Energy Changes in Chemical Reactions - SS1 Chemistry Past Questions and Answers - page 4
Define heat of formation and explain how it is related to enthalpy changes and Hess's Law.
The heat of formation (ΔHf) is the enthalpy change that occurs when one mole of a compound is formed from its constituent elements in their standard states. It is a measure of the energy released or absorbed during the formation of a compound.
The heat of formation is closely related to enthalpy changes and Hess's Law. According to Hess's Law, the enthalpy change of a reaction can be calculated by considering the enthalpy changes of other reactions.
Which of the following best defines spontaneity in a chemical reaction?
The reaction occurs rapidly.
The reaction occurs without any external influence.
The reaction occurs with a decrease in entropy.
The reaction occurs at a constant temperature.
Spontaneous reactions are characterised by:
A positive change in Gibbs free energy (ΔG).
A negative change in Gibbs free energy (ΔG).
A zero change in Gibbs free energy (ΔG).
An unpredictable change in Gibbs free energy (ΔG).
Which of the following statements is true regarding the Gibbs free energy (ΔG) of a spontaneous reaction?
ΔG is always positive.
ΔG is always negative.
ΔG can be positive or negative depending on the reaction conditions.
ΔG is always zero.
The relationship between the change in Gibbs free energy (ΔG), enthalpy change (ΔH), and entropy change (ΔS) is given by:
ΔG = ΔH + ΔS
ΔG = ΔH - ΔS
ΔG = ΔH/ΔS
ΔG = ΔS/ΔH
Which of the following conditions indicates that a reaction is spontaneous at all temperatures?
ΔH > 0 and ΔS > 0
ΔH < 0 and ΔS < 0
ΔH > 0 and ΔS < 0
ΔH < 0 and ΔS > 0
The spontaneity of a reaction can be determined by:
The initial concentrations of reactants.
The activation energy of the reaction.
The sign of ΔG.
At equilibrium, the value of ΔG for a reaction is:
Negative
Zero
Cannot be determined
Which of the following conditions indicates a non-spontaneous reaction?
ΔG = 0
ΔG < 0
ΔG > 0
ΔG is not defined for non-spontaneous reactions.
The spontaneity of a reaction can be determined by comparing:
Activation energy with enthalpy change.
Enthalpy changes with entropy change.
Gibbs free energy change with activation energy.
Entropy changes with activation energy.