Problem 11 - Introductory Chemistry Atoms First

Question

Which of the following represent a chemical transformation? (a) \(4 \mathrm{P}(s)+5 \mathrm{O}_{2}(g) \rightarrow 2 \mathrm{P}_{2} \mathrm{O}_{5}(s)\) (b) \(\mathrm{H}_{2} \mathrm{O}(\mathrm{g}) \rightarrow \mathrm{H}_{2} \mathrm{O}(l)\) (c) \(3 \mathrm{O}_{2} \rightarrow 2 \mathrm{O}_{3}\) Answer: (a) and (c) because the products are different from the reactants; (b) is a physical change because we have water on both sides of the arrow.

Step-by-Step Solution

Answer

Answer: Reactions (a) and (c) represent chemical transformations because the products have different chemical compositions than the reactants. Reaction (b) is a physical change because only the state of matter, not the chemical composition, changes.

Step 1: Reaction (a)

In this reaction, we have solid phosphorous (P) reacting with oxygen gas (O₂) to form solid phosphorous pentoxide (P₂O₅): \(4 P(s) + 5 O_2(g) \rightarrow 2 P_2O_5(s)\) Here, solid phosphorous and oxygen gas are the reactants, and solid phosphorous pentoxide is the product. The product P₂O₅ has a different chemical composition than the reactants, so this is a chemical transformation.

Step 2: Reaction (b)

In this reaction, we have water vapor (H₂O) condensing to form liquid water (H₂O): \(H_2O(g) \rightarrow H_2O(l)\) In this case, both the reactants and the products are chemically identical (H₂O). Only the state of matter changes from gas to liquid. This is a physical change and not a chemical transformation.

Step 3: Reaction (c)

In this reaction, we have oxygen gas (O₂) reacting to form ozone (O₃): \(3 O_2 \rightarrow 2 O_3\) Here, oxygen gas (O₂) is the reactant, and ozone (O₃) is the product. The product O₃ has a different chemical composition than the reactant O₂, so this is a chemical transformation.

Step 4: Conclusion

Reactions (a) and (c) represent chemical transformations because the products have different chemical compositions than the reactants. Reaction (b) is a physical change because only the state of matter, not the chemical composition, changes.

Key Concepts

Chemical Reactions

Chemical reactions are processes where substances, known as reactants, undergo a transformation to create new substances called products. A chemical reaction involves breaking the bonds in reactants and forming new ones in the products.

• Reactants are the starting materials of a reaction.
• Products are the substances formed as a result of a reaction.

A reaction is classified as a chemical transformation if the products are chemically different from the reactants, indicating a new substance has been formed. For example, when phosphorus reacts with oxygen to create phosphorous pentoxide, new chemical identities are emerged, signifying a chemical reaction.

Physical Changes

Physical changes are transformations that affect the form, but not the chemical composition, of a substance. Common examples include changes in state such as melting, freezing, or vaporization.

• The substance's chemical structure remains intact.
• Only the physical form or state of matter changes.

In physical changes, no new substances are formed. For example, converting water vapor into liquid water involves a change in physical state but does not alter water's chemical composition, making it a physical change rather than a chemical reaction.

Reactants and Products

Reactants and products are key concepts in understanding chemical reactions. Reactants are substances that start a reaction, while products are the substances that result from the reaction.

• The composition and properties of products differ from those of reactants in a chemical reaction.
• The arrangement of atoms in the products is different from that in the reactants.

In the reaction where oxygen gas turns into ozone, the reactant is oxygen gas (O₂), and the product is ozone (O₃). This change signifies a chemical reaction, as the molecular structure and properties of ozone differ from those of oxygen gas.

State of Matter Changes

State of matter changes involve a substance transitioning from one state to another, such as solid to liquid or liquid to gas. These changes are physical in nature and do not involve a change in the chemical identity of a substance.

• Common states of matter are solids, liquids, gases, and plasmas.
• State changes occur under varying temperature and pressure conditions.

When a substance changes state, such as ice melting to form water, the molecules may move differently or become more spread out, but the chemical composition remains unchanged. This is evident when water vapor condenses into liquid water, maintaining its chemical identity as H₂O.