NEB Class 12 Science Chemistry (Technical Stream) Question Paper 2082 Nepal

Ethanoic acid is a weak acid and NaOH is a strong base, so the equivalence point is on the basic side (pH > 7). Phenolphthalein (range 8.3-10) is the suitable indicator.

A negative enthalpy change ($\Delta H < 0$) means heat is released, so the reaction is exothermic.

$HCO_3^-$ can donate a proton (acting as an acid to give $CO_3^{2-}$) and accept a proton (acting as a base to give $H_2CO_3$), so it is amphiprotic.

Below ~1000°C copper forms black CuO, but above ~1100°C CuO decomposes to red cuprous oxide $Cu_2O$. So $Cu_2O$ is formed.

The Bessemer converter blows air through molten pig iron to remove impurities and produce steel.

A flux combines with the infusible gangue to form an easily fusible slag.

Benzenediazonium chloride ($C_6H_5N_2^+Cl^-$) on boiling with water gives phenol: $C_6H_5N_2Cl + H_2O \rightarrow C_6H_5OH + N_2 + HCl$. Chlorobenzene needs harsh conditions (Dow process), so the standard 'boiled with water' answer is benzenediazonium chloride.

Fehling's solution is reduced by aliphatic aldehydes. Aromatic aldehydes like benzaldehyde do not reduce Fehling's solution.

Rectified spirit (95.6% ethanol) is an azeotrope, so ordinary distillation cannot give absolute alcohol; azeotropic distillation (with benzene) is used.

Ozonolysis cleaves the C=C double bond into two carbonyl fragments. Ethanal ($CH_3CHO$) + methanal ($HCHO$) recombine across the broken double bond to give $CH_3-CH=CH_2$, i.e. propene.

An antipyretic is a drug used to reduce fever (body temperature).

The law of chemical equivalence states that whenever two substances react chemically, they always do so in the ratio of their chemical equivalents. In other words, one gram-equivalent of a substance reacts exactly with one gram-equivalent of another substance.

Applied to titration, this means at the end point the number of gram-equivalents (or milliequivalents) of the acid is exactly equal to the number of gram-equivalents of the base:

By definition, the number of gram-equivalents of a solute is related to its normality $(N)$ and volume $(V)$:

Let an acid (normality $N_1$, volume $V_1$) be titrated against a base (normality $N_2$, volume $V_2$). By the law of equivalence, at the end point:

Cancelling the common factor gives the normality equation:

The pH of a solution is defined as the negative logarithm (base 10) of the hydrogen ion (or hydronium ion) concentration expressed in mol L$^{-1}$:

It is a measure of the acidity or basicity of a solution; lower pH means more acidic.

pH has many practical uses in daily life:

• Agriculture: testing soil pH to decide which crops to grow and whether to add lime/fertilizer.
• Medicine/health: monitoring the pH of blood and body fluids (blood pH must stay near 7.4); treating acidity of the stomach with antacids.
• Food and beverages: controlling acidity in foods, soft drinks and preservation.
• Personal care products: ensuring shampoos, soaps and cosmetics have a skin-friendly pH.
• Water and waste-water treatment: adjusting pH for safe drinking water.

pOH is the negative logarithm (base 10) of the hydroxide ion concentration (in mol L$^{-1}$):

Relation with pH: In water, the ionic product is $[H^+][OH^-] = K_w = 10^{-14}$ at 25 °C. Taking negative logarithm of both sides:

Zero Order Reaction

A reaction is zero order when its rate is independent of the concentration of the reactant.

Consider: $A \rightarrow \text{Products}$

Integrated Rate Equation

For a zero order reaction the rate equals the rate constant $k$:

Separating variables and integrating between limits ($[A]_0$ at $t=0$ and $[A]$ at time $t$):

Explanation: This is a straight-line equation. A plot of $[A]$ versus $t$ gives a straight line with slope $= -k$ and intercept $= [A]_0$. The concentration falls linearly with time. Also $k = \dfrac{[A]_0 - [A]}{t}$, whose units are mol L$^{-1}$ s$^{-1}$.

Half Life ($t_{1/2}$)

Half life is the time taken for the concentration to fall to half its initial value, i.e. $[A] = \dfrac{[A]_0}{2}$. Substituting in the integrated equation:

Conclusion: For a zero order reaction, the half life is directly proportional to the initial concentration $[A]_0$.

A standard hydrogen electrode (SHE) is a reference electrode consisting of a platinised platinum plate dipped in a 1 M solution of H$^+$ ions (1 M HCl), through which pure hydrogen gas at 1 atm pressure is bubbled, all maintained at 25 °C (298 K). Its electrode potential is arbitrarily taken to be zero ($E^\circ = 0.00$ V).

Electrode reaction: $2H^+ + 2e^- \rightleftharpoons H_2$

The major application of the standard hydrogen electrode is that it is used as a reference (standard) electrode to measure the standard electrode (reduction) potentials of other electrodes/half-cells, since its own potential is fixed at zero.

Standard Hydrogen Electrode (description of the labelled diagram):

The diagram should show:

• A glass jacket/tube with a side inlet for H$_2$ gas at 1 atm pressure.
• A platinum wire carrying a platinised platinum (Pt black) plate at its lower end.
• The Pt plate dipped in 1 M HCl (1 M H$^+$) solution.
• Hydrogen gas bubbling over the Pt plate and escaping through holes at the bottom of the glass tube.
• A salt bridge / connecting tube to complete the circuit with the other half-cell.

        H2 gas in (1 atm)
              |
        ______v______
       |  glass tube |
       |   | wire |  |
       |   |      |  |---> to external circuit
       |  [Pt black plate]
       |  o o o (H2 bubbles)
       |_____________|
     1 M HCl (1 M H+) solution

(Label: H$_2$ gas inlet at 1 atm, platinised Pt plate, 1 M H$^+$ / HCl solution, hydrogen bubbles, connection to circuit.)

The standard electrode potential ($E^\circ$) of an electrode is the potential developed at the electrode when it is in contact with its ions at unit concentration (1 M), with any gases at 1 atm pressure and the temperature at 25 °C (298 K), measured relative to the standard hydrogen electrode (whose potential is taken as zero).

Derivation of $\Delta G = \Delta H - T\Delta S$

Gibbs free energy is defined as:

where $H$ = enthalpy, $S$ = entropy, $T$ = absolute temperature.

Consider a process in which the state changes from initial state 1 to final state 2 at constant temperature $T$:

Subtracting:

This is the Gibbs–Helmholtz relation, used to predict the spontaneity of a process ($\Delta G < 0$ → spontaneous).

Why useful work is called free energy

For a process carried out at constant temperature and pressure, it can be shown that the decrease in Gibbs energy equals the maximum useful (non-expansion / net) work obtainable from the system:

Thus the quantity $G$ represents the portion of the system's energy that is "free" or available to be converted into useful work. The remaining energy ($T\Delta S$) is bound/unavailable. Hence the useful work of the system is called free energy.

Froth Floatation Process

• Used chiefly for the concentration of sulphide ores (e.g. galena, zinc blende, copper pyrites).
• It works on the difference in wettability of ore and gangue. The powdered ore is mixed with water, a frothing agent (pine oil) and a collector in a tank, and air is blown through.
• The ore particles become wetted by oil and rise up with the froth (which is skimmed off), while the gangue is wetted by water and settles at the bottom.

Magnetic Separation Process

• Used when either the ore or the gangue is magnetic while the other is non-magnetic.
• The powdered ore is dropped on a belt moving over two rollers, one of which is an electromagnet.
• The magnetic component is attracted and falls near the magnetic roller, while the non-magnetic component is thrown farther away, forming two separate heaps.
• Example: separating magnetic tin ore (cassiterite/wolframite) or iron ore from non-magnetic impurities.

Functional Isomers of $C_3H_6O$

The two functional isomers are:

1. Propanal — $CH_3CH_2CHO$ (an aldehyde)
2. Propanone (acetone) — $CH_3COCH_3$ (a ketone)

They have the same molecular formula but different functional groups, hence are functional isomers.

Distinction by Tollen's Reagent

Tollen's reagent is ammoniacal silver nitrate, $[Ag(NH_3)_2]^+$.

• Propanal (aldehyde) reduces Tollen's reagent and gives a shining silver mirror:

• Propanone (ketone) does not reduce Tollen's reagent — no silver mirror is formed.

Thus the aldehyde gives a silver mirror while the ketone does not.

Preparation by Dehydrogenation of Alcohols

Passing alcohol vapour over heated copper at 300 °C removes hydrogen (dehydrogenation):

• Primary alcohol → aldehyde:

• Secondary alcohol → ketone:

a) Dow's process (chlorobenzene → phenol):

b) Wurtz reaction (alkyl halide + Na in dry ether → alkane):

c) 2,4-DNP test (aldehydes/ketones give orange-yellow 2,4-dinitrophenylhydrazone):

d) Preparation of alcohol from ester (reduction with LiAlH$_4$, or hydrolysis):

e) Phenol from benzene sulphonic acid (alkali fusion):

Or (alternative)

Compound (A) is chloroform, $CHCl_3$ (its vapour is an anaesthetic causing loss of consciousness; with conc. HNO$_3$ it forms chloropicrin $CCl_3NO_2$, a tear gas component).

i) Preparation of (A) from an isomer of $C_2H_6O$: Ethanol ($C_2H_5OH$, an isomer of $C_2H_6O$) is warmed with bleaching powder (or Cl$_2$ + alkali) — the haloform reaction:

Overall: $2C_2H_5OH + 6Cl_2 + Ca(OH)_2 \rightarrow 2CHCl_3 + (HCOO)_2Ca + \ldots$

ii) Reactions of chloroform:

(a) Exposed in air (light): slowly oxidised to the poisonous gas phosgene:

(b) Heated with Ag powder: gives acetylene:

(c) Heated with aqueous (dilute) alkali: gives formate (sodium/potassium formate):

Rate of reaction = change in concentration of reactant or product per unit time. Factors: increasing concentration increases rate (more collisions); increasing temperature increases rate (more molecules cross activation energy); a catalyst increases rate by lowering activation energy; greater surface area of reactant increases rate. Numerical: 87.5% completion means 3 half lives (50% → 75% → 87.5%). $t = 3 \times t_{1/2} = 3 \times 3 = 9$ hours.

Haematite ($Fe_2O_3$) and magnetite ($Fe_3O_4$). (Also limonite, siderite.)

Labelled diagram of the Blast Furnace (description):

The blast furnace is a tall (~30 m) steel chimney lined with fire-bricks. From top to bottom, the following must be shown and labelled:

• Charging cup-and-cone hopper (top): for feeding the charge (roasted ore + coke + limestone).
• Flue/waste gas outlet (top): hot gases (mainly CO, CO$_2$, N$_2$) leave here.
• Zone of reduction (upper, ~400–700 °C): where Fe$_2$O$_3$ is reduced by CO.
• Zone of slag formation / fusion (middle, ~800–1000 °C): CaCO$_3$ decomposes; CaO + SiO$_2$ form slag.
• Zone of combustion / fusion (lower, ~1500–1900 °C): coke burns in the hot air blast.
• Tuyeres (bottom sides): pipes that blow in a hot air blast.
• Tap holes (bottom): an upper one for molten slag and a lower one for molten iron (pig iron).

          Charge (ore+coke+limestone) + waste gas out
        ===========  cup & cone hopper  ===========
        |   Zone of Reduction   (~400-700°C)       |
        |   Zone of Slag Formation (~800-1000°C)   |
        |   Zone of Fusion/Combustion (~1500°C+)   |
   air->|--[tuyeres]                  [tuyeres]--<-air
        |   slag tap -->  molten iron tap -->      |
        ============================================

(Label each zone with its temperature, the tuyeres, the hot air blast, the slag and molten iron tap holes, and the flue-gas outlet.)

Reactions in the different zones of the blast furnace:

1. Zone of Combustion / Fusion (bottom, ~1500 °C): coke burns in the hot air blast and CO$_2$ is reduced to CO:

2. Zone of Reduction (upper, ~400–700 °C): the iron oxide ore is reduced by CO to spongy iron:

(stepwise: $3Fe_2O_3 + CO \rightarrow 2Fe_3O_4 + CO_2$; $Fe_3O_4 + CO \rightarrow 3FeO + CO_2$; $FeO + CO \rightarrow Fe + CO_2$).

3. Zone of Slag Formation / Fusion (~800–1000 °C): limestone decomposes and CaO removes the silica gangue as fusible slag:

Iron is used in construction (rods, beams, bridges), making machinery and tools, automobiles, railways, utensils, and as a raw material for steel and various alloys.

(Or-alternative: extraction of zinc from zinc blende — roasting ZnS to ZnO, then reduction with coke in a retort; $2ZnS + 3O_2 \rightarrow 2ZnO + 2SO_2$, $ZnO + C \rightarrow Zn + CO$. When zinc is exposed to moist air it forms a protective layer of basic zinc carbonate $ZnCO_3·Zn(OH)_2$.)

Radioactivity is the spontaneous disintegration of an unstable atomic nucleus with the emission of alpha, beta or gamma radiation. Its SI unit is the becquerel (Bq); the older unit is the curie (Ci).

Artificial (induced) radioactivity is the radioactivity produced in a normally stable element by bombarding it with high-energy particles. Example: bombarding aluminium with alpha particles gives radioactive phosphorus-30: $^{27}_{13}Al + ^{4}_{2}He \rightarrow ^{30}_{15}P + ^{1}_{0}n$.

i) Insecticide: DDT — used to kill insects/mosquitoes. ii) Synthetic polymer: polythene (polyethene) — used for making bags, bottles, packaging. iii) Antiseptic: dettol (or tincture of iodine) — applied on wounds to kill/prevent microbes. iv) Fungicide: Bordeaux mixture (copper sulphate + lime) — used to kill fungi on crops.