Download - Section 1 Reversible Reactions and Equilibrium Completion Reactions and Reversible Reactions Do all reactants change into products during a reaction?

Section 1Reversible Reactions and Equilibrium

Completion Reactions and Reversible Reactions

Do all reactants change into products during a reaction? Sometimes onlya trace of reactants remain after the reaction is over.

Completion Reaction – the reactants are used to make products and the products are very stable and do not reverse and make any reactants.

2 C8H18 + 25O2 18 H2O + 16 CO2

Gasoline + oxygen water + carbon dioxide

This reaction is one directional. The gasoline will burn with oxygen andcreate water and carbon dioxide. None of the water and carbon dioxide will react and make gasoline and oxygen. (notice the one directional arrowbetween the reactants and products)

Reversible Reactions Reach Equilibrium

A chemical reaction in which the products re-form the original reactants is called a reversible reaction

In the equation below the reactants form the products and someof the product react to form reactants. Take note of the arrowsbetween the products and reactants.

CaCl2(aq) + Na2SO4(aq) CaSO4(s) + 2NaCl(aq)

Chemical equilibrium is a state of balance in which the rate of a forward reaction equals the rate of the reverse reaction and the concentrations of products and reactants remain unchanged

Opposing Reaction Rates Are Equal at Equilibrium

When CO2 is dissolved in water to make carbonic acid (the fizz in soda),the reaction is written as seen below.

CO2(g) + H2O(l) H2CO3(aq)

If the soda is left unopened some carbonic acid will go back to form carbon dioxide and water, as seen in the reaction below.

H2CO3(aq) CO2(g) + H2O(l)

This should be written as a reversible reaction. as written below.

CO2(g) + H2O(l) H2CO3(aq)

When the equilibrium is reached the amount of product and reactants remainsthe same but both reactions continue as the same rate.

Section 2 - SkipSection 3 - Equilibrium Systems and Stress

Le Châtelier’s Principle

Le Châtelier’s principle the principle that states that a system in equilibrium will oppose a change in a way that helps eliminate the change.This principle is used to explain how a stress (change) move the point of equilibrium.

Chemical equilibria respond to three kinds of stress: changes in the concentrations of reactants or products, changes in temperature, and changes in pressure

Before we get started we need to define some terms.

Stress: any change in condition that will effect an equation that is at equilibrium.

Shift: Shifts are said to be to the right or the left: Shift to the right favors the forward reaction causing more products to be formed and the amount of reactants to decrease. Shift to the left favors the reverse reaction causing more reactants to be formed and the amount of products to decrease.

N2(g) + 3H2(g) 2NH3(g) + 91.8 kilojoules

When we study the effects of stress on equilibrium we will apply it tothe equation below. This equation shows the formation of ammonia.

Push away / Pull toward If you push away the reaction shifts away from the stress. If you pull toward the reaction shift toward the stress

Change in Concentration

If a chemical’s concentration is increased there are more particles of that chemical to react and push the reaction away from the chemical. If a chemical’s concentration is decreased there are fewer particles of that chemical to react and the reaction will pull toward the chemical.

For example:


What happens if I increase the concentration of Nitrogen gas?

More nitrogen will be available to react with hydrogen gas to form more ammonia and heat.

The equilibrium will shift to the right.

The concentration of hydrogen will go down, ammonia up, and an increasein heat production.

An increase in concentration causes the reaction to push away.

Another example:


What happens if I decrease the amount of ammonia?

Less ammonia will be available to combine with heat to form hydrogen gasand nitrogen gas.

The equilibrium will shift to the right.

The concentration of hydrogen will go down, nitrogen down, and an increasein heat production.

A decrease in concentration causes the reaction to pull toward.

Will the stress favor the forward or reverse reaction?

Will this result in a shift to the right or left?

Answer the following question if I decrease the concentration of hydrogen gas.


Will the amount of each substance increase or decrease?

Nitrogen gas

Ammonia

Heat

Increase

Decrease

Decrease

Reverse

Left

Change in Temperature

Temperature will effect the equilibrium of a chemical reaction. Rememberan endothermic reaction absorbs energy and has a positive H. An Exothermic reaction releases energy and has a negative H. Energy is located on the left (reactant) for endothermic reactions causing the reactionto cool, with exothermic reactions having energy on the right (products) causing the reaction to warm.

The rules for applying Le Châtelier’s Principle to changes in temperatureare the same for concentration.If the temperature in reaction is increased the reaction will push away from the energy. If the temperature in reaction is decreased the reaction will pull toward the energy.

For example:


Is this reaction endothermic or exothermic?

Which direction will this reaction shift if I increase the temperature?

What will happen to the concentration of ammonia if I decrease the temperature?

What will happen to the concentration of hydrogen if I increase the temperature?

What will happen to the concentration of nitrogen if I decrease the temperature?

Decrease

Left

Increase

Increase

Exothermic

Pressure has almost no effect on equilibrium reactions that are in solution.Gases in equilibrium, however, may be affected by changes in pressure.

Change in Pressure

If you increase pressure the side of the reaction with few moles of gas willbe favored and will shift in that direction.

If you decrease pressure the side of the reaction with more moles of gas willbe favored and will shift in that direction.


The above reaction has 4 moles of reactants and 2 moles of products that are gases.

Increasing pressure will favor the forward reaction and decreasing pressure will favor the reverse reaction.

If I increase the pressure how will it affect each of the following reactionsand the amount of each reactant and product?

2Al(s) + 3HCl(aq) → 2AlCl3(aq) + 3H2(g)

Shift to the left.

Increase in aluminum and hydrochloric acid, with a decrease in aluminumchloride and hydrogen gas.

H2(g) + I2(g) → 2HI(g)

No shift or change in concentration, due to equal moles of gas on both product and reactant side.

N2(g) + 3H2(g) 2NH3(g)

Shift to the right

Increase in production of ammonia and decrease in nitrogen and hydrogen

The Common-Ion Effect

The reduction of the solubility of a salt in the solution due to the addition of a common ion is called the common-ion effect.

CuCl(s) Cu+(aq) + Cl−(aq)

If in a beak we establish an equilibrium between copper(I) chloride and dissolved copper(I) ions and chloride ions in solution as seen in the equation Below.

If we add some NaCl to the beaker it will cause the production of Na+ ions and Cl- ions in solution as seen in the equation below.

NaCl(s) Na+(aq) + Cl−(aq)

Both equations have a common ion, Cl-, This concentration will combine To create the effect of an increase in concentration of Cl-.

The copper(I) chloride reaction will shift to the left causing a decrease in concentration of Cu+(aq) and an increase in concentration of CuCl(s).