Anomalous physicochemical properties
The water molecule has a specific structure that determines its behavior in nature. Its high dissolving capacity is explained by the polarity of $H_2O$ molecules and their tendency to bind with each other via hydrogen bridges.
Among the main anomalies are:
- Density: unlike most substances, water expands during cooling in the range from +4 °C to 0 °C. Upon transitioning to a solid state (ice), its density decreases, which is why ice does not sink.
- Boiling point: for a compound with such a small molecular weight, it is paradoxically high.
- Heat capacity: water is capable of storing colossal amounts of thermal energy. On a global scale, this property regulates the planet's climate (especially in coastal areas) and maintains the stability of biological processes.
Chemical interactions with simple substances
Water actively enters into reactions with both non-metals and metals. The outcome of the reaction depends on the reactivity of the substance and environmental conditions.
Reactions with non-metals:
- Interaction with chlorine results in disproportionation with the formation of hydrochloric and hypochlorous acids: $Cl_2 + H_2O \rightleftarrows HCl + HClO$.
- Heating with carbon produces the so-called synthesis gas (a mixture of carbon monoxide and hydrogen): $C + H_2O \xrightarrow{t} CO + H_2$.
Reactions with metals:
- Active metals (e.g., calcium) react to form an alkali and release gas: $Ca + 2H_2O = Ca(OH)_2 + H_2\uparrow$.
- Moderately active metals (e.g., iron) require heating. The resulting products are a metal oxide and hydrogen: $Fe + H_2O \xrightarrow{t} FeO + H_2$.
Interaction with oxides, acids, and bases
In protolytic reactions, water exhibits a dual nature (amphotericity), acting either as a proton donor or a proton acceptor.
Protolytic reactions:
- With acids: the $H_2O$ molecule acts as a base (acceptor). Example: $HF + H_2O \rightleftarrows F^- + H_3O^+$.
- With bases: water acts as an acid (proton donor). Example: $NH_3 + H_2O \rightleftarrows NH_4^+ + OH^-$.
Reactions with oxides:
- Acidic oxides react to form acids: $SO_3 + H_2O = H_2SO_4$.
- Basic oxides react to form alkalis: $CaO + H_2O = Ca(OH)_2$.
- Non-salt-forming oxides can react upon heating: $CO + H_2O \xrightarrow{t} CO_2 + H_2$.
Reactions with salts and organic compounds
Water plays a crucial role in the transformations of salts and organic molecules.
Interaction with salts:
- Ionization (dissociation): the crystal lattice is disrupted, and ions are hydrated. $NaCl_{(cr)} + (m + n)H_2O = Na^+ \cdot mH_2O + Cl^- \cdot nH_2O$.
- Hydration: water molecules embed into the structure to form crystalline hydrates: $MgSO_4 + 7H_2O = MgSO_4 \cdot 7H_2O$.
- Hydrolysis: salt ions enter into an exchange reaction with water: $Na_2CO_3 + H_2O \rightleftarrows NaHCO_3 + NaOH$.
Organic substances and photolysis:
- Water can add across multiple bonds of organic molecules (hydration). For example, ethylene yields ethanol: $C_2H_4 + H_2O \xrightarrow{t, p} C_2H_5OH$.
- Under the influence of light ($h\nu$), photochemical decomposition of water occurs with the formation of active radicals: $H_2O = OH + H$.
Briefly: answers to topical questions
What is the mechanism of water autoprotolysis, and what is its ion product?
The mechanism of water autoprotolysis involves the transfer of a proton from one water molecule to another, forming a hydronium ion ($H_3O^+$) and a hydroxide ion ($OH^-$). The ion product of water ($K_w$) at standard temperature (25 °C) is $1 \cdot 10^{-14} \text{ mol}^2/\text{l}^2$. This is a constant value representing the product of the concentrations of hydrogen ions and hydroxide ions in water and dilute aqueous solutions.
What is the spatial geometry of the water molecule, and what is the H-O-H bond angle?
The spatial geometry of the water molecule is bent (angular). The bond angle between the $O-H$ bonds in the molecule is $104.5^\circ$. This asymmetrical arrangement of atoms determines the polarity of the water molecule. Various models are used for the graphical representation of its spatial structure: space-filling, deformed, ball-and-stick, and structural formula.
What types of salt hydrolysis are distinguished depending on the strength of their constituent acids and bases?
Depending on the strength of the acids and bases, three main types of salt hydrolysis are distinguished:
- Anion hydrolysis — occurs in salts formed by a weak acid and a strong alkali (the medium reaction is alkaline).
- Cation hydrolysis — characteristic of salts formed by a strong acid and a weak base (the medium reaction is acidic).
- Cation and anion hydrolysis — occurs in salts formed by a weak base and a weak acid (the medium is approximately neutral, and hydrolysis is most extensive).
Salts formed by cations of strong bases and anions of strong acids do not undergo hydrolysis.
To avoid confusing the role of water in protolytic reactions, remember: water always behaves opposite to its partner. With an acid, it acts as a base (accepts a proton), and with a base, it acts as an acid (donates a proton).
Class and exam questions
Why does ice float on the surface of liquid water?
In the temperature range from +4 °C to 0 °C upon cooling, water expands anomalously. Because of this, the solid phase (ice) has a lower density than the liquid phase and is lighter.
Due to what does water conduct electric current?
Electrical conductivity is provided by autoprotolysis, a reaction in which water molecules exchange a proton with each other.
What is formed during the reaction of water with carbon upon heating?
Synthesis gas is formed, which is a mixture of carbon monoxide ($CO$) and hydrogen gas ($H_2$).
What else to study on this topic
- Mechanism of hydrogen bond formation between $H_2O$ molecules
- Ion hydration processes and disruption of crystal lattices
- Quantitative characteristics of water autoprotolysis
- Patterns of hydrolysis for various classes of salts
- Photochemical decomposition of water and radical reactions
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