Question

a. Distinguish between enthalpies of reaction, formation, and combustion. b. On what basis are enthalpies of formation and combustion defined?

Short answer

Enthalpy of reaction is the heat change during a reaction. Enthalpy of formation is the heat change when forming one mole of a compound from its elements. Enthalpy of combustion is the heat change when one mole of a substance burns in oxygen. Formation and combustion values are defined under standard conditions.

Step-by-step solution

- Define Enthalpy of Reaction

The enthalpy of reaction, \(\u0394H_{\text{reaction}}\), is the heat change associated with a chemical reaction. It is the difference between the enthalpy of the products and the enthalpy of the reactants at constant pressure.

- Define Enthalpy of Formation

The enthalpy of formation, \( \u0394H_{\text{f}}^o \), is the heat change that occurs when one mole of a compound is formed from its elements in their standard states. It is usually measured under standard conditions (298 K, 1 atm).

- Define Enthalpy of Combustion

The enthalpy of combustion, \( \u0394H_{\text{c}}^o \), is the heat change when one mole of a substance is completely burned in oxygen under standard conditions. It also typically refers to the reaction of a substance with oxygen to form CO2 and H2O.

- Basis for Enthalpy of Formation

Enthalpy of formation is defined based on the formation of one mole of a compound from its constituent elements in their standard states. For example, the enthalpy change for the formation of water from hydrogen and oxygen gas.

- Basis for Enthalpy of Combustion

Enthalpy of combustion is defined based on the complete combustion of one mole of substance with oxygen, yielding products like CO2 and H2O, all under standard conditions.

Key concepts

enthalpy of reaction

In chemistry, the enthalpy of reaction, represented as \(\Delta H_{\text{reaction}}\), is a crucial concept. It refers to the heat change that happens during a chemical reaction. Basically, it tells us how much heat is absorbed or released when the reactants turn into products at constant pressure.

For instance, consider a simple reaction like the combustion of methane: \[ CH_4 + 2 O_2 \rightarrow CO_2 + 2 H_2O \] The enthalpy of reaction for this process would be the difference in enthalpy between the carbon dioxide and water produced and the methane and oxygen that reacted.

This value is very helpful because it helps us understand how energy is involved in different chemical processes, which is essential for system design and safety in chemical engineering.

enthalpy of formation

The enthalpy of formation, denoted as \(\Delta H_{\text{f}}^o\), is another important concept in thermodynamics. This measures the heat change when one mole of a compound is created from its basic elements, all in their standard states (usually at 298 K and 1 atm pressure).

For example, let's look at the formation of water from hydrogen and oxygen: \[ H_2(g) + \frac{1}{2} O_2(g) \rightarrow H_2O(l) \] The enthalpy of formation for water would be the energy released or absorbed during this specific reaction.

This metric helps chemists understand and estimate the stability and reactivity of compounds. It also plays a critical role in calculating the energy needed for various reactions.

enthalpy of combustion

Enthalpy of combustion, symbolized as \(\Delta H_{\text{c}}^o\), refers to the heat change when one mole of a substance is burned completely in the presence of oxygen, all under standard conditions. This typically means the substance reacts with oxygen to form common combustion products like CO2 and H2O.

Take the combustion of ethanol as an example: \[ C_2H_5OH(l) + 3 O_2(g) \rightarrow 2 CO_2(g) + 3 H_2O(l) \] The enthalpy of combustion for ethanol would be the energy change when one mole of ethanol combusts in oxygen.

Knowing the enthalpy of combustion is very useful for energy considerations. It helps in understanding how much energy a fuel can release, which is vital for designing engines, furnaces, and even in food stuff to gauge calorific values.