Question

The false statement among the following is (1) \(\Delta H\) for the thermal decomposition process is always positive. (2) Bond-breaking cnergy of a molecule is always positive. (3) Conversion of oxygen into ozone is endothermic reaction and hence it is more stable than oxygen. (4) The heat change in a chemical reaction is represented by enthalpy change.

Short answer

Statement 3 is false.

Step-by-step solution

Analyze each statement

Examine the validity of each statement given the principles of thermodynamics and chemical reactions.

Evaluate Statement 1

Statement 1: \(\Delta H\) for the thermal decomposition process is always positive. This statement is true. Thermal decomposition requires energy input to break bonds, making the enthalpy change positive.

Evaluate Statement 2

Statement 2: Bond-breaking energy of a molecule is always positive. This statement is true. Breaking chemical bonds requires energy, hence bond dissociation energy is always positive.

Evaluate Statement 3

Statement 3: Conversion of oxygen into ozone is endothermic reaction and hence it is more stable than oxygen. This statement is false. While the conversion is endothermic, ozone is actually less stable than oxygen due to the high energy of ozone's bonds.

Evaluate Statement 4

Statement 4: The heat change in a chemical reaction is represented by enthalpy change. This statement is true. Enthalpy change \(\Delta H\) represents the heat absorbed or released during a chemical reaction at constant pressure.

Key concepts

enthalpy change

Enthalpy change, denoted as \(\Delta H\), refers to the amount of heat absorbed or released in a chemical reaction at constant pressure. It is an important concept in thermodynamics and helps us understand energy changes during reactions.
For instance, if a reaction absorbs heat from its surroundings, the enthalpy change is positive, indicating an endothermic process. Conversely, if a reaction releases heat, the enthalpy change is negative, indicating an exothermic process.
Understanding \(\Delta H\) allows chemists to predict whether a reaction will proceed spontaneously and how much energy is required or released. This concept is crucial in designing experimental conditions and understanding natural processes.

thermal decomposition

Thermal decomposition is a chemical process where a compound breaks down into simpler substances due to heat. This process typically requires the input of energy, making it endothermic.
During thermal decomposition, bonds within the compound are broken, which consumes energy. For example, when calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide:
\[ \text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2 \]
This reaction requires heat to proceed, highlighting that thermal decomposition processes usually have positive enthalpy changes (\( \Delta H \) > 0). Such reactions are essential in various industrial applications, like producing lime from limestone.

bond dissociation energy

Bond dissociation energy is the energy required to break a specific chemical bond in a molecule, resulting in the formation of neutral atoms. This energy is always positive because breaking a bond consumes energy.
Consider the classic example of breaking a hydrogen molecule (H₂) into two hydrogen atoms:
\[ \text{H}_2 \rightarrow 2\text{H} \]
The energy required to break the H-H bond is defined as its bond dissociation energy. Higher bond dissociation energy indicates a stronger bond, meaning more energy is needed to break it. This concept is vital in understanding reaction mechanisms and the stability of molecules.

endothermic reactions

Endothermic reactions are chemical reactions that absorb heat from their surroundings. During such reactions, the system requires energy input, resulting in a positive enthalpy change (\( \Delta H \) > 0).
An example of an endothermic reaction is the conversion of oxygen gas (O₂) into ozone (O₃):
\[ 3\text{O}_2 \rightarrow 2\text{O}_3 \]
This reaction absorbs energy from its surroundings to form ozone, hence it is endothermic. Although endothermic reactions require energy to proceed, they are essential in various natural and industrial processes. For instance, photosynthesis in plants is an endothermic reaction where sunlight provides the necessary energy to convert carbon dioxide and water into glucose and oxygen.