The given statement “the overall chemical reaction is exothermic” is true regarding the enthalpy diagram and enthalpies of intermediate and overall chemical reactions.
Further explanation:
Properties are categorized into two types:
1. Intensive properties:
These properties depend on the nature of the substance and not on the size of the system. If the system is further divided into a number of subsystems, the values of intensive properties remain unchanged. Temperature, refractive index, molarity, concentration, pressure, and density are some of the examples of intensive properties.
2. Extensive properties:
These properties depend on the amount of the substance. These are additive in nature if a single system is divided into several subsystems. Mass, enthalpy, volume, energy, size, weight, and length are some of the examples of extensive properties.
Enthalpy:
It is a thermodynamic property that is defined as the sum of internal energy and the product of pressure (P) and volume (V) of the system. It is a state function, an extensive property, and is independent of the path followed by the system while moving from initial to the final point. The total enthalpy of the system cannot be measured directly so its change 
is usually measured.
The heat of reaction 
can have two values:
Case I: If the reaction is endothermic, more energy needs to be supplied to the system than that released by it. So 
comes out to be positive.
Case II: If the reaction is exothermic, more energy is released by the system than that supplied to it. So comes out to be negative.
In the given enthalpy diagram (refer to the attached image), line 1 shows the final or the product state. Line 2 indicates the initial or the reactant state. Line 3 or the dotted line indicates the formation of the first intermediate during the progress of the chemical reaction. Line 4 represents the formation of the second intermediate whose energy is higher than that of first intermediate.
According to the enthalpy diagram, the stable products are formed from the highly energetic unstable intermediate and energy is released during the process. Therefore the overall chemical reaction is exothermic.
In this system, the reactants (line 2) are converted to the first intermediate, represented by line 3. This intermediate is then converted into another higher energy intermediate shown by line 4. The second intermediate is then converted into products, shown by line 1. Therefore three intermediate reactions occur in this system.
The enthalpy of the third intermediate reaction is represented as . This reaction shows the conversion of highly energetic unstable intermediate into products. Since unstable intermediate is to be converted into stable products, this change is accompanied by the release of energy. Therefore the third intermediate reaction is exothermic, not endothermic in nature.
The longest arrow represents only the conversion of higher energy intermediate into products, not the overall reaction. Therefore the longest arrow does not represent the overall chemical reaction.
Learn more:
1. Calculate the enthalpy change using Hess’s Law: brainly.com/question/11293201
2. Find the enthalpy of decomposition of 1 mole of MgO: brainly.com/question/2416245
Answer details:
Grade: Senior School
Subject: Chemistry
Chapter: Thermodynamics
Keywords: enthalpy, exothermic, endothermic, negative, positive, longest arrow, overall reaction, third intermediate reaction, unstable, stable.
