Equilibrium

Write down on separate post-it notes what you think each of the following terms means. Open system Closed system Reversible reaction Equilibrium Stress in a system

Open or closed system? Why?

What will happen to it over time?

H 2 O (l)  H 2 O (g) Evaporation!

H 2 O (l)  H 2 O (g) Endothermic or Exothermic?ΔH > 0 or ΔH < 0?

H 2 O (l)  H 2 O (g) Endothermic, ΔH > 0

Now what?

H 2 O (l)  H 2 O (g) Endothermic, ΔH > 0 STILL TRUE!!!

H 2 O (l)  H 2 O (g) Endothermic, ΔH > 0 STILL HAPPENING EVEN IF YOU DON’T SEE IT!!! BUT… Evaporation!

H 2 O (l)  H 2 O (g) Endothermic, ΔH > 0 Huh….? H 2 O (g)  H 2 O (l) or H 2 O (l)  H 2 O (g) Evaporation! Condensation!

Evaporation and condensation are reversible processes.

H 2 O (l) H 2 O (g)

Reversible reaction Liquid water molecules are turning to vapor, and vapor molecules are turning to liquid. A reversible reaction is a reaction that can take place in both the forward and reverse directions.

H 2 O (l) H 2 O (g) Forward (Evaporation)   Reverse (Condensation)

Dynamic equilibrium Liquid water molecules are turning to vapor (forward), and vapor molecules are turning to liquid (reverse). Equilibrium means that both forward and reverse reactions are happening at the same rate (dynamic equilibrium). Dynamic = ongoing Equilibrium = equal rate

Why can’t a dynamic equilibrium be achieved in an open system?

Graph – Let’s draw. Time Concentration

Initially, only H 2 O (l) is present. System is then closed. Sketch the shape of the two curves as time passes. Time Concentration H 2 O (l) H 2 O (g)

Initially, only H 2 O (l) is present. System is then closed. Sketch the shape of the two curves as time passes. Time Concentration H 2 O (l) H 2 O (g) Concentration does not change anymore, but not necessary end at the same level!

What if we now introduce some kind of “stress” into the system? Time Concentration H 2 O (l) H 2 O (g)

What if we started with only H2O (g)? Time Concentration H 2 O (g)

What if we started with only H2O (g)? Time Concentration H 2 O (g)

Going up the down escalator rJMoeko rJMoeko

Equilibrium Which ever side of the reaction you start from the end point is the same Which ever side you start from, the reaction reaches the same point (somewhere in the middle) H 2 O (l) H 2 O (g) Forward (Evaporation)   Reverse (Condensation)

Equilibrium can only happen at the following conditions: 1.Closed system – none of the products or reactants can escape the system 2.Constant temperature – T does not change throughout the reaction 3.Constant pressure (for gases) 4.No change in macroscopic properties – colour, pH, concentration… In a dynamic equilibrium, although the reaction never goes to completion the concentration of the products and reactants remains constant It is because the rate of the forward reaction equals the rate of the reverse reaction. The dynamic equilibrium can occur in both physical and chemical systems, i.e. during the changes of state or chemical reactions

Rate-time graph vs Concentration-time graph

CO 2 (g) ⇌ CO 2 (aq) Equilibria are all around, eg between the ocean and air

CO 2 in the atmosphere is in a reversible reaction with CO 2 in the sea Phytoplankton in the sea use CO 2 (aq) for photosynthesis. This shifts the reaction to the right CO 2 (aq) + H 2 O(l) ⇌ HCO 3 - (aq) + H + (aq) Some dissolved CO 2 reacts with water. This is why fizzy water and rain is slightly acidic. The equilibrium lies very much to the left, so CO 2 only forms weakly acidic solutions CO 2 (g) ⇌ CO 2 (aq) Gas dissolving… Physical system! Chemical system!

Many chemical reactions are reversible. Reversible reactions often reach a state of balance between the two sides. This is called equilibrium. The state of equilibrium can be approached from either side of the reaction. At equilibrium, the overall concentration of substances does not change, however… At a molecular level, it is still changing. The forward reaction is happening at the same rate as the backward reaction. This is called dynamic equilibrium.

Reversible biochemical reactions

What “stress” does to a system that was in equilibrium? Application of Le Châtelier’s principle to predict the qualitative effects of changes of temperature, pressure and concentration on the position of equilibrium.

Restoring balance: Position of equilibrium “Equilibrium shifts to the left.  Products will convert back to reactants. ” “Equilibrium shifts to the right.  Reactants will form products.”

Practical: Investigating changes to the position of an equilibrium Iron (III) + Thiocyanate  Iron thiocyanate How does different stresses affect the position of the equilibrium?

Setting dynamic equilibrium The position of dynamic equilibrium is not always at a half-way point, i.e. when there are equal amounts of products and reactants. It may be at a position where there are mainly reactants with a little product, or vice versa. The position of equilibrium is influenced by two main factors: temperature concentration (or pressure for reactions involving gases) Adding a catalyst speeds up the time it takes to reach equilibrium, but does not change the position of equilibrium.

Le Châtelier’s Principle “When a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change” Increasing the temperature shifts the equilibrium in the direction that takes in heat Increasing the concentration of a substance shifts the equilibrium in the direction that produces less of that substance Increasing the pressure shifts the equilibrium in the direction that produces less gas Temperature Concentration Pressure Condition Effect

Exothermic and endothermic reactions All reactions are exothermic in one direction and endothermic in the other If the temperature is increased: If the temperature is decreased: equilibrium shifts to decrease the temperature equilibrium shifts in the endothermic direction equilibrium shifts to increase the temperature equilibrium shifts in the exothermic direction

Opposing changes in temperature Nitrogen dioxide is in constant equilibrium with dinitrogen tetroxide. The forward reaction is exothermic and the backwards reaction is endothermic. What will happen if the temperature is increased? The equilibrium will shift to decrease the temperature, i.e. to the left (endothermic). If the temperature is decreased, more N 2 O 4 will be produced. N 2 O 4 (g) 2NO 2 (g) nitrogen dioxidedinitrogen tetroxide More NO 2 will be produced.

Concentration and equilibrium Changing the concentration of a substance affects the equilibrium of reversible reactions involving solutions. increasing the concentration of substance A equilibrium shifts to decrease the amount of substance A = decreasing the concentration of substance A equilibrium shifts to increase the amount of substance A =

Opposing changes in concentration (1) Bismuth chloride reacts with water to produce a white precipitate of bismuth oxychloride and hydrochloric acid. What will happen if more H 2 O is added? If H 2 O is removed, more BiCl 3 and H 2 O will be produced. The equilibrium will shift to decrease the amount of water, i.e. to the right. bismuth oxychloride bismuth chloride water hydrochloric acid ++ BiOCl (s)BiCl 3 (aq)H 2 O (l)2HCl (aq) ++ More BiOCl and HCl will be produced.

Opposing changes in concentration It will become more yellow. Chlorine gas reacts with iodine chloride to produce iodine trichloride. What effect will adding more Cl 2 have on the colour of the mixture? What effect will removing Cl 2 have on the colour of the mixture? It will become more brown. iodine trichloride chlorine iodine chloride + ICl 3 (s)Cl 2 (g) + ICl (l) pale greenbrownyellow

Pressure and equilibrium Changing the pressure has an effect on the equilibrium of reversible reactions involving gases. If the pressure is increased: equilibrium shifts to decrease the pressure equilibrium shifts in the direction of fewest molecules If the pressure is decreased: equilibrium shifts to increase the pressure equilibrium shifts in the direction of most molecules

Opposing changes in pressure Nitrogen dioxide is in constant equilibrium with dinitrogen tetroxide. Two molecules of nitrogen dioxide react to form one molecule of dinitrogen tetroxide. If the pressure is decreased, more NO 2 will be produced. N 2 O 4 (g) 2NO 2 (g) dinitrogen tetroxide nitrogen dioxide The equilibrium will shift to reduce the number of molecules, i.e. to the right (only 1 molecule). What will happen if the pressure is increased? More N 2 O 4 will be produced.

Dissociation of dinitrogen tetroxide

Dynamic equilibrium and change

Deduction of the equilibrium constant expression (K c ) from an equation of a homogeneous reaction. Homogeneous reaction = all reactants and products are in the same state – All aqueous solutions – All gas

1.Increasing temperature favours the endothermic (heat absorbing) direction. - Which reaction direction (forward or reverse) is endothermic? 2. Increasing pressure favours the direction resulting in less gas. - Which reaction direction would result in less gas? Stresses that would cause the position of the equilibrium to shift to the right. Heat is released (exothermic)

1.Increasing temperature favours the endothermic (heat absorbing) direction. - Which reaction direction (forward or reverse) is endothermic? 2. Increasing pressure favours the direction resulting in less gas. - Which reaction direction (forward or reverse) would result in less gas? Stresses that would cause the position of the equilibrium to shift to the right. Heat is released (exothermic)

Is the forward reaction endothermic or exoterthmic? Think Topic 5 Energetics. Is this bond breaking or bond forming? Which one is endothermic or exothermic?

How do we know the concentration of products vs reactants at equilibrium? What are the relative positions of the lines?

Equilibrium constant expression (K c )

Only a change in TEMPERATURE can cause the K c of a reaction to change!!! Addition of a catalyst only speeds up the reaction rate by lowering the activation energy. Changing the pressure or concentration of a system at equilibrium will cause the system to restore the equilibrium, not changing Kc.

Watch the following videos Writing Kc bhoE bhoE Applying Le Chatelier’s Principle oLis oLis Then do the worksheets.