Problem 60 - Chemistry The Molecular Science

Question

$$ \begin{aligned} &\text { Use these bond enthalpy values to answer Question } { . }\\\ &\begin{array}{lclc} \hline \text { Bond } & \begin{array}{c} \text { Bond Enthalpy } \\ (\mathrm{k}\rfloor / \mathrm{mol}) \end{array} & \text { Bond } & \begin{array}{c} \text { Bond Enthalpy } \\ (\mathrm{k} / / \mathrm{mol}) \end{array} \\ \hline \mathrm{H}-\mathrm{F} & 566 & \mathrm{~F}-\mathrm{F} & 158 \\ \mathrm{H}-\mathrm{Cl} & 431 & \mathrm{Cl}-\mathrm{Cl} & 242 \\ \mathrm{H}-\mathrm{Br} & 366 & \mathrm{Br}-\mathrm{Br} & 193 \\ \mathrm{H}-\mathrm{I} & 299 & \mathrm{I}-\mathrm{I} & 151 \\ \mathrm{H}-\mathrm{H} & 436 & & \\ \hline \end{array} \end{aligned} $$ Which molecule, \(\mathrm{F}_{2}, \mathrm{Cl}_{2}, \mathrm{Br}_{2},\) or \(\mathrm{I}_{2}\), has the weakest chemical bond?

Step-by-Step Solution

Answer

\( \mathrm{I}_2 \) has the weakest bond.

Step 1: Understand the Problem

We need to find the molecular bond that requires the least amount of energy to break. This is represented by the smallest bond enthalpy value among the given diatomic molecules: \( \mathrm{F}_2, \mathrm{Cl}_2, \mathrm{Br}_2, \mathrm{I}_2. \)

Step 2: Examine Given Bond Enthalpies

Check the bond enthalpy values provided for \( \mathrm{F}-\mathrm{F}, \mathrm{Cl}-\mathrm{Cl}, \mathrm{Br}-\mathrm{Br}, \mathrm{I}-\mathrm{I} \):- \( \mathrm{F}-\mathrm{F} = 158 \: \mathrm{kJ/mol} \)- \( \mathrm{Cl}-\mathrm{Cl} = 242 \: \mathrm{kJ/mol} \)- \( \mathrm{Br}-\mathrm{Br} = 193 \: \mathrm{kJ/mol} \)- \( \mathrm{I}-\mathrm{I} = 151 \: \mathrm{kJ/mol} \)

Step 3: Identify the Smallest Enthalpy

Compare the bond enthalpy values:- \( 158 \: \mathrm{kJ/mol} \) for \( \mathrm{F}-\mathrm{F} \)- \( 242 \: \mathrm{kJ/mol} \) for \( \mathrm{Cl}-\mathrm{Cl} \)- \( 193 \: \mathrm{kJ/mol} \) for \( \mathrm{Br}-\mathrm{Br} \)- \( 151 \: \mathrm{kJ/mol} \) for \( \mathrm{I}-\mathrm{I} \)The smallest enthalpy value is \( 151 \: \mathrm{kJ/mol} \).

Step 4: Conclude Which Molecule Has the Weakest Bond

Since \( \mathrm{I}-\mathrm{I} \) has the lowest bond enthalpy of \( 151 \: \mathrm{kJ/mol} \), it means that \( \mathrm{I}_2 \) has the weakest bond among the diatomic molecules listed.

Key Concepts

Diatomic Molecules

Diatomic molecules are molecules composed of only two atoms. These atoms can be either of the same element or different elements. Examples of diatomic molecules include oxygen ( O_2 ), nitrogen ( N_2 ), and each of the halogen molecules such as fluorine ( F_2 ), chlorine ( Cl_2 ), bromine ( Br_2 ), and iodine ( I_2 ). In the context of this exercise, we are looking at diatomic molecules that are formed from the same type of atom, making them homonuclear diatomic molecules.

Understanding diatomic molecules is fundamental in chemistry because they illustrate how atoms can come together to form a chemical bond. The bonds in these molecules are characterized by the distance between atoms and the energy required to break them. Bond enthalpy, which is the focus of the exercise, gives us a quantitative measure of this energy. By studying diatomic molecules, students can get a clearer idea of how chemical bonds are formed and broken. This knowledge further extends to more complex compounds, making diatomic molecules a basic yet crucial concept in chemistry.

Chemical Bond Strength

Chemical bond strength is determined by the amount of energy needed to break a bond between two atoms. This energy is specified as bond enthalpy, measured in kilojoules per mole ( kJ/mol ). In simple terms, the larger the bond enthalpy, the stronger the bond between the two atoms in a molecule, and vice versa.

In our exercise, we compared bond enthalpies of various diatomic halogen molecules. Here's how bond enthalpy values relate to bond strength:

• A bond enthalpy of 151 kJ/mol for I-I in iodine indicates a weaker bond because it requires less energy to break.
• F-F bond in fluorine with a bond enthalpy of 158 kJ/mol reflects relatively weak bond strength for its size.
• The Cl-Cl bond is significantly stronger than I-I as indicated by its bond enthalpy of 242 kJ/mol.
• Br-Br falls in between those two with a bond enthalpy of 193 kJ/mol.

These values help us understand that iodine ( I_2 ) has the weakest bond among F_2 , Cl_2 , Br_2 , and I_2 . Studying chemical bond strength guides chemists in predicting the reactivity and stability of various substances.

Thermochemistry

Thermochemistry is a branch of chemistry that studies the energy changes involved in chemical reactions, primarily in terms of heat. It connects the concepts of temperature, energy, and heat flow to chemical processes. Bond enthalpy is a key concept in thermochemistry as it allows us to quantify the energy required to break or form bonds during chemical reactions.

Enthalpy changes are calculated when bonds form and break, marking whether a reaction is endothermic (absorbs heat) or exothermic (releases heat). For instance, when a diatomic molecule like I_2 is broken into separate iodine atoms, energy is absorbed, reflecting an endothermic process.

Here's why understanding thermochemistry is useful:

• It enables chemists to predict the course and energy changes of reactions.
• Understanding energy flow helps in designing processes that minimize energy waste.
• It assists in determining reaction conditions favoring efficiency and yield.

Thus, learning thermochemistry helps students to bridge the gap between theoretical chemistry and practical applications, emphasizing the significance of energy considerations in chemical transformations.