Gases

Laboratory preparation of ammonia

   Clamp stand
      |
  [Hard glass test tube]  (mouth slightly downward)
  NH4Cl + Ca(OH)2 mixture  <-- heat (burner)
        \
         \ delivery tube
          \__ [Lime tower: CaO / quicklime] __
                                              \
                                          +---------+
                                          | inverted|  <- gas jar (downward
                                          | gas jar |     displacement of air)
                                          | NH3 gas |
                                          +---------+

Reaction: A mixture of ammonium chloride and slaked lime is heated.

$2NH_4Cl + Ca(OH)_2 \xrightarrow{\Delta} CaCl_2 + 2H_2O + 2NH_3\uparrow$

The gas is dried by passing it through a lime (CaO) tower and collected by downward displacement of air, because it is lighter than air.

Test of the gas in the gas jar:

• Bring a moist red litmus paper into the jar. It turns blue, because ammonia dissolves in the moisture to form basic ammonium hydroxide.
• (Also, a glass rod dipped in conc. HCl held at the mouth of the jar gives dense white fumes of NH$_4$Cl.)
• The pungent smell is a further identification.

Diagram:

   Clamp stand
      |
  [Hard glass test tube]  (mouth slightly downward)
  NH4Cl + Ca(OH)2 mixture  <-- heat (burner)
        \
         \ delivery tube
          \__ [Lime tower: CaO / quicklime] __
                                              \
                                          +---------+
                                          | inverted|  <- gas jar (downward
                                          | gas jar |     displacement of air)
                                          | NH3 gas |
                                          +---------+

Label (any one): Lime tower (CaO) is placed between the test tube and the gas jar to dry the gas. (Other parts: hard glass test tube with NH$_4$Cl + Ca(OH)$_2$; burner; inverted gas jar.)

Function of lime tower: It contains quicklime (CaO), which absorbs the water vapour present in the ammonia gas, so dry ammonia is obtained. CaO is basic, so it does not react with ammonia (acidic drying agents like conc. H$_2$SO$_4$ or CaCl$_2$ cannot be used).

(i) Ammonia gas (NH$_3$) is collected in the gas jar.

(ii) Ammonia is used to make fertilizers such as urea and ammonium sulphate (also used in refrigeration, and to make nitric acid).

(iii) CaO (quicklime) is used to dry the gas. It absorbs the moisture from ammonia and, being basic, does not react with ammonia.

(iv) The wet red litmus paper turns blue, because ammonia dissolves in water to form ammonium hydroxide, which is a base (alkali): $NH_3 + H_2O \rightarrow NH_4OH$.

(a) Ammonia gas (NH$_3$) is collected in B (by downward displacement of air).

(b) The chemical at C (quicklime, CaO) dries the gas by absorbing the water vapour. It does not react with ammonia as it is basic.

(c) Ammonia reacts with dilute sulphuric acid to form ammonium sulphate (a neutralisation reaction):

$2NH_3 + H_2SO_4 \rightarrow (NH_4)_2SO_4$

(a) Ammonia gas (NH$_3$) is collected in the gas jar.

(b) Tower X contains quicklime (CaO), the drying agent. Without it, the gas would contain water vapour, so moist (wet) ammonia would be collected. It would be impure and would dissolve in the water droplets, forming ammonium hydroxide solution instead of dry gas.

(c) $2NH_4Cl + Ca(OH)_2 \xrightarrow{\Delta} CaCl_2 + 2H_2O + 2NH_3\uparrow$

Errors:

1. The mouth of the hard-glass test tube is tilted upward. The water formed in the reaction condenses and flows back to the hot glass, cracking it.
2. The drying agent used is calcium chloride (CaCl$_2$). It reacts with ammonia ($CaCl_2 + 8NH_3 \rightarrow CaCl_2\cdot 8NH_3$), so ammonia is lost.

Correct setting:

• Clamp the test tube with its mouth slightly downward.
• Use a lime tower containing quicklime (CaO) to dry the gas.
• Collect ammonia in an inverted gas jar (downward displacement of air), as the gas is lighter than air.

(iv) To dry the gas. Lime tower contains quicklime (CaO), which absorbs the moisture from ammonia without reacting with it.

Ammonia is highly soluble in water (it dissolves to form ammonium hydroxide), so it cannot be collected over or passed through water.

Ammonia (NH$_3$) gas is formed.

$2NH_4Cl + Ca(OH)_2 \xrightarrow{\Delta} CaCl_2 + 2H_2O + 2NH_3$

Ammonia has a pungent (sharp, choking) odour, like that of urine.

Ammonia dissolves in the moisture on the litmus paper and forms ammonium hydroxide ($NH_3 + H_2O \rightarrow NH_4OH$), which is basic. Bases turn red litmus blue.

(i) Carbon dioxide (CO$_2$) forms potassium carbonate with KOH solution:

$2KOH + CO_2 \rightarrow K_2CO_3 + H_2O$

(ii) Ammonia (NH$_3$) is used to prepare a weak alkali. It dissolves in water to give ammonium hydroxide, $NH_3 + H_2O \rightarrow NH_4OH$ (a weak base).

Laboratory preparation of carbon dioxide

 Thistle funnel (dil. HCl added)
      |
      |      delivery tube
  +---|---+     ________
  |   |   |____/        |
  |  Flask         +----|----+
  | CaCO3 (marble  |  gas jar |  (erect, mouth up)
  |  chips)        |   CO2    |
  +-------+        +----------+
   (dil. HCl is poured through the funnel, its end dips in the acid)

Salt used: Calcium carbonate (CaCO$_3$), as marble chips or limestone.

$CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2\uparrow$

(a)

 Thistle funnel (dil. HCl added)
      |
      |      delivery tube
  +---|---+     ________
  |   |   |____/        |
  |  Flask         +----|----+
  | CaCO3 (marble  |  gas jar |  (erect, mouth up)
  |  chips)        |   CO2    |
  +-------+        +----------+
   (dil. HCl is poured through the funnel, its end dips in the acid)

$CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2\uparrow$

(b) When CO$_2$ is passed into lime water for a short time, the lime water turns milky because insoluble calcium carbonate forms: $Ca(OH)_2 + CO_2 \rightarrow CaCO_3\downarrow + H_2O$ When passed for a long time, the milkiness disappears (the solution becomes clear again) as soluble calcium bicarbonate forms: $CaCO_3 + H_2O + CO_2 \rightarrow Ca(HCO_3)_2$

(c) Plants use CO$_2$ for photosynthesis to make glucose (food) in the presence of sunlight and chlorophyll.

(i) $CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2\uparrow$

(ii) No. Dil. H$_2$SO$_4$ reacts with CaCO$_3$ to form calcium sulphate, which is almost insoluble: $CaCO_3 + H_2SO_4 \rightarrow CaSO_4 + H_2O + CO_2$. It forms a coating over the marble chips and stops further contact with the acid, so the reaction soon stops.

(iii) CO$_2$ is heavier than air (molecular mass 44 against about 29 for air), so it settles down in an erect jar and pushes the air out (upward displacement of air).

(i) Carbon dioxide (CO$_2$).

(ii) Test: Pass the gas through lime water. It turns milky: $Ca(OH)_2 + CO_2 \rightarrow CaCO_3\downarrow + H_2O$. (Alternatively, a burning splinter is extinguished in the gas.)

(iii) Use: it is used in fire extinguishers (also for soft drinks, and for dry ice for cooling, and by plants in photosynthesis).

(iv) The gas jar is kept erect because CO$_2$ is heavier than air, so it collects at the bottom of the jar by upward displacement of air.

(a) Carbon dioxide (CO$_2$).

(b) CO$_2$ is heavier than air, so it is collected by upward displacement of air in an erect jar, where it settles at the bottom.

(c) Burning magnesium continues to burn in CO$_2$ and forms white magnesium oxide and black carbon: $2Mg + CO_2 \rightarrow 2MgO + C$

(d) Urea is formed when CO$_2$ reacts with ammonia at high temperature and pressure: $CO_2 + 2NH_3 \xrightarrow{high\ T,\ P} CO(NH_2)_2 + H_2O$

(a) Carbon dioxide (CO$_2$) is produced. $CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2\uparrow$

(b) Carbonic acid, H$_2$CO$_3$ ($CO_2 + H_2O \rightarrow H_2CO_3$).

(c) Test: Pass the gas through lime water. It turns milky because calcium carbonate is formed: $Ca(OH)_2 + CO_2 \rightarrow CaCO_3\downarrow + H_2O$

(a) $CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2\uparrow$

(b) CO$_2$ is heavier than air (and fairly soluble in water), so it is collected by upward displacement of air in an erect jar.

(c) It is used in fire extinguishers (also in aerated drinks and for making dry ice).

(d) Excess CO$_2$ is a greenhouse gas. It traps the heat radiated from the Earth, causing global warming and climate change, such as melting of glaciers and sea-level rise.

(a) Carbon dioxide (CO$_2$).

(b) Calcium carbonate (marble chips/limestone) and dilute hydrochloric acid.

(c) Photosynthesis: $6CO_2 + 6H_2O \xrightarrow[chlorophyll]{sunlight} C_6H_{12}O_6 + 6O_2$

(d) It turns blue litmus paper red (moist), because CO$_2$ with water forms acidic carbonic acid, H$_2$CO$_3$.

Egg shells are mainly calcium carbonate, so on reaction with dilute HCl they give carbon dioxide.

(i) $Ca(OH)_2 + CO_2 \rightarrow CaCO_3\downarrow + H_2O$ (lime water turns milky)

(ii) Solid CO$_2$ (dry ice) is used in cryosurgery, for example to remove warts and skin growths, and to keep organs and medicines cold.

(iii) The gas of molecular mass 17 is ammonia (NH$_3$). With CO$_2$ it forms urea, CO(NH$_2$)$_2$ (molecular formula CH$_4$N$_2$O): $CO_2 + 2NH_3 \rightarrow CO(NH_2)_2 + H_2O$.

(iv) Pass the gas through lime water: it turns milky. (A burning splinter is also extinguished.)

Carbon dioxide is heavier than air, so it settles at the bottom and displaces the air upward out of an erect jar.

Because it dissolves in water to form carbonic acid ($CO_2 + H_2O \rightarrow H_2CO_3$), and it turns blue litmus red. It is also an oxide of a non-metal.

H$_2$CO$_3$ (carbonic acid): $CO_2 + H_2O \rightarrow H_2CO_3$.

Why dil. HCl, not dil. H$_2$SO$_4$: H$_2$SO$_4$ reacts with CaCO$_3$ to form insoluble calcium sulphate, which coats the marble chips and stops the reaction. HCl forms soluble CaCl$_2$, so the reaction continues until the chips are used up.

Tests before entering a well: CO$_2$ may have collected in the well (it is heavy and does not support breathing). So:

1. Lower a burning candle into the well. If it is extinguished, CO$_2$ (or lack of oxygen) is present, and it is unsafe to enter.
2. Lower a vessel of lime water and bring it up; if it turns milky, CO$_2$ is present.

(i) The litmus paper changes colour first in the set-up where the blue litmus paper is moist (wet). The gas is carbon dioxide, which dissolves in the water on the paper to form carbonic acid ($CO_2 + H_2O \rightarrow H_2CO_3$); this acid turns blue litmus red. Dry litmus paper shows no change because dry CO$_2$ is not acidic.

(ii) No. Dil. H$_2$SO$_4$ forms calcium sulphate, which is almost insoluble and coats the marble chips, so the acid cannot reach the CaCO$_3$ and the reaction soon stops: $CaCO_3 + H_2SO_4 \rightarrow CaSO_4 + H_2O + CO_2\uparrow$ Dil. HCl forms soluble CaCl$_2$, so the reaction continues.

(iii) CO$_2$ reacts with ammonia at high temperature and pressure to form urea: $CO_2 + 2NH_3 \xrightarrow{high\ T,\ P} CO(NH_2)_2 + H_2O$

Carbon dioxide changes to solid dry ice at about $-78^\circ C$ (more exactly $-78.5^\circ C$) at normal pressure.

Ammonia ($NH_3$) gas is used in making blue prints (copies) of maps.

Ammonia ($NH_3$). It dissolves in water to form the alkali ammonium hydroxide, $NH_4OH$.

The gas is ammonia.

(a) Calcium hydroxide and ammonium chloride are used in the mole ratio 1 : 2.

(b) $2NH_4Cl + Ca(OH)_2 \xrightarrow{Heat} CaCl_2 + 2H_2O + 2NH_3\uparrow$

OR Ammonia dissolves in water to form ammonium hydroxide, a weak alkali: $NH_3 + H_2O \to NH_4OH$

(a) $NH_3$ is extremely soluble in water, so it would dissolve instead of being collected over it.

(b) $CO_2$ is heavier than air, so it would sink and could not be collected in an inverted jar; it is collected by upward displacement of air.

(a) The gas is carbon dioxide, $CO_2$. It forms weak carbonic acid in water (sour taste) and does not support burning.

(b) Burning magnesium continues to burn in a jar of $CO_2$ and forms magnesium oxide (with black carbon): $2Mg + CO_2 \to 2MgO + C$

(a) Calcium bicarbonate reacts with dilute HCl to give carbon dioxide: $Ca(HCO_3)_2 + 2HCl \to CaCl_2 + 2H_2O + 2CO_2\uparrow$

(b) Lower a burning magnesium ribbon into a jar of the gas. It keeps burning and leaves a white powder (MgO) and black specks of carbon on the walls of the jar: $2Mg + CO_2 \to 2MgO + C$.

OR (a) Carbon dioxide ($CO_2$) is produced.

The acid is hydrochloric acid (HCl), which gives $H^+$ and $Cl^-$ ions.

(a) X is carbon dioxide, $CO_2$.

(b) Burning magnesium reacts with $CO_2$ to liberate carbon: $2Mg + CO_2 \to 2MgO + C$

OR Lime water turns milky because insoluble calcium carbonate forms: $Ca(OH)_2 + CO_2 \to CaCO_3\downarrow + H_2O$

(a) Being heavier than air, $CO_2$ forms a blanket over a fire and cuts off the supply of oxygen, so it is used in fire extinguishers.

(b) $6CO_2 + 6H_2O \xrightarrow[\text{chlorophyll}]{\text{sunlight}} C_6H_{12}O_6 + 6O_2$

OR Carbon dioxide and water take part in photosynthesis.

(a) X is carbon dioxide ($CO_2$), obtained by strongly heating a carbonate such as limestone. Solid $CO_2$ is dry ice. Y is ammonia, used in making map copies (blue prints).

(b) A cold store uses X (as dry ice) as a refrigerant to keep food, meat and ice cream frozen and to prevent them from spoiling.

(c) Y (ammonia) is manufactured by the Haber process under a high pressure (about 200 atm) and a temperature of about $450^\circ C$ to $500^\circ C$.

(d) In the fertilizer industry, ammonia and carbon dioxide combine to form urea: $2NH_3 + CO_2 \to NH_2CONH_2 + H_2O$

Laboratory preparation of carbon dioxide

 Thistle funnel (dil. HCl added)
      |
      |      delivery tube
  +---|---+     ________
  |   |   |____/        |
  |  Flask         +----|----+
  | CaCO3 (marble  |  gas jar |  (erect, mouth up)
  |  chips)        |   CO2    |
  +-------+        +----------+
   (dil. HCl is poured through the funnel, its end dips in the acid)

Marble chips (CaCO$_3$) react with dil. HCl: $CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2\uparrow$ The gas is heavier than air, so it is collected by upward displacement of air in an erect gas jar.

Test with litmus paper: Put a moist (wet) blue litmus paper into the gas jar. It turns red. CO$_2$ dissolves in the water of the paper to form carbonic acid ($CO_2 + H_2O \rightarrow H_2CO_3$), which is acidic and turns blue litmus red. This shows the gas is acidic.

Lime water turns milky because insoluble calcium carbonate is formed: $Ca(OH)_2 + CO_2 \rightarrow CaCO_3\downarrow + H_2O$.

(a) Ammonia gas (NH$_3$) is collected in the gas jar (by downward displacement of air in an inverted jar), prepared by heating NH$_4$Cl with Ca(OH)$_2$.

(b) A is ammonia gas (collected in the gas jar). B is the mixture of ammonium chloride and calcium hydroxide (slaked lime), which is heated in the hard glass test tube.

(c) Red litmus paper (moist) is used. It turns blue, because ammonia forms basic ammonium hydroxide with water.