DP Chemistry (first assessment 2025)

Question 19M.2.SL.TZ1.5

Date	May 2019	Marks available	[Maximum mark: 7]	Reference code	19M.2.SL.TZ1.5
Level	SL	Paper	2	Time zone	TZ1
Command term	Deduce, Draw, Outline, Suggest	Question number	5	Adapted from	N/A

[Maximum mark: 7]

19M.2.SL.TZ1.5

Both vinegar (a dilute aqueous solution of ethanoic acid) and bleach are used as cleaning agents.

Bleach reacts with ammonia, also used as a cleaning agent, to produce the poisonous compound chloramine, NH₂Cl.

(a)

Outline why ethanoic acid is classified as a weak acid.

[1]

Markscheme

partial dissociation «in aqueous solution» [✔]

Examiners report

The definition of a weak acid was generally correct.

(b)

A solution of bleach can be made by reacting chlorine gas with a sodium hydroxide solution.

Cl2 (g) + 2NaOH (aq) $\rightleftharpoons$ NaOCl (aq) + NaCl (aq) + H₂O (l)

Suggest, with reference to Le Châtelier’s principle, why it is dangerous to mix vinegar and bleach together as cleaners.

[3]

Markscheme

ethanoic acid/vinegar reacts with NaOH [✔]

moves equilibrium to left/reactant side [✔]

releases Cl₂ (g)/chlorine gas
OR
Cl₂ (g)/chlorine gas is toxic [✔]

Note: Accept “ethanoic acid produces H⁺ ions”.

Accept “ethanoic acid/vinegar reacts with NaOCl”.

Do not accept “2CH₃COOH + NaOCl + NaCl → 2CH₃COONa + Cl₂ + H₂O” as it does not refer to equilibrium.

Accept suitable molecular or ionic equations for M1 and M3.

Examiners report

Explaining why it was dangerous to mix chlorine with vinegar was not well answered but most students gained at least one mark for stating that “chlorine gas will be produced”, but couldn’t link it to equilibrium ideas.

(c(i))

Draw a Lewis (electron dot) structure of chloramine.

[1]

Markscheme

[✔]

Note: Accept any combination of dots/crosses or lines to represent electron pairs.

Examiners report

The Lewis structure of chloramine was correct for strong candidates, but many made the mistake of omitting electron pairs on N and Cl.

(c(ii))

Deduce the molecular geometry of chloramine and estimate its H–N–H bond angle.

Molecular geometry:

H–N–H bond angle:

[2]

Markscheme

Molecular geometry:
«trigonal» pyramidal [✔]

H–N–H bond angle:
107° [✔]

Note: Accept angles in the range of 100–109.

Examiners report

The molecular geometry and bond angles often did not correspond to each other with quite a few candidates stating trigonal planar and then 107 for the angle.

Syllabus sections

Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.1—Proton transfer reactions » Reactivity 3.1.6—Strong and weak acids and bases differ in the extent of ionization. Recognize that acid–base equilibria lie in the direction of the weaker conjugate. and are strong acids, and group 1 hydroxides are strong bases.

Reactivity 2. How much, how fast and how far? » Reactivity 2.3—How far? The extent of chemical change » Reactivity 2.3.4—Le Châtelier’s principle enables the prediction of the qualitative effects of changes in concentration, temperature and pressure to a system at equilibrium. Apply Le Ch.telier’s principle to predict and explain responses to changes of systems at equilibrium.

Structure 2. Models of bonding and structure » Structure 2.2—The covalent model » Structure 2.2.1—A covalent bond is formed by the electrostatic attraction between a shared pair of electrons and the positively charged nuclei. The octet rule refers to the tendency of atoms to gain a valence shell with a total of 8 electrons. Deduce the Lewis formula of molecules and ions for up to four electron pairs on each atom.

Structure 2. Models of bonding and structure » Structure 2.2—The covalent model » Structure 2.2.4—The valence shell electron pair repulsion (VSEPR) model enables the shapes of molecules to be predicted from the repulsion of electron domains around a central atom. Predict the electron domain geometry and the molecular geometry for species with up to four electron domains.

Question 19M.2.SL.TZ1.5

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