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Q1.
Cobalt forms coloured compounds.
A pink cobalt compound reacts with hydrochloric acid.
The reaction can be represented as:
pink cobalt compound + hydrochloric acid ⇌ blue cobalt compound + water
The forward reaction is endothermic.
When both cobalt compounds are present in a solution at equilibrium, the equilibrium mixture is purple.
(a) What is meant by equilibrium?
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(2)
(b) The equilibrium mixture is cooled.
Explain what happens to the concentration of the pink cobalt compound.
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(3)
(c) More hydrochloric acid is added.
Explain what happens to the colour of the equilibrium mixture
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(3)
(d) Why does cobalt form different coloured compounds?
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(1)
(e) An oxide of cobalt has the formula Co2O3
Which cobalt ion is present in this oxide?
Tick (✔) one box.
| Co+ |  |
| Co2+ | |
| Co3+ | |
| Co4+ | |
(1)
(f) Cobalt compounds can act as catalysts.
Which two statements about cobalt compounds are correct?
Tick (✔) two boxes.
| They allow reactions to reach equilibrium more quickly. | |
| They are reactants in reactions catalysed by cobalt compounds. | |
| They are used up when acting as catalysts. | |
| They increase the equilibrium yield of reactions. | |
| They provide a different reaction pathway. | |
(2)
(g) The reaction of hydrogen with carbon monoxide is catalysed by cobalt metal.
Balance the equation for the reaction.
H2 + CO ⟶ C6H14 + H2O
(1)
(h) C6H14 is an alkane.
What is the formula of an alkane containing 18 hydrogen atoms?
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(i) The graph shows a reaction profile diagram for a reaction without a catalyst.
On the graph:
• draw the reaction profile diagram for a catalysed reaction
• draw and label an arrow to show the activation energy for the reaction without a catalyst.
(2)
(Total 16 marks)
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(a)
What is meant by equilibrium?Equilibrium happens in a closed system.
It is when the rate of the forward reaction and the rate of the backward reaction are the same.
(b)
The equilibrium mixture is cooled. Explain what happens to the concentration of the pink cobalt compound.Cooling the mixture increases the concentration of the pink cobalt compound.
This is because the equilibrium shifts to the left, towards the reactants.
The reverse reaction is exothermic, so it is favoured at lower temperatures.
(c)
More hydrochloric acid is added. Explain what happens to the colour of the equilibrium mixture.The mixture becomes blue.
This is because the equilibrium shifts to the right, towards the products.
The concentration of the blue cobalt compound increases, so the colour gets more blue.
(d)
Why does cobalt form different coloured compounds?Because cobalt is a transition metal, and it has ions with different charges.
(e)
Which cobalt ion is present in Co₂O₃?
Co³⁺(f)
Which two statements about cobalt compounds are correct?
They allow reactions to reach equilibrium more quickly
They provide a different reaction pathway(g)
Balance the equation:
13H₂ + 6CO → C₆H₁₄ + 6H₂O(h)
What is the formula of an alkane with 18 hydrogen atoms?
C₈H₁₈(i)
Reaction profile diagram:draw a lower curve to show the catalysed reaction.
draw a vertical arrow from the reactants to the top of the original curve to show activation energy.
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