Question

State whether each of these statements is true or false. (a) The longer the bond, the larger the bond enthalpy. (b) \(C-C\) bonds are stronger than \(C-H\) bonds. (c) A typical single bond length is in the \(5-10\) Ä range. (d) If you break a chemical bond, energy is released.(e) Energy is stored in chemical bonds.

Short answer

(a) False - Bond enthalpy is higher for shorter and stronger bonds. (b) False - \(C-H\) bonds are generally stronger than \(C-C\) bonds. (c) False - Typical single bond lengths are in the range of \(1-2\) Å. (d) False - Energy is absorbed when a chemical bond is broken. (e) True - Energy is stored in chemical bonds.

Step-by-step solution

(a) The longer the bond, the larger the bond enthalpy.

False. Bond enthalpy refers to the energy required to break a chemical bond. Generally, shorter bonds (which are stronger) require more energy to break and have higher bond enthalpies. In contrast, longer bonds (which are weaker) have smaller bond enthalpies.

(b) \(C-C\) bonds are stronger than \(C-H\) bonds.

False. In general, \(C-H\) bonds have higher bond enthalpies compared to \(C-C\) bonds. The bond enthalpy of the \(C-H\) bond is approximately \(413\,\text{kj/mol}\), while the bond enthalpy for the \(C-C\) bond is about \(348\,\text{kj/mol}\). This means that \(C-H\) bonds are stronger and require more energy to break.

(c) A typical single bond length is in the \(5-10\) Å range.

False. The given range of \(5-10\) Å is too large for typical single bond lengths. A more accurate range for single bond lengths would be \(1-2\) Å. For example, the bond length of the \(C-C\) single bond is about \(1.54\) Å, and the bond length of the \(C-H\) single bond is approximately \(1.09\) Å.

(d) If you break a chemical bond, energy is released.

False. This statement confuses bond breaking with bond forming. When a chemical bond is broken, energy is actually absorbed, not released. Bond forming, on the other hand, releases energy.

(e) Energy is stored in chemical bonds.

True. Energy is stored within the chemical bonds of a molecule, and this energy can be released by breaking these bonds during chemical reactions. This energy release is the driving force behind many chemical processes, including combustion and metabolism.

Key concepts

Bond Length and Strength

The bond length between two atoms within a molecule is distinctive and correlates with the bond's strength. To fully understand how these two properties are interconnected, it is crucial to clarify what is meant by strength in a chemical context. The strength of a bond typically refers to its bond enthalpy, or the energy required to break the bond.

Contrary to what might seem intuitive, a longer bond is not stronger. In fact, it's quite the opposite: longer bonds are usually weaker. This is because shorter bonds are formed by atoms that are closer together, which results in a stronger attractive force between them. Consequently, more energy is needed to overcome this attraction and break the bond.

In reviewing bond length and strength, it's also important to address misconceptions. For example, the assertion in our example exercise that 'the longer the bond, the larger the bond enthalpy' is false. A longer bond actually indicates a weaker interaction and thus a smaller bond enthalpy.

Chemical Bond Energy

Chemical bond energy, or bond enthalpy, represents an essential concept in the study of chemistry. It is the measure of the amount of energy needed to break a mole of bonds in molecules into its constituent atoms while the substance is in the gaseous state. This is a vital principle because it helps us understand the stability of compounds and the energy involved in chemical reactions.

An important point to underscore is that bond breaking requires the absorption of energy, which can be a common point of confusion. For instance, the statement 'If you break a chemical bond, energy is released' is inaccurate. Instead, energy must be supplied to break the bonds, which is often seen in endothermic reactions where heat is absorbed from the surroundings.

Conversely, the formation of chemical bonds releases energy, which is why exothermic reactions, such as combustion, generate heat. Indeed, energy is stored in these bonds as potential energy, which can later be released during chemical reactions, contributing to the overall enthalpy change.

Single Bond Length

The length of a single bond is a specific distance that defines the space between the nuclei of two atoms bonded together. It is measured in angstroms (Å), where 1 Å is equivalent to 10^-10 meters. The length of a bond can affect its properties, including the bond energy and overall molecule stability.

The range for typical single bond lengths should be narrow and precise. A range of '5-10 Å' is much too large to accurately represent most single bond lengths, as suggested in the exercise. Single bonds in molecules such as hydrocarbons, where carbon is a central atom, typically exhibit lengths in the range of '1-2 Å'. For example, the average length of a carbon-carbon (C-C) single bond, like in ethane, is approximately 1.54 Å, and for a carbon-hydrogen (C-H) bond, like in methane, it is about 1.09 Å.

It's essential for students to grasp that single bond lengths are relatively short due to the nature of the overlapping atomic orbitals involved, which allows for a firmer and more stable connection compared to what longer bond lengths would suggest.