Are you ready to take this lesson's quiz? These questions will help you find out. Make sure you understand why each correct answer is correct--if you don't, review that part of the lesson.
1. Chemical equilibrium is defined by what being equal?
- the concentrations of the reactants and products
- the masses of the reactants and products
- the rates of the forward and reverse reactions
- the temperatures of the system and the surroundings
The concentrations of the reactants and products remain constant at equilibrium. They are not necessarily equal to each other.
According to the law of conservation of mass, the total mass of the reactants must equal the total mass of the products. This is always true, regardless of whether a reaction is at equilibrium or not.
Equilibrium is defined as the rate of the forward reaction being equal to the rate of the reverse reaction.
When the temperatures of the system and the surroundings are equal to each other, they have reached thermal equilibrium, not chemical equilibrium.
2. What is the equilibrium constant for the reaction represented by this equation?
2NO (g) ⇌ N2 (g) + O2 (g)
- K= [N2][O2][NO]
- K= [N2][O2][NO]2
- K= [NO]2[N2][O2]
- K= [N2][O2]2[NO]
Remember that the coefficients from the balanced equation become exponents when writing the equilibrium constant expression.
The equilibrium constant expression is a ratio of the concentrations of the products over the reactants, with coefficients becoming exponents.
The equilibrium constant expression is a ratio of the concentration of the products over the reactants, not the other way around.
When writing an equilibrium constant expression, the coefficients from the balanced equation become exponents, not multipliers.
3. What does the value of the equilibrium constant (K) mean about a reaction at equilibrium?
- A large K value means the reaction has not yet reached completion. A small K value means the reaction has reached completion.
- A large K value means the reaction has reached completion. A small K value means the reaction has not yet reached completion.
- A large K value means there are more reactants present at equilibrium. A small K value means there are more products present at equilibrium.
- A large K value means there are more products present at equilibrium. A small K value means there are more reactants present at equilibrium.
A reversible reaction never reaches “completion.” Instead, it can reach equilibrium, where the rates of the forward and reverse reactions are equal.
A reversible reaction never reaches “completion.” Instead, it can reach equilibrium, where the rates of the forward and reverse reactions are equal.
K is a ratio of products over reactants. If there are more products present, then the numerator is large, resulting in a large K value overall. Conversely, if there are more reactants present, then the denominator is large, resulting in a small K value overall.
K is a ratio of products over reactants. If there are more products present, then the numerator is large, resulting in a large K value overall. Conversely, if there are more reactants present, then the denominator is large, resulting in a small K value overall.
4. Use Le Chatelier’s Principle to predict how the equilibrium of this reaction will shift with the addition of more carbon dioxide gas (CO2).
CO2 (g) + C (s) ⇌ 2 CO (g)
- shift towards products (forward direction)
- shift towards reactants (reverse direction)
- no shift
- shift towards products and then back towards reactants
Increasing the concentration of a reactant will shift the reaction to use up the excess chemical and make more products to get back to the equilibrium ratio.
Increasing the concentration of a reactant will shift the reaction to use up the excess chemical and make more products to get back to the equilibrium ratio.
Changing the concentration of a gaseous or aqueous substance will always cause the reaction to shift in order to get back to the equilibrium ratio.
Le Chatelier’s Principle predicts how a reaction will shift to accommodate an external stress in order to get back to the equilibrium ratio. Shifting in only one direction (either forward or reverse) is required for this.
5. Use Le Chatelier’s Principle to predict how the equilibrium of this reaction will shift when the pressure is increased, or the temperature is increased.
2 SO2 (g) + O2 (g) ⇌ 2 SO3 (g) + heat
- Increasing the pressure will cause a shift towards the reactants (reverse direction). Increasing the temperature will cause a shift towards the products (forward direction).
- Increasing the pressure will cause a shift towards the products (forward direction). Increasing the temperature will cause a shift towards the reactants (reverse direction).
- Increasing the pressure or increasing the temperature would each cause a shift towards the reactants (reverse direction).
- Increasing the pressure or increasing the temperature would each cause a shift towards the products (forward direction).
The reaction would shift towards the reactants if the pressure were decreased, because the reactant side makes more gas molecules, which would make up for the deficit in pressure. The reaction would shift towards the products if the temperature were decreased because that is the exothermic direction, which would produce more heat to make up for the deficit in heat energy.
Increasing the pressure will cause the reaction to shift in the direction that will make fewer gas molecules in order to reduce the overall pressure. Increasing the temperature will cause the reaction to shift in the endothermic direction to use up the excess heat energy.
The reaction would shift towards the reactants if the pressure were decreased because the reactant side makes more gas molecules, which would make up for the deficit in pressure.
The reaction would shift towards the products if the temperature were decreased because that is the exothermic direction, which would produce more heat to make up for the deficit in heat energy.
Summary
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