Topic 17.2: The equilibrium law

17.2.1 Solve homogeneous equilibrium problems using the expression for Kc.

There are three different types of equilibrium questions the IBO can ask.


Calculating the equilibrium constant from initial and equilibrium concentrations.

If we know the equilibrium concentations of all reactants and products in a reaction, we can simply substitute these into the equilibrium expression to calculate Kc. The first step in such a calculation is always to write the equilibrium expression from the chemical equation.

Once you have your equation, just plug in the numbers of the equilibrium concentrations and you will receive Kc. If you were given only the initial concentration, we just need to convert it back to the equilibrium concentration for it to work.


Calculating equilibrium concentration from the equilibrium constant

If we know the value of Kc and the equilibrium concentrations of all but one of the components, we can calculate the remaining equilibrium concentration simply by substituting the values into the equilibrium expression.

Finding out what x is equal to would show the equilibrium concentrations. To find x, simply equate the Kc value with the equation containing the x. Simple maths and some rearrangement will provide the answer for x.

Topic 17.1: Liquid-vapour equilibrium

17.1.1 Describe the equilibrium established between a liquid and its own vapour and how it is affected by temperature changes

If we place some liquid in a container in which none of its molecules is in the vapour phase, evaporation will occur as vapour particles escape from the surface. In an open system, the vapour particles will leave so the liquid will continue to evaporate and equilibrium will not be reached. In this situation, the liquid will eventually all be converted into vapour - this is what happens, for example, when a puddle of water dries up or clothes are hung up to dry outside.

Temperature

The kinetic theory of matter gives us a model to describe the behaviour of liquids and gases. As temperature increases, there will be more vapour particles due to the increase rate of evaporation. Thus vapour pressure increases,



17.1.2 Sketch graphs showing the relationship between vapour pressure and temperature and explain them in terms of kinetic theory

17.1.3 State and explain the relationship between enthalpy of vapourization, boiling point and intermolecular forces

Stronger intermolecular forces
Higher enthalpy of vapourization
Lower vapour pressure
Higher boiling point

Weaker intermolecular forcese
Lower enthalpy of vapourization
Higher vapour pressure
Lower boiling point

The vapour pressure of a substance depends on its temperature and on the strength of the intermolecular forces present. The higher the temperature and the weaker the intermolecular forces, the higher the vapour pressure.

Topic 17: Equilibrium

Topic 17 of the IB HL Chemistry syllabus is the Equilibrium. IBO recommends to spend 4 hours on this topic.

This topic has 2 sub-chapters: "Liquid-vapour equilibrium" and "The equilibrium law". Each are separated with numerical values in order of mentioned.

These are advanced HL syllabus statements, it is recommended to bring a Casio Graphical Calculator instead of Texas. Casio Calculators have the periodic table installed already.