Technology: notes and practice questions

A student investigates the neutralisation reaction between dilute ethanoic acid, $\text{CH}_3\text{COOH(aq)}$, and sodium hydroxide solution, $\text{NaOH(aq)}$.

The student considers two methods to monitor the progress of the reaction:

• Method A: Adding universal indicator solution to the reaction mixture and visually observing the colour change.

• Method B: Using a digital pH probe connected to a data logger to continuously record the pH as sodium hydroxide solution is added from a burette.

(a) Outline one advantage of using a data logger to continuously record the pH in Method B rather than recording values manually.

(b) Evaluate the suitability of using the digital pH probe in Method B compared to universal indicator in Method A for identifying the equivalence point of this titration.

A sample of chlorine consists of two isotopes, $^{35}Cl$ and $^{37}Cl$.

(a) Contrast the sub-atomic structure of these two isotopes.

(b) (i) The sample of chlorine is analysed in a mass spectrometer, producing a spectrum for the $Cl_2^+$ ion. The spectrum shows three peaks at m/z values of 70, 72 and 74. Explain the origin and relative heights of these three peaks, given that the abundance of $^{35}Cl$ is approximately three times that of $^{37}Cl$.

(ii) A more precise measurement finds the composition by mass to be: $^{35}Cl$: 75.76%, $^{37}Cl$: 24.24%. Calculate the relative atomic mass of chlorine from this sample, giving your answer to two decimal places. (Use isotopic masses of 35.0 and 37.0 for this calculation).

Magnesium chloride, $MgCl_2$, and manganese(II) chloride, $MnCl_2$, are two ionic compounds.

(c) (i) Deduce the type of bonding in magnesium chloride, $MgCl_2$, using electronegativity values from section 9 of the data booklet.

(ii) Determine the lattice enthalpy of magnesium chloride, assuming the bonding is purely ionic. Use sections 9, 10 and 12 of the data booklet and the following data:

Enthalpy of formation of magnesium chloride = $-641 \text{ kJ mol}^{-1}$

(iii) Explain, with reference to electron configurations, why the ionic radii of $Mg^{2+}$, $Mn^{2+}$ and $Cl^-$ are different. Use section 10 of the data booklet.

(iv) Predict, with a reason, which has the stronger ionic bonding, manganese(II) chloride, $MnCl_2$, or magnesium chloride.

Magnesium chloride is white, but manganese(II) chloride is pale pink.

(d) (i) State the condensed electron configuration of a manganese atom.

(ii) State the reason, in terms of electron configuration, why manganese(II) chloride is coloured.

(iii) Manganese(II) chloride absorbs light with a wavelength of approximately 530 nm. Describe why this is consistent with the observed colour of the compound. Use sections 2 and 15 of the data booklet.

A copper key is to be electroplated with manganese using an aqueous solution of manganese(II) chloride as the electrolyte.

(e) (i) Deduce the half-equations for the reactions occurring at the anode (made of pure manganese) and the cathode (the copper key).

(ii) Deduce a balanced chemical equation for the reaction of fluorine gas with the aqueous chloride ions in the electrolyte.

A student investigates the enthalpy change of neutralisation by reacting $50.0\text{ cm}^3$ of $1.00\text{ mol dm}^{-3}\text{ HCl(aq)}$ with $50.0\text{ cm}^3$ of $1.00\text{ mol dm}^{-3}\text{ NaOH(aq)}$ in an insulated polystyrene cup.

The student considers two methods for recording the temperature of the reaction mixture over time:

• Method A: Using a liquid-in-glass thermometer and manually recording the temperature at $30\text{ s}$ intervals.

• Method B: Using a digital temperature probe connected to a datalogger recording the temperature continuously every $1\text{ s}$.

(a) Outline why the digital temperature probe should be calibrated before the experiment is carried out.

(b) Evaluate the suitability of Method B compared to Method A for determining the maximum temperature change in this reaction.

When investigating the trend in reactivity of the Group 1 elements, the reactions of lithium and sodium with water are often demonstrated in a school laboratory. However, the reactions of rubidium and caesium are commonly investigated using computer simulations.

(a) Explain why a computer simulation is preferred over a physical demonstration for the reaction of caesium with water in terms of laboratory safety.

(b) Aside from safety considerations, explain two advantages of using computer simulations or dynamic 3D models to investigate chemical reactions.

Computer modelling software is frequently used to visualize the three-dimensional (3D) structures of molecules and simulate chemical reactions.

(a) Explain why a 3D virtual model is more effective than a two-dimensional (2D) Lewis formula on paper when predicting whether a molecule such as dichloromethane, $\text{CH}_2\text{Cl}_2$, has an overall molecular dipole moment.

(b) Explain one reason why chemists use computer simulations to study reaction pathways at the molecular level rather than relying solely on direct laboratory observations.

A student synthesises a sample of aspirin (acetylsalicylic acid) in the laboratory and purifies it by recrystallisation.

(a) Describe how a database of physical properties can be used to assess the purity of the synthesised product.

(b) Describe how a spectral database can be used to confirm the identity of the synthesised product.

A student investigates the kinetics of a reaction in which an acid is consumed. At regular time intervals, samples of the reaction mixture are withdrawn, quenched, and titrated against standard sodium hydroxide solution to determine the amount of remaining acid. The student records the time of sampling and the raw titre volume for each sample in two columns of a spreadsheet.

(a) Describe how the spreadsheet can be used to manipulate the raw titre volumes into concentrations of acid for each time point.

(b) Describe how the spreadsheet can be used to represent these processed data in a graphical form to determine the initial rate of the reaction.

Butan-2-ol, $CH_3CH(OH)CH_2CH_3$, is a secondary alcohol that can be oxidized using acidified potassium dichromate(VI) solution to produce an organic compound, X.

(a) Identify the organic compound X and state the colour change observed for the oxidizing agent.

(b) The structure of a compound can be confirmed using ¹H NMR spectroscopy. Predict the number of signals and the ratio of the areas under the signals for the ¹H NMR spectrum of butan-2-ol.

(c) Predict the number of signals and the ratio of the areas under the signals for the ¹H NMR spectrum of compound X.