past Paper 2021: 1,2 And 3

Question 1

Magnesium chloride dissolves in water. Give an equation giving state symbols to represent the process that occurs when the Enthalpy of solution of magnesium chloride is measured

Answer 1

MgCl2 (s) —> Mg2+ (as) + 2Cl-(aq)

Question 2

SUGGEST WHY THE ENTHALPY OF HYDRATION of Ca2+ is less exothermic than Mg2+

Answer 2

Ca2+ ion bigger , there is a lower charge to size ratio, weaker attraction to Od- in water

Question 3

Define the mass number of an atom

Answer 3

Number of protons and neutrons in the nucleus of an atom

Question 4

Give an equation including state symbols for the reaction of sodium with water, suggest why it is dangerous to react a similar piece of sodium with a lot more water in a boiling tube

Answer 4

2Na(s) + 2H2O(l) —> 2NaOH(aq) + H2(g)
Temperature will go up more causing reactants to shoot out or explode

Question 5

Give an equation foe the reaction of phosphorous oxide with water and suggest a pH for this

Answer 5

P4O10 + 6H2O —> 4H3PO4
1

Question 6

Explain in terms of crystal structure and bonding, why silicon oxide has a higher melting point than phosphorous oxide

Answer 6

SiO2 is macromolecular with strong covalent bonds between atoms or covalent bonds need a lot of energy to be overcome
P4O10 is molecular or simple covalent with weak induced dipole dipoles between molecules- require little energy to overcome

Question 7

Am element in period 3 forms an oxide that is insoluble in water- it reacts with sulfuric acid and aqueous potassium hydroxide, give the formula of this oxide and an equation for its reaction with sulfuric acid

Answer 7

Al2O3
Al2O3 + 3H2SO4 —> Al2(SO4)3 + 3H2O

Question 8

Give the formula of a hydroxide ion of an element in period 3 using in medicine

Answer 8

Mg(OH)2

Question 9

identity the element in period 3, from sodium to chlorine, that has the largest atomic radius

Answer 9

Na

Question 10

2SO2 + O2 = 2SO3
State and explain the effect of a decrease in overall pressure of the equilibrium yield of SO3

Answer 10

Decrease, equilibrium shifts to the left to oppose decrease in pressure

Question 11

Explain why the value of Kw increases as temperature increases

Answer 11

Forward reaction is endothermic, equilibrium shifts to the right to oppose increase in temperature so concentration of H+ increases

Question 12

Suggest why the ph probe is washed with distilled water

Answer 12

Different solutions must not contaminate each other

Question 13

Explain why the volume of sodium hydroxide added between each pH measurement is smaller as the end point of titration is approached

Answer 13

To avoid missing the endpoint

Question 14

How to determine if a an indicator is suitable

Answer 14

Changes pH within the vertical region

Question 15

Explain why the standard entropy for CO2 is greater than that of carbon

Answer 15

CO2 is a gas so there is more disorder

Question 16

Name the type of compound shown by formula RCOOK ans state its use

Answer 16

Carboxylic salt- soap

Question 17

Suggest why aqueous ethanol is a suitable solvent when heating the coconut oil with KOH, give a safety precaution used when heating the mixture

Answer 17

It dissolves both oil and KOH
Use a water bath as ethanol is flammable

Question 18

State the meaning of fraction as used in the term petrol fraction

Answer 18

Group of hydrocarbons with similar boiling point

Question 19

Give a reason why carbon dioxide absorbs infrared radiation

Answer 19

Vibrate at the same frequency as IR

Question 20

Bioethanol is used as an alternative to fossil fuels
Evaluate the statement: “the fact that bioethanol is a carbon-neutral fuel outweighs the environmental disadvantages of producing bioethanol”

Answer 20

Photosynthesis: 6CO2 + 6H2O —> C6H2O6 + 6O2
Fermentation: C6H12O6 —> 2C2H5OH + 2CO2
Combustion: 2C2H5OH + 6O2 —> 6H2O 4CO2
Environmental impacts: deforestation, loss of biodiversity, 6CO2 in and 6CO2 out isn’t carbon neutral as fuel is used in production

Question 21

The student considered using a glass beaker on a tripod and gauze instead of the clamped copper calorimeter
Suggest two disadvantages of using a glass beaker beaker on a tripodd gauze

Answer 21

Tripod and gauze so reduced heat transfer
Doesn’t conduct heat as well as copper

Question 22

Suggest two reasons why the value of Enthalpy of combustion for this experiment is less exothermic than the data value

Answer 22

Heat lost, incomplete combustion may have occured

Question 23

Suggest one addition to apparatus to improve accuracy

Answer 23

Use a lid to prevent heat loss

Question 24

Explain why benzene is more thermodynamically stable

Answer 24

Electrons delocalised

Question 25

Suggest the importance of fewer steps and higher percentage atom economy in experiments

Answer 25

Fewer steps-better yield Less waste

Question 26

State why the ozone layer is beneficial for living organism

Answer 26

Prevents harmful UV

Question 27

State how chlorofluorocarbons form chlorine in the upper atmosphere

Answer 27

C-Cl bonds broken homolytically

Question 28

Suggest why the 1cm portions of the reaction mixture are added to an excess of NaHCO3 solution

Answer 28

The sodium Hydrogencarbonate solution neutralises the acid so stops the reaction

Question 29

Suggest why the order of this reaction in respect to propanone can be ignored

Answer 29

Concentration is almost constant The concentration is much larger than the concentration of iodine

Question 30

Explain how the graph shows 0 order

Answer 30

Has a constant gradient so the rate of reaction does not change as the concentration of iodine changes

Question 31

Describe how the crystalline solid is separated and purified

Answer 31

Filter, dissolve in minimum volume of hot solvent cool and filter under reduced pressure, wash with cold solvent and dry

Question 32

Suggest a reason why KCN is used rather than HCN

Answer 32

KCN dissociates to provide the CN- nucleophile

Question 33

1.dilute nitric acid is added to the solution 2.aqueous silver nitrate is added to the solution 3. A pale yellow ppt forms Excess dilute aqueous ammonia is added to the mixture Some of the precipitate dissolves and a darker yellow ppt remains Give a reason for the use and explain the observations using equation

Answer 33

HNO3 to remove hydroxide ions that may give other pts with AgNO3 AgNO3 produces pts with ions Ag+ + Cl- —> AgCl NH3 dissolves AgCl AgCl + 2NH3 —> Ag(NH3)2+ + Cl-