Problem 30 - A Complete Resource Book in Chemistry for JEE Main

Question

Standard molar enthalpy of formation of \(\mathrm{CO}_{2}\) is equal to (a) zero (b) the standard molar enthalpy of combustion of gaseous carbon. (c) the sum of standard molar enthalpies of formation Of \(\mathrm{CO}\) and \(\mathrm{CO}_{2}\) (d) the standard molar enthalpy of combustion of carbon (graphite)

Step-by-Step Solution

Answer

The answer is (d), it equals the standard molar enthalpy of combustion of carbon (graphite).

Step 1: Understanding Standard Molar Enthalpy of Formation

The standard molar enthalpy of formation is defined as the heat change that results when one mole of a compound is formed from its elements in their standard states under standard conditions (1 atm pressure and 298 K temperature). For a compound like \(\mathrm{CO}_{2}\), this process involves the reaction of carbon in its standard state (graphite) with oxygen.

Step 2: Write the Equation for the Formation of CO2

The formation of \(\mathrm{CO}_{2}\) from its elements is given by the chemical equation:\[\text{C(graphite)} + \text{O}_2(g) \rightarrow \text{CO}_2(g)\]This equation represents the formation of one mole of \(\mathrm{CO}_{2}\) from carbon (graphite) and oxygen gas.

Step 3: Recognize Relation to Combustion

The reaction described in Step 2 is also the combustion of carbon (graphite) in the presence of oxygen, which is defined as the standard molar enthalpy of combustion of carbon (graphite). This aligns with answer choice (d).

Key Concepts

Combustion

Combustion is a chemical reaction that occurs when a substance combines with oxygen to release energy in the form of heat and light. This process is often associated with burning. In most everyday instances, combustion involves the rapid reaction of a fuel with oxygen, leading to the production of

1. carbon dioxide,
2. water,
3. heat, and
4. sometimes light.

In the context of chemistry and standard molar enthalpy, the combustion process is particularly significant because it allows us to measure the energy change involved when one mole of a substance reacts completely with oxygen under standard conditions. For example, when carbon (in its graphite form) combusts, it reacts with oxygen to form carbon dioxide (\[ ext{C(graphite)} + ext{O}_2(g) ightarrow ext{CO}_2(g) \] ). This reaction is a classic example of combustion, and the enthalpy change for this reaction is known as the standard molar enthalpy of combustion.

Standard Conditions

Standard conditions in chemistry refer to a set of specific conditions under which measurements are made to ensure consistency and comparability in data. These conditions typically include a pressure of 1 atmosphere (atm) and a temperature of 298 Kelvin (K), which is approximately 25°C or room temperature.
This is important because the enthalpy change of a reaction can vary depending on the conditions under which it occurs.

• By using standard conditions, chemists can ensure that measurements like the standard molar enthalpy of formation are consistent.
• This makes it easier to compare values from different experiments and understand how energy is transferred during chemical reactions.

Enthalpy changes, such as those found in combustion reactions or formation processes, are recorded at these standard conditions to provide meaningful data for scientists and students as they study different substances and their reactions.

Chemical Reactions

Chemical reactions involve the breaking of bonds in reactants and the formation of new bonds in products. This process depends on the rearrangement of atoms and typically results in a transformation of energy.
Understanding chemical reactions is vital in predicting the behavior of substances and in practical applications such as energy production, material synthesis, and biological processes.

• Each step of a reaction must follow the law of conservation of mass, meaning the number of atoms of each element is conserved throughout the process.
• Reactions are often represented by balanced chemical equations which show the reactants converting into products.

For instance, the reaction of carbon with oxygen to form carbon dioxide is represented as \[ ext{C(graphite)} + ext{O}_2(g) ightarrow ext{CO}_2(g) \]
This balanced equation demonstrates the conversion of reactants (carbon and oxygen) to a product (carbon dioxide), involving an enthalpy change that can be measured under standard conditions. Recognizing these transformations underlies many basic and advanced studies in chemistry.