9.1 Chemical Equilibrium Objectives S1:a-e; S2 Section 1 - chemistry

Essential Questions What is a reversible reaction? What is a chemical equilibrium? How does a reaction(s) come to equilibrium? What does Le Chatelier’s Principle say? What are ways to affect chemical equilibrium?

Reversible Reactions What is a reversible reaction? Example p 552: A reaction that can go backwards so that products are reforming as reactants. Example p 552: Forward reaction 2 NO2 (g) → N2O4 (g) Reverse Reaction N2O4 (g) → 2 NO2 (g) Write reversible RXN using a double headed arrow.

Reversible Reactions (cont)

SO3 decomposes to SO2 and O2 18.2 Reversible Reactions SO3 decomposes to SO2 and O2 SO2 and O2 react to give SO3 At equilibrium, all three types of molecules are present. Molecules of SO2 and O2 react to give SO3 . Molecules of SO3 decompose to give SO2 and O2 . At equilibrium, all three types of molecules are present in the mixture.

Chemical Equilibrium What is a chemical equilibrium? Rate of forward reaction equals rate of reverse reaction What happens to the concentration (how much you have) of the products and reactants? Concentration of reactants and products stay constant because they are being formed as soon as they are being consumed

Chemical Equilibrium Any system involving a reversible reaction will reach equilibrium if left alone Called a dynamic equilibrium because its always active

Graphing equilibrium Concentrations What happens to the concentration of the reactants at first? Decreases What happens to the concentration of products at first? Increases What happens to both as time passes? They do not change Are the concentrations in all equal in equilibrium?

Graphing Equilibrium Rates What happens to the rate of the forward reaction at first? Decreases How about the Reverse reaction? Increases What happens to both as time passes? They become equal to each other How does a concentration graph compare to a rate graph?

Le Chatelier’s Principle How does a see-saw work? Weight be placed on one end causes the see-saw to move as it tries to get balanced How do you think that principle applies to a chemical equilibrium? When there is a change in conditions, the chemical equilibrium will shift Like a see-saw, the equilibrium will shift to maintain balance

Le Chatelier’s Principle (cont) Le Chatelier’s Principle states that when conditions of a chemical equilibrium are changed the equilibrium will shift towards the direction that will reduce that change The 3 conditions that affect equilibrium are concentration of reactants/products, pressure, or temperature Adding a catalyst affects each reaction equally and no overall impact

Changes in Concentration When you add concentration the reaction shifts to the opposite side When you remove the reaction shifts to the same side What will happen if NO2 is added? What will happen if N2O4 is added? What will happen if N2O4 is taken away?

Changes in Pressure For gasses if you increase pressure then volume decreases causing the equilibrium to shift to side with fewest number of gas molecules so they will take up less space N2 (g) + 3H2 (g) ↔ 2NH3 (g) If I wanted more ammonia should I increase or decrease the pressure?

Changes in Temperature Depends on if reaction is Endothermic or Exothermic Treat heat term as a reactant (left side) for endothermic and as product (right side) for exothermic

Changes in Temperature Example: H2 (g) + I2 (g) ↔ 2 HI (g) + heat What will happen if you increase the temperature? If I wanted more HI, what would you change?

Changes in Temperature (cont) For endothermic reaction which direction will the equilibrium shift if heat is added? To the right (products) How about for an exothermic reaction? To the left (reactants) How would removing heat affect an exo? How about endo?

Essential Questions What is a reversible reaction? What is a chemical equilibrium? How does a reaction(s) come to equilibrium? What does Le Chatelier’s Principle say? What are ways to affect chemical equilibrium?

9.1 Tracked Assignment worksheet