Chemical Energetics

Define the term 'enthalpy change'.

• Enthalpy change is the heat absorbed or released by a system at constant pressure during a chemical reaction.

• Enthalpy change is the change in color of a system during a chemical reaction.

• Enthalpy change is the change in volume of a system during a chemical reaction.

• Enthalpy change is the change in temperature of a system during a chemical reaction.

Explain the difference between endothermic and exothermic reactions in terms of energy changes.

• Endothermic reactions and exothermic reactions have the same energy changes.

• Endothermic reactions release energy, exothermic reactions absorb energy.

• Endothermic reactions absorb energy, exothermic reactions release energy.

• Endothermic reactions do not involve energy changes, exothermic reactions absorb energy.

What is the standard enthalpy of formation?

• The energy required to form one mole of a compound from its elements in their standard states.

• Enthalpy change when one mole of a compound is formed from its elements in their standard states.

• The enthalpy change when one mole of a compound is formed from its elements in their non-standard states.

• Enthalpy change when one mole of a compound is decomposed into its elements in their standard states.

Describe how Hess's Law is used to calculate the enthalpy change of a reaction.

• Hess's Law calculates the enthalpy change by multiplying the enthalpy changes of other reactions

• Hess's Law is used to calculate the enthalpy change of a reaction by summing up the enthalpy changes of other reactions that add up to the desired reaction.

• Hess's Law is only applicable to gaseous reactions

• Hess's Law cannot be used to calculate enthalpy changes

What is meant by the term 'bond enthalpy'?

• Bond enthalpy is the energy released when a bond is formed.

• Bond enthalpy is the energy needed to form a bond between two atoms.

• Bond enthalpy is the energy required to break one mole of a bond in a solid substance.

• Bond enthalpy is the energy required to break one mole of a particular type of bond in a gaseous substance.

Discuss the concept of entropy and its relationship to spontaneity of a reaction.

• Entropy is related to the spontaneity of a reaction as an increase in entropy often leads to spontaneity.

• Decrease in entropy always leads to spontaneity

• Entropy has no impact on spontaneity of a reaction

• Entropy is a measure of energy in a reaction

How does temperature affect the rate of a chemical reaction?

• Temperature affects the rate of a chemical reaction by increasing the kinetic energy of molecules, leading to more frequent and energetic collisions.

• Temperature only affects the color of the chemicals, not the rate of reaction

• Lowering the temperature increases the rate of a chemical reaction

• Temperature has no effect on the rate of a chemical reaction

Explain the significance of Gibbs free energy in determining the spontaneity of a reaction.

• Gibbs free energy is only applicable to exothermic reactions

• Gibbs free energy is used to determine the rate of a reaction

• Gibbs free energy is crucial in predicting the spontaneity of a reaction based on its value (negative or positive).

• Gibbs free energy is not affected by temperature changes

What is the difference between a state function and a path function in thermodynamics?

• State functions depend only on the current state of the system, while path functions depend on the path taken to reach that state.

• State functions and path functions are interchangeable in thermodynamics.

• State functions depend on the path taken to reach the state, while path functions depend on the current state of the system.

• State functions are only applicable to closed systems, while path functions are for open systems.

Calculate the standard enthalpy change for the combustion of methane (CH4) given the following data: C(s) + 2H2(g) -> CH4(g) ΔH = -74.8 kJ/mol, O2(g) -> 2O(g) ΔH = 495 kJ/mol, CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l) ΔH = -890.3 kJ/mol.

• -74.8 + 495 + (-890.3) = -469.1 kJ/mol

• -74.8 - 495 - 890.3 = -1460.1 kJ/mol

• -74.8 + 495 + 890.3 = 1311.5 kJ/mol

• -74.8 + 495 - 890.3 = -469.1 kJ/mol