Unit 4 Chemical Equilibrium

Completion of reactions Only dependant on limiting reactant? Do all reactions go to completion? Why or why not?

Reversible reactions Sometimes products of a reaction react with each other to reform the reactants (the reverse reaction) A reversible reaction is one that proceeds in both forward and reverse directions.

Example Consider nitrogen dioxide in a closed system (a flask). NO 2, a reddish-brown gas, reacts with heat to form colourless dinitrogen tetroxide, N 2 O 4 : 2 NO 2 (g) → N 2 O 4 (g) But the reaction can also go the other way - dinitrogen tetroxide also readily breaks down to form nitrogen dioxide: N 2 O 4 (g) → 2 NO 2 (g) We typically write a reaction that can go in both directions by using a double arrow (which will sometimes appear as ↔ in these online notes): 2 NO 2 (g) N 2 O 4 (g) Because the reaction continues in both directions at the same time, we never run out of either NO 2 or N 2 O 4. NO 2 is continually being used up to form N 2 O 4, but at the same time N 2 O 4 is forming more NO 2

Chemical equilibrium As the reaction goes forward, the [NO 2 ] decreases and [N 2 O 4 ] increases but after some time, the concentration of either substances does not change significantly (however, this does not mean they are the same). Voila! Chemical equilibrium has been reached.

Equilibrium cont’d….. A dynamic equilibrium is reached when the rate of the forward reaction equals the rate of the reverse reaction. The reaction appears to stop at the macroscopic scale but continues at the atomic scale. Explains falling short of theoretical yield (conversion back to reactants)

Characteristics of a chemical system in equilibrium Observable (macroscopic) properties are constant. Equilibrium can only be reached in a closed system. A system in equilibrium will remain in equilibrium as long as the system remains closed. Equilibrium can be approached from either the forward or reverse direction.

Dynamic equilibrium Indicators or properties of equilibrium at the macroscopic scale include: concentration, colour, pressure, temperature and pH When the system is changing at the atomic level, it is in dynamic equilibrium even though the macroscopic properties remain constant (i.e. no more colour change)

Representing equilibrium equations N 2 + 3H 2 ⇄ 2NH 3

Conditions for equilibrium 1.Closed system – no matter enters of leaves system (e.g. equilibrium between H 2 O (l) and H 2 O (g) ) 2. Reaction must be directly reversible; usually single step reactions (and not combustion ones) e.g. CaCO 3 ⇄ CaO(s) + CO 2 (g) (equilibrium can be approached from either direction) 3.Identical reaction conditions Youtube clip

Types of equilibrium reactions 1.Phase (change in states) – vapour above a pure liquid, vapour above a solid, solid in solution, etc. 2.Solubility (more later in the unit…) 3.Reaction equilibria – homogeneous and heterogeneous

Types of chemical equilibria a)Homogeneous equilibrium – a reaction in which reactants and products are in the same phase (e.g. two gases) b)Heterogeneous equilibrium – a reaction in which reactants and products are in different phases (e.g. liquid and solid)

Equilibrium or not? A puddle of water after rain. Hemoglobin and oxygen. A sealed jar of vinegar. A sauna…

Determining [ equilibria Use ICE for the following equilibria reaction: 3H P ⇄ 2 PH 3 e.g mols of each of H 2 (g) and P (g) are placed in a 1.00 L container at 500oC. If the reaction proceeds until there is 0.74 M PH 3, calculate the concentration of all other substances.