Question

Predict the type of reaction (if any) that occurs between each pair of substances. Write balanced molecular equations for each. If no reaction occurs, write NO REACTION. (a) aqueous potassium hydroxide and aqueous acetic acid (b) aqueous hydrobromic acid and aqueous potassium carbonate (c) gaseous hydrogen and gaseous oxygen (d) aqueous ammonium chloride and aqueous lead(II) nitrate

Short answer

(a) Neutralization: KOH(aq) + CH3COOH(aq) -> H2O(l) + CH3COOK(aq), (b) Acid-base/Gas Evolution: 2HBr(aq) + K2CO3(aq) -> 2KBr(aq) + H2O(l) + CO2(g), (c) Combination: 2H2(g) + O2(g) -> 2H2O(g), (d) Precipitation: 2NH4Cl(aq) + Pb(NO3)2(aq) -> 2NH4NO3(aq) + PbCl2(s)

Step-by-step solution

- Predicting the reaction type for (a)

For part (a), potassium hydroxide (KOH) is a strong base and acetic acid (CH3COOH) is a weak acid. When an acid reacts with a base, they undergo an acid-base neutralization reaction to form water and an ionic compound called a salt.

- Writing the balanced equation for (a)

The balanced molecular equation for the reaction between KOH and CH3COOH is: \[ KOH(aq) + CH3COOH(aq) \rightarrow H_2O(l) + CH3COOK(aq) \]

- Predicting the reaction type for (b)

For part (b), hydrobromic acid (HBr) is a strong acid and potassium carbonate (K2CO3) is a salt which can act as a base. When an acid reacts with a base, it also undergoes an acid-base reaction to form water and a salt.

- Writing the balanced equation for (b)

The balanced molecular equation for the reaction between HBr and K2CO3 is: \[ 2HBr(aq) + K2CO3(aq) \rightarrow 2KBr(aq) + H_2O(l) + CO_2(g) \]

- Predicting the reaction type for (c)

For part (c), hydrogen (H2) and oxygen (O2) are both diatomic gases. When they react together, they undergo a combination reaction to form water.

- Writing the balanced equation for (c)

The balanced molecular equation for the reaction between H2 and O2 is: \[ 2H_2(g) + O_2(g) \rightarrow 2H_2O(g) \]

- Predicting the reaction type for (d)

For part (d), ammonium chloride (NH4Cl) and lead(II) nitrate (Pb(NO3)2) both dissolve in water to produce ions. A reaction will occur if an insoluble salt, a gas, or a weakly dissociating compound can be formed. In this case, lead(II) chloride is known to be an insoluble salt.

- Writing the balanced equation for (d)

The balanced molecular equation for the reaction between NH4Cl and Pb(NO3)2 is: \[ 2NH4Cl(aq) + Pb(NO3)_2(aq) \rightarrow 2NH4NO3(aq) + PbCl_2(s) \]

Key concepts

Acid-Base Neutralization

Chemical reactions are fascinating events where substances, known as reactants, transform into new products. An intriguing kind of reaction is the acid-base neutralization. It usually occurs when an acid and a base, substances with contrasting pH values, mix together. Picture an acid, it typically has a sharp taste like vinegar and turns blue litmus paper red, while a base feels slippery, like soap, and turns red litmus paper blue. When they combine, they typically produce a salt and water, bringing the solution closer to a neutral pH.

In our exercise examples, both (a) and (b) are classic cases of neutralization. For instance, the reaction between potassium hydroxide, a base, and acetic acid, an acid, results in water and potassium acetate, a salt. The balanced molecular equation \( KOH(aq) + CH3COOH(aq) \rightarrow H_2O(l) + CH3COOK(aq) \) paints the full picture. Similarly, hydrobromic acid reacts with potassium carbonate to yield water, potassium bromide, and carbon dioxide gas following the equation \( 2HBr(aq) + K2CO3(aq) \rightarrow 2KBr(aq) + H_2O(l) + CO_2(g) \).

The examples make it clear that during a neutralization reaction, the hydrogen ion (H+) from the acid and the hydroxide ion (OH-) from the base work together to form water (H2O), while the remaining parts of the acid and base form the salt.

Balanced Molecular Equations

Balancing molecular equations is akin to solving a puzzle where the number of atoms from the reactants side must exactly match the number from the products side. This respects the law of conservation of mass, which states that mass cannot be created or destroyed in a chemical reaction. Consider it like a recipe; to maintain the proper flavor, the amount of each ingredient must be precise.

For example, balancing the reaction between hydrogen and oxygen to produce water, the challenge is to ensure that the number of hydrogen and oxygen atoms is conserved. By placing a '2' before the hydrogen gas and water in the equation \( 2H_2(g) + O_2(g) \rightarrow 2H_2O(g) \), we accomplish atomic equality on both sides—four hydrogen atoms and two oxygen atoms are present in both reactants and products. Without balance, our 'recipe' wouldn't accurately represent the reaction.

Balancing equations is critical in providing a proper stoichiometric relationship between the reactants and products, ensuring the chemical equation is not only descriptive but quantitative as well. This allows chemists and students alike to predict the amounts of reactants needed and products formed in a given reaction.

Combination Reaction

In the realm of chemical reactions, combination reactions are straightforward yet significant; they involve two or more reactants coming together to form a single product. It's like a team coming together to build something greater than the sum of its parts. An everyday example is the synthesis of water when hydrogen gas combusts with oxygen gas from the air—an essential chemical process that even powers some rockets!

In example (c) from our exercise, we witness a perfect illustration of a combination reaction: \( 2H_2(g) + O_2(g) \rightarrow 2H_2O(g) \). Here, two hydrogen molecules (H2) marry an oxygen molecule (O2) to create water. It's a simple reaction, but it's profoundly important for life on our planet and is used in numerous industrial applications.

The beauty of a combination reaction lies in its simplicity and the transformation of less complex substances into a more complex one. It is a synthesis process that takes us from the building blocks to a completed structure, giving us insights into how more complex molecules are formed from simpler ones in nature and industry.