# 11.2 Nitrification and Denitrification This section explores two critical, opposing processes in the nitrogen cycle: nitrification (oxidation of ammonia to nitrate) and denitrification (reduction of nitrate to nitrogen gas). ## 11.2.1 Nitrification Nitrification is a two-step **oxidation process** that is a crucial stage in the nitrogen cycle. It involves the conversion of ammonium ions into nitrates by specialized microorganisms. **Definition:** The biological oxidation of ammonium ($NH_4^+$) to nitrite ($NO_2^-$), followed by the oxidation of nitrite to nitrate ($NO_3^-$). **Bacteria:** The process is carried out by nitrifying bacteria. *Nitrosomonas* converts ammonia into nitrites. Other bacteria, such as *Nitrobacter*, then convert nitrites to nitrates. **Significance:** This is the first and essential stage of the nitrogen cycle, making nitrogen available to plants in a usable form (nitrates). **Chemical Reactions** The overall process can be broken down into two main steps: 1. **Oxidation of Ammonium to Nitrite:** $$2NH_4^+ + 3O_2 \rightarrow 2NO_2^- + 4H^+ + 2H_2O$$ 2. **Oxidation of Nitrite to Nitrate:** $$2NO_2^- + O_2 \rightarrow 2NO_3^-$$ **Change in Oxidation State** During nitrification, the oxidation state of nitrogen increases from -3 to +5: - In $NH_4^+$, nitrogen's oxidation state is **-3**. - In $NO_2^-$, nitrogen's oxidation state is **+3**. - In $NO_3^-$, nitrogen's oxidation state is **+5**. ## 11.2.2 Denitrification Denitrification is a biological **reduction process** that completes the nitrogen cycle by returning nitrogen gas to the atmosphere. **Definition:** The microbial process of reducing nitrate and nitrite into gaseous molecular nitrogen ($N_2$). **Conditions:** This process is carried out by denitrifying bacteria in anoxic (oxygen-deficient) conditions. **Bacteria:** Common denitrifying bacterial species include *Clostridium* and *Pseudomonas*. **Significance:** This is the final stage of the nitrogen cycle, returning inert nitrogen gas to the atmosphere and preventing excessive nitrate buildup in ecosystems. **Overall Chemical Reaction** The overall reduction reaction is: $$2NO_3^- + 10e^- + 12H^+ \rightarrow N_2 + 6H_2O$$ ## Possible Questions/Answers This section is based on the "Concept Assessment Exercise 11.2". **1.** In many countries, new cars have to comply with regulations which are intended to reduce pollutants from their internal combustion engines. Two pollutants that may be formed are carbon monoxide, CO, and nitrogen monoxide, NO. **(a)(i) Outline how each of these pollutants may be formed in an internal combustion engine.** - **Carbon Monoxide (CO):** Formed from the incomplete combustion of hydrocarbon fuels when the oxygen supply is insufficient. - **Nitrogen Monoxide (NO):** Formed when nitrogen and oxygen gases react under the high temperature and pressure conditions inside the engine. $$N_2(g) + O_2(g) \rightleftharpoons 2NO(g)$$ **(a)(ii) State the main hazard associated with each of these pollutants.** - **CO:** It is a toxic gas that binds irreversibly to hemoglobin in red blood cells, reducing their ability to transport oxygen. - **NO:** It contributes to the formation of acid rain and photochemical smog. **(b)(i) What metal is most commonly used as the catalyst in a catalytic converter?** Platinum (Pt), Palladium (Pd), and Rhodium (Rh) are commonly used. **(b)(ii) Construct one balanced equation for the reaction in which both CO and NO are removed from the exhaust gases by a catalytic converter.** $$2CO(g) + 2NO(g) \xrightarrow{catalyst} 2CO_2(g) + N_2(g)$$ **2. Differentiate between nitrification and denitrification.** | Feature | Nitrification | Denitrification | | :--- | :--- | :--- | | **Process Type** | Oxidation | Reduction | | **Starting Compound** | Ammonium ($NH_4^+$) | Nitrate ($NO_3^-$) | | **End Product** | Nitrate ($NO_3^-$) | Nitrogen Gas ($N_2$) | | **Oxygen Requirement** | Aerobic (Requires Oxygen) | Anaerobic (Absence of Oxygen) | | **Nitrogen State** | Converts N from a reduced state (-3) to an oxidized state (+5). | Converts N from an oxidized state (+5) back to its elemental state (0). | | **Function in Cycle** | Makes nitrogen available to plants. | Returns nitrogen to the atmosphere. |