Question

Unlike \(\mathrm{CaCO}_{3}, \mathrm{Na}_{2} \mathrm{CO}_{3}\) does not readily yield \(\mathrm{CO}_{2}\) when heated. On the other hand, \(\mathrm{NaHCO}_{3}\) undergoes thermal decomposition to produce \(\mathrm{CO}_{2}\) and \(\mathrm{Na}_{2} \mathrm{CO}_{3}\). (a) Write a balanced equation for the reaction. (b) How would you test for the \(\mathrm{CO}_{2}\) evolved? [Hint: Treat the gas with limewater, an aqueous solution of \(\left.\mathrm{Ca}(\mathrm{OH})_{2} .\right]\)

Short answer

The balanced equation is \( 2\text{NaHCO}_3 \rightarrow \text{Na}_2\text{CO}_3 + \text{CO}_2 + \text{H}_2\text{O} \). Test for \( \text{CO}_2 \) by passing it through limewater to observe a milky precipitate.

Step-by-step solution

Understand the Chemical Reaction

The thermal decomposition of sodium bicarbonate, \( \text{NaHCO}_3 \), involves heating the compound until it decomposes into carbon dioxide, \( \text{CO}_2 \), sodium carbonate, \( \text{Na}_2\text{CO}_3 \), and water, \( \text{H}_2\text{O} \). This reaction is a common example of a thermal decomposition process.

Write the Unbalanced Equation

First, we'll write the unbalanced chemical equation representing the decomposition. \[ \text{NaHCO}_3 \rightarrow \text{Na}_2\text{CO}_3 + \text{CO}_2 + \text{H}_2\text{O} \] This equation shows the reactant \( \text{NaHCO}_3 \) decomposing into the products \( \text{Na}_2\text{CO}_3 \), \( \text{CO}_2 \), and \( \text{H}_2\text{O} \).

Balance the Chemical Equation

To balance the chemical equation, we need equal numbers of each type of atom on both sides. Start by balancing the sodium (Na) atoms. There are 2 sodium atoms in \( \text{Na}_2\text{CO}_3 \), so we need 2 \( \text{NaHCO}_3 \) units. Let's balance the equation: \[ 2\text{NaHCO}_3 \rightarrow \text{Na}_2\text{CO}_3 + \text{CO}_2 + \text{H}_2\text{O} \] Check other atoms: 2 sodium, 2 carbon, 4 oxygen, and 2 hydrogen on both sides.

Finalize Balanced Equation

The balanced equation is: \[ 2\text{NaHCO}_3 \rightarrow \text{Na}_2\text{CO}_3 + \text{CO}_2 + \text{H}_2\text{O} \] This equation confirms the stoichiometry of the reaction, showing the moles of each reactant and product involved.

Test for CO2 Evolved

The presence of \( \text{CO}_2 \) gas can be tested using limewater, which is a solution of \( \text{Ca(OH)}_2 \). When \( \text{CO}_2 \) is bubbled through limewater, it reacts to form calcium carbonate \( \text{CaCO}_3 \), which is insoluble and precipitates out, turning the solution milky. The reaction is: \[ \text{Ca(OH)}_2 (aq) + \text{CO}_2 (g) \rightarrow \text{CaCO}_3 (s) + \text{H}_2\text{O} (l) \] The formation of a white precipitate of \( \text{CaCO}_3 \) confirms the presence of \( \text{CO}_2 \).

Key concepts

Sodium Bicarbonate

Sodium bicarbonate, commonly known as baking soda, is a versatile chemical compound with the formula \(\text{NaHCO}_3\). It appears as a white crystalline powder with a slightly salty taste. In households, it's primarily used for baking as a leavening agent. When heated, sodium bicarbonate undergoes a process called thermal decomposition at temperatures around 100 to 200 degrees Celsius. This reaction is crucial in various applications, such as baking, where it helps dough rise.

• Thermal decomposition transforms \(\text{NaHCO}_3\) into \(\text{Na}_2\text{CO}_3\), \(\text{H}_2\text{O}\), and \(\text{CO}_2\).
• The release of \(\text{CO}_2\) gas is what causes dough to expand.

Understanding the decomposition behavior of sodium bicarbonate can also aid students in grasping basic concepts in chemistry such as reactions involving heat and gas production.

Balancing Chemical Equations

Balancing chemical equations is an essential skill in chemistry, enabling the depiction of a chemical reaction accurately. It ensures the law of conservation of mass is satisfied, with equal numbers of each type of atom on both sides of the equation. In the case of sodium bicarbonate's thermal decomposition, the unbalanced equation is:\[ \text{NaHCO}_3 \rightarrow \text{Na}_2\text{CO}_3 + \text{CO}_2 + \text{H}_2\text{O} \]To balance it, we must ensure that all atoms are evenly distributed:

• Begin by balancing the sodium atoms; notice \(\text{Na}_2\text{CO}_3\) has two sodium atoms.
• Use two \(\text{NaHCO}_3\) to balance sodium: \(2\text{NaHCO}_3\).
• Ensure carbon, oxygen, and hydrogen are balanced across products and reactants.

The final balanced equation:\[ 2\text{NaHCO}_3 \rightarrow \text{Na}_2\text{CO}_3 + \text{CO}_2 + \text{H}_2\text{O} \]This helps achieve an understanding of stoichiometry, showing the proportions of reactants and products involved.

Limewater Test for CO2

The limewater test is a simple and effective method for detecting the presence of carbon dioxide gas \(\text{CO}_2\). Limewater is simply an aqueous solution of calcium hydroxide, \(\text{Ca(OH)}_2\). When \(\text{CO}_2\) is introduced to limewater, a chemical reaction occurs:

• The \(\text{CO}_2\) reacts with \(\text{Ca(OH)}_2\), forming calcium carbonate, \(\text{CaCO}_3\), which is insoluble in water.
• As \(\text{CaCO}_3\) precipitates, the limewater turns cloudy, providing a visual indication of \(\text{CO}_2\) presence.

The equation for this test is:\[ \text{Ca(OH)}_2 (aq) + \text{CO}_2 (g) \rightarrow \text{CaCO}_3 (s) + \text{H}_2\text{O} (l) \]This test is valuable in educational settings to confirm \(\text{CO}_2\) is produced in reactions, demonstrating practical applications of chemical reactions and analysis.