DHS Y4 2023 Haber Process Worksheet (Answers)

DUNMAN HIGH SCHOOL DHP CHEMISTRY WORKSHEET – AMMONIA (HABER PROCESS) NAME..............................................…....( ) YEAR ............... DATE........................ Answer the following questions with reference to “Ammonia (Haber Process)” notes. 1 Draw a ‘dot-and-cross’ diagram to show the bonding in a nitrogen molecule, thus explain why nitrogen is quite unreactive. [3] Note: All electrons must be drawn since question did not state “show outer electrons only”. Explanation: A lot of energy is needed to break the very strong triple covalent bond between the atoms in the molecule before nitrogen can react. [1] Bond Bond energy (kJ/mol) N-N 167 N=N 418 N≡N 942 2 Complete the flow diagram for the manufacture of ammonia. [7] N2 (1 volume) – obtained from ………………………………..of ………… air H 2 ( …… volume) – obtained from …………....... of alkanes from crude oil Converter …….....°C ………… catalyst Equation: ...…….…..…..…………… Compressor (……… atm) Cooling Chamber Unreacted N2 and H2 gases N2, H2 N2, H2 & NH3 Liquid ammonia fractional distillation liquid 3 cracking 200 450 iron N2(g) + 3H2(g) ⇌ 2NH3(g) N N electron of N electron of another N key Hence, ammonia has a higher boiling point than nitrogen and hydrogen.

3 Suggest two reasons why less energy is needed to maintain the temperature for reaction C than for reaction B in page C2 of “Ammonia (Haber Process)” notes. [2] Reaction B: methane reacts with steam to make hydrogen. CH4(g) + H2O(g) ⇌ CO(g) + 3H2(g) ∆H = +210 kJ Reaction C : nitrogen reacts with hydrogen to make ammonia N2(g) + 3H2(g) ⇌ 2NH3(g) ∆H = -92 kJ • The temperature required for reaction C is lower. • Reaction C is exothermic , heat released during the reaction can help to maintain the operating temperature of 450°C. 4 Using ideas related to stoichiometry and Avogadro’s Law, e xplain why the nitrogen and hydrogen are mixed in a ratio of 1:3 by volume. (N2(g) + 3H2(g) ⇌ 2NH3(g)) [1] • From the equation, 1 mole of nitrogen reacts with 3 moles of hydrogen to produce 2 moles of ammonia. • According to Avogadro’s Law, 1 volume of nitrogen will react with 3 volumes of hydrogen when the gases are under the same temperature and pressure. Some students compared by chemical formula of NH3. No of atoms of N: No of atoms of H is 1:3. Therefore, volume is 1:3. Why is this wrong? Another eg. 2CO + O2  2CO2. Why was CO and O2 reacted in a 2:1 volume ratio? Is there any way to explain in terms of chemical formula? Compare by moles of reactants, not of atoms in the chemical formula. 5 Explain why the iron catalyst is finely divided. [1] A finely divided catalyst has a large surface area , leading to a faster rate of reaction. 6 Give two reasons why the unreacted nitrogen and hydrogen are fed back into the converter. [2] • To recycle the nitrogen and hydrogen to save cost • To increase the yield of ammonia Conditions: 30 atm nickel oxide catalyst 800°C Conditions: 200 atm iron catalyst 450°C

Use Diagram 1 on page C3 of “Ammonia (Haber Process)” notes to answer questions 7 and 8. 7 Deduce the effect of the following on the percentage yield of ammonia: [2] (a) increasing the pressure The percentage yield of ammonia increases. (b) increasing the temperature The percentage yield of ammonia decreases. 8 Estimate the percentage yield of ammonia if the conditions are 200 atm and 450°C. [1] Any answer from 25% to 31%

9 Suggest one advantage a nd one disadvantage of using 350 oC rather than 450 oC in manufacturing ammonia. [2] Advantage The percentage yield of ammonia obtained is higher. [how much you get/ the amount] Disadvantage The reaction will be slower. 10 Suggest why the process is not operated at pressures higher than 200 atm. [1] • Operating cost will be higher as expensive equipment such as special pump and stronger pipes are needed. OR • Risk of explosion will be higher 11 Assuming that an overall of 98% of the nitrogen and hydrogen is converted into ammonia, calculate the volume of hydrogen, measured at r.t.p, required to produce 8.50 tonnes of ammonia. (1 tonne = 1 x 106 g) [3] No. of moles of NH3 = mass ÷ molar mass = 8.50 x 106 ÷ (14.0 + 3.0) = 5.00 x 105 mol^ N2(g) + 3H2(g) ⇌ 2NH3(g) No. of moles of H2 = 3/2 x no. of moles of NH3 = 3/2 x 5.00 x 105 = 7.50 x 105 mol^ (^both --------- [1]) Volume of H2 converted to NH3 = no. of moles of H2 x molar volume = 7.50 x 105 x 24 = 1.80 x 107 dm3 [1] Actual volume of H2 used = 1.80 x 107 x 𝟏𝟏𝟏𝟏𝟏𝟏% 𝟗𝟗𝟗𝟗% = 1.84 x 107 dm3 (3 s.f.) [1] 12 With reference to reaction C on page C2 of “Ammonia (Haber Process)” notes, state the enthalpy change (∆H) for the reaction, 2NH3(g) ⇌ N2(g) + 3H2(g). [1] ∆H = + 92 kJ

13 In the laboratory, ammonia is usually prepared by heating an ammonium salt with an alkali. The apparatus set up for the preparation is shown below. Diagram 2 (a) Write the equation for the reaction if ammonium chloride and calcium hydroxide are used to prepare ammonia gas. [1] 2NH4Cl + Ca(OH)2 → CaCl2 + 2H2O + 2NH3 (b) Why is the test tube containing the reagents tilted downwards? [1] This prevents condensed water at the mouth of the test tube from running back to the hot part of the test tube and cracking the glass. (c) Concentrated sulfuric acid (acidic) is a common drying agent, can it be used in place of calcium oxide to dry ammonia gas (alkaline)? Explain your answer. [1] No. Concentrated sulfuric acid reacts with ammonia. H2SO4 + 2NH3 → (NH4)2SO4 (d) Ammonia is less dense than air. Name the method for collecting ammonia as shown in Diagram 2. [1] Upward delivery of gas or downward displacement of air. Upward delivery of gas or downward displacement of air.