Review Equilibrium

Rate of reaction The rate of the forward reaction = the rate of the reverse reaction N02 is being used up at the same rate that it is being formed N2O4 2 NO2 Rate of reaction does not change because both N202 and NO2 are constant

Equilibrium When this happens we say the system is at EQUILIBRIUM We have reached a state of DYNAMIC EQUILIBRIUM Key points Rate of the Forward Reaction = Rate of Reverse Reaction Dynamic Equilibrium – Reactants are changing to products, products are changing to reactants but at a microscopic level All observable properties are constant (macroscopic properties) If no changes were made to the condition a system would stay at equilibrium forever A system at equilibrium is a closed system

Calculations Equilibrium calculations We can figure out what the concentrations are for the products and reactants when they are at equilibrium We use: Keq It tells us the ratio of products: reactants when the reaction is at equilibrium

Calculations to figure out the concentration of the products and reactants at equilibrium we use: Keq = [products] [reactants] A + B C + D Keq = [C] [D] [A] [B]

Calculations Keq Calculations – Write expressions – Find Keq given concentration at equilibrium – Find concentration at equilibrium given Keq and all other concentrations at equilibrium – Find Keq given initial concentration and 1 concentration at equilibrium ICE – Find concentrations at equilibrium given initial concentration and Keq ICEEE & Trial Keq

Le Chatelier’s Principle If a system that is at equilibrium is changed (temperature, concentration, pressure) processes will occur that tend to counteract that change – This means the rxn will shift in a way to try and “undo” what was added Add heat  rxn will shift so it will use up added heat Remove heat  rxn will shift so it can produce heat Increase [ ]  rxn will shift so it will decrease [ ]

Temperature temperature is increased, the equilibrium will shift away from the side with the heat term – A + B + heat2NO2 Shift right When the temperature is decreased, the equilibrium will shift toward the side with the heat term. – A + B + heat2NO2 Shift left

Concentration concentration is increased the equilibrium will shift toward the other side of H2(g) + I2(g) 2HI(g) – Add H2 equilibrium will shift right concentration decreased the equilibrium will shift toward that substance – H2(g) + I2(g) 2HI(g) Remove H2 equilibrium will shift left

Partial Pressure – If the pressure of a substance is increased, equilibrium shifts toward the other side of the equation H2(g) + I2(g) 2HI(g) Increase pressure of H2 shifts right – If the pressure of a substance is decreased, equilibrium shifts toward the side of the equation with that substance H2(g) + I2(g) 2HI(g) Decrease pressure H2 shifts left

Total Pressure total pressure increased (or volume decreased), the equilibrium will shift toward the side with less moles of gas (as shown by coefficients) N2(g) + 3H2(g) 2NH3(g) – shift to the right (the side with fewer moles of gas) total pressure decreased (volume increased) the equilibrium will shift toward the side with more moles of gas (as shown by coefficients) – N2(g) + 3H2(g) 2NH3(g) – shift to the left (the side with more moles of gas)

Keq The larger the value for Keq the closer to completion the reaction is at equilibrium. – Most of the reactants have been completely converted to products A very small value (< 1) for Keq means that there is very little product and lots of reactant at equilibrium. – Most of the reaction has not occurred most of reactants remained unchanged and there is very little product

Keq When Keq is not large or small (close to 1) – This means that there is about the same amount of products as reactants. – At equilibrium, this reaction has proceeded to "about half way" to completion

Keq Temperature When the temperature changes, the value of Keq also changes Endothermic (heat on reactant side) – A + B + heat C – Increasing temp – Shifts right – Higher concentration C (product/numerator) – Higher Keq value – Decreasing Temp – Shift left – Higher concentration A + B (reactants/denomenator) – Lower Keq

Keq + Temperature Exothermic (heat on product side) – A + B C + heat – Increasing temp – Shifts left – Higher concentration A + B (reactants/denomenator) – Lower Keq – Decreasing Temp – Shifts Right – Higher concentration C (product/numerator) – Higher Keq value

Keq Concentration Pressure Cataylist – Do not change Keq significantly

Graphs A + B  AB + Heat Temp Increase Shifts LEFT GRADUAL INCREASE/DECREASE A + B  AB + Heat INCREASE [A] Shifts RIGHT Sudden increase (decrease) A Gradual increase A + B  AB + Heat Incrase Pressure Shifts RIGHT (less moles) Sudden increase in both A gradually decrease

Practical Applications Le Chatelier’s Principle Haber Process – N2 + 3H2 2NH3 + heat Able to produce large amounts of ammonia using Le Chatalier’s Principle – Increased [N2]  shift RIGHT – Decreased Temp  shift RIGHT – Increased Pressure  shift RIGHT – Added a catalyst  speeds up rate of RXN

Enthalpy & Entropy Enthalpy – Energy it takes for a reaction to occur – Potential energy – Favours side of rxn where minimum enthalpy is PCL5 + HEAT  Cl2(g) + PCl3(g) Entropy – Disorder – Favours side of rxn where entropy is maximum – Entropy of a Solid < Entropy of a Liquid < Entropy of an Aqueous Solution < Entropy of a Gas CaCO3(s) + 2 HCl(aq)  CaCl2(aq) + CO2(g) + H2O(l)

Enthalpy & Entropy Equilibrium occurs when two oppose each other" Minimum enthalpy favours reactant – Maximum entropy favours product When both tendencies favour the products, this reaction will go to completion. When both favour the reactants, the reaction will not occur at all!

Terms Solubility – The ability of a substance (solute) to dissolve Soluble – substance will completely dissolve Partially Soluble – substance will dissolve but there will be some precipitate left Insoluble – will dissolve but the amount that dissolves is very small and almost ALL the substance precipitates. Solubility Equilibrium – The rate of dissolving = The rate of precipitation