Problem 22 - Chemistry The Central Science In Si Units

Question

Balance the following equations and indicate whether they are combination, decomposition, or combustion reactions: (a) \(\mathrm{NaClO}_{3}(s) \longrightarrow \mathrm{NaCl}(s)+\mathrm{O}_{2}(g)\) (b) \(\mathrm{NH}_{4} \mathrm{OH}(a q) \longrightarrow \mathrm{H}_{2} \mathrm{O}(l)+\mathrm{NH}_{3}(g)\) (c) \(\mathrm{K}(s)+\mathrm{Cl}_{2}(g) \longrightarrow \mathrm{KCl}(s)\) (d) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(l)+\mathrm{O}_{2}(g) \longrightarrow \mathrm{CO}_{2}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(l)\)

Step-by-Step Solution

Answer

(a) Decomposition: \( 2 \; \mathrm{NaClO}_{3} \rightarrow 2 \; \mathrm{NaCl} + 3 \; \mathrm{O}_{2} \); (b) Decomposition: \( \mathrm{NH}_{4} \mathrm{OH} \rightarrow \mathrm{H}_{2} \mathrm{O} + \mathrm{NH}_{3} \); (c) Combination: \( 2 \; \mathrm{K} + \mathrm{Cl}_{2} \rightarrow 2 \; \mathrm{KCl} \); (d) Combustion: \( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH} + 3 \; \mathrm{O}_{2} \rightarrow 2 \; \mathrm{CO}_{2} + 3 \; \mathrm{H}_{2} \mathrm{O} \).

Step 1: Balance Equation (a)

For the decomposition reaction \( \mathrm{NaClO}_{3}(s) \rightarrow \mathrm{NaCl}(s)+\mathrm{O}_{2}(g) \), you must ensure that the number of each type of atom is the same on both sides of the equation. Start by balancing the oxygen atoms. Since there are 2 oxygen molecules on the product side \( (\mathrm{O}_{2}(g)) \), you need 3 molecules of \( \mathrm{NaClO}_{3} \) to produce 3 molecules of \( \mathrm{O}_{2} \). This gives you: \[ 2 \; \mathrm{NaClO}_{3}(s) \rightarrow 2 \; \mathrm{NaCl}(s) + 3 \; \mathrm{O}_{2}(g) \].

Step 2: Balance Equation (b)

For the decomposition reaction \( \mathrm{NH}_{4} \mathrm{OH}(aq) \rightarrow \mathrm{H}_{2} \mathrm{O}(l) + \mathrm{NH}_{3}(g) \), check the number of each atom. There is 1 nitrogen, 4 hydrogens (from \( \mathrm{NH}_{4} \)), and 1 oxygen on each side, making this equation already balanced. Thus, the balanced equation is: \( \mathrm{NH}_{4} \mathrm{OH}(aq) \rightarrow \mathrm{H}_{2} \mathrm{O}(l) + \mathrm{NH}_{3}(g) \).

Step 3: Balance Equation (c)

The combination reaction \( \mathrm{K}(s) + \mathrm{Cl}_{2}(g) \rightarrow \mathrm{KCl}(s) \) involves combining elements to form a compound. Each \( \mathrm{K} \) atom needs 1 \( \mathrm{Cl} \) atom to form \( \mathrm{KCl} \). Thus, you need 2 \( \mathrm{K} \) atoms and 1 \( \mathrm{Cl}_{2} \) molecule to form 2 \( \mathrm{KCl} \) molecules: \[ 2 \; \mathrm{K}(s) + \mathrm{Cl}_{2}(g) \rightarrow 2 \; \mathrm{KCl}(s) \].

Step 4: Balance Equation (d)

The combustion reaction \( \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(l)+\mathrm{O}_{2}(g) \rightarrow \mathrm{CO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(l) \) involves hydrocarbon and oxygen to form carbon dioxide and water. Start by balancing the carbon atoms, then the hydrogen atoms, and finally oxygen. This leads to: \[ \mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}(l) + 3 \; \mathrm{O}_{2}(g) \rightarrow 2 \; \mathrm{CO}_{2}(g) + 3 \; \mathrm{H}_{2} \mathrm{O}(l) \].

Key Concepts

Decomposition Reactions

Decomposition reactions are fascinating chemical processes where a single compound breaks down into two or more simpler substances. It's like taking apart a puzzle to reveal its individual pieces.

Imagine you have a big LEGO brick structure (the compound) and you break it down into its smaller bricks (the simpler substances). These reactions often involve the application of heat, light, or electricity to trigger the breakdown.

Examples of decomposition reactions include the heating of limestone (\( \text{CaCO}_3 \) ) to produce lime (\( \text{CaO} \) ) and carbon dioxide (\( \text{CO}_2 \) ), and in our exercise, the breakdown of sodium chlorate (\( \text{NaClO}_3 \) ) into sodium chloride (\( \text{NaCl} \) ) and oxygen gas (\( \text{O}_2 \) ). Balancing these reactions requires an understanding that the number of each type of atom must be equal on both sides of the equation to preserve the law of conservation of mass.

• Ensure that both sides of the equation have an equal number of atoms of each element.
• The compound is separated into smaller, often simpler substances.
• Used in processes like metal extraction and recycling.

In this example, noticing that one molecule of \( \text{O}_2 \) contains two oxygen atoms helps in determining that three \( \text{NaClO}_3 \) molecules yield three \( \text{O}_2 \) molecules when balanced correctly.

Combination Reactions

Combination reactions are the opposite of decomposition reactions. Instead of breaking down a compound, you start with simpler substances that come together to form a more complex compound. These are also called synthesis reactions. Think of it as mixing different ingredients together to bake a cake.

For example, in the combination reaction of potassium and chlorine to form potassium chloride, \( \text{K}(s) + \text{Cl}_2(g) \to \text{KCl}(s) \), you combine two elements to create a new compound. These reactions are straightforward in terms of balancing since the product is typically one compound.

• Two or more simple substances combine to form a more complex product.
• Often involve elements forming a compound.
• Commonly used in industrial processes like the formation of water from hydrogen and oxygen gases.

In balancing this type of reaction, it's key to ensure that for each element, the atoms on the reactant side match with those on the product side. For instance, since chlorine is diatomic, you need two potassium atoms for every molecule of chlorine gas to form two molecules of potassium chloride.

Combustion Reactions

Combustion reactions are a group of exothermic reactions where a substance combines with oxygen, releasing energy in the form of heat and light. When you see a bright flame or a familiar candle burning, you're witnessing a combustion reaction.

These reactions typically involve organic compounds like hydrocarbons (for example, ethanol or gasoline) reacting with oxygen to produce carbon dioxide and water. A key example in our exercise is the combustion of ethyl alcohol, \( \text{C}_2\text{H}_5\text{OH}(l) + \text{O}_2(g) \to \text{CO}_2(g) + \text{H}_2\text{O}(l) \). The challenge in balancing these reactions lies in managing the multiple types of atoms present.

• Always involves oxygen as a reactant and often results in the formation of \( \text{CO}_2 \) and \( \text{H}_2\text{O} \).
• Releases energy, typically in the form of heat and light.
• Commonly used in engines and energy production.

For successful balancing, start with the carbon atoms, followed by hydrogen, and finally adjust the oxygen. In this example, ensuring that there are two carbon atoms, six hydrogen atoms, and adequate oxygen atoms on both sides will achieve a balanced equation.