Question

Which equations represent chemical reactions: (a) \(\mathrm{CO}_{2}(\mathrm{~s}) \rightarrow \mathrm{CO}_{2}(g)\) (b) \(3 \mathrm{O}_{2}(g) \rightarrow 2 \mathrm{O}_{3}(g)\) (c) \(\mathrm{NaCl}(s)+\mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{NaOH}(a q)+\mathrm{HCl}(a q)\) (d) \(\mathrm{H}_{2} \mathrm{O}(g)+\mathrm{CH}_{4}(g) \rightarrow \mathrm{CH}_{4}(g)+\mathrm{H}_{2} \mathrm{O}(g)\)

Short answer

In summary, equations \(b\) (\(3 \mathrm{O}_{2}(g) \rightarrow 2 \mathrm{O}_{3}(g)\)) and \(c\) (\(\mathrm{NaCl}(s)+\mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{NaOH}(a\mathrm{q})+\mathrm{HCl}(a \mathrm{q})\)) represent chemical reactions, as they involve the formation of new substances with different chemical properties. Equations \(a\) (\(\mathrm{CO}_{2}(\mathrm{~s}) \rightarrow \mathrm{CO}_{2}(g)\)) and \(d\) (\(\mathrm{H}_{2} \mathrm{O}(g)+\mathrm{CH}_{4}(g) \rightarrow \mathrm{CH}_{4}(g)+\mathrm{H}_{2} \mathrm{O}(g)\)) do not represent chemical reactions, as they involve only physical changes or simple mixing of substances.

Step-by-step solution

(a) Sublimation of CO2

In the given equation, \(\mathrm{CO}_{2}(\mathrm{~s}) \rightarrow \mathrm{CO}_{2}(g)\), solid carbon dioxide (dry ice) is changing phase from solid to gas. This change in state, known as sublimation, is a physical change, and there's no new substance being formed. So, this equation doesn't represent a chemical reaction.

(b) Formation of ozone

In the given equation, \(3 \mathrm{O}_{2}(g) \rightarrow 2 \mathrm{O}_{3}(g)\), we see that oxygen gas (O₂) is being transformed into ozone gas (O₃). Since the reactant (oxygen) and the product (ozone) have different chemical properties, this equation represents a chemical reaction.

(c) Reaction between NaCl and H₂O

In the given equation, \(\mathrm{NaCl}(s)+\mathrm{H}_{2} \mathrm{O}(l) \rightarrow \mathrm{NaOH}(a\mathrm{q})+\mathrm{HCl}(a \mathrm{q})\), we have sodium chloride (NaCl) and water (H₂O) reacting to form sodium hydroxide (NaOH) and hydrochloric acid (HCl). Since new substances with distinct chemical properties are formed, this equation represents a chemical reaction.

(d) Mixing of water vapor and methane

In the given equation, \(\mathrm{H}_{2} \mathrm{O}(g)+\mathrm{CH}_{4}(g) \rightarrow \mathrm{CH}_{4}(g)+\mathrm{H}_{2} \mathrm{O}(g)\), there is no change in the substances present – water vapor (H₂O) and methane (CH₄) remain unchanged on both sides. This equation represents a simple mixing of gases, and not a chemical reaction since there's no new substance being formed. In summary, option (b) and (c) represent chemical reactions, while option (a) and (d) do not.

Key concepts

Phase Changes

Phase changes involve the transformation of a substance from one state of matter to another. These include the transitions between solid, liquid, and gas.
In the exercise example of

• Sublimation, which is a direct transition from solid to gas, as demonstrated by carbon dioxide (\(\mathrm{CO}_2\mathrm{(s)} \rightarrow \mathrm{CO}_2\mathrm{(g)}\)) disappearing from dry ice form to become a gas without going through a liquid phase.

This transformation is a physical change because the chemical identity of the substance doesn't change, only its physical form.
Importantly, phase changes can be reversed, showing that the processes are physical, not chemical.
Phase changes are crucial for various applications, including refrigeration and the natural water cycle, where processes like freezing, melting, evaporation, condensation, and sublimation play key roles.

Ozone Formation

Ozone (\(\mathrm{O}_3\)) is a molecule composed of three oxygen atoms and is found in two main areas on Earth: the stratosphere and troposphere.
Its formation is a chemical reaction where oxygen gas (\(\mathrm{O}_2\mathrm{(g)}\)) is converted into ozone (\(\mathrm{O}_3\mathrm{(g)}\)).
The exercise provides an example:

• This reaction, \(3\mathrm{O}_2(\mathrm{~g}) \rightarrow 2\mathrm{O}_3(\mathrm{~g})\), represents a change where the molecular structures and properties of oxygen evolve from a diatomic to a triatomic form.

This transformation requires energy input, such as ultraviolet light, and highlights the chemical change because the end product has different characteristics and molecular structure from the reactants.
Ozone plays an essential role in absorbing harmful UV radiation in the stratosphere, but it can be a pollutant at ground level.

Sublimation

Sublimation is the process where a solid transforms directly into a gas without passing through the liquid state.
This process is best illustrated by substances like dry ice, which is solid carbon dioxide.
In the equation

• \(\mathrm{CO}_2\mathrm{(s)} \rightarrow \mathrm{CO}_2\mathrm{(g)}\), where solid \(\mathrm{CO}_2\) sublimates into gaseous \(\mathrm{CO}_2\), you witness a phase change.

The primary feature of sublimation is that the original chemical identity of the substance remains unchanged, which makes it a physical change.
Applications of sublimation include freeze-drying of food and materials, facilitating storage and preservation without the use of liquid phases.

Physical and Chemical Changes

Physical and chemical changes are two distinct processes in the transformation of substances.
A physical change involves changes in the state or appearance of a substance without altering its chemical composition.

• An example is the physical change of water from liquid to ice or steam, where the water molecules remain \(\mathrm{H}_2\mathrm{O}\) regardless of the state.

Chemical changes, however, involve the rearrangement of atoms and the formation of new substances with different chemical properties.
An example from the exercise is the equation

• \(\mathrm{NaCl}(s) + \mathrm{H}_2\mathrm{O}(l) \rightarrow \mathrm{NaOH}(a\mathrm{q}) + \mathrm{HCl}(a\mathrm{q})\), where new molecules \(\mathrm{NaOH}\) and \(\mathrm{HCl}\) with different properties are formed.

Recognizing the difference is crucial for understanding chemical processes and reactions, which are integral to everything from cooking to the functioning of the human body.