Equilibria

Define Le Chatelier’s principle.

Reaction 1 describes the reversible reaction between yellow $\text{Fe}^{3+}(\text{aq})$ and colourless $\text{SCN}^-(\text{aq})$ to produce red $\text{FeSCN}^{2+}(\text{aq})$.

A mixture of $\text{Fe}^{3+}(\text{aq})$, $\text{SCN}^-(\text{aq})$ and $\text{FeSCN}^{2+}(\text{aq})$ is at equilibrium at 20 °C.

The temperature of this mixture is then increased to 50 °C and allowed to reach equilibrium.

Deduce the changes that occur, if any, in the equilibrium mixture at 50 °C compared to the equilibrium mixture at 20 °C.

• change in appearance:

• change in relative concentration of $\text{FeSCN}^{2+}(\text{aq})$:

• change in value of the equilibrium constant, $K_c$:

Calculate the initial amount, in mol, of $\text{Fe}^{3+}(\text{aq})$ added to $\text{SCN}^-(\text{aq})$ to produce this mixture.

initial amount of $\text{Fe}^{3+}(\text{aq})$ = .............................. mol

Calculate $K_c$ for reaction 1 and state its units.

Show your working.

$K_c$ = ..............................

units: ..............................

Ammonia is described as a weak base and sulfuric acid as a strong acid.

By using an equation, explain clearly what is meant by the term weak base.

Sulfuric acid is produced from sulfur trioxide which is made by the Contact process.

State three important operating conditions for the Contact process for the manufacture of sulfur trioxide.

For each of your conditions, you should avoid the use of vague phrases such as 'high temperature'.

condition 1 .................................................................................................................

condition 2 .................................................................................................................

condition 3 .................................................................................................................

How is the sulfur trioxide produced converted into sulfuric acid?

State Le Chatelier's Principle.

From your understanding of Le Chatelier’s Principle, state the conditions of temperature and pressure that could be used in order to produce an increased yield of methanol in this process.
In each case, explain why the yield would increase.

temperature ...............................................................................................................

explanation ................................................................................................................

pressure .....................................................................................................................

explanation ................................................................................................................

The carbon monoxide for use in the production of methanol may be formed by reacting carbon dioxide with hydrogen.

A mixture containing $0.70\text{ mol}$ of $\text{CO}_2$, $0.70\text{ mol}$ of $\text{H}_2$, $0.30\text{ mol}$ of CO and $0.30\text{ mol}$ of $\text{H}_2\text{O}$ was placed in a $1\text{ dm}^3$ flask and allowed to come to equilibrium at $1200\text{ K}$.

Calculate the amount, in moles, of each substance present in the equilibrium mixture at $1200\text{ K}$.

Hydrogen iodide can be made by heating together hydrogen gas and iodine vapour. The reaction is incomplete.

Write an expression for $K_c$ and state the units.

increasing the pressure applied to the equilibrium

decreasing the temperature of the equilibrium

A mixture of $0.02\text{ mol}$ of hydrogen and $0.02\text{ mol}$ of iodine was placed in a $1\text{ dm}^3$ flask and allowed to come to equilibrium at $650\text{ K}$.

Calculate the amount, in moles, of each substance present in the equilibrium mixture at $650\text{ K}$.

Write an expression for the equilibrium constant, $K_c$, for this reaction, clearly stating the units.

$K_c =$

units .................................................

Use your results from (b) to calculate the amount, in moles, of ethanoic acid present at equilibrium. Hence complete the table below.

Use your results to calculate a value for $K_c$ for this reaction.

What would be the effect, if any, on the amount of ester present if all of the water were removed from the flask and the flask kept for a further week at 25 °C?

Explain your answer.

Hydrogen iodide can be made by heating together hydrogen gas and iodine vapour. The reaction is incomplete.

Write an expression for $K_c$ and state the units.

$K_c =$ ........................................... units ...........................................

increasing the pressure applied to the equilibrium

decreasing the temperature of the equilibrium

A mixture of $0.02\text{ mol}$ of hydrogen and $0.02\text{ mol}$ of iodine was placed in a $1\text{ dm}^3$ flask and allowed to come to equilibrium at $650\text{ K}$.

Calculate the amount, in moles, of each substance present in the equilibrium mixture at $650\text{ K}$.

Describe what is meant by dynamic equilibrium.

Deduce the changes that occur, if any, in the equilibrium mixture after $\text{KSCN(s)}$ is added compared to the original equilibrium mixture.

• change in appearance
• change in relative concentration of $\text{Fe}^{3+}\text{(aq)}$
• change in value of the equilibrium constant, $K_\text{c}$

The expression for the equilibrium constant, $K_\text{c}$, for reaction 4 is shown.

$5.00 \times 10^{-5}\text{ mol}$ of $\text{Fe}^{3+}\text{(aq)}$ and $5.00 \times 10^{-5}\text{ mol}$ of $\text{SCN}^-\text{(aq)}$ are added together and allowed to reach equilibrium. The total volume of the mixture is $25.0\text{ cm}^3$.

At equilibrium the concentration of $\text{FeSCN}^{2+}\text{(aq)}$ is $4.23 \times 10^{-4}\text{ mol dm}^{-3}$.

Calculate the equilibrium constant, $K_\text{c}$, for reaction 4.

Include the units in your answer.

State and explain the effect of increasing temperature on the percentage of PCl₅(g) that decomposes.

Explain the meaning of the term dynamic equilibrium and the conditions necessary for it to become established.

Calculate the partial pressure of phosphorus(V) chloride, $p\text{PCl}_5$, in this equilibrium mixture.

Write the expression for the equilibrium constant, $K_p$, for the decomposition of PCl₅(g).

The partial pressures of PCl₃(g) and of Cl₂(g) in this equilibrium mixture are both $2.86 \times 10^4 \text{ Pa}$.

Calculate the value of $K_p$ and state its units.

State one difference in the appearance of the initial reaction mixture compared to the mixture at equilibrium.

Deduce the expression for equilibrium constant $K_c$ for reaction 1.

$K_c =$

Calculate the concentration of $\text{I}_2$ present in the reaction mixture at equilibrium.

Show your working.

concentration of $\text{I}_2 = \text{.............................. mol dm}^{-3}$

Describe the difference in the composition of the equilibrium mixture at 500 K compared to 764 K.

Use Le Chatelier's principle to deduce whether the decomposition of $\text{HI(g)}$ is endothermic or exothermic. Explain your answer.

When a sample of butane is heated to $373\text{ K}$, in the presence of a catalyst, and allowed to reach equilibrium the following reaction occurs.

State and explain the effect on the composition of this equilibrium mixture when the temperature is increased to $473\text{ K}$.

Label the graph with a $t$ to show the time taken to reach dynamic equilibrium.

Use the graph to find the concentration of butane and methylpropane in the mixture at equilibrium.

concentration of butane = ................................... $\text{mol dm}^{-3}$

concentration of methylpropane = ...................... $\text{mol dm}^{-3}$

Write an expression for $K_c$ for this reaction.

Calculate a value for $K_c$ and state its units.

$K_c$ = .............................. units = ..............................