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Tuesday, April 5 th : “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.
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Tuesday, April 5th: “A” DayAgenda • 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) + SO42- (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) + SO42- (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.
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, orcoordination compound: the name given to any metal atom or ion that is bonded to more than one atom or molecule. • Ligands: amolecule 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.
Homework • Section 14.1 review, pg. 501: #1-5 • Concept Review: “Reversible Reactions and Equilibrium” Quiz over this section next time…