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OCR A-Level Chemistry A > Module 3 - Periodic Table and Energy - Exam Questions > Flashcards

Module 3 - Periodic Table and Energy - Exam Questions Flashcards

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1
Q

Why does Xenon have a lower first ionisation energy than neon? (3)

A

Xe has a bigger atomic radius
Xe has more shielding
Lower nuclear attraction

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2
Q

Na melting point: 98°C
Mg melting point: 639°C
Explain the difference in melting point (3)

A

Mg ions have a greater charge
Mg has more electrons
Mg has greater attraction between ions and electrons

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3
Q

Sulphur exists as S8 molecules, and chlorine as Cl2. Explain why S has a higher melting point than Cl (2)

A

S8 and Cl2 have intermolecular forces

S8 has stronger intermolecular forces than Cl2

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4
Q

Why does atomic radius decrease across a period? (3)

A

atomic radius decrease because…
Nuclear charge increases
Shielding remains the same
Greater attraction

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5
Q

Why does first ionisation energy increase across a period? (3)

A

Atomic radius decreases
Nuclear charge increases
Shielding remains the same

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6
Q

Define first ionisation energy (3)

A

Energy change when each atom in 1 mole of gaseous atoms loses an electron

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7
Q

Explain why first ionisation energy of B is less than that of Be (2)

A

In B, electron being removed is at a higher energy

An s electron is lost in Be and a p electron is lost in B

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8
Q

Explain why first ionisation energy of O is less than that of N (2)

A

In N, all 2p electrons are unpaired

In O, 2p electron removed is paired so has repulsion that makes removal easier.

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9
Q

Describe what would be seen when a magnesium ribbon is added to excess dilute HCl (2)

A

Effervescence

Mg dissolves

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10
Q

Describe the difference between Mg and Sr reacting with HCl (1)

A

Sr would be a quicker reaction

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11
Q

Predict the formulas of:

Barium oxide and barium nitride (2)

A

BaO

Ba3N2

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12
Q

Describe and explain the trend down group 2 of reactivity with water (5)

A 
Reactivity increases down the group
Atomic radii increase
More shielding
Nuclear attraction decreases
Easier to remove outer electrons
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13
Q

A student reacts calcium exactly with HCl in solution. After adding more calcium it still reacts, explain this (2)

A

Calcium reacts with water producing calcium hydroxide

Ca(s) + H2O(l) -> Ca(OH)2(aq) + H2(g)

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14
Q

Explain why samples of magnesium oxide become contaminated with magnesium carbonate over time (1)

A

MgO reacts with CO2

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15
Q

CO2 can be prepared by which reaction of carbonates (1)

A

Thermal decomposition

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16
Q

How do indigestion tablets containing CaCO3 work? (2)

A

CaCO3 neutralises HCl

CaCO3 + 2HCl → CaCl2 + H2O + CO2

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17
Q

Write the equation and conditions for chlorine reacting with sodium hydroxide to form bleach (2)

A

2NaOH + Cl2 -> NaCl + NaClO + H2O
Cold and dilute NaOH

NaClO is bleach

18
Q

Define disproportionation (1)

A

The simultaneous oxidation and reduction of the same element

19
Q

Which reagent would need to be added to calcium iodide to prove it contained iodide ions? (1)

A

Silver nitrate

20
Q

A solution contains CaBr2 and CaCl2, along with silver nitrate, which other reagent is added to prove iodide ions are present? (1)

A

Concentrated NH3

21
Q

Which reaction type is forming AgCl from adding silver nitrate to chloride ions? (1)

A

Precipitation

22
Q

Use bond breaking and bond making to explain why some reactions are exothermic (2)

A

Bond breaking absorbs energy and bond making releases energy

More energy released then absorbed

23
Q

Suggest three reasons why standard enthalpy changes of combustion are lower experimentally than theoretically (3)

A

Heat released to the surroundings
Incomplete combustion
Non-standard conditions

24
Q

Define enthalpy change of combustion (2)

A

The enthalpy change for the complete combustion of 1 mol of a substance

25
Q

What are standard conditions? (1)

A

101kPa and 298K

26
Q

Why is it difficult to determine enthalpy change of formation for hydrocarbons? (1)

A

Many different hydrocarbons would form

27
Q

Why do bond enthalpies have positive values?

A

Bond breaking is endothermic

28
Q

Define enthalpy change of formation (2)

A

Change in enthalpy when 1 mole of substance is formed from its elements

29
Q

Define bond enthalpy (2)

A

Energy required to break 1 mole of bonds in the substance in the gaseous state

30
Q

Explain why increasing the temperature increases reaction rate (Boltzmann distribution) (1)

A

More molecules have energy above activation energy

31
Q

CO(g) + 2H2(g) -> CH3OH(g) ∆H = –91 kJ mol–1

Why would high pressure and low temperature give maximum yield? (2)

A

High pressure as fewer moles of gas on RHS

Low temperature as forward reaction is exothermic

32
Q

Why is high pressure and low temperature often not used in chemical industry for reaction conditions? (2)

A

Too expensive to use a high pressure

Too slow to use a low temperature

33
Q

Suggest two ways in which catalyst use in industry makes processes more sustainable (4)

A

Reactions take place at lower temperatures
Use less fossil fuel
Less CO2 emitted into atmosphere

Catalyst not used up in reaction

34
Q

State le Chatelier’s principle (2)

A

When a system in dynamic equilibrium is subjected to an external change, the system readjusts itself to minimise the effect of that change

35
Q

Describe the effect on equilibrium of adding a catalyst (2)

A

No change in equilibrium position

Catalyst speeds up forward and reverse reactions by same amount

36
Q

Give two features of a reversible reaction, when a dynamic equilibrium has been set up (2)

A

Rate of forward reaction = rate of reverse reaction

Concentrations remain constant

37
Q

What is meant by a catalyst? (1)

A

A substance that lowers the activation energy by providing an alternative route

38
Q

What is the difference between homogeneous and heterogeneous catalysts? (2)

A

Homogeneous: catalyst in same state as reactants
Heterogeneous: catalyst in different state to reactants

39
Q

State the pressure and temperature used in the Haber process (2)

A

Pressure: 200 atm
Temperature: 450°C

40
Q

Describe and explain why the conditions of the Haber process are a compromise between rate and equilibrium (9)

A

Increased pressure increases rate because molecules are closer together
Increased temperature increases rate because molecules have more energy
Increased pressure pushes equilibirum to RHS because fewer moles on RHS
Increased temperature pushes equilibrium to LHS because forward reaction is exothermic
If temperature is too high, yield is low
If temperature is too low, rate is slow
If pressure is too high, increased costs

OCR A-Level Chemistry A (33 decks)

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