Gas Evolution Equations: Videos & Practice Problems

Understanding gas evolution reactions is crucial in chemistry, particularly when dealing with the formation of gases like ammonia (NH₃), carbon dioxide (CO₂), and sulfur dioxide (SO₂). These gases typically arise when intermediate products, which are not yet the final substances, lose a water molecule. For instance, when hydroxide ions react with ammonium ions, the intermediate product is NH₄OH, which upon losing water, forms NH₃, ammonia gas. Similarly, when hydrogen ions react with bicarbonate ions, carbonic acid (H₂CO₃) forms as an intermediate. The loss of water from carbonic acid yields CO₂. In the case of sulfurous acid, removing water results in SO₂. Notably, hydrogen sulfide (H₂S) is already in its final gas form and does not require the loss of water. These reactions are pivotal in predicting the final gaseous products in chemical equations.

Gas Evolution Reactions

A gas evolution equation describes a chemical reaction that produces specific gases, notably ammonia (NH₃), carbon dioxide (CO₂), and sulfur dioxide (SO₂). These gases are generated when medium products lose a water molecule during the reaction process. A medium product refers to an intermediate form of a product before it fully converts into its final state. The final product can be determined by subtracting water from the medium product.

To illustrate this, consider the following reactions involving reactive ions that combine to form medium products:

1. The combination of hydroxide ions (OH^-) and ammonium ions (NH₄⁺) results in the medium product NH₄OH. When water is removed, the final product is ammonia (NH₃).

2. The reaction between hydrogen ions (H⁺) and bicarbonate ions (HCO₃^-) yields carbonic acid (H₂CO₃). Upon losing water, the final product is carbon dioxide (CO₂).

3. When hydrogen ions (H⁺) react with sulfate ions (SO₄^2-), sulfuric acid (H₂SO₄) is formed. Removing water from this medium product results in sulfur dioxide (SO₂).

4. The combination of hydrogen ions (H⁺) and sulfide ions (S₂^-) produces hydrogen sulfide (H₂S). In this case, there is no water to remove, so the final product remains hydrogen sulfide (H₂S).

In summary, gas evolution reactions typically yield one or more of these gases as final products. Understanding the formation of medium products and the subsequent loss of water is crucial for predicting the outcomes of these reactions. As you practice with example questions, you'll become more familiar with identifying these gas evolution products.

Gas Evolution Reaction Example

In this example, we explore the reaction between sodium carbonate and hydrobromic acid, focusing on predicting whether a reaction occurs and writing the balanced molecular equation. The first step involves breaking down the reactants into their ionic forms. Sodium carbonate consists of sodium ions (Na⁺) and carbonate ions (CO₃^2-), while hydrobromic acid is made up of hydrogen ions (H⁺) and bromide ions (Br^-).

Next, we apply the principle of charge attraction to swap the ionic partners. The positive sodium ion will combine with the negative bromide ion to form sodium bromide (NaBr). For the combination of hydrogen ions and carbonate ions, we need to crisscross the charges, resulting in carbonic acid (H₂CO₃).

Following this, we refer to solubility rules. Sodium bromide is soluble in water due to sodium being a group 1A ion. Carbonic acid, however, is unstable and will decompose into water (H₂O) and carbon dioxide gas (CO₂). Thus, we replace carbonic acid in our equation with its decomposition products.

Finally, we balance the molecular equation. Initially, we have 2 sodium ions on the reactant side, so we place a coefficient of 2 in front of sodium bromide. The carbon and oxygen counts remain consistent, but we also need to balance the hydrogen and bromine atoms. Since we have 2 bromide ions from the sodium bromide, we adjust the coefficient in front of hydrobromic acid to 2, ensuring that both sides of the equation are balanced.

The final balanced equation for the reaction is:

2 HBr + Na₂CO₃ → 2 NaBr + H₂O + CO₂ (g)

Predict the products formed from the following gas evolution equation.
CaS (aq) + HNO₃ (aq) →

• A. CaS (aq) + 2 HNO₃ (aq) → Ca(NO₃)₂ (aq) + 2 H₂S (g)
• B. CaS (aq) + HNO₃ (aq) → CaNO₃ (aq) + H₂S (g)
• C. CaS (aq) + 2 HNO₃ (aq) → Ca(NO₃)₂ (aq) + H₂S (g)
• D. No reaction

Answer: (C)

Predict the products formed from the following gas evolution equation.
NH₄Cl (aq) + NaOH (aq) →

• A. NH₄Cl (aq) + NaOH (aq) → NH₄OH (aq) + NaCl (aq)
• B. NH₄Cl (aq) + NaOH (aq) → NH₃ (g) + H₂O (l) + NaCl (aq)
• C. NH₄Cl (aq) + NaOH (aq) → NH₄OH (g) + NaCl (s)
• D. No reaction

Answer: (B)

Predict the products formed from the following gas evolution equation
K₂SO₃ (aq) + H₂SO₄ (aq) →

• A. K₂SO₃ (aq) + H₂SO₄ (aq) → K₂SO₄ (aq) + H₂O (l) + SO₂ (g)
• B. K₂SO₃ (aq) + H₂SO₄ (aq) → K₂SO₄ (aq) + H₂SO₃ (aq)
• C. K₂SO₃ (aq) + H₂SO₄ (aq) → K₂SO₄ (aq) + SO₂ (g)
• D. No reaction

Answer: (A)

Here’s what students ask on this topic:

What is a gas evolution reaction?

A gas evolution reaction is a type of chemical reaction where the reactants interact to form one or more gaseous products. This process often results in the release of a gas, which can be observed as bubbles or effervescence in a liquid reaction mixture. Gas evolution reactions are common in various fields of chemistry, including inorganic and organic chemistry, and can be involved in processes such as photosynthesis, respiration, and industrial chemical production.

For example, when you mix hydrochloric acid (HCl) with sodium bicarbonate (NaHCO₃), you get sodium chloride (NaCl), water (H₂O), and carbon dioxide gas (CO₂) as products. The carbon dioxide gas forms bubbles and escapes from the mixture, which is a classic example of a gas evolution reaction:

${\mathrm{HCl}}_{\text{(aq)}}+{\mathrm{NaHCO}}_3(s)\; \to {\mathrm{NaCl}}_{\text{(aq)}}+H_{2}O(l)\; +{\mathrm{CO}}_2(g)$

Gas evolution reactions are important to identify because the gases produced can sometimes be hazardous, and knowing when a gas is going to be released can be crucial for conducting safe experiments or industrial processes.