17272348834723380015126120954427.jpg

## Enthalpy 1. The enthalpy of formation for all elements in their standard states is **zero**. 2. The exothermic reaction is **C(s) → C(g)**. 3. The enthalpy of vaporization of ethanol is **38.9 kJ mol-1**. 4. The standard enthalpy of formation of HCl is **-93 kJ mol-1**. 5. **Comment on the...

## Enthalpy 1. The enthalpy of formation for all elements in their standard states is **zero**. 2. The exothermic reaction is **C(s) → C(g)**. 3. The enthalpy of vaporization of ethanol is **38.9 kJ mol-1**. 4. The standard enthalpy of formation of HCl is **-93 kJ mol-1**. 5. **Comment on the statement: no work is involved in the expansion of gas in vacuum.** - The expansion of a gas in a vacuum is an example of a process that is **irreversible**. - Work is done by the system during the expansion, but it is not **reversible**. - Therefore, we can say that no work is done **in the expansion**, but there is work done **by the system**. 6. **State the first law of thermodynamics.** - The first law of thermodynamics states that the **total energy of a system remains constant** in any process. - This law can be written as **ΔU = Q + W**, where ΔU is the change in internal energy, Q is heat transfer, and W is work done. 7. **What is enthalpy of fusion?** - Enthalpy of fusion is the **amount of heat required to melt one mole of a substance** at its melting point. 8. **What is the standard state of a substance?** - The standard state of a substance is its **most stable form at 1 atm pressure and 298 K**. 9. **State whether ΔS is positive, negative or zero for the reaction 2H(g) → H₂(g). Explain.** - In this reaction, two gaseous hydrogen atoms combine to form one gaseous hydrogen molecule. - This process involves a decrease in the number of particles. - This decrease in the number of particles leads to a **decrease in entropy**. - Therefore, **ΔS is negative**. 10. **State second law of thermodynamics in terms of entropy.** - The second law of thermodynamics states that **the entropy of an isolated system always increases over time**. - This law can also be stated that **spontaneous processes always involve an increase in the total entropy of the universe**. 11. **If the enthalpy change of a reaction is ΔH how will you calculate entropy of surroundings?** - Entropy change of surroundings can be calculated using the following equation: - ΔSsurroundings = -ΔH/T - Where T is the temperature of the surroundings. 12. **Comment on spontaneity of reactions for which ΔH is positive and ΔS is negative.** - For a reaction to be spontaneous, the Gibbs free energy change (ΔG) must be negative. - ΔG is given by the equation: ΔG = ΔH - TΔS. - If ΔH is positive and ΔS is negative, ΔG will be positive. - Therefore, reactions with **positive ΔH and negative ΔS are not spontaneous**. ## Enthalpy (Continued) 1. **Obtain the relationship between ΔGo of a reaction and the equilibrium constant.** - The relationship between ΔGo and the equilibrium constant, K, can be expressed using the following equation: - **ΔGo = -RTlnK** - Where, R is the ideal gas constant, T is the temperature in Kelvin. 2. **How will you calculate reaction enthalpy from data on bond enthalpies?** - Reaction enthalpy can be calculated from bond enthalpies using the following equation: - ΔHreaction = Σ(bond energies of reactants) - Σ(bond energies of products) 3. **What is entropy? Give its units.** - Entropy is a thermodynamic property that measures **the degree of disorder or randomness in a system**. - Units of entropy are **J K-1 mol-1** 4. **ΔH = 40.63 kJ mol-1 and ΔS = 108.8 J K-1 mol-1. At what temperature, ΔG = 0 ?** - The temperature at which ΔG=0 can be calculated using the following equation: - **T = ΔH/ ΔS** - Substituting the values we get: - T = (40.63 kJ mol-1) / (108.8 J K-1 mol-1) - T = 373.4 K 5. **What is the standard enthalpy of combustion? Give an example.** - The standard enthalpy of combustion is **the enthalpy change when 1 mol of a substance is completely burned in oxygen under standard conditions**. - An example is the combustion of methane: - CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l), ΔHocombustion = -890 kJ mol-1. 6. **What is the enthalpy of atomization? Give an example.** - The enthalpy of atomization is **the enthalpy change when 1 mol of a substance in its standard state is broken down into its atoms in the gaseous state**. - An example is the atomization of oxygen: - O2(g) -> 2O(g), ΔHoatomization = +498 kJ mol-1. 7. **Obtain the expression for work done in chemical reaction.** - The expression for work done in a chemical reaction is: - **W = - PΔV** - Where, P is the external pressure, ΔV is the change in volume. Note that, due to the image's quality some of the answer might be slightly different from the actual ones.