Chemical Thermodynamics - SS2 Chemistry Past Questions and Answers - page 3
In a chemical reaction at equilibrium,
the concentrations of reactants and products remain constant.
the forward reaction rate is always equal to the reverse reaction rate.
the reaction has come to a complete stop.
the concentrations of reactants are always higher than the concentrations of products.
The equilibrium constant (K) for a reaction is 1.0 × 103. What can be said about the position of the equilibrium?
The reaction favours the reactants.
The reaction favours the products.
The reaction is at equilibrium.
The position of the equilibrium cannot be determined from K alone.
The value of the equilibrium constant (K) is less than 1.0 for a reaction. What can be inferred about the relative concentrations of reactants and products at equilibrium?
The concentration of reactants is higher than the concentration of products.
The concentration of products is higher than the concentration of reactants.
The concentrations of reactants and products are equal.
The concentrations of reactants and products cannot be determined from K alone.
Which of the following factors can affect the position of equilibrium?
Temperature
Pressure
Concentration of reactants and products
All of the above
Explain the concept of spontaneity in chemical reactions. What factors determine whether a reaction is spontaneous or nonspontaneous? Provide examples.
Spontaneity refers to the tendency of a chemical reaction to occur without any external influence. A spontaneous reaction proceeds on its own, releasing energy and progressing toward equilibrium. The spontaneity of a reaction depends on two main factors: enthalpy (ΔH) and entropy (ΔS).
If a reaction has a negative ΔH (exothermic, releases heat) and a positive ΔS (increase in disorder), it will be spontaneous at all temperatures. For example, the combustion of gasoline (exothermic) and the dissolution of sugar in water are spontaneous processes.
Conversely, a reaction with a positive ΔH (endothermic, absorbs heat) and a negative ΔS (decrease in disorder) is non-spontaneous at all temperatures. An example is the decomposition of water into hydrogen and oxygen gas.
However, if a reaction has a positive ΔH and a positive ΔS, it can still be spontaneous at high temperatures. The increase in temperature allows the entropy term to dominate, driving the reaction forward. An example is the Haber-Bosch process, where nitrogen and hydrogen react to form ammonia.
Define chemical equilibrium and discuss the key principles associated with it. How do changes in concentration, pressure, and temperature affect the equilibrium position? Provide examples.
Chemical equilibrium refers to the state in a reversible reaction where the forward and reverse reactions occur at equal rates. At equilibrium, the concentrations of reactants and products remain constant, although the individual molecules continue to undergo reactions.
The principles associated with equilibrium include the law of mass action, which states that the ratio of the concentrations of reactants and products at equilibrium is constant and can be expressed as the equilibrium constant (K). Le Chatelier's principle is also crucial, stating that a system at equilibrium will respond to changes in concentration, pressure, or temperature to reestablish equilibrium.
Changes in concentration: If the concentration of a reactant or product is increased, the equilibrium will shift in the direction that consumes or reduces that substance. Conversely, if the concentration is decreased, the equilibrium will shift in the direction that produces or increases that substance. For example, increasing the concentration of CO2 in the reaction CO2 + H2O ⇌ H2CO3 will shift the equilibrium to the right, favouring the formation of carbonic acid.
Changes in pressure: For reactions involving gases, changes in pressure can affect the equilibrium position. An increase in pressure shifts the equilibrium towards the side with fewer moles of gas, while a decrease in pressure favours the side with more moles of gas. The reaction N2(g) + 3H2(g) ⇌ 2NH3(g) illustrates this principle.
Changes in temperature: Altering the temperature affects the equilibrium position differently depending on whether the reaction is exothermic or endothermic. Increasing the temperature favours the endothermic reaction, while decreasing the temperature favours the exothermic reaction. An example is the equilibrium between nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O4).
Hess's Law is based on the principle of:
Conservation of energy
Conservation of mass
Conservation of momentum
Conservation of enthalpy
Which of the following is a correct statement regarding Hess's Law?
It states that the enthalpy change of a reaction depends on the physical state of the reactants.
It states that the enthalpy change of a reaction is independent of the pathway taken.
It states that the enthalpy change of a reaction is equal to the standard enthalpy of formation.
It states that the enthalpy change of a reaction is determined by the bond enthalpies of the reactants.
Which of the following is a correct statement regarding Hess's Law?
It states that the enthalpy change of a reaction depends on the physical state of the reactants.
It states that the enthalpy change of a reaction is independent of the pathway taken.
It states that the enthalpy change of a reaction is equal to the standard enthalpy of formation.
It states that the enthalpy change of a reaction is determined by the bond enthalpies of the reactants.
Which of the following best describes a thermochemical equation?
It represents the balanced chemical equation for a reaction.
It includes the enthalpy change associated with a chemical reaction.
It shows the stoichiometric coefficients of the reactants and products.
It represents the equilibrium constant for a chemical reaction.