Lecture one

TERMS THAT YOU NEED TO KNOW Closed System Dynamic Equilibrium Forward Reaction Reverse Reaction Solubility Equilibrium Phase Equilibrium Chemical Reaction Equilibrium Dissolution

Quick Reminder Steps about Equations The way you write a chemical equation is very important. It allows to you to transmit detailed information to others using symbols. These symbols are very specific and the appropriate ones must be used. For a chemical reaction that is complete, a single arrow is used:

Quick Reminder Steps about Equations For a chemical reaction that proceeds in both directions, two full arrows in opposite directions is used: For a chemical reaction that is in equilibrium, two arrows with half heads in opposite directions is used:

Quick Reminder Steps about Equations To indicate resonance structures (but not a reaction) between two molecules, a single arrow with heads on both sides is used: Square brackets [ ] are used to represent concentration. For example, the concentration of NaCl would be noted as [NaCl].

Chemical Equilibrium A chemical reaction is said to be in equilibrium if the reactants react together to form the products, and the products then react together to form the reactants and these two processes are equal in their rates.

Chemical Equilibrium Consider the following equation. A + B C+D At the beginning of this reaction, the concentrations of reactants A and B are very high. So, the reaction goes from left to right (A, B to C, D). Gradually the reactants are consumed and the direct reaction (to the right) slows down. The concentrations of C and D increase. The inverse reaction (to the left) begins very slowly and accelerates. After a certain time the direct reaction and the inverse reaction occur at the same rate. This is chemical equilibrium.

Characteristics of the Equilibrium Condition: 1. A system at equilibrium is dynamic, with both forward and reverse reactions occurring at the same rate. Contrary to a static system, where no reactions are taking place, in a dynamic equilibrium there is no observable change even though reactions are occurring in order to maintain the state of equilibrium. 2. Macroscopically, no observable property changes are occurring. Concentration, pressure, temperature, colour, etc. remain constant. 3. Equilibrium is reached spontaneously from either direction. 4. A system reacts spontaneously to reduce an imposed stress in order to re-establish a state of equilibrium.

Requirements for Chemical Equilibrium: In order to attain chemical equilibrium: 1. The temperature must remain constant. 2. The external pressure must remain constant. 3. Once the reaction has begun, no substances are added to the system. So, the system must be isolated and sealed

1. Effect of concentration changes Fe(No 3 ) 3 + 3KSCN Fe(SCN) 3 + 3KNO 3 Yellow Colorless Red Colorless The compound Fe(SCN) 3 is the only compound with a dark colour. So, the red colour is a direct measure of the concentration of this compound in the solution. If KSCN is added to the system in equilibrium, the reaction will proceed from left to right. The [Fe(NO 3 ) 3 ] decreases while the [KSCN], [Fe(SCN) 3 ] and [KNO 3 ] increase. So, the red colour will get darker. This indicates that the reaction is proceeding towards the right.

2. Effect of temperature changes N 2 O 4(g) + Heat 2 NO 2(g) Colorless Brown When adding energy to this system in equilibrium, the reaction will go from left to right. The [N 2 O 4 ] decreases and the [NO 2 ] increases. The colour in this system will become a darker brown, which means that there are more NO 2 molecules. Once the temperature stabilises, the system will again be in the equilibrium state.

3. Effect of external pressure changes Changing the pressure of a system only affects the substances involved that are gases N 2 O 4(g) + Heat 2NO 2(g) Colorless Brown There is 1 mol of reactants on the left and 2 mol of products on the right. Increasing the pressure of the system will decrease the volume.

3. Effect of external pressure changes The side with the lowest number of moles of gases will be favoured. So the reaction will produce more N 2 O 4 and the colour will become lighter. A variation of external pressure does not necessarily affect a system in equilibrium

Example H 2(g) + I 2(g) 2HI (g) In this reaction, 1 mol of H 2 and 1 mol of I 2 are needed to make 2 mol of HI. This means that there is the same amount of moles per litre, on each side of the equation. Therefore, the gas pressure inside the system is not affected. So, since there are an equal number of moles of gases on either side, the external pressure does not affect this system's equilibrium state

4. Effect of a catalyst A catalyst does not affect the equilibrium state of a system. It only permits the system to attain its equilibrium state faster.

Problems: Sodium chromate, Na 2 CrO 4, can be dissolved in water to form a yellow solution. Sodium dichromate, Na 2 Cr 2 O 7, can be dissolved in water to produce an orange solution. The equilibrium equation is: 2CrO H + Cr 2 O H 2 O A) What happens when you add hydrochloric acid to a solution of sodium chromate? B) What happens if Cr 2 O 7 2- is added to the system?

Problems What happens when the system described by the following reaction is heated? N 2 +3H 2(g) 2NH 3(g) KJ