Question

Write a balanced chemical equation for the reaction that occurs when (a) aluminum metal undergoes a combination reaction with \(\mathrm{O}_{2}(g) ;\) (b) copper(II) hydroxide decomposes into copper(II) oxide and water when heated; (c) heptane, \(\mathrm{C}_{7} \mathrm{H}_{16}(l)\), burns in air; (d) the gasoline additive MTBE (methyl tert-butyl ether), \(\mathrm{C}_{5} \mathrm{H}_{12} \mathrm{O}(l)\), burns in air.

Short answer

(a) 2Al + 3O2 -> 2Al2O3 (b) Cu(OH)2 -> CuO + 2H2O (c) C7H16 + 11O2 -> 7CO2 + 16H2O (d) C5H12O + 6O2 -> 5CO2 + 12H2O

Step-by-step solution

(a) Aluminum metal and O2 combination reaction

Identify the reactants and products: In this case, the reactants are aluminum metal (Al) and oxygen gas (O2). The product will be aluminum oxide (Al2O3). Write the unbalanced equation: Al + O2 -> Al2O3 Now, balance the equation. We have two oxygen atoms on the right side, but only one on the left. To balance this, place a 3/2 coefficient in front of O2: Al + 3/2 O2 -> Al2O3 However, it is more convenient to have whole numbers as coefficients. You can achieve this by multiplying all coefficients by 2: 2Al + 3O2 -> 2Al2O3 The balanced equation for the combination reaction of aluminum metal and oxygen gas is: 2Al + 3O2 -> 2Al2O3

(b) Copper(II) hydroxide decomposition

Identify the reactants and products: In this case, the reactant is copper(II) hydroxide (Cu(OH)2) and the products are copper(II) oxide (CuO) and water (H2O). Write the unbalanced equation: Cu(OH)2 -> CuO + H2O To balance the equation, note that there are two hydroxide (OH) groups on the left side and only one water molecule on the right. To balance this, place a 2 in front of H2O: Cu(OH)2 -> CuO + 2H2O Now, the equation is balanced: Cu(OH)2 -> CuO + 2H2O

(c) Heptane combustion

Identify the reactants and products: Here, heptane (C7H16) is the reactant and it burns in the presence of oxygen (O2). The products will be carbon dioxide (CO2) and water (H2O). Write the unbalanced equation: C7H16 + O2 -> CO2 + H2O To balance the equation, place a 7 in front of CO2 for carbon atoms and 16 in front of H2O for hydrogen atoms: C7H16 + O2 -> 7CO2 + 16H2O Now, to balance the oxygen atoms, place a 11 in front of O2: C7H16 + 11O2 -> 7CO2 + 16H2O The balanced equation for the combustion of heptane is: C7H16 + 11O2 -> 7CO2 + 16H2O

(d) Methyl tert-butyl ether combustion

Identify the reactants and products: In this case, methyl tert-butyl ether (C5H12O) is the reactant and burns in the presence of oxygen (O2). The products will be carbon dioxide (CO2) and water (H2O). Write the unbalanced equation: C5H12O + O2 -> CO2 + H2O To balance the equation, place a 5 in front of CO2 for carbon atoms and 12 in front of H2O for hydrogen atoms: C5H12O + O2 -> 5CO2 + 12H2O Now, to balance the oxygen atoms, place a 6 in front of O2: C5H12O + 6O2 -> 5CO2 + 12H2O The balanced equation for the combustion of methyl tert-butyl ether is: C5H12O + 6O2 -> 5CO2 + 12H2O

Key concepts

Combination Reaction

A combination reaction, also known as a synthesis reaction, involves two or more substances combining to form a single product. In educational terms, think of it as a gathering where individual guests (reactants) come together to form a group (product).

For example, when aluminum metal reacts with oxygen gas, the product is aluminum oxide. This process can be visualized as each aluminum atom joining hands with oxygen atoms to create a more complex structure, like building blocks coming together to build a larger toy.

Visualizing the Reaction

Imagine two separate piles of blocks—one red (Aluminum) and one blue (Oxygen). Through the combination reaction, they connect to form a new structure with both colors (Aluminum Oxide). The key is to ensure the structure is stable, meaning we have the right ratio of red to blue blocks. In chemical terms, this requires balancing the chemical equation to ensure the number of aluminum and oxygen atoms is equal on both sides of the equation.

Decomposition Reaction

In contrast to the previous concept, a decomposition reaction is the opposite—it's like a team dispersing. We start with one compound that breaks down into two or more simpler substances.

This concept can be imagined as a complex toy that, upon heating, falls apart into simpler pieces. For instance, copper(II) hydroxide decomposes into copper(II) oxide and water when heated, similar to a toy breaking down into water molecules and smaller copper oxide blocks when exposed to warmth.

Importance of Heat

Heat often acts as the external force that breaks the bonds within a compound, allowing it to decompose. It's like the energy that causes the toy to break apart into smaller pieces, initiating the decomposition. This heat factor is vital for students to remember when observing or predicting decomposition reactions.

Combustion Reaction

A combustion reaction is an exothermic reaction (it releases heat) that occurs when a substance combines with oxygen, often resulting in a flame. This is the type of reaction you witness when lighting a candle or burning gasoline.

Using the burning of heptane as an example, which is present in fuels, we see that it reacts with oxygen to produce water and carbon dioxide. Picture a log (heptane) in a fireplace. As it burns (reacts with oxygen), it produces flame and smoke (carbon dioxide and water in gaseous form).

Environmental Note

While the heat generated from combustion reactions is useful, the carbon dioxide by-product contributes to greenhouse gases, factoring into environmental discussions on energy sources. It's crucial for students to understand both the chemical and environmental ramifications of these reactions.

Chemical Reaction Balancing

Balancing chemical equations is a fundamental skill in chemistry. It's akin to solving a puzzle where each piece (atom of each element) must fit perfectly on either side of the equation.

The rule of the game, based on the Law of Conservation of Mass, is that we must have the same number of each type of atom on both the reactant and product sides of the equation. This involves adding coefficients to molecules, just as you might rearrange puzzle pieces to make everything fit together. The process requires practice, attention to detail, and patience, much like putting together a challenging puzzle.

Real-World Connection

When students grasp the concept of balancing chemical reactions, they're not just learning chemistry; they're sharpening analytical skills that apply to real-world problem-solving where balance and proportion are key. Each balanced equation provides a clear picture of the chemical process at hand.