8.6: Some Common Reducing Agents

Learning Objectives

• Know the occurrence and properties of hydrogen.
• Know reactions involving common reducing agents such as carbon, hydrogen, and antioxidants.

Metal Extraction

Smelting is a process of applying heat to ore in order to extract a base metal. It uses heat and a chemical reducing agent to decompose the ore, leaving the metal base behind. It is used to extract many metals from their ores, including silver, iron, copper, and other base metals. The reducing agent is commonly a source of carbon, such as coke—or, in earlier times, charcoal. Coke is a grey, hard, and porous fuel with a high carbon content and few impurities, made by heating coal or oil in the absence of air.

Coke is used in smelting iron ore for example in a blast furnace. The reaction can be represented as follows:

$2Fe_2O_3 + 3C → 4 Fe + 3 CO_2$

Antioxidants

In food chemistry, the substances known as antioxidants are reducing agents. Ascorbic acid (vitamin C; $$\ce{C6H8O6}$$) is thought to retard potentially damaging oxidation of living cells. In the process, it is oxidized to dehydroascorbic acid ($$\ce{C6H6O6}$$). In the stomach, ascorbic acid reduces the nitrite ion ($$\ce{NO_2^{−}}$$) to nitric oxide ($$\ce{NO}$$):

$\ce{C_6H_8O_6 + 2H^{+} + 2NO_2^{−} \rightarrow C_6H_6O_6 + 2H_2O + 2NO} \label{Eq7}$

If reaction in Equation $$\ref{Eq7}$$ did not occur, nitrite ions from foods would oxidize the iron in hemoglobin, destroying its ability to carry oxygen.

Tocopherol (vitamin E) is also an antioxidant. In the body, vitamin E is thought to act by scavenging harmful by-products of metabolism, such as the highly reactive molecular fragments called free radicals. In foods, vitamin E acts to prevent fats from being oxidized and thus becoming rancid. Vitamin C is also a good antioxidant (Figure $$\PageIndex{2}$$).

Hydrogen as a Reducing Agent

Large quantities of H2 are needed in the petroleum and chemical industries. The largest application of H2 is for the processing ("upgrading") of fossil fuels, and in the production of ammonia. Mass production of Ammonia mostly uses the Haber–Bosch process, reacting hydrogen (H2) and nitrogen (N2) at a moderately-elevated temperature (450 °C) and high pressure (100 standard atmospheres (10,000 kPa)):[

$3 H_2(g)+ N_2(g) → 2 NH_3(g)$

H2 has several other important uses. H2 is used as a hydrogenating agent (Chapter 17), particularly in increasing the level of saturation of unsaturated fats and oils (found in items such as margarine), and in the production of methanol. It is similarly the source of hydrogen in the manufacture of hydrochloric acid. H2 is also used as a reducing agent of metallic ores.

Hydrogen gas is highly flammable producing a large amount of heat when it reacts with oxygen gas as shown below.

$2 H_2(g) + O_2(g) → 2 H_2O(l) + 572 kJ$

Hydrogen gas forms explosive mixtures with air in concentrations from 4–74%[16] and with chlorine at 5–95%. The explosive reactions may be triggered by spark, heat, or sunlight.

A Closer Look at Hydrogen

Hydrogen is among the ten most abundant elements on the planet, but very little is found in elemental form due to its low density and reactivity. Much of the terrestrial hydrogen is locked up in water molecules and organic compounds like hydrocarbons.

Hydrogen is the fuel for reactions of the Sun and other stars (fusion reactions). Hydrogen is the lightest and most abundant element in the universe. About 70%- 75% of the universe is composed of hydrogen by mass. All stars are essentially large masses of hydrogen gas that produce enormous amounts of energy through the fusion of hydrogen atoms at their dense cores. In smaller stars, hydrogen atoms collided and fused to form helium and other light elements like nitrogen and carbon(essential for life). In the larger stars, fusion produces the lighter and heavier elements like calcium, oxygen, and silicon.

On Earth, hydrogen is mostly found in association with oxygen; its most abundant form being water (H2O). Hydrogen is only .9% by mass and 15% by volume abundant on the earth, despite water covering about 70% of the planet. Because hydrogen is so light, there is only 0.5 ppm (parts per million) in the atmosphere, which is a good thing considering it is EXTREMELY flammable.

Hydrogen gas can be prepared by reacting a dilute strong acid like hydrochloric acids with an active metal. The metal becomes oxides, while the H+ (from the acid) is reduced to hydrogen gas. This method is only practical for producing small amounts of hydrogen in the lab, but is much too costly for industrial production:

$Zn_{(s)} + 2H^+_{(aq)} \rightarrow Zn^{2+}_{(aq)} + H_{2(g)}$

The purest form of H2(g) can come from electrolysis of H2O(l), the most common hydrogen compound on this plant. This method is also not commercially viable because it requires a significant amount of energy ($$\Delta H = 572 \;kJ$$):

$2H_2O_{(l)} \rightarrow 2H_{2(g)} + O_{2(g)}$

$$H_2O$$ is the most abundant form of hydrogen on the planet, so it seems logical to try to extract hydrogen from water without electrolysis of water. To do so, we must reduce the hydrogen with +1 oxidation state to hydrogen with 0 oxidation state (in hydrogen gas).

Three commonly used reducing agents are carbon (in coke or coal), carbon monoxide, and methane. These react with water vapor form H2(g):

$C_{(s)} + 2H_2O_{(g)} \rightarrow CO(g) + H_{2(g)}$

$CO_{(g)} + 2H_2O_{(g)} \rightarrow CO2 + H_{2(g)}$

Reforming of Methane:

$CH_{4(g)} + H_2O_{(g)} \rightarrow CO(g) + 3H_{2(g)}$

These three methods are most industrially feasible (cost effective) methods of producing H2(g).

Summary

• Common reducing agents include carbon (in the form of coke or coal), hydrogen gas, as well as those substances referred to in the food chemistry as antioxidants (e.g. ascorbic acid and vitamin E).
• Various reactions involving reducing agents were given.