2023 SOTA Yr 6 Chem HL Prelim Exam MS3

2023 Prelim Chemistry HL Paper 3 Mark Scheme Section A 1a Ag+(aq) + I-(aq) à AgI(s) Correct products and balancing ✔ Correct state symbols ✔ bi Oxidising agent ; ✔ ii To provide an acidic medium /H+ ions needed for the redox reaction ; ✔ Do not accept catalyst ci (5.20 + 5.30) / 2 = 5.25 cm3 ✔ Reject other values ii % uncertainty = 0.10 / 5.25 x 100% = 1.9% ; ✔ ECF from c(i) iii Dilute/lower the concentration of sodium thiosulfate solution OR Increase the mass of fish OR Increase volume of iodine solution used OR more repeats ✔ Reject: “bigger” or “larger” sample 2a P ¡water = 3.2 kPa ✔ Accept any answer from 3.1 kPa to 3.3 kPa. b The vapour pressure will increase with temperature. ✔ c 7 kPa ✔ Accept any answer from 6.8 kPa to 7.2 kPa. d Pure ethanol has a higher vapour pressure than water. ✔ e(i) Award [2 max] for the following: There is strong intermolecular forces of attraction/hydrogen bonding between the ethanol molecules. ✔ The ethanol molecules has a significant size and occupies space. ✔ The collisions between ethanol molecules are not elastic. ✔

e(ii) Award [2 max] for each of the following: Starting point at origin (P = 0 kPa) Ending point at P = 8.0 kPa A straight line from these two points.

Section B 3a TD50 is the amount / dose that negatively affects / produce toxic effects in 50% of the population. ü LD50 is the amount / dose that kills 50% of the population. ü Award 1 mark max: TD50 is used in human studies but LD50 is used in animal studies. b The risk of overdose is high because of the low therapeutic index. ü Accept: The drug must be used with caution. c intravenous / IV <<injection>> OR injection into the bloodstream ü d Any one of: • «negative» side-effects of medication on patient / volunteers ü • effects on environment «from all materials used and produced» ü • potential for abuse ü • drugs may be developed that are contrary to some religious doctrines ü • animal testing ü • risk to benefit ratio ü • appropriate consent of patient volunteers ü

4a ethanoic anhydride / (CH3CO)2O OR ethanoyl chloride / CH3COCl ü Accept: ethanoic acid / CH3COOH b It has a <<large>> benzene ring / arene ring which is non-polar / hydrophobic / cannot form hydrogen bonds with water. ü It has carboxyl / COOH / hydroxyl / OH <<and ester group>> which is polar / hydrophilic / can form hydrogen bonds with water. ü c OR C6H4(COOCH3)COOH + NaOH → C6H4(COOCH3)COONa + H2O ü Accept: ionic equation, any other strong base Charges (O– and Na+) are not necessary to score the mark. d Similarities: Award 2 marks max • Both have <<strong>> absorption from 1700-1750 cm-1 for C=O <<in carboxylic acid>>. ü • Both have <<strong and very broad>> absorption from 2500-3000 cm-1 for O–H <<in carboxylic acid>>. ü • Both have <<strong>> absorption from 1050-1410 cm-1 for C–O <<in alcohol/phenol>>. ü • Both have <<<<strong>> absorption from 2850-3090 cm-1 for C–H. ü Difference: Award 1 mark max • Salicylic acid has a <<strong and broad>> absorption from 3200-3600 cm-1 for O–H <<in alcohol/phenol>> but not aspirin. ü Accept: Aspirin has 2 absorptions in 1700-1750 cm-1 due to 2 different C=O, but salicylic acid only has 1 absorption.

5 They <<temporarily>> bind / bond to <<opioid>> receptors in the brain / CNS. ü This prevents / block the transmission of pain impulses. ü 6a It blocks histamine from binding to the <<H2>> receptor. OR it binds to the same <<H2>> receptors <<as histamine>>. OR it competes with histamine for binding. ü b proton pump OR H+/K+ ATPase enzyme ü Accept: <<secretary surface of>> parietal cells Do not accept: “stomach/stomach wall” 7 Any two of: • It prevents the virus from attaching to host cell/ binding to cellular receptors targeted by viruses. ü • It alters the cell’s genetic material / DNA <<so that virus cannot use it to multiply>>. ü • It blocks enzyme activity in the host cell <<so that virus cannot use it to multiply>>. ü • It prevents the release of <<replicated>> viruses from host cell. ü • It prevents the removal of protein coat / capsid. ü • It prevents the injection of viral DNA / RNA into cell. ü Accept: “prevents synthesis of virus by host cell”, “alters genetic material / DNA / RNA of virus”

8a It gives off small / low amounts of radiation for a short time. ü Accept: “weakly ionizing radiation” instead of “small amounts of radiation”, “short half-lives” instead of “for a short time”. b It is stored in shielded containers until radiation drops <<to a safe level>>. OR shipped to central/specific site for specialized disposal. OR Incineration/burn. OR Buried underground. ü 9a It contains many stereoisomers / optical isomers / chiral carbons / chiral centres. ü Do not accept just “chiral”. b Any one of: • use of a chiral auxiliary to form the desired enantiomer ü • asymmetric synthesis ü • biosynthesis / using genetically modified bacteria / microorganisms ü c Any two of: • use of immiscible solvents ü • partitioning of Taxol between the two solvents ü • Taxol is more soluble in one solvent. ü • The extraction is carried out multiple times «to improve extraction». ü • shaking / stirring the mixture ü • separating the two layers ü • The solvent is evaporated from the final solution «to obtain pure Taxol». ü

10a Any two of: Advantages: • It can be readily “tagged” to a variety of biologically active carriers «which will deliver it to specific locations for imaging uses». ü • The frequency of radiation is compatible with existing X-ray detection apparatus. ü • The product of decay has low radioactivity / relatively short half-life / low total exposure to patient. ü • It produces only low-level waste / LLW. ü • It is a transition metal which forms compounds in a variety of oxidation states. ü • Gamma-radiation «can escape the body and» be detected by external sensors. ü Limitations: • There is a «small» increased risk of cancer to patient. ü • It must be made on site. ü • Activity decreases quickly, so dose must be calculated prior to each injection. ü b = 100 x 0.515.0/6.0 ü = 18 % ü Award 1 mark: Correct substitution of values Award 2 marks: Correct final answer

11a gas chromatography / GC OR high-performance liquid chromatography / HPLC ü Accept: chromatography / thin-layer chromatography / paper chromatography / extraction Do not accept just “mass spectrometry / MS”, but do not penalize combined techniques such as GC-MS or HPLC-MS. b Water contains O-H bonds <<and is found in the breath>>. ü