Chemistry 19. Equilibrium

Dooley (19.1-19.3)

What is equilibrium constant Kc?

It is a value that quantifies the ratio of product concentrations to reactant concentrations for a reversible chemical reaction at dynamic equilibrium.

What are the 3 rules of equilibrium?

1. Equilibrium happens in a closed system

2. The concentraton of reactants and products remain constant in equilibrium

3. The reverse reaction = forward reaction

How is Kc expressed?

[products]/ [reaction]

Define homogeneous and heterogeneous equilibria.

Homogeneous equilibria= equilibrium species that all have the same phase/ state

Heterogeneous equilibria= equilibrium species that have different state or phases

What and why are some states/ phases of species included in Kc?

Gas and aqueous

Adding more gas at a fixed volume increases the concentration

Adding more solute increases moles without a proportional increase in solution volume

What and why are some states/ phases of species not included in Kc?

Solid and Liquid are omitted from the Kc expression

This is because adding solid or liquid will not affect the equilibrium constant as they will have constant concentrations.

They have a fixed density at a given temp and pressure

When moles of these phases are changed, their volume would also change proportionally.

What is ICE tables use for and what does ICE represent?

ICE tables are used to calculate Kc values from experimental data

I represents the inital value

C represents the change

E represents the equilibrium

What 3 types of practicals that can help to determine the Kc value?

1. Acid- base titration

2. Colorimeter

3. Gas volume

Describe the method for the acid- base titration to acheive experimental data for calcultaing Kc. (7)

1. Mix known stating volume and concentration of reactants in a flask at a constant temperature in a closed system (usually carboxylic acids and alcohol mixing)

2. Allow equilibium to be reached

3. Quench the solution to stop further reactions to make sure composition is fixed

4. Add a suitable indicator like phenolphthalein and titrate the sample against a standard solution of alkali like NaOH

5. The volume of NaOH used to neutralised will be used to calculate the moles of acid using stoichiometry (n =c x v)

6. Then calculate the equilibrium concentration by dividing the each equilibrium value by the volume given (of the acids)

7. Finally sub in the equilibrium concentration to equilibrium expressions to calculate Kc

How can the acid- base titration be accurate? (5)

1. Repeat titrations until concordant results are obtained

2. Make sure enough time allow equilibrium to be reached

3. Make sure there are no side reactions

4. Make sure to keep temp constant

5. Quenching is needed to keep the composition fixed

Describe the method of using colorimetry to acheive experimental data for calcultaing Kc. (8)

1. Mix known stating volume and concentration of reactants in a flask at a constant temperature in a closed system

2. Allow equilibium to be reached

3. Place a sample of its equilibrium mixture into a cuvette

4. Use a colorimeter to measure its absorbance

5. Plot a calibration curve (absorbance against conc)

6. Prepare known concentrations solutions to determine the equilibrium conc of the coloured solution

7. Use the initial values and stoichiometry ratios (ICE Table) to calculate the equilibrium conc of other substances

8. Sub these conc into the Kc expression

Describe the method of using colorimetry to acheive experimental data for calcultaing Kc. (6)

1. Mix known stating volume and concentration of reactants in a flask at a constant temperature in a closed system

2. Allow equilibium to be reached

3. Measure the volume of gas present at equilibrium using a gas syringe

4. Use the ideal gas equation to find moles- PV=nRT

5. Use c=n/v to find the equilirbium conc of all species

6. Sub the equilibrium conc nto Kc expression to find Kc.

What is equilibrium constant Kp?

It is the equilibrium constant in terms of partial pressures/ involving gases

Define mole fraction.

The ratio of number of moles of a single component in a mixture to the total number of moles of all components.

Give the equation of mole fraction.

Mole fraction x (A) = number of moles of A/ total number of moles in gas mixture

State the Avogadro’s Law.

Under the same temp and pressure, equal volumes of different gases have the same number of moles/ molecules

What is a rule for mole fractions?

The sum of the mole fractions in a gas mixture must equal 1

Define partial pressure

The pressure an individual gas in a mixture of gases would exert if it filled the entire volume by itself at the same temperature

Give the equation for partial pressure.

Partial pressure (A) = Mole fraction of A x Total pressure

What is the rule for partial pressures?

The sum of partial pressure of each gas equals the total pressure.

How can Kp be expressed?

(products)/ (reactants)

What are some units for pressure? (3)

kPa (kilopascals), Pa (pascals) and atm (atmosphere)

What does the Le Chatelier’s principle suggests?

When a dynamic equilibrium system experienced a change in conc/ pressure/ temp, the system will shift its position to oppose or to counteract the change

How does the magnitude of Kc or Kp relies on the product-to- reactant ratio?

The bigger the ratio, the larger the Kc or Kp

The more product (the top part of the ratio), the bigger the ratio

How does a catalyst affect the equilibrium constants?

It doesn’t - this is because it speeds up both reverse and forward reaction in equilibrium by the same factor (equally)

It can only change the rate of a chemical reaction or how dast equilibrium can be reached.

However, equilibrium yield/ position or constant will not be affected

How do changes in concentration affect equilibrium constant?

• Changes in constant do not affect Kc/ Kp

• As Kc/ Kp always stays the same, the reaction will shift in order to restore the equilibrium when there are changes in concentration

What happens if the reactant concentration increases/ product conc decreases? (3)

• The product-to-reactant ratio would be smaller than K

• The reaction shifts (temporarily) forward to the product so more products will be made or the reactant will have to decrease

• So that the ratio would equal to K again (we say the equilibrium is restored here)

What happens if the reactant concentration decreases/ product conc increases? (3)

• The product-to-reactant ratio would be bigger than K

• Either the concentration of reactant increase or the product decrease to restore the equilibrium

• So the ratio = K again

How do changes in pressure affect equilibrium constant?