Tuesday, April 5th: “A” Day Agenda ACT sample test #3 Begin chapter 14: “Chemical Equilibrium” Homework: Section 14.1 review, pg. 501: #1-5 Concept Review: “Reversible Reactions and Equilibrium” Be ready for a quiz covering section 14.1 next time!

Completion Reactions If enough oxygen gas is provided for the following reaction, almost all of the sulfur will react: S8(s) + 8 O2(g) → 8 SO2(g) Reactions such as this one, in which almost all of the reactants react, are called completion reactions.

Reversible Reactions In other reactions, called reversible reactions, the products can re-form reactants. Reversible reaction: a chemical reaction in which the products re-form the original reactants. Another way to think about reversible reactions is that they form both products and reactants.

Reversible Reactions Reach Equilibrium One reversible reaction occurs when you mix solutions of calcium chloride and sodium sulfate: CaCl2(aq) + Na2SO4(aq) → CaSO4(s) + 2 NaCl(aq) Because Na+ and Cl- are spectator ions, the net ionic equation best describes what happens. Ca 2+ (aq) + SO4 2- (aq) CaSO4 (s)

Reversible Reactions Reach Equilibrium Solid calcium sulfate, the product, can break down to make calcium ions and sulfate ions in a reaction that is the reverse of the previous one. CaSO4 (s) Ca2+ (aq) + SO42- (aq) Use arrows that point in opposite directions when writing a chemical equation for a reversible reaction. Ca 2+ (aq) + SO4 2- (aq) CaSO4 (s)

Chemical Equilibrium The reactions occur at the same rate after the initial mixing of CaCl2 and Na2SO4. The amounts of the products and reactants do not change. Reactions in which the forward and reverse reaction rates are equal are at chemical equilibrium. Chemical Equilibrium: 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 remains unchanged.

Opposing Reaction Rates are Equal at Equilibrium The reaction of hydrogen, H2, and iodine, I2, to form hydrogen iodide, HI, reaches chemical equilibrium. H2 (g) + I2 (g) 2 HI (g) At first, only a very small fraction of the collisions between H2 and I2 result in the formation of HI. H2(g) + I2(g) → 2 HI(g)

Opposing Reaction Rates are Equal at Equilibrium After some time, the concentration of HI goes up. As a result, fewer collisions occur between H2 and I2 molecules, and the rate of the forward reaction drops. Similarly, in the beginning, few HI molecules exist in the system, so they rarely collide with each other.

Opposing Reaction Rates are Equal at Equilibrium As more HI molecules are made, they collide more often and form H2 and I2 by the reverse reaction. 2 HI(g) → H2(g) + I2(g) The greater the number of HI molecules that form, the more often the reverse reaction occurs.

Rate Comparison for H2(g) + I2(g) 2 HI(g)

Opposing Reaction Rates are Equal at Equilibrium When the forward rate and the reverse rate are equal, the system is at chemical equilibrium. If you repeated this experiment at the same temperature, starting with a similar amount of pure HI instead of the H2 and I2, the reaction would reach chemical equilibrium again and produce the same concentrations of each substance.

Chemical Equilibria are Dynamic If you drop a ball into a bowl, it will bounce. When the ball comes to a stop it has reached static equilibrium, a state in which nothing changes. Chemical equilibrium is different from static equilibrium because it is dynamic. In a dynamic equilibrium, there is no net change in the system. Two opposite changes occur at the same time.

Chemical Equilibria are Dynamic In equilibrium, an atom may change from being part of the products to part of the reactants many times. But the overall concentrations of products and reactants stay the same. For chemical equilibrium to be maintained, the rates of the forward and reverse reactions must be equal. Arrows of equal length also show equilibrium. Reactants Products

Chemical Equilibria are Dynamic In some cases, the equilibrium has a higher concentration of products than reactants. This type of equilibrium is also shown by using two arrows. Reactants Products The forward reaction has a longer arrow to show that the products are favored.

Another Example of Equilibria Even when systems are not in equilibrium, they are continuously changing to try to reach equilibrium. For example, combustion produces carbon dioxide, CO2, and poisonous carbon monoxide, CO. As CO and CO2 cool after combustion, a reversible reaction produces soot, solid carbon. 2 CO(g) C(s) + CO2(g) This reaction of gases and a solid will reach chemical equilibrium. Equilibria can involve any state of matter, including aqueous solutions.

Equilibria Involving Complex Ions Complex ion, or coordination compound: the name given to any metal atom or ion that is bonded to more than one atom or molecule. Ligands: a molecule or anion that readily bonds to a metal ion. (Ex: NH3, CN-) Complex ions may be positively charged cations or negatively charged anions. (Remember, in order to be an ION, an atom or group of atoms has to have a CHARGE.)

Equilibria Involving Complex Ions In this complex ion, [Cu(NH3)4]2+, ammonia molecules bond to the central copper(II) ion. What is the ligand in this complex ion? NH3

Equilibria Involving Complex Ions Complex ions formed from transition metals are often deeply colored.

Equilibria Involving Complex Ions The charge on a complex ion is a sum of the charges on the species from which the complex ion forms. For example, when the cobalt ion, Co2+, bonds with four Cl− ligands, the total charge is (+2) + 4(−1) = −2. Metal ions and ligands can form complexes that have no charge. These are not complex ions. Why not? Complex ions often form in systems that reach equilibrium.

Equilibria Involving Complex Ions Consider zinc nitrate dissolving in water: Zn(NO3)2 (s) Zn2+ (aq) + 2 NO3- (aq) In the absence of other ligands, water molecules bond with zinc ions. So, this reaction can be written: Zn(NO3)2(s) + 4H2O [Zn(H2O)4]2+ (aq) + 2NO3- (aq) (complex ion)

Equilibria Involving Complex Ions If another ligand, such as CN−, is added, the new system will again reach chemical equilibrium. Both water molecules and cyanide ions “compete” to bond with zinc ions, as shown in the equation below. [Zn(H2O)4]2+(aq) + 4CN-(aq) [Zn(CN)4]2-(aq) + 4H2O(l) All of these ions are colorless, so you cannot see which complex ion has the greater concentration.

Equilibria Involving Complex Ions In the chemical equilibrium of nickel ions, ammonia, and water, the complex ions have different colors. You can tell which ion has the greater concentration based on color: [Ni(H2O)6]2+(aq) + 6NH3(aq) [Ni(NH3)6]2+(aq) + 6H20(l) Green Blue-violet The starting concentration of NH3 will determine which one will have the greater concentration.

Quiz over this section next time… Homework Section 14.1 review, pg. 501: #1-5 Concept Review: “Reversible Reactions and Equilibrium” Quiz over this section next time…