Question

Classify the reactions represented by the following unbalanced equations by as many methods as possible. Balance the equations. a. \(\mathrm{C}_{3} \mathrm{H}_{8} \mathrm{O}(l)+\mathrm{O}_{2}(g) \rightarrow \mathrm{CO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(g)\) b. \(\mathrm{HCl}(a q)+\mathrm{AgC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}(a q) \rightarrow\) \(\mathrm{AgCl}(s)+\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}(a q)\) c. \(\mathrm{HCl}(a q)+\mathrm{Al}(\mathrm{OH})_{3}(s) \rightarrow \mathrm{AlCl}_{3}(a q)+\mathrm{H}_{2} \mathrm{O}(l)\) d. \(\mathrm{H}_{2} \mathrm{O}_{2}(a q) \rightarrow \mathrm{H}_{2} \mathrm{O}(l)+\mathrm{O}_{2}(g)\) e. \(\mathrm{N}_{2} \mathrm{H}_{4}(l)+\mathrm{O}_{2}(g) \rightarrow \mathrm{N}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(g)\)

Short answer

(a) Combustion reaction. Balanced equation: \(C_3H_8O(l) + 5 O_2(g) \rightarrow 3 CO_2(g) + 4 H_2O(g)\) (b) Double displacement and acid-base reaction. Balanced equation: \(HCl(aq) + AgC_2H_3O_2(aq) \rightarrow AgCl(s) + HC_2H_3O_2(aq)\) (c) Double displacement and acid-base reaction. Balanced equation: \(3 HCl(aq) + Al(OH)_3(s) \rightarrow AlCl_3(aq) + 3 H_2O(l)\) (d) Decomposition and redox reaction. Balanced equation: \(2 H_2O_2(aq) \rightarrow 2 H_2O(l) + O_2(g)\) (e) Redox reaction. Balanced equation: \(N_2H_4(l) + O_2(g) \rightarrow N_2(g) + 2 H_2O(g)\)

Step-by-step solution

(a) Classify the Reaction#a

This is a combustion reaction as it involves a fuel (C3H8O) and oxygen (O2) to produce carbon dioxide (CO2) and water (H2O).

(a) Balance the Equation#a

To balance C3H8O + O2 → CO2 + H2O, we follow these steps: 1. Balance the carbon atoms: (3 CO2) 2. Balance the hydrogen atoms: (4 H2O) 3. Balance the oxygen atoms: (5 O2) Balanced equation: C3H8O(l) + 5 O2(g) → 3 CO2(g) + 4 H2O(g)

(b) Classify the Reaction#b

This is a double displacement (double replacement) reaction as well as an acid-base (neutralization) reaction, where HCl and AgC2H3O2 exchange ions to form AgCl and HC2H3O2.

(b) Balance the Equation#b

To balance HCl(aq) + AgC2H3O2(aq) → AgCl(s) + HC2H3O2(aq), no additional changes need to be made. Balanced equation: HCl(aq) + AgC2H3O2(aq) → AgCl(s) + HC2H3O2(aq)

(c) Classify the Reaction#c

This is a double displacement (double replacement) reaction as well as an acid-base (neutralization) reaction, where HCl and Al(OH)3 exchange ions to form AlCl3 and H2O.

(c) Balance the Equation#c

To balance HCl(aq) + Al(OH)3(s) → AlCl3(aq) + H2O(l), we follow these steps: 1. Balance the aluminum atoms: (1 AlCl3) 2. Balance the chloride atoms: (3 HCl) 3. Balance the hydroxide ions: (3 H2O) Balanced equation: 3 HCl(aq) + Al(OH)3(s) → AlCl3(aq) + 3 H2O(l)

(d) Classify the Reaction#d

This is a decomposition reaction, as H2O2 breaks down into simpler products (H2O and O2). It is also a redox reaction, as the oxidation state of oxygen changes.

(d) Balance the Equation#d

To balance H2O2(aq) → H2O(l) + O2(g), we follow these steps: 1. Balance the hydrogen atoms: (2 H2O) 2. Balance the oxygen atoms: (2 H2O2) Balanced equation: 2 H2O2(aq) → 2 H2O(l) + O2(g)

(e) Classify the Reaction#e

This is a redox reaction, where N2H4 and O2 transfer electrons and change their oxidation states.

(e) Balance the Equation#e

To balance N2H4(l) + O2(g) → N2(g) + H2O(g), we follow these steps: 1. Balance the nitrogen atoms: (1 N2) 2. Balance the hydrogen atoms: (2 H2O) 3. Balance the oxygen atoms: (1 O2) Balanced equation: N2H4(l) + O2(g) → N2(g) + 2 H2O(g)

Key concepts

Balancing Chemical Equations

Mastering the balancing of chemical equations is essential for students to understand the law of conservation of mass, which states that matter cannot be created or destroyed in a closed system. Take, for example, the combustion of C3H8O:

Initially, visually inspect the equation and count the atoms of each element on both sides. The next step is to adjust coefficients—these are the numbers placed in front of compounds—to ensure the same number of each type of atom appears on both sides of the equation. You might need to balance one element at a time, iterating through to check that adjusting one doesn't unbalance another. In our exercise, we started with carbon, moved on to hydrogen, and finished with oxygen.

Often, finding the correct coefficients is a trial-and-error process, and it’s perfectly normal to backtrack and adjust until you find that balance. Remember to only change the coefficients, not the subscripts in the chemical formulas. Adjusting subscripts would change the substance’s identity, which isn't allowed.

Combustion Reaction

A combustion reaction is a high-energy reaction involving the rapid combination of a substance (often an organic compound) with oxygen to produce oxides and release energy, typically in the form of heat and light. Most of us encounter combustion in everyday life through the burning of fuels. In our example, C3H8O serves as a fuel, reacting with oxygen (O2) to form carbon dioxide (CO2) and water vapor (H2O). An indication that a combustion reaction is taking place is the presence of oxygen as a reactant and the production of CO2 and H2O if the fuel contains carbon and hydrogen.

Double Displacement Reaction

In a double displacement reaction, or double replacement reaction, ions from two compounds exchange places to form two new compounds. This reaction type often occurs in aqueous solution and can result in the formation of a precipitate, a gas, or a molecular compound such as water. For instance, AgC2H3O2 and HCl switch ions in our example, resulting in the formation of AgCl, which precipitates out of the solution, and HC2H3O2. One helpful cue that a double displacement reaction has occurred is the presence of an insoluble substance precipitating out from the solution or a weak electrolyte forming from stronger electrolytes.

Decomposition Reaction

Decomposition reactions are processes in which a compound breaks down into two or more simpler products. These reactions typically require an input of energy in the form of heat, light, or electricity. An easy way to recognize a decomposition reaction is by seeing a single reactant breaking down into multiple products. In our example, hydrogen peroxide (H2O2) decomposes into water (H2O) and oxygen gas (O2). It's important to reiterate that though they may seem straightforward, decomposition reactions can sometimes follow complex pathways depending on the substance and conditions.

Redox Reaction

Redox reactions, or oxidation-reduction reactions, encompass all chemical processes in which elements undergo changes in oxidation state. These reactions involve the transfer of electrons between species. A useful tip is that in every redox reaction, there is always one component being oxidized (losing electrons) and another being reduced (gaining electrons). For example, in the reaction between N2H4 and O2, N2H4 is oxidized to N2, while O2 is reduced to H2O. Identifying redox reactions requires looking at how the oxidation states of atoms change from reactants to products.