Unit 12: Hydrogen, Oxygen & Their Compounds
Understanding the chemistry of the two most fundamental elements for life and their important compounds.
12.17 Hydrogen (H₂) & Hydrogen Ions (H⁺)
Hydrogen Gas (H₂)
Hydrogen is a diatomic gas ($H_2$) with a simple molecular structure. It is the lightest of all elements, making it much less dense than air. It is highly flammable and reacts explosively with oxygen to form water. The characteristic test for hydrogen gas is that it extinguishes a lit splint with a "squeaky pop" sound.
Hydrogen Ions (H⁺)
The hydrogen ion ($H^+$) is a proton. Its presence in an aqueous solution is what defines an
acid. Strong acids dissociate completely to release a high concentration of
H⁺ ions. These ions are responsible for the characteristic reactions of acids, such as
turning blue litmus paper red and reacting with reactive metals to produce hydrogen gas.
$$ Zn(s) + 2H^+(aq) \rightarrow Zn^{2+}(aq) + H_2(g) $$
Solved Examples:
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What is the test for hydrogen gas?
Solution: Place a lit splint into a test tube of the gas. If hydrogen is present, it will extinguish the splint with a distinctive "squeaky pop" sound. -
Why is hydrogen gas no longer used in airships?
Solution: Due to its high flammability and the risk of explosion, as famously demonstrated by the Hindenburg disaster. It has been replaced by the non-flammable noble gas, helium.
12.18 Hydroxide Ions (OH⁻)
The hydroxide ion ($OH^-$) is a polyatomic ion. Its presence in an aqueous solution is what defines an alkali (a soluble base). Strong alkalis, like sodium hydroxide (NaOH), dissociate completely in water to release a high concentration of OH⁻ ions. These ions are responsible for the characteristic properties of alkalis, such as turning red litmus paper blue and feeling soapy to the touch.
Solved Examples:
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What is the difference between a base and an alkali?
Solution: A base is a substance that can neutralize an acid. An alkali is a base that is soluble in water and produces hydroxide ions ($OH^-$) in solution. All alkalis are bases, but not all bases are alkalis.
12.19 Hydrogen Peroxide (H₂O₂)
Hydrogen peroxide ($H_2O_2$) is another oxide of hydrogen. In this molecule, oxygen has an unusual oxidation state of -1. It is a powerful oxidising agent and is known for its bleaching properties, used in hair dyes and for whitening teeth.
Hydrogen peroxide is unstable and slowly decomposes into water and oxygen. This
decomposition is greatly accelerated by a catalyst, such as manganese(IV) oxide ($MnO_2$).
This reaction is a convenient way to produce oxygen gas in the laboratory.
$$ 2H_2O_2(aq) \xrightarrow{MnO_2} 2H_2O(l) + O_2(g) $$
Solved Examples:
-
What is the role of manganese(IV) oxide in the decomposition of hydrogen
peroxide?
Solution: It acts as a catalyst, speeding up the rate of decomposition without being consumed in the reaction. -
What is the oxidation state of oxygen in hydrogen peroxide?
Solution: -1.
12.20 Oxygen (O₂) (Preparation, Uses, Combustion)
Oxygen exists as a diatomic gas ($O_2$) and makes up about 21% of the atmosphere. It is essential for aerobic respiration and is a key reactant in combustion (burning) reactions. The standard test for oxygen is that it will relight a glowing splint.
Industrial Preparation
Oxygen is prepared industrially by the fractional distillation of liquid air. Air is cooled to about -200 °C until it liquefies. The liquid air is then warmed slowly in a fractionating column. Nitrogen has a lower boiling point (-196 °C) than oxygen (-183 °C), so it boils first and is collected at the top of the column, leaving behind liquid oxygen.
Combustion
Combustion is a rapid reaction between a substance with an oxidant, usually oxygen, to produce heat and light. For a fire to occur, three things are needed: fuel, oxygen, and heat (the fire triangle). Removing any one of these will extinguish the fire.
Solved Examples:
-
Why does nitrogen boil before oxygen during the fractional distillation of
liquid air?
Solution: Nitrogen has a lower boiling point (-196 °C) than oxygen (-183 °C). -
A firefighter sprays water on a wood fire. Which part of the fire triangle
is being removed?
Solution: The water absorbs a large amount of heat as it turns to steam, thus removing the heat from the triangle and extinguishing the fire.
12.21 Oxides of Other Elements (Acidic, Basic, Amphoteric, Neutral)
Oxides can be classified based on their acid-base properties:
- Basic Oxides: These are typically oxides of metals. They react with acids to form a salt and water. Most are insoluble in water, but those that do dissolve form alkaline solutions (e.g., $CaO + H_2O \rightarrow Ca(OH)_2$). Example: Copper(II) oxide (CuO).
- Acidic Oxides: These are typically oxides of non-metals. They react with bases to form a salt and water. Many dissolve in water to form acids (e.g., $SO_3 + H_2O \rightarrow H_2SO_4$). Example: Carbon dioxide (CO₂).
- Amphoteric Oxides: These oxides can react with both acids and strong bases. Examples include aluminum oxide ($Al_2O_3$) and zinc oxide (ZnO).
- Neutral Oxides: These non-metal oxides do not react with either acids or bases. Examples include carbon monoxide (CO) and dinitrogen monoxide (N₂O).
Solved Examples:
-
Classify sulfur dioxide ($SO_2$) as acidic, basic, amphoteric, or
neutral.
Solution: Sulfur is a non-metal, so its oxide, sulfur dioxide, is an acidic oxide. -
Write an equation for the reaction of the basic oxide, magnesium oxide
(MgO), with hydrochloric acid (HCl).
Solution: $MgO(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2O(l)$ - What is the defining characteristic of an amphoteric
oxide?
Solution: It can react with and be neutralized by both acids and strong bases.