Equilibrium

The story so far… So far, we learnt about the kinetics of a rxn –In other words, how fast or how slow it goes, what factors affect this rate and how we can calculate that rate given data. Today, we are only focused on one specific condition of rates “Equilibrium”

Equilibrium A system/rxn is at equilibrium when the forward and reverse reactions occur at equal rates. Chemists draw arrows that look like this to show a system at equilibrium.

Equilibrium Many chemical reactions are reversible if….. the activation energy is low enough, the system is closed and temperature is kept constant. *Closed system= one where nothing can enter or escape (ideal) Reactants ⇌ Products Forward Rate Reverse Rate reaction rate= k forward. [reactants] k is a rate constant that tells you how fast or slow the reaction is normally.

Approaching Equilibrium Reactants ⇌ Products Forward Reaction is left to right Reverse Reaction is right to left

Let’s see what happens to some reactants if they are placed in a reaction vessel and allowed to react.

Reactants Products Approaching Equilibrium Forward Rate Reverse Rate

Reactants Products Reverse Rate Forward Rate Approaching Equilibrium

Reactants Products Reverse Rate Forward Rate Approaching Equilibrium

Reactants Products Reverse Rate Forward Rate Approaching Equilibrium

Reactants Products Reverse Rate Forward Rate Approaching Equilibrium

Reactants Products Reverse Rate Forward Rate Approaching Equilibrium

Reactants Products Reverse Rate Forward Rate Are Equal [concentration] is constant, but NOT EQUAL Approaching Equilibrium

Reactants Products Reverse Rate Forward Rate Rates are equal, but concentrations are constant( unchanged) Approaching Equilibrium

How come the concentration of the reactants at equilibrium does not change even though the forward reaction continues to occur???? The reverse reaction continues at the same rate as the forward reaction in an equilibrium. In other words, the products that are made in the forward reaction are used up as fast as they are made due to the reverse reaction taking place at the same rate too! So all concentrations remain the same!

Rates and Concentrations at equilibrium 1- Rate of consumption = Rate of production 2- The forward and reverse rates are equal and remain constant unless the system is interrupted. 3-[reactants] differs from [products] but…. 4- Concentrations of [reactants] and [products] is now CONSTANT/unchanged in time. In general, any system which is not at equilibrium will tend to move towards establishing equilibrium

Recognize the difference Forward and reverse rates : equal & constant Concentrations of reactants and products: constant, but not necessarily equal.

How equilibrium is established Stage 1 As a reaction begins, high[reactants] generate a fast forward rate.

How equilibrium is established Stage 2 As the [reactants] falls so does the forward rate, meanwhile the reverse rate rises along with the [products]

How equilibrium is established Stage 3 This continues until the reverse rate equals the forward rate, thereby establishing equilibrium.

Concentration vs time Concentration Time reactant product

Equilibrium Reactant concentrations start high and decrease as the reaction proceeds. The forward rate, which depends on collisions of the reactants, also decreases. Product concentrations start at zero and increase as the reaction proceeds. The reverse rate, which depends on collisions of the products, also increases. Eventually when the forward rate is equal to the reverse rate AND the concentrations are constant ( but not necessarily the same)  This is equilibrium.

Rate vs time Rate Time Forward rate Reverse rate

Characteristics of a System at Equilibrium 1.The Forward rate = The Reverse rate The equilibrium can be approached from starting with reactants or starting with products. 2.The Reactant and Product concentrations are constant/unchanging [ ] 3.The Macroscopic (observable) properties are constant Large enough properties to be observed with the naked eyes. Ex; colour changes, total pressure,etc Yet microscopic changes occur at the atomic or molecular level. 4.The system is in a Dynamic Equilibrium as the forward and reverse reactions continue. 5. Equilibrium will Only shift or change when conditions change -->later topic.

Reactants Products Reverse Rate Forward Rate Approaching Equilibrium from Products

Reactants Products Reverse Rate Forward Rate Approaching Equilibrium from Products

Reactants Products Reverse Rate Forward Rate Approaching Equilibrium from Products

Reactants Products Reverse Rate Forward Rate Approaching Equilibrium from Products

Reactants Products Reverse Rate Forward Rate Approaching Equilibrium from Products

Reactants Products Reverse Rate Forward Rate Rates Are Equal [conc]’s are constant Approaching Equilibrium from Products

If you start with products all aspects of the approach to equilibrium are reversed Reactants and Forward Rate increases Products and Reverse Rate decrease

Equilibrium (looking at reverse reaction) Concentration Time reactant(originally products) product

Rate Vs Time /Equilibrium Rate Time Forward rate Reverse rate

Conditions Necessary for Equilibrium Ea is low so the reaction is reversible Constant temperature Closed system

Think... 2NH 3(g) N 2(g) + 3H 2(g) This is how the above reaction occurs at equilibrium. It means: _________________________________________

2NH 3(g) N 2(g) + 3H 2(g) If there was a gas mixture of NH 3, N 2, H 2 put in a container and after a few mins it was found that 2 moles of NH 3 react for every 3 moles of products which react. Is this mixture at equilibrium?

Question 6 & 7 on page 40

Clearing out misconceptions What we refer to is technically called “Dynamic Equilibrium” as in, it is active and both the reactions continue to occur. They do not stop.

Hebden Practice Problems Solve q’s 3-5 on page 39 Solve q’s on pages 42 and 43 I am here for help!