Percent Yield & Le Châtelier’s Principle

Percent Yield

The theoretical yield of a chemical reaction is the maximum amount of product that can be produced from a given amount of reactant. –It is calculated from the balanced chemical equation The actual yield is the amount of product produced

Practice Problems Solid silver chromate (Ag 2 CrO 4 ) forms when excess potassium chromate (K 2 CrO 4 ) is added to a solution containing g of silver nitrate (AgNO 3 ). Determine the theoretical yield of Ag 2 CrO 4. Calculate the percent yield if the reaction yields g of Ag 2 CrO 4.

Zinc can be removed from bronze by placing bronze in hydrochloric acid. The zinc reacts with the hydrochloric acid producing zinc chloride and hydrogen gas, and leaving the copper behind. A) If 25.0 g of zinc are in a sample of bronze, determine the theoretical yield of hydrogen gas. Zn + 2HCl → ZnCl 2 + H 2 B) If the reaction yields g H 2, determine the percent yield.

When copper wire is placed into a silver nitrate solution (AgNO 3 ), silver crystals and copper (II) nitrate (Cu(NO 3 ) 2 ) solution form. A) Write the balanced chemical equation for the reaction. B) If a 20.0-g sample of copper is used, determine the theoretical yield of silver. C) If 60.0 g of silver is recovered from the reaction, determine the percent yield of the reaction.

Le Châtelier’s Principle In 1889, French chemist Henri-Louis Le Châtelier discovered that there are ways to control equilibria to make reactions more productive. If a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress. –A stress is any kind of change in a system at equilibrium that upsets the equilibrium

Adding reactants Adding more reactants to a reaction will cause the reaction to shift towards the right (producing more products) CO (g) + 3H 2 (g) ↔ CH 4 (g) + H 2 O (g)

Removing Products Removing products will cause the reaction to shift towards the right (results in the production of more product) CO (g) + 3H 2 (g) ↔ CH 4 (g) + H 2 O (g)

Adding Products Adding products will cause the reaction to shift towards the left (changing products into reactants) –If one of the reactants is removed, a similar shift to the left will occur CO (g) + 3H 2 (g) ↔ CH 4 (g) + H 2 O (g)

Changes in Volume & Pressure Remember what type of relationship pressure and volume have. CO (g) + 3H 2 (g) ↔ CH 4 (g) + H 2 O (g) What happens if you decrease the volume?

Moles of reactant versus moles of product CO (g) + 3H 2 (g) ↔ CH 4 (g) + H 2 O (g) Compare the number of moles of gaseous reactants in the equation to the number of moles of gaseous products. The equilibrium can relieve the stress of increased pressure by shifting to the right. Changing the volume (and pressure) of an equilibrium system shifts the equilibrium only if the number of moles of gaseous reactants is different from the number of moles of gaseous products.

Changes in Heat If heat is added to an equilibrium system, the equilibrium shifts in the direction in which heat is used up. CO (g) + 3H 2 (g) ↔ CH 4 (g) + H 2 O (g) Lowering the temperature shifts the equilibrium to the right because the forward reaction liberates heat and relieves the stress.

Catalysts and Equilibrium A catalyst speeds up a reaction, but it does so equally in both directions. A catalyzed reaction reaches equilibrium more quickly but with no change in the amount of product formed.