Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Hydrogen
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Hydrogen
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Subject: Chemistry
Class: Senior Secondary 2
Term: 1st Term
Week: 2
Theme: Chemistry And Environment
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Hydrogen (H) is the simplest element.
Atomic Number (Z):
1. This means it has 1 proton in its nucleus.
Mass Number (A):
1. This means its most common isotope has 1 proton and 0 neutrons.
Electron Count: In a neutral atom, the number of electrons equals the number of protons, so hydrogen has 1 electron.
Electron Configuration: The single electron of hydrogen occupies the first energy shell (K shell) or the 1s subshell.
Shell notation: K1 Orbital notation: 1s1 Drawing the Electron Configuration: To draw, represent the nucleus and the electron shell(s). Draw a central circle representing the nucleus, indicating "1p" (1 proton) and "0n" (0 neutrons for protium).
Laboratory preparations typically focus on small-scale, convenient generation. Reaction of Dilute Acids with Electropositive Metals: Principle: More reactive metals (above hydrogen in the electrochemical series) displace hydrogen from dilute mineral acids.
Common Reactants: Zinc (granulated) and dilute hydrochloric acid (HCl) or dilute sulphuric acid (H2SO4). Magnesium (Mg) can also be used.
Apparatus: Kipp's apparatus (for continuous supply) or a conical flask with a thistle funnel and delivery tube.
Collection: Downward displacement of water: Hydrogen is sparingly soluble in water, so this is a common method. Upward delivery/downward displacement of air: Hydrogen is lighter than air, so it rises and displaces air downwards.
Equations: Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g) Mg(s) + H2SO4(aq) → MgSO4(aq) + H2(g)
Procedure: Granulated zinc is placed in the flask. Dilute HCl or H2SO4 is added gradually through the thistle funnel. Effervescence (gas evolution) is observed. The gas is passed through a wash bottle containing water to remove acid spray, then collected.
Note: Iron can be used but reacts slowly. Less reactive metals like copper will not react. Dilute nitric acid is generally not used because it acts as an oxidising agent, producing oxides of nitrogen instead of hydrogen.
Electrolysis of Acidified Water: Principle: Passing an electric current through water breaks it down into hydrogen and oxygen. Acid (e.g., dilute H2SO4) is added to increase conductivity, as pure water is a poor conductor.
Apparatus: Hoffman voltameter or a simple setup with electrodes (e.g., platinum or carbon) immersed in acidified water.
Reactions: Overall: 2H2O(l) \xrightarrow{\text{electrolysis}} 2H2(g) + O2(g)
At Cathode (negative electrode): 2H+(aq) + 2e− → H2(g) (or 2H2O + 2e− → H2 + 2OH−)
At Anode (positive electrode): 4OH−(aq) → 2H2O(l) + O2(g) + 4e− (or 2H2O → O2 + 4H+ + 4e−)
Observation: Hydrogen gas collects at the cathode (negative electrode) in twice the volume of oxygen gas collected at the anode (positive electrode). Reaction of Amphoteric Metals with Strong Alkalis: Principle: Certain metals that can react with both acids and bases (amphoteric metals) can displace hydrogen from strong solutions of alkalis (like NaOH or KOH).
Common Reactants: Aluminium (Al) or Zinc (Zn) with concentrated sodium hydroxide (NaOH) solution.
Equations: 2Al(s) + 2NaOH(aq) + 6H2O(l) → 2Na[Al(OH)4](aq) + 3H2(g) (Sodium tetrahydroxoaluminate(III)) Zn(s) + 2NaOH(aq) → Na2ZnO2(aq) + H2(g) (Sodium zincate)
Note: This method is less common for general lab preparation due to the corrosive nature of strong alkalis. Industrial production focuses on large-scale, cost-effective generation.
Steam Reforming of Natural Gas (Methane): Principle: Methane (the main component of natural gas) reacts with steam at high temperatures over a catalyst. This is the most common industrial method globally.
Equation: CH4(g) + H2O(g) \xrightarrow{\text{Ni catalyst, 700-1100°C}} CO(g) + 3H2(g)
Products: This reaction produces "synthesis gas" or "syngas" (a mixture of CO and H2). Further Hydrogen Production (Water-Gas Shift Reaction): To increase hydrogen yield and remove carbon monoxide (which can poison catalysts), the CO is further reacted with steam: CO(g) + H2O(g) \xrightarrow{\text{Fe/Cr catalyst, ~400°C}} CO2(g) + H2(g) The CO2 is then removed, typically by absorption in solutions like potassium carbonate. Electrolysis of Brine (Chlor-Alkali Process): Principle: Electrolysis of a concentrated aqueous solution of sodium chloride (brine) produces chlorine gas, sodium hydroxide, and hydrogen gas.
Equation: 2NaCl(aq) + 2H2O(l) \xrightarrow{\text{electrolysis}} 2NaOH(aq) + Cl2(g) + H2(g)
Significance: Hydrogen is a co-product in this process, which is primarily designed for the production of chlorine and sodium hydroxide, both important industrial chemicals. From Cracking of Hydrocarbons (Petroleum Refining): Principle: In the petroleum refining industry, larger hydrocarbon molecules are broken down (cracked) into smaller, more valuable molecules. Hydrogen is often produced as a byproduct in these processes.
Example: C10H22 (decane) → C5H10 (pentene) + C5H12 (pentane) + H2 (this is a simplified example, actual cracking produces various hydrocarbons and hydrogen).
From Water Gas/Syngas: (Already covered partly under steam reforming, but can be distinct when starting from coal gasification)
Production of Water Gas: Red-hot coke reacts with steam. C(s) + H2O(g) \xrightarrow{\text{1000°C}} CO(g) + H2(g) (Water gas)
Separation: Hydrogen is then separated from CO using various methods, e.g., pressure swing adsorption (PSA) or cryogenic distillation. The water-gas shift reaction can also be employed as described above.
State: Gaseous at room temperature and pressure. Colour, Odour, Taste: Colourless, odourless, and tasteless gas.
Density: Lightest known substance. Its density is 0.08988 g/L at STP, which is significantly less than air (average 1.29 g/L).
Solubility: Sparingly soluble in water.
Boiling Point: -252.87 °C Melting Point: -259.16 °C (extremely low due to very weak intermolecular forces)
Thermal Conductivity: High thermal conductivity.
Diffusivity: Diffuses rapidly through porous materials and even some metals due to its small molecular size.
Toxicity: Non-toxic. Hydrogen has a single electron and can achieve stability by either losing that electron (forming H+) or gaining one electron (forming H−). It is relatively unreactive at room temperature but becomes highly reactive when heated or in the presence of catalysts.
Combustibility: Burns with a pale blue, almost invisible flame in air or oxygen.
Equation: 2H2(g) + O2(g) → 2H2O(l)
Explosive Mixture: A mixture of hydrogen and air (or oxygen) is highly explosive. This is why hydrogen-filled balloons are dangerous. (e.g., Hindenburg disaster). The reaction is highly exothermic.
Reducing Agent: Hydrogen acts as a reducing agent, especially at high temperatures. It can reduce oxides of less reactive metals (those below hydrogen in the electrochemical series) and some non-metallic oxides.
Examples: CuO(s) + H2(g) \xrightarrow{\text{heat}} Cu(s) + H2O(g) (Reduces black copper(II) oxide to red-brown copper metal) Fe2O3(s) + 3H2(g) \xrightarrow{\text{heat}} 2Fe(s) + 3H2O(g) WO3(s) + 3H2(g) \xrightarrow{\text{heat}} W(s) + 3H2O(g) (Used in the extraction of tungsten)
Reaction with Halogens (Halogenation): Reacts with halogens to form hydrogen halides.
Chlorine: H2(g) + Cl2(g) \xrightarrow{\text{light or heat}} 2HCl(g) (Explosive in direct sunlight)
Bromine: H2(g) + Br2(g) \xrightarrow{\text{heat}} 2HBr(g) (Slower reaction)
Iodine: H2(g) + I2(g) \xrightarrow{\text{heat, catalyst}} 2HI(g) (Reversible reaction)
Fluorine: Reacts explosively even in the dark and at low temperatures.
Reaction with Nitrogen (Haber Process): Reacts with nitrogen under specific conditions (high temperature, high pressure, iron catalyst) to form ammonia.
Equation: N2(g) + 3H2(g) \xrightarrow{\text{Fe catalyst, 400-500°C, 150-350 atm}} 2NH3(g) This is a crucial industrial process.
Hydrogenation of Unsaturated Compounds: Addition of hydrogen across double or triple bonds in organic molecules, usually in the presence of a catalyst (e.g., Ni, Pt, Pd).
Example (Oils to Fats): Used to convert unsaturated vegetable oils (liquid) into saturated fats (solid or semi-solid), like in the production of margarine. R-CH=CH-R' (unsaturated oil) + H2(g) \xrightarrow{\text{Ni catalyst, heat}} R-CH2-CH2-R' (saturated fat)
Formation of Hydrides: With highly electropositive metals (Group 1 and Group 2, except Be and Mg), it forms ionic hydrides (e.g., NaH, CaH2), where hydrogen exists as H− (hydride ion). 2Na(s) + H2(g) \xrightarrow{\text{heat}} 2NaH(s) With non-metals, it forms covalent hydrides (e.g., CH4, NH3, H2O, HCl).
Agriculture and Food Security (Nigeria): Application: Hydrogen is a key reactant in the Haber process for producing ammonia (N2 + 3H2 → 2NH3). Ammonia is then processed into nitrogenous fertilisers like urea and ammonium sulphate.
Relevance to Nigeria: Agriculture is a cornerstone of the Nigerian economy. The availability and affordability of fertilisers directly impact crop yields, food production, and food security. Understanding hydrogen's role helps learners appreciate the chemical basis of modern agricultural practices and national economic development.
Integration: Discuss the types of crops grown in Nigeria that benefit from nitrogenous fertilisers (e.g., maize, rice, yam, cassava) and the government's initiatives to boost fertiliser production. Energy Transition and Climate Change (Global and Nigeria's Future): Application: Hydrogen is being increasingly considered as a clean energy carrier, especially for fuel cells. Fuel cells combine hydrogen and oxygen to produce electricity, with water as the only byproduct, offering a zero-emission alternative to fossil fuels.
Relevance to Nigeria: Nigeria, as a major oil and gas producer, is also grappling with environmental issues and the need for sustainable energy solutions. Exploring hydrogen as a future fuel (e.g., for vehicles, power generation) aligns with global efforts to combat climate change and could provide a pathway for Nigeria's energy diversification and economic stability in a post-fossil fuel era.
Integration: Discuss the concept of a "hydrogen economy," comparing hydrogen with traditional fuels (petrol, diesel) in terms of emissions and efficiency. Explore potential for Nigeria to produce "green hydrogen" from its abundant renewable energy resources (solar, hydro).
Food Processing and Industry (Nigeria): Application: Hydrogenation, the addition of hydrogen to unsaturated fats and oils, is crucial in the production of margarine and other solid cooking fats from liquid vegetable oils.
Relevance to Nigeria: Margarine and cooking oils are staple food items in Nigerian households. Understanding the chemical process of hydrogenation reveals how chemistry contributes to transforming raw agricultural products into marketable consumer goods, impacting the local food industry and consumer choices.
Integration: Discuss common Nigerian cooking oils (e.g., palm oil, groundnut oil, soybean oil) and how hydrogenation alters their properties to create different food products. ---