Studied by 3 peopleExplain why sodium chloride solution cannot be used as the electrolyte to produce sodium metal.
hydrogen would be produced
sodium is more reactive than hydrogen
Aluminium is produced by electrolysing a molten mixture of aluminium oxide and cryolite.
Explain why a mixture is used as the electrolyte instead of using only aluminium oxide
mixture has a lower mp
so less energy needed
This question is about chemical cells. A student investigated the voltage produced by different chemical cells.
Suggest two control variables used in this investigation.
concentration of electrolyte
temperature of solution
compound in electrolyte
Copper nitrate solution is blue. Suggest why the blue colour of the copper nitrate solution fades during the electrolysis.
copper ions are discharged
Some of the copper produced did not stick to the negative electrode but fell to the bottom of the beaker. Suggest how the students could find the total mass of copper produced
filter the mixture
wash and dry the copper
weigh the copper collected
add the increase in mass of the electrode
methyl orange
red to yellow (acidic to basic)
Suggest one improvement that could be made to the apparatus used that would give more accurate results. Give a reason for your answer.
use a pipette to measure the acid because it is more accurate than a measuring cylinder
The student compares the rates of the reaction of zinc carbonate with:
• 0.01 mol/dm3 methanoic acid
• 0.01 mol/dm3 ethanoic acid.
The rate of the reaction with methanoic acid is greater than the rate of the reaction with ethanoic acid. Explain why.
You should refer to ions in your answer.
methanoic acid has a lower pH
methanoic acid has a higher concentration of hydrogen ions
therefore more collisions per unit time
Explain why the pH of an acid depends on:
• the strength of the acid
• the concentration of the acid.
pH depends on H+ conc
the higher the conc, the lower the pH
strong acid = greater ionisation and lower pH
the higher the concentration of an acid, the more acid in the same volume so lower pH
Explain why the electrical conductivity of the mixture was zero when the sulfuric acid had just been neutralised.
H2SO4(aq) + Ba(OH)2(aq) → BaSO4(s) + 2H2O(l)
there are no ions free to move because barium sulfate is solid
hydrogen ions have reacted with hydroxide ions to produce water
The higher the concentration of a sample of dilute sulfuric acid, the greater the volume of sodium hydroxide needed to neutralise the acid.
The student tested two samples of dilute sulfuric acid, P and Q.
Describe how the student could use titrations to find which sample, P or Q, is more concentrated.
measure volume of acid using pipette and put in conical flask
add indicator to acid
add sodium hydroxide solution from the burette until colour changes, while swirling the conical flask
record volume of sodium hydroxide added
this is a rough titre and do another titration and do step 1-3 but add dropwise near the endpoint and do a few and find a mean
repeat procedure with the other acid
compare the 2 volumes of sodium hydroxide to find which sample is more concentrated
Describe a method to make pure, dry crystals of magnesium sulfate from a metal oxide and a dilute acid.
use mgo and sulfuric acid
add sulfuric acid to a beaker and warm gently
add mgo and stir
continue adding until mgo is in excess
filter using filter paper and funnel to remove excess magnesium oxide
heat solution in an evaporating basin to crystallisation point
leave to crystallise and pat dry with filter paper
Describe a safe method for making pure crystals of copper sulfate from copper carbonate and dilute sulfuric acid. Use the information in the figure above to help you.
In your method you should name all of the apparatus you will use.
sulfuric acid in a beaker
add copper carbonate one spatula at a time until copper carbonate is in excess or until no more effervescence occurs
filter excess copper carbonate using filter paper and funnel
pour solution into an evaporating dish and heat using bunsen burner
leave to crystallise
pat dry
wear safety spectacles
Dilute hydrochloric acid is a strong acid. Explain why an acid can be described as both strong and dilute.
strong because completely ionised in aq solution
dilute because small amount of acid per unit volume
The result for 0.29 g of sodium carbonate is anomalous.
Suggest what may have happened to cause this anomalous result.
bung not firmly put in
One student said that the results of the experiment were wrong because the first few bubbles of gas collected were air.
A second student said this would make no difference to the results.
Explain why the second student was correct.
the bubbles should be collected because co2 is left in the flask at the end and it has the same volume as the air collected
strong acid
completely ionised in an aqueous solution (has H+)
methyl orange colour change (alkali → acid)
yellow to red
Explain why the student should use a pipette to measure the dilute sulfuric acid and a burette to measure the sodium hydroxide solution.
pipette for a fixed volume
burette for variable volume
The student makes two conclusions.
Conclusion 1: ‘All metal oxides produce alkaline solutions.’
Conclusion 2: ‘All non-metal oxides produce acidic solutions.’
Explain why the student’s conclusions are only partly correct. Use information from the table above.
conclusion 1
pH values above 7 are alkaline
sodium oxide, calcium oxide and magnesium oxide do form alkaline so the conclusion is correct for those
zinc oxide did not form the solution and so not all metals oxides form solutions
conclusion 2
pH values below 7 are acidic
carbon dioxide, sulfur dioxide and phosphorus oxide do form acidic solution so the conclusion is correct for those
silicon oxide did not form the solution and so not all non metals oxides form solutions
Give an improved conclusion for metal oxides
metal oxides produce alkaline solutions if they dissolve in water
A student wants to obtain sodium chloride crystals from the sodium chloride solution.
This is the method used.
1. Add solid charcoal to the sodium chloride solution to remove the indicator colour.
2. Remove the solid charcoal.
3. Evaporate the solution to dryness over a Bunsen burner.
evaporate some of the solution and leave to cool
Explain why the scientist adds excess zinc rather than excess iodine.
make sure all the iodine reacts as excess iodine would remain in the solution
if excess iodine remains, it cannot be filtered off
Suggest one reason why the percentage yield in this reaction is not 100%
incomplete reaction
weak acid
produces H+ ions in aqueous solution but is only partially ionised
The reaction between copper carbonate and the acid produced a gas. What was the gas?
carbon dioxide
Explain, in terms of particles, how and why the rate of reaction changes during the reaction of calcium carbonate with hydrochloric acid
acids particles used up so concentration decreases and so less frequent collisions per second so rate decreases
Explain the difference between the processes in electrolysis and in a chemical cell.
electrolysis uses electricity to produce a chemical reaction but cells use chemical reaction to produce electricity
Some of the copper produced did not stick to the negative electrode but fell to the bottom of the beaker.
Suggest how the students could find the total mass of copper produced.
filter the mixture
wash and dry the copper
weigh the copper collected
add to the increase in mass of the electrode
How do the results in Figure 2 support the conclusion that the total mass of copper produced is directly proportional to the current?
when current doubles, mass doubles as well e.g. from 0.3A to 0.6 A the mass increased from 0.18 to 0.36 which is double
Explain why electrolysis would not take place with a solid
the salt has to be in a solution or molten because ions can only move in a liquid
Explain why graphite conducts electricity.
Answer in terms of the structure and bonding in graphite.
each carbon forms 3 covalent bonds
one electron per carbon is delocalised
the delocalised electrons carry charge through the structure
Describe the trends shown in the results.
the volume of hydrogen collected is directly proportional to the time
the rate of collection is 0.45 cm^3/min
up to 8 minutes, chlorine is collected at an increasing rate
Explain why zinc ions move towards the negative electrode.
zinc ions are positive and opposite charges attract
Magnesium chloride contains magnesium ions (Mg2+) and chloride ions (Cl⁻).
Describe, in terms of electrons, what happens when a magnesium atom reacts with chlorine atoms to produce magnesium chloride.
magnesium loses 2 electrons and chlorine gains one electron
there is one magnesium and 2 chlorines
now both magnesium and chlorine have 8 electrons in the outer shell
Explain why magnesium chloride must be molten or dissolved in water to be electrolysed.
because ions can move to the electrodes and carry charge
The temperature change depends on the reactivity of the metal.
The student’s results are used to place copper, iron, magnesium and zinc in order of their reactivity.
Describe a method to find the position of an unknown metal in this reactivity series.
Your method should give valid results.
add unknown metal to copper sulfate solution
measure the temperature change
place the metals in order of temperature change
make sure to have the same concentration of solution
Describe and explain the trends shown in the student’s results
concentration of copper sulfate increases when temperature change increases
there is an anomalous result, experiment 3
the reaction is exothermic
the temperature change increases then remains constant
after experiment 7, the temp change remains constant
the trend changes after experiment 7
temperature change increases because rate increases
temperature change levels off because the reaction is complete
as more copper sulfate reacts, more heat energy is given off
once copper sulfate is in excess, no further heat energy is produced
Copper is extracted from low-grade ores by phytomining. Describe how copper metal is produced by phytomining.
growing plants on low-grade ores
plants are burnt to produce ash
ash dissolved in acid to produce solution of a copper compound
electrolysis to extract copper
Alloys can be bent. Some alloys return to their original shape when heated. What name is used for these alloys?
shape memory alloys
give one reason why it is important for the percentage atom economy of a reaction to be as high as possible
economic reasons
Note
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