Problem 81 - Chemistry The Central Science In Si Units

Question

Without doing any calculations, predict the sign of \(\Delta H\) for each of the following reactions: (a) \(\mathrm{NaCl}(s) \longrightarrow \mathrm{Na}^{+}(g)+\mathrm{Cl}^{-}(\mathrm{g})\) (b) \(2 \mathrm{H}(g) \longrightarrow \mathrm{H}_{2}(g)\) (c) \(\mathrm{Na}(g) \longrightarrow \mathrm{Na}^{+}(g)+\mathrm{e}^{-}\) (d) \(\mathrm{I}_{2}(s) \longrightarrow \mathrm{I}_{2}(l)\)

Step-by-Step Solution

Answer

(a) Positive, (b) Negative, (c) Positive, (d) Positive.

Step 1: Analyze Reaction (a)

The reaction involves dissociating solid NaCl into gaseous ions. This process requires breaking strong ionic bonds, which typically requires energy. Therefore, the enthalpy change, \( \Delta H \), is likely to be positive.

Step 2: Analyze Reaction (b)

Here, diatomic hydrogen molecules form from gaseous hydrogen atoms. When bonds are formed, energy is usually released, suggesting that the enthalpy change, \( \Delta H \), is negative.

Step 3: Analyze Reaction (c)

This reaction involves removing an electron from gaseous sodium to form a sodium ion, which requires energy input to overcome the attraction between the electron and the nucleus. Thus, \( \Delta H \) is expected to be positive.

Step 4: Analyze Reaction (d)

Going from a solid to a liquid phase involves weakening intermolecular forces, which typically requires energy. As a result, the enthalpy change, \( \Delta H \), is likely to be positive.

Key Concepts

Reaction Energetics

Reaction energetics involves understanding how energy is absorbed or released during a chemical reaction. The quantity that measures this energy change is called enthalpy change, represented as \(\Delta H\). Whether \(\Delta H\) is positive or negative indicates the behavior of the reaction energetically.

• If \(\Delta H\) is positive, the reaction is endothermic, meaning it absorbs energy from the surroundings. This typically happens when bonds are broken, requiring an input of energy.
• If \(\Delta H\) is negative, the reaction is exothermic, releasing energy into the surroundings. This occurs when new bonds are formed, a process that typically releases energy.

Understanding the signs of enthalpy change can help predict the energy requirements or output of a reaction without performing detailed calculations.

Phase Transitions

Phase transitions involve a substance changing from one state of matter to another, such as from solid to liquid or gas. These transitions are accompanied by enthalpy changes due to the energy required to overcome intermolecular forces.

• During a transition from solid to liquid, as in melting, energy is needed to break the structured arrangement of molecules in a solid. This requires energy, leading to a positive \(\Delta H\).
• Conversely, when a gas transitions back to a liquid or solid, energy is typically released as the molecules settle into a more stable, intermolecular arrangement, resulting in a negative \(\Delta H\).

By understanding these transitions, one can predict the likelihood of \(\Delta H\) being positive or negative, helping in the analysis of energy changes in reactions.

Bond Formation and Breaking

In chemical reactions, bonds must be broken and then formed anew to create different substances. The process of bond formation and breaking is central to understanding reaction energetics and often dictates whether a reaction is endothermic or exothermic.

• Breaking bonds, such as during the dissociation of molecules like NaCl, requires energy. Hence, reactions where bonds are broken are usually endothermic with a positive \(\Delta H\).
• On the other hand, when bonds are formed, such as when two hydrogen atoms form \(\text{H}_2\), energy is released. This results in an exothermic reaction with a negative \(\Delta H\).

By studying the types of bonds involved in a reaction, one can easily predict whether the overall enthalpy change will be positive or negative, thus informing us about the energetic nature of the reaction.

Ionic and Covalent Bonds

Ionic and covalent bonds represent two major types of chemical bonds, each involving different kinds of interactions between atoms. Understanding these bonds is crucial when analyzing reactions like those in the provided exercise.

• Ionic bonds form between metal and non-metal ions, such as Na\(^+\) and Cl\(^{-}\) in NaCl. These bonds are formed through electrostatic attraction and are generally very strong. Breaking them, as in converting NaCl\(_{(s)}\) to Na\(^+\)\(_{(g)}\) and Cl\(^{-}\)\(_{(g)}\), requires substantial energy, resulting in a positive \(\Delta H\).
• Covalent bonds, by contrast, involve the sharing of electrons between atoms. When such bonds are formed, as in the case where two hydrogen atoms bond to form \(\text{H}_2\), energy is released, making the \(\Delta H\) of the reaction negative.

By clearly understanding the nature of ionic and covalent bonds, students can more accurately predict the enthalpy changes in various chemical reactions.