the student would expect to make.
Give equations, including state symbols, for any reactions that would take place.
[6 marks]
Stage 1 Suggested tests
1a Add named acid to all 3
1b Add water / make into a solution
1c Add AgNO3
Stage 2 Expected observations -
2a Na2CO3 will fizz with acid
2b NaCl gives white ppt with AgNO3
2c NaF shows no (visible) change / no ppt
Stage 3 Equations – state symbols must match
method
3a Na2CO3 + 2HNO3 → 2NaNO3 + CO2 + H2O
… or ionic
3b AgNO3 + NaCl → AgCl + NaNO3
… or ionic
3c correct state symbols
2018
Some of the liquid injected did not evaporate because it dripped into the gas syringe nozzle outside the oven.
Explain how this would affect the value of the Mr of Y calculated from the experimental
results.
2 marks
M1- lower volume recorded
M2-greater mr
2018
Cl2 + H2O ⇌ HClO + HCl
Give two half-equations to show the oxidation and reduction processes that occur in this redox reaction.
2 marks
Oxidation:
Cl2 + 2H2O 2HClO + 2H+ + 2e-
Reduction:
Cl2 + 2H+ + 2e- 2HCl
2018
Give an equation for this reaction between chlorine and sodium hydroxide.
[1 mark]
2NaOH + Cl2 NaCl + NaClO + H2O
2018
A sample of bromine was analysed in a time of flight (TOF) mass spectrometer and found to contain two isotopes, 79Br and 81Br
After electron impact ionisation, all of the ions were accelerated to the same kinetic energy (KE) and then travelled through a flight tube that was 0.950 m long.
The 79Br+ ions took 6.69 x 10–4 s to travel through the flight tube.
Calculate the mass, in kg, of one ion of 79Br+
Calculate the time taken for the 81Br+ ions to travel through the same flight tube.
The Avogadro constant, L = 6.022 x 1023 mol−1
KE = 1/2mv 2 where m = mass (kg) and v = speed (m s–1)
d= s x t
where d = distance (m) and t = time (s)
5 marker
mass = 1.3 x 10-25
Accept 6.77 – 6.80 x10-4 s
2018
Explain how ions are detected and relative abundance is measured in a TOF mass spectrometer.
2 marker
M1- ion hits the detector / negative plate and gains an electron (relative) M2- abundance is proportional to (the size of) the current
2018
There are two lone pairs of electrons on the oxygen atom in a molecule of
oxygen difluoride (OF2).
Explain how the lone pairs of electrons on the oxygen atom influence the bond angle
in oxygen difluoride.
2 marker
M1- Lone pairs repel more than bond pairs
M2- bond angle will be lower (than regular tetrahedral angle) / bond angle of
103-106o
