Problem 68 - Chemistry

Question

Baking soda (sodium bicarbonate) undergoes thermal decomposition as follows: $$2 \mathrm{NaHCO}_{3}(s) \rightleftharpoons \mathrm{Na}_{2} \mathrm{CO}_{3}(s)+\mathrm{CO}_{2}(g)+\mathrm{H}_{2} \mathrm{O}(g)$$ Would we obtain more \(\mathrm{CO}_{2}\) and \(\mathrm{H}_{2} \mathrm{O}\) by adding extra baking soda to the reaction mixture in (a) a closed vessel or (b) an open vessel?

Step-by-Step Solution

Answer

Adding extra baking soda (sodium bicarbonate) in a closed vessel would result in more carbon dioxide and water, as the gases remain within the system. In contrast, in an open vessel, the formed gases would escape into the surroundings, hence their concentration in the reaction mixture would not increase significantly.

Step 1: Understand Le Chatelier’s Principle

Le Chatelier’s principle states that if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change. In this case, adding extra sodium bicarbonate (reactant) should shift the equilibrium to the right, forming more products.

Step 2: Consider a closed system

In a closed vessel, the system can achieve equilibrium as no gases escape from it. As extra sodium bicarbonate is added to the system according to Le Chatelier's principle, this will shift the equilibrium to the right leading to an increase in concentration of carbon dioxide and water vapor. The equilibrium shift counteracts the change and tries to consume the extra reactant.

Step 3: Consider an open system

In an open vessel, carbon dioxide and water vapor (both are gases) can escape into the surroundings as they are formed. Hence, adding extra baking soda in an open system won't increase the concentration of carbon dioxide and water vapor much, as these gases would dissipate into the surrounding air.

Key Concepts

Thermal Decomposition

Thermal decomposition is a chemical reaction where a compound breaks down into two or more substances when heated. In the case of sodium bicarbonate, also known as baking soda, it decomposes into sodium carbonate, carbon dioxide, and water vapor upon heating.

Such reactions are usually endothermic, meaning they absorb energy from their surroundings. This plays a critical role in understanding how added heat affects the reaction. As heat is added to sodium bicarbonate, the process of decomposition is facilitated, leading to increased production of the decomposition products.

To exemplify thermal decomposition, consider toasting bread: the heat causes the bread to transform, often turning it brown and altering its chemical makeup. Just as toasting alters bread, heat alters sodium bicarbonate, explicating the essence of thermal decomposition.

Dynamic Equilibrium

In chemistry, dynamic equilibrium occurs in a closed system when the rate of the forward reaction equals the rate of the reverse reaction, and the concentrations of reactants and products remain constant over time. It's important to note that this doesn't mean the reactions have stopped; rather, they are continuously occurring but at the same rate in both directions.

Think of it like a busy downtown intersection, where equal numbers of cars enter and exit at the same rate, keeping the number of cars within the intersection steady. Similarly, in a dynamic equilibrium of a chemical reaction, substances transform back and forth at equal rates, resulting in no net change in concentration.

Enhancing the Concept

In our baking soda scenario, reaching a dynamic equilibrium means the reaction has stabilized, and adding more baking soda can disturb this balance, thereby invoking Le Chatelier's Principle.

Sodium Bicarbonate Reaction

Sodium bicarbonate, a common household substance, undergoes a reaction known as thermal decomposition when exposed to heat. This specific reaction is central to many baking processes, contributing to the rise of doughs and batters by releasing carbon dioxide gas.

In a closed vessel where a dynamic equilibrium exists, adding more sodium bicarbonate will push the equilibrium toward the formation of more products, that is, more carbon dioxide and water vapor according to Le Chatelier's principle. In an open vessel, however, the reaction doesn't truly reach equilibrium because the products, being gaseous, escape into the atmosphere.

Practical Implications

In baking, this reaction allows for leavening; similarly, in various industrial processes, such as the manufacture of fire extinguishers, it serves a different function. But in both instances, understanding the sodium bicarbonate reaction can optimize outcomes, whether creating the perfect cake or designing effective safety equipment.