Question

Many oxidation-reduction reactions can be balanced by inspection. Try to balance the following reactions by inspection. In each reaction, identify the substance reduced and the substance oxidized. a. \(\mathrm{Al}(s)+\mathrm{HCl}(a q) \rightarrow \mathrm{AlCl}_{3}(a q)+\mathrm{H}_{2}(g)\) b. \(\mathrm{CH}_{4}(g)+\mathrm{S}(s) \rightarrow \mathrm{CS}_{2}(l)+\mathrm{H}_{2} \mathrm{~S}(g)\) c. \(\mathrm{C}_{3} \mathrm{H}_{8}(g)+\mathrm{O}_{2}(g) \rightarrow \mathrm{CO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(l)\) d. \(\mathrm{Cu}(s)+\mathrm{Ag}^{+}(a q) \rightarrow \mathrm{Ag}(s)+\mathrm{Cu}^{2+}(a q)\)

Short answer

The balanced reactions and identified substances are as follows: a. \(\mathrm{Al}(s)+3\mathrm{HCl}(a q) \rightarrow \mathrm{AlCl}_{3}(a q)+3\mathrm{H}_{2}(g)\) Al is oxidized, H in HCl is reduced. b. \(2 \mathrm{CH}_{4}(g)+2 \mathrm{S}(s) \rightarrow \mathrm{CS}_{2}(l)+2 \mathrm{H}_{2} \mathrm{S}(g)\) C in CH4 is oxidized, S is reduced. c. \(\mathrm{C}_{3} \mathrm{H}_{8}(g)+5 \mathrm{O}_{2}(g) \rightarrow 3 \mathrm{CO}_{2}(g)+4 \mathrm{H}_{2} \mathrm{O}(l)\) C in C3H8 is oxidized, O in O2 is reduced. d. \(\mathrm{Cu}(s)+\mathrm{Ag}^{+}(a q) \rightarrow \mathrm{Ag}(s)+\mathrm{Cu}^{2+}(a q)\) Cu is oxidized, Ag+ is reduced.

Step-by-step solution

Reaction a: Balancing the atoms

The given reaction is: $\mathrm{Al}(s)+\mathrm{HCl}(a q) \rightarrow \mathrm{AlCl}_{3}(a q)+\mathrm{H}_{2}(g)$ We have 1 Al atom and 3 Cl atoms on LHS (Left Hand Side) while on RHS (Right Hand Side), we have 1 Al atom, 3 Cl atoms, and 2 H atoms. To balance the equation, we need to multiply HCl by 3: \(\mathrm{Al}(s)+3\mathrm{HCl}(a q) \rightarrow \mathrm{AlCl}_{3}(a q)+3\mathrm{H}_{2}(g)\)

Reaction a: Identifying oxidized and reduced substances

In reaction a, Al is oxidized (loses electrons) while H in HCl is reduced (gains electrons).

Reaction b: Balancing the atoms

The given reaction is: \(\mathrm{CH}_{4}(g)+\mathrm{S}(s) \rightarrow \mathrm{CS}_{2}(l)+\mathrm{H}_{2} \mathrm{S}(g)\) To balance the equation, we will multiply CH4 by 2, S by 2, and H2S by 2: \(2 \mathrm{CH}_{4}(g)+2 \mathrm{S}(s) \rightarrow \mathrm{CS}_{2}(l)+2 \mathrm{H}_{2} \mathrm{S}(g)\)

Reaction b: Identifying oxidized and reduced substances

In reaction b, C in CH4 is oxidized (loses electrons) while S is reduced (gains electrons).

Reaction c: Balancing the atoms

The given reaction is: \(\mathrm{C}_{3} \mathrm{H}_{8}(g)+\mathrm{O}_{2}(g) \rightarrow \mathrm{CO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(l)\) To balance the equation, we will multiply CO2 by 3, O2 by 5 and H2O by 4: \(\mathrm{C}_{3} \mathrm{H}_{8}(g)+5 \mathrm{O}_{2}(g) \rightarrow 3 \mathrm{CO}_{2}(g)+4 \mathrm{H}_{2} \mathrm{O}(l)\)

Reaction c: Identifying oxidized and reduced substances

In reaction c, C in C3H8 is oxidized (loses electrons) while O in O2 is reduced (gains electrons).

Reaction d: Balancing the atoms

The given reaction is: $\mathrm{Cu}(s)+\mathrm{Ag}^{+}(a q) \rightarrow \mathrm{Ag}(s)+\mathrm{Cu}^{2+}(a q)$ The equation is already balanced as there are 1 Cu atom and 1 Ag atom on both LHS and RHS.

Reaction d: Identifying oxidized and reduced substances

In reaction d, Cu is oxidized (loses electrons) while Ag+ is reduced (gains electrons).

Key concepts

Balancing Chemical Equations

Balancing chemical equations is a fundamental skill in chemistry that ensures the law of conservation of mass is upheld. This law states that matter cannot be created or destroyed in a chemical reaction. Therefore, the number of atoms for each element must be the same on both the Left Hand Side (LHS) and the Right Hand Side (RHS) of the equation. For instance, in reaction a, to achieve balance, HCl was multiplied by 3 to equalize hydrogen and chlorine atoms on both sides of the equation. Balancing equations often requires making sure that the number of atoms of each element is equal on each side by adjusting coefficients—numbers placed in front of compounds or elements in the reaction.

• Always begin with compounds that contain elements that are not oxygen or hydrogen.
• Next, balance polyatomic ions as a whole if they appear unchanged on both sides of the equation.
• Then, balance oxygen and hydrogen atoms, typically found in water and oxygen gas in combustion reactions.

Balancing does not alter the nature of the chemicals involved; it simply ensures that the equation accurately represents the conservation of atoms.

Identifying Oxidized and Reduced Substances

Oxidation-reduction (redox) reactions involve the transfer of electrons between substances. During these reactions, one substance loses electrons (oxidation) while another gains electrons (reduction). To identify the oxidized and reduced substances, one must track the movement of electrons. In our examples, Aluminum (Al) loses three electrons when reacting with hydrochloric acid (HCl), making it the oxidized substance, while hydrogen (H) gains electrons, being reduced.

Oxidation

• Oxidation involves the loss of electrons.
• The substance that loses electrons gets oxidized.
• Oxidation often corresponds with an increase in oxidation state.

Reduction

• Reduction involves the gain of electrons.
• The substance that gains electrons gets reduced.
• Reduction often corresponds with a decrease in oxidation state.

Keeping in mind these points helps to determine which species are oxidized and reduced in a given reaction.

Chemical Reaction Types

Chemical reactions can be categorized into various types, depending on the nature of the change that occurs. The reactions in our examples are oxidation-reduction reactions, but it's important to recognize other types such as synthesis, decomposition, single replacement, double replacement, and combustion. Each type follows specific patterns:

• Synthesis reactions combine elements or simpler compounds into more complex ones.
• Decomposition reactions break down compounds into simpler products.
• Single replacement reactions involve an element reacting with a compound, displacing another element in the process.
• Double replacement reactions involve the exchange of components between two compounds.
• Combustion reactions, often involving oxygen and a hydrocarbon, produce water and carbon dioxide.

Recognizing the type of reaction allows chemists to predict the products that might form and the conditions necessary for the reaction to occur. In educational contexts, being able to categorize and predict outcomes of these reactions builds a strong foundation for understanding chemical processes.