Syllabus Edition
First teaching 2014
Last exams 2024
Define the terms Lewis acid and Lewis base and state the type of bond formed between a Lewis acid and base.
Identify which of the following are Lewis acids, Lewis bases or neither:
Explain why aqueous ions of transition metals can act as Lewis acids.
State an alternative name for a species that acts as a Lewis base in organic chemistry mechanisms.
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On the axes below, draw a sketch graph to show the neutralisation of ethanoic acid by sodium hydroxide:
Write an equation for the reaction between ethanoic acid and sodium hydroxide and identify the species acting as a Lewis base in the reaction.
Identify the type of titration taking place from the curve and indicate where the buffer region is found on this curve.
Identity on the graph the point at which pKa= pH and find the pKa of the acid.
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Explain how an acid-base indicator works.
Phenolphthalein, C20H14O4, is an acid-base indicator. State the formula and colour of the conjugate base of phenolphthalein.
Explain how suitable indicators are chosen for titrations.
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Outline what is meant by a buffer solution.
Outline how a buffer solution can be made starting from 1.0 mol dm-3 ethanoic acid and 1.0 mol dm-3 sodium hydroxide.
Use suitable equations to explain how the buffer in b) functions when a small quantity of acid is added.
State the composition of a basic buffer
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Explain what is meant by the term hydrolysis in acids and bases.
Salts can be acidic, basic or neutral. Explain how you can predict whether a salt is likely to be acidic. Include an equation in your answer.
Deduce which of the following salts are acidic, basic or neutral:
CH3COONa; NH4Cl; KCl
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Ammonia reacts with boron trifluoride to form an adduct, a molecule made from the combination of two others.
NH3 + BF3 → NH3BF3
Identify the Lewis acid and base and the type of bond formed between a Lewis acid and base.
Explain the role of water, in terms of Lewis acid-base theory, in the following equations:
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NH4+ (aq) + OH- (aq)'All Brønsted-Lowry acids are Lewis acids but not all Lewis acids are Brønsted Lowry acids.'
Evaluate whether this statement is true, giving an appropriate example.
In the nitration of benzene, identify a species which acts as a Lewis base.
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Sketch the titration curve when 50 cm3 of 0.1 mol dm-3 HNO3 (aq) is titrated against 25 cm3 of 0.1 mol dm-3 NH3 (aq).
Select a suitable indicator for the titration from table 22 of the Data booklet.
5 marksCalculate the pH of 0.1 mol dm-3 ammonia using section 21 of the Data booklet.
Deduce, using section 21 of the Data booklet whether the pH of 0.1 mol dm-3 ethylamine would be higher or lower than 0.1 mol dm-3 ammonia solution.
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Indicators are solutions of weak acids or bases. Methyl red has the molecular formula C15H15N2O2.
Draw the structure of the conjugate base of methyl red.
What will be seen if a few drops of methyl red are added during a titration of 50 cm3 of 0.1 mol dm-3 HCl (aq) against 25 cm3 of 0.1 mol dm-3 NaOH ( aq).
The pKa of methyl red is 5.1. Explain how this relates to the acid-base character of methyl red when added to water.
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Using section 21 of the Data booklet, discuss the relationship between the chemical structures and acidity of chloroethanoic acid, dichloroethanoic acid and trichloroethanoic acid.
This question is about acid buffers.
[1]
3 marks20 cm3 of 0.05 mol dm-3 dichloroethanoic acid was reacted with 10 cm3 of 0.10 mol dm-3 sodium hydroxide. Suggest, with a reason, a pH value for the resulting solution.
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Iron(III) ions can react with cyanide ions to form ferricyanide ion via the following equation
Fe3+ + 6CN- → [Fe(CN)6]3-
State which of the reactants is behaving as a Lewis base and justify your answer.
State the definition of a Brønsted-Lowry acid and the equation which demonstrates how ethanoic acid, CH3COOH (aq) , behaves as a Brønsted-Lowry acid when reacting with ammonia, NH3 (aq).
Sketch a graph to indicate the change in pH during a titration of 25.0 cm3 of 0.100 mol dm-3 hydrochloric acid, HCl (aq) , with 0.100 mol of ammonia, NH3 (aq).
The end point in a titration can be identified using a suitable indicator.
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Ethanoic acid, CH3COOH (aq) , is titrated with 0.16 mol dm-3 potassium hydroxide and the following graph is obtained.
Explain why the equivalence point is greater than pH 7 for this titration.
Explain what is meant by a buffer solution and describe where the ‘buffer region’ on the graph would occur.
Explain the shape of the pH curve up to the equivalence point.
Explain why potassium hydroxide can act as a Brønsted-Lowry base and Lewis base.
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This question is about Brønsted-Lowry acids and bases.
When an acid and a base react they produce a conjugate base and a conjugate acid.
acid + base ⇋ conjugate base + conjugate acid
Write an equation to show how hydrochloric acid behaves as a strong acid when it reacts with water, and state the role of water in this reaction.
Ethanoic acid is a weak acid. Hydrogen carbonate ions can also act as a weak acid if in an aqueous solution.
Explain how a solution containing ethanoic acid and ethanoate ions can act as a buffer.
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A student performed a titration of 25.0 cm3 of 0.100 mol dm-3 hydrochloric acid, HCl (aq) , with 0.100 mol of sodium hydroxide, NaOH (aq).
Explain why the salt produced in this reaction is neutral.
The student repeated the titration using two different chemicals, 25.0 cm3 of 0.100 mol dm-3 nitric acid, HNO3 (aq) , and 0.100 mol dm-3 ammonia, NH3 (aq).
State the equation for reaction between nitric acid and water, this reaction and identify the conjugate acid formed in the reaction.
The image below shows the hexaaquaaluminium ion, [Al(H2O)6]3+. Explain why this can behave as an acid.
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25.0 cm3 of 0.100 mol dm-3 propanoic acid, CH3CH2COOH (aq) , is titrated with of 0.100 mol dm-3 sodium hydroxide. The pH curve for this titration is shown below.
The student used the pH range of the indicators to determine which was the best one to use for this titration.
The end point of an indicator depends on its pKa
A buffer solution contains a mixture of propanoic acid and its salt. A small amount of nitric acid is added to the buffer.
Write an equation, including state symbols, showing how this buffer can resist the change in pH.
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