SL Paper 3
A class was determining the concentration of aqueous sodium hydroxide by titrating it with hydrochloric acid, whilst monitoring the pH of the solution. The sodium hydroxide solution was added into a glass beaker from a measuring cylinder and the hydrochloric acid added using a burette. One group of students accidentally used a temperature probe rather than a pH probe. Their results are given below.
Volume of aqueous NaOH = 25.0 ± 0.5 cm3
Concentration of HCl = 1.00 ± 0.01 mol dm−3
The graph of temperature against titre can be used to calculate the concentration of alkali without knowing the concentration of the hydrochloric acid, using the enthalpy of neutralization.
Explain how the concentration may be calculated in this way.
Heat losses would make this method less accurate than the pH probe method. Outline why the thermometric method would always give a lower, not a higher, concentration.
Suggest how heat loss could be reduced.
State one other assumption that is usually made in the calculation of the heat produced.
Suggest why scientists often make assumptions that do not correspond to reality.
Outline why the thermochemical method would not be appropriate for 0.001 moldm−3 hydrochloric acid and aqueous sodium hydroxide of a similar concentration.
Polymers are made up of repeating monomer units which can be manipulated in various ways to give structures with desired properties.
(i) Draw the structure of 2-methylpropene.
(ii) Deduce the repeating unit of poly(2-methylpropene).
Deduce the percentage atom economy for polymerization of 2-methylpropene.
(i) Suggest why incomplete combustion of plastic, such as polyvinyl chloride, is common in industrial and house fires.
(ii) Phthalate plasticizers such as DEHP, shown below, are frequently used in polyvinyl chloride.
With reference to bonding, suggest a reason why many adults have measurable levels of phthalates in their bodies.
Vegetable oils, such as that shown, require conversion to biodiesel for use in current internal combustion engines.
State two reagents required to convert vegetable oil to biodiesel.
Deduce the formula of the biodiesel formed when the vegetable oil shown is reacted with the reagents in (a).
Explain, in terms of the molecular structure, the critical difference in properties that makes biodiesel a more suitable liquid fuel than vegetable oil.
Determine the specific energy, in kJg−1, and energy density, in kJcm−3, of a particular biodiesel using the following data and section 1 of the data booklet.
Density = 0.850 gcm−3; Molar mass = 299 gmol−1;
Enthalpy of combustion = 12.0 MJmol−1.
Powdered zinc was reacted with 25.00 cm3 of 1.000 mol dm−3 copper(II) sulfate solution in an insulated beaker. Temperature was plotted against time.
Estimate the time at which the powdered zinc was placed in the beaker.
State what point Y on the graph represents.
The maximum temperature used to calculate the enthalpy of reaction was chosen at a point on the extrapolated (dotted) line.
State the maximum temperature which should be used and outline one assumption made in choosing this temperature on the extrapolated line.
Maximum temperature:
Assumption:
To determine the enthalpy of reaction the experiment was carried out five times. The same volume and concentration of copper(II) sulfate was used but the mass of zinc was different each time. Suggest, with a reason, if zinc or copper(II) sulfate should be in excess for each trial.
The formula q = mcΔT was used to calculate the energy released. The values used in the calculation were m = 25.00 g, c = 4.18 J g−1 K−1.
State an assumption made when using these values for m and c.
Predict, giving a reason, how the final enthalpy of reaction calculated from this experiment would compare with the theoretical value.
Red supergiant stars contain carbon-12 formed by the fusion of helium-4 nuclei with beryllium-8 nuclei.
Mass of a helium-4 nucleus = 4.002602 amu
Mass of a beryllium-8 nucleus = 8.005305 amu
Mass of a carbon-12 nucleus = 12.000000 amu
State the nuclear equation for the fusion reaction.
Explain why fusion is an exothermic process.
Beryllium-8 is a radioactive isotope with a half-life of 6.70 × 10−17 s.
Calculate the mass of beryllium-8 remaining after 2.01 × 10−16 s from a sample initially containing 4.00 g of beryllium-8.