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Chapter 6: Problem 51

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.

Short Answer

Expert verified
Fe (s) + 2HCl (aq) 鈫� FeCl鈧� (aq) + H鈧� (g), 螖H = -89.1 kJ.

Step by step solution

01

Write the chemical equation

Iron (Fe) reacts with hydrochloric acid (HCl) producing iron(II) chloride (FeCl鈧�) and hydrogen gas (H鈧�). The balanced chemical equation is: \[ \text{Fe (s) + 2HCl (aq) } \rightarrow \text{ FeCl}_2 \text{(aq) + H}_2 \text{(g)} \]
02

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.
03

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: \[ \text{Fe (s) + 2HCl (aq) } \rightarrow \text{ FeCl}_2 \text{(aq) + H}_2 \text{(g)}, \quad \Delta H = -89.1 \text{ kJ} \]

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

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.

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Most popular questions from this chapter

Under what condition is the enthalpy change equal to the heat of reaction?

What is meant by the reference form of an element? What is the standard enthalpy of formation of an element in its reference form?

When ice at \(0^{\circ} \mathrm{C}\) melts to liquid water at \(0^{\circ} \mathrm{C}\), it absorbs \(0.334 \mathrm{~kJ}\) of heat per gram. Suppose the heat needed to melt \(31.5 \mathrm{~g}\) of ice is absorbed from the water contained in a glass. If this water has a mass of \(0.210 \mathrm{~kg}\) and a temperature of \(21.0^{\circ} \mathrm{C}\), what is the final temperature of the water? (Note that you will also have \(31.5 \mathrm{~g}\) of water at \(0^{\circ} \mathrm{C}\) from the ice.)

Hydrogen, \(\mathrm{H}_{2}\), is used as a rocket fuel. The hydrogen is burned in oxygen to produce water vapor. $$ 2 \mathrm{H}_{2}(g)+\mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{H}_{2} \mathrm{O}(g) ; \Delta H=-484 \mathrm{~kJ} $$ What is the enthalpy change per gram of hydrogen?

The carbon dioxide exhaled in the breath of astronauts is often removed from the spacecraft by reaction with lithium hydroxide. $$ 2 \mathrm{LiOH}(s)+\mathrm{CO}_{2}(g) \longrightarrow \mathrm{Li}_{2} \mathrm{CO}_{3}(s)+\mathrm{H}_{2} \mathrm{O}(l) $$ Estimate the grams of lithium hydroxide required per astronaut per day. Assume that each astronaut requires \(2.50 \times 10^{3}\) kcal of energy per day. Further assume that this energy can be equated to the heat of combustion of a quantity of glucose, \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}\), to \(\mathrm{CO}_{2}(g)\) and \(\mathrm{H}_{2} \mathrm{O}(l)\). From the amount of glucose required to give \(2.50 \times 10^{3}\) kcal of heat, calculate the amount of \(\mathrm{CO}_{2}\) produced and hence the amount of LiOH required. The \(\Delta H_{f}^{\circ}\) for glucose \((s)\) is \(-1273 \mathrm{~kJ} / \mathrm{mol}\).
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