Problem 51 - General Chemistry

Question

When \(1 \mathrm{~mol}\) of iron metal reacts with hydrochloric acid at constant temperature and pressure to produce hydrogen gas and aqueous iron(II) chloride, \(89.1 \mathrm{~kJ}\) of heat evolves. Write a thermochemical equation for this reaction.

Step-by-Step Solution

Answer

Fe (s) + 2HCl (aq) → FeCl₂ (aq) + H₂ (g), ΔH = -89.1 kJ.

Step 1: Write the chemical equation

Iron (Fe) reacts with hydrochloric acid (HCl) producing iron(II) chloride (FeCl₂) and hydrogen gas (H₂). The balanced chemical equation is: \[ \text{Fe (s) + 2HCl (aq) } \rightarrow \text{ FeCl}_2 \text{(aq) + H}_2 \text{(g)} \]

Step 2: Identify the heat of reaction

The problem states that \(89.1 \text{ kJ}\) of heat is evolved when \(1 \text{ mol}\) of iron reacts, indicating the reaction is exothermic. This means the value of heat will be shown with a negative sign because energy is released.

Step 3: Write the thermochemical equation

Include the heat change in the chemical equation. Since \( -89.1 \text{ kJ}\) of heat is released when \(1 \text{ mol}\) of iron reacts, the thermochemical equation is: \[ \text{Fe (s) + 2HCl (aq) } \rightarrow \text{ FeCl}_2 \text{(aq) + H}_2 \text{(g)}, \quad \Delta H = -89.1 \text{ kJ} \]

Key Concepts

Exothermic Reactions

In chemical reactions, energy can be absorbed or released. When energy is released, the reaction is termed "exothermic." This kind of reaction is typical when bonds that are being formed are stronger than the bonds that were broken. As a result, the excess energy is given off, often as heat.

In the context of the exercise, the reaction of iron with hydrochloric acid evolves heat, indicating it's exothermic.

• Exothermic reactions often feel warm or hot to the touch because they produce heat.
• Common examples include combustion and certain oxidation reactions.

The release of 89.1 kJ of energy as heat during this chemical reaction confirms it is exothermic, and this energy is typically expressed with a negative sign in thermochemical equations.

Enthalpy Change

Enthalpy change, denoted by the symbol \( \Delta H \), is a measure of the overall heat energy transferred in a chemical reaction at constant pressure. It helps us understand whether a reaction absorbs or releases energy.

For exothermic reactions, like the one with iron and hydrochloric acid, \( \Delta H \) is negative. This is because energy is being released to the surroundings.

• An exothermic reaction has \( \Delta H < 0 \).
• An endothermic reaction, which requires energy, has \( \Delta H > 0 \).

Enthalpy change is crucial in calculating the energy aspects of chemical processes and assists in predicting the spontaneity of reactions and understanding reaction environments.

Chemical Reactions

Chemical reactions involve the rearrangement of atoms and molecules to form new substances. In these reactions, bonds between atoms in reactants are broken, and new bonds are formed to create the products.

The reaction between iron and hydrochloric acid is a type of double displacement reaction, where iron displaces hydrogen in hydrochloric acid to form iron chloride and hydrogen gas.

• Reactants: Substances that start a reaction (iron and hydrochloric acid in this case).
• Products: New substances formed as a result of the reaction (iron(II) chloride and hydrogen gas).

Balancing chemical equations is essential to ensure the same number of each type of atom is present in both reactants and products, following the law of conservation of mass. This concept ensures no matter is lost or gained during the reaction.