7.2. Brønsted–Lowry theory of acids and bases
A subsection of Chemistry, 9701, through 7. Equilibria
Listing 10 of 133 questions
The three substances shown all have some acidic properties. O phenol propan-1-ol C H H H H O C H H C propanoic acid C H H H C C H H H O O H H H Write an equation for the reaction between propan-1-ol and sodium metal. Give the order of the relative acidities of propanoic acid, propan‑1‑ol and phenol, stating the most acidic first. Explain your answer to . Methanoic acid, HCO2H, has a similar acid strength to propanoicacid. Describe a chemical test to distinguish between these two acids. Name the acid which gives a positive result in this test and describe the observations that would be made. The ester phenyl propanoate, C2H5CO2C6H5, can be made from phenol and propanoicacid in a two-step synthesis. The first step produces an acyl chloride. For this two-step synthesis, ● draw the structure of the product of the first step, ● state the reagents and conditions needed for each step of the synthesis. An unknown compound, Z, is propan‑1‑ol, propanal or propanoic acid. The proton NMR spectrum of Z dissolved in CDCl 3 is shown. δ / ppm –2 From the proton NMR spectrum, identify Z. State one feature that would be seen, and why, in the proton NMR spectra of each of the two compounds that are not Z.
9701_s18_qp_43
THEORY
2018
Paper 4, Variant 3
View
Group17 elements are commonly referred to as the halogens. State and explain the trend in volatility of chlorine, bromine and iodine down the group. Hydrogen gas reacts with the different halogens under different conditions. State the conditions required for chlorine to react with hydrogen at room temperature. On heating, iodine reacts with hydrogen in a reversible reaction. Give the equation for this reaction. Include state symbols. Hydrogen chloride reacts with water. HCl + H2O H3O+ + Cl – In this reaction, one of the reactants behaves as a Brønsted-Lowry acid. What is meant by the term Brønsted-Lowry acid? Identify the reactant behaving as an acid and its conjugate base. acid conjugate base  Name the type of bond formed between H+ and H2O to make H3O+. For H3O+, predict its shape and the H–O–H bond angle. shape bond angle  
9701_s19_qp_22
THEORY
2019
Paper 2, Variant 2
View
Amino acids can be separated by thin-layer chromatography. A mixture of amino acids is analysed using this technique. The chromatogram obtained is shown, drawn to scale. The table shows some Rf values for different amino acids in the solvent used. cm baseline solvent front amino acid Rf value alanine 0.40 glutamic acid 0.29 leucine 0.71 valine 0.61 A B Use the chromatogram and the Rf values to deduce the amino acid responsible for spotA and spotB. amino acid responsible for spotA amino acid responsible for spotB  A second chromatogram of the same mixture is taken using a more polar solvent. Predict the effect on the Rf values of the amino acids. Explain your reasoning. Glycine, H2NCH2CO2H, is the simplest amino acid. Complete the equations to show the acid-base properties of glycine. H2NCH2CO2H+ HCl H2NCH2CO2H+ NaOH In aqueous solution, amino acids exist as zwitterions. Draw the zwitterionic structure of glycine. Explain how the zwitterion for glycine is formed.  Apart from glycine, all naturally occurring amino acids have a chiral centre and exhibit stereoisomerism. Draw the two stereoisomers of alanine, CH3CH(NH2)CO2H. C C  The amino acid alanine can be synthesised from 2‑chloropropanoic acid, CH3CHCl CO2H. State the reagents and conditions and name the mechanism for this reaction. reagents and conditions name of mechanism  State and explain the relative acidities of trichloroethanoic acid, chloroethanoic acid and ethanoic acid. Serine, HOCH2CH(NH2)CO2H, can react with alanine, CH3CH(NH2)CO2H, to form three different structural isomers, each with the molecular formula C6H12N2O4. Draw the structures of these three structural isomers. isomer 1 (C6H12N2O4) isomer 2 (C6H12N2O4) isomer 3 (C6H12N2O4)  
9701_s19_qp_42
THEORY
2019
Paper 4, Variant 2
View
Compare and explain the relative acidities of 2‑chloropropanoicacid, 3‑chloropropanoicacid, and propanoicacid. Explain your answer. > > most acidic least acidic explanation  The numerical values of Ka for methanoic acid, HCO2H, and pyruvicacid, CH3COCO2H, are given. acid Ka HCO2H 1.78 × 10–4 CH3COCO2H 4.07 × 10–3 An equilibrium mixture containing the two acid-base pairs is formed. HCO2 – + CH3COCO2H HCO2H + CH3COCO2 – Use the Ka values to calculate the equilibrium constant, Keq, for this equilibrium.  Keq = Use your value of Keq to predict the position of this equilibrium. Indicate this by placing a tick () in the appropriate box in the table. Explain your answer. equilibrium lies to the left equilibrium lies in the middle equilibrium lies to the right  Ethanedioic acid, HO2CCO2H, has two dissociation constants, Ka1 and Ka2, whose pKa values are 1.23 and 4.19. Suggest equations to show the two dissociations that give rise to these pKa values. pKa1 1.23 pKa2 4.19  State the mathematical relationship between pKa and the acid dissociation constant Ka. Three tests were carried out on separate samples of the organic acids shown in the table. The following results were obtained. = observed change = no observed reaction test reagentand conditions HCO2H CH3COCO2H HO2CCO2H observed change          Complete the table with the reagentand conditions and the observed change for each test. Assume these organic acids all have a similar acid strength. A sample of pyruvicacid, CH3COCO2H, is analysed by carbon‑13 NMR spectroscopy. Three peaks are observed. Complete the table by: ● ● circling the carbon atom responsible for the chemical shift ● ● stating the hybridisation of the circled carbon atom. chemical shift (δ) carbon atom responsible for chemical shift hybridisation of the circled carbon atom O O C C O H H H C H O O C C O H H H C H O O C C O H H H C H  An ester of pyruvicacid, F, is dissolved in CDCl 3 and analysed by proton NMR spectroscopy. C C H3C O F O CH2 O CH3 The proton NMR spectrum of F is shown. δ / ppm Use the proton NMR spectrum of F to complete the table. chemical shift (δ) group responsible for the peak splitting pattern number of 1H atoms responsible for the peak 1.3 2.2 4.0  Deuterium oxide, D2O, where D is 2H, can be used as a solvent in proton NMR spectroscopy. The proton NMR spectrum of alanine in CDCl 3 has 4 peaks. The proton NMR spectrum of alanine in D2O has 2 peaks. H3C C NH2 H alanine COOH On the diagram of alanine, circle the protons that show peaks in both NMR spectra. Explain your answer. The ionic product, Kw, for D2O has a value of 1.35×10–15 mol2 dm–6 at 298 K. Write the expression for the Kw of D2O. Kw =  Calculate the pH of pure, neutral D2O at 298 K. Assume [D+] is equivalent to [H+] for pH calculations.  pH = 
9701_s20_qp_41
THEORY
2020
Paper 4, Variant 1
View
Compare and explain the relative acidities of 2‑chloropropanoicacid, 3‑chloropropanoicacid, and propanoicacid. Explain your answer. > > most acidic least acidic explanation  The numerical values of Ka for methanoic acid, HCO2H, and pyruvicacid, CH3COCO2H, are given. acid Ka HCO2H 1.78 × 10–4 CH3COCO2H 4.07 × 10–3 An equilibrium mixture containing the two acid-base pairs is formed. HCO2 – + CH3COCO2H HCO2H + CH3COCO2 – Use the Ka values to calculate the equilibrium constant, Keq, for this equilibrium.  Keq = Use your value of Keq to predict the position of this equilibrium. Indicate this by placing a tick () in the appropriate box in the table. Explain your answer. equilibrium lies to the left equilibrium lies in the middle equilibrium lies to the right  Ethanedioic acid, HO2CCO2H, has two dissociation constants, Ka1 and Ka2, whose pKa values are 1.23 and 4.19. Suggest equations to show the two dissociations that give rise to these pKa values. pKa1 1.23 pKa2 4.19  State the mathematical relationship between pKa and the acid dissociation constant Ka. Three tests were carried out on separate samples of the organic acids shown in the table. The following results were obtained. = observed change = no observed reaction test reagentand conditions HCO2H CH3COCO2H HO2CCO2H observed change          Complete the table with the reagentand conditions and the observed change for each test. Assume these organic acids all have a similar acid strength. A sample of pyruvicacid, CH3COCO2H, is analysed by carbon‑13 NMR spectroscopy. Three peaks are observed. Complete the table by: ● ● circling the carbon atom responsible for the chemical shift ● ● stating the hybridisation of the circled carbon atom. chemical shift (δ) carbon atom responsible for chemical shift hybridisation of the circled carbon atom O O C C O H H H C H O O C C O H H H C H O O C C O H H H C H  An ester of pyruvicacid, F, is dissolved in CDCl 3 and analysed by proton NMR spectroscopy. C C H3C O F O CH2 O CH3 The proton NMR spectrum of F is shown. δ / ppm Use the proton NMR spectrum of F to complete the table. chemical shift (δ) group responsible for the peak splitting pattern number of 1H atoms responsible for the peak 1.3 2.2 4.0  Deuterium oxide, D2O, where D is 2H, can be used as a solvent in proton NMR spectroscopy. The proton NMR spectrum of alanine in CDCl 3 has 4 peaks. The proton NMR spectrum of alanine in D2O has 2 peaks. H3C C NH2 H alanine COOH On the diagram of alanine, circle the protons that show peaks in both NMR spectra. Explain your answer. The ionic product, Kw, for D2O has a value of 1.35×10–15 mol2 dm–6 at 298 K. Write the expression for the Kw of D2O. Kw =  Calculate the pH of pure, neutral D2O at 298 K. Assume [D+] is equivalent to [H+] for pH calculations.  pH = 
9701_s20_qp_43
THEORY
2020
Paper 4, Variant 3
View
Compare and explain the relative acidities of butanoicacid, ethanol, ethanoicacid and water. > > > most acidic least acidic Three carboxylic acids, methanoic acid, HCO2H, ethanedioic acid, HO2CCO2H, and butanedioicacid, HO2CCH2CH2CO2H, are compared. Two tests were carried out on separate samples of each organic acid, as shown. The following results were obtained.  = observed change  = no observed reaction test reagents and conditions HCO2H HO2CCO2H HO2CCH2CH2CO2H observed change       Complete the table with the reagents and conditions and the observed change for a positive test. Assume these organic acids all have a similar acid strength. Each compound, HCO2H, HO2CCO2H and HO2CCH2CH2CO2H, is dissolved seperately in CDCl 3. Proton (1H) NMR and carbon‑13 (13C) NMR spectra are then obtained. Complete the table. compound number of peaks in proton NMR number of peaks in carbon-13 NMR HCO2H HO2CCO2H HO2CCH2CH2CO2H  The proton NMR spectrum of HCO2H in D2O is obtained. Describe and explain the difference observed between this spectrum and the proton NMR spectrum of HCO2H in . 1,4-dibromobutane, Br(CH2)4Br, is used in the synthesis of the dicarboxylic acid J and diamineK as shown. step 1 step 2 G step 3 H J K HO2CCH2CH2CO2H H2NCH2CH2CH2CH2CH2CH2NH2 step 4 BrCH2CH2CH2CH2Br Draw the structures of G and H in the boxes. Suggest reagents and conditions for each of steps 1 to 4. step 1 step 2 step 3 step 4  PolyamideL can be synthesised from dicarboxylic acidJ, HO2C(CH2)2CO2H, and diamineK, H2N(CH2)6NH2. Draw the repeat unit of the polymer formed in the box. Any functional groups should be shown displayed. polyamide L  
9701_s21_qp_41
THEORY
2021
Paper 4, Variant 1
View
Compare and explain the relative acidities of butanoicacid, ethanol, ethanoicacid and water. > > > most acidic least acidic Three carboxylic acids, methanoic acid, HCO2H, ethanedioic acid, HO2CCO2H, and butanedioicacid, HO2CCH2CH2CO2H, are compared. Two tests were carried out on separate samples of each organic acid, as shown. The following results were obtained.  = observed change  = no observed reaction test reagents and conditions HCO2H HO2CCO2H HO2CCH2CH2CO2H observed change       Complete the table with the reagents and conditions and the observed change for a positive test. Assume these organic acids all have a similar acid strength. Each compound, HCO2H, HO2CCO2H and HO2CCH2CH2CO2H, is dissolved seperately in CDCl 3. Proton (1H) NMR and carbon‑13 (13C) NMR spectra are then obtained. Complete the table. compound number of peaks in proton NMR number of peaks in carbon-13 NMR HCO2H HO2CCO2H HO2CCH2CH2CO2H  The proton NMR spectrum of HCO2H in D2O is obtained. Describe and explain the difference observed between this spectrum and the proton NMR spectrum of HCO2H in . 1,4-dibromobutane, Br(CH2)4Br, is used in the synthesis of the dicarboxylic acid J and diamineK as shown. step 1 step 2 G step 3 H J K HO2CCH2CH2CO2H H2NCH2CH2CH2CH2CH2CH2NH2 step 4 BrCH2CH2CH2CH2Br Draw the structures of G and H in the boxes. Suggest reagents and conditions for each of steps 1 to 4. step 1 step 2 step 3 step 4  PolyamideL can be synthesised from dicarboxylic acidJ, HO2C(CH2)2CO2H, and diamineK, H2N(CH2)6NH2. Draw the repeat unit of the polymer formed in the box. Any functional groups should be shown displayed. polyamide L  
9701_s21_qp_43
THEORY
2021
Paper 4, Variant 3
View
Questions Discovered
133