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Reaction rates, Equilibrium, Acids/Bases, Redox Reactions Measure of disorder or randomness in a system Natural tendency for system to increase entropy (more random) EXAMPLE – Diffusion ◦ As molecules are dispersed, entropy increases ◦ Continued dispersal leads to a uniform solution Remember, things tend towards an increase in entropy Spontaneous reaction favors the products (exothermic) and releases free energy C + O2 CO2 ◦ Exothermic ◦ Solid gas increases entropy Gibbs free energy – max amt of E that can be used in another process Entropy never decreases in a system and instead will increase over time UNLESS you change the surroundings ◦ Spraying air freshener ◦ Spray it into a collapsible box Study of reaction rates (rate at which a chemical reaction takes place) Measured by: ◦ Rate of formation of products ◦ Rate of disappearance of reactants ◦ Changes in concentration of reactants or products Concentration Pressure Temperature Surface Area All of the above have a DIRECT relationship When reactants collide Normally, molecules bounce off each other b/c of electron clouds repulsion BUT, if those molecules have a LARGE amount of energy, they can overcome the repulsion and react Molecules also must collide in the right orientation Energy required to start a chemical reaction ◦ A nudge, a spark ◦ Potential E Activated complex – “speed bump” of the reaction – point at which it could go either way H2O + CO2 H2CO3 H+ + HCO3- Another factor that affects reaction rate Speeds the reaction by lowering the activation energy Not used up by reaction Two basic categories for reactions 1. Completion reactions – 1-way (combustion, decomp, rusting) 2. Reversible reactions – products can re-form original reactants Reversible reactions often use 2 arrows b/c reactions occur at the same time Chemical equilibrium is DYNAMIC, not STATIC Chemical equilibrium – reactions in which the forward and reverse reaction rates are equal Every reaction has a condition of equilibrium at a given temperature That means that 2 reactants will react to form products until a state is reached where the amounts of products and reactants no longer change ◦ CO2 in a half-filled, sealed soda bottle Things will stay that way until the system is somehow altered Equilibrium constant, Keq – a number that expresses the necessary concentrations of reactants and products for the reaction to be at equilibrium aA + bB cC + dD Keq = [C]c [D]d [A]a [B]b If Keq >1, the reaction favors the products If Keq <1, the reaction favors the reactants Calculate the Keq of the following equation CO2 (g) + H2 (g) CO (g) + H2O (g) If the [CO2] = 1.5 M, [ H2 ] = 1.5 M, [ CO ] = 0.6 M, [ H2O] = 0.6 M Keq= [CO]1 [H2O]1 = [0.6] [0.6] = 0.16 [CO2]1 [H2]1 [1.5] [1.5] So this reaction favors the…. When a system at equilibrium is disturbed, the system adjusts in a way to reduce the change. Chemical equilibria responds to 3 kinds of stress or change 1. Change in concentration 2. Change in temperature 3. Change in pressure Increasing concentration of reactant will make the rate of the forward reaction faster than the reverse ◦ Called a shift right ◦ Continues until new equilibrium H3O+ + HCO3 2H2O + CO2 Increasing concentration of product leads to shift left Remember that endothermic & exothermic are opposites Increasing the temp adds E so the endothermic will go faster to use it If it is exothermic forward, increasing the temp favors the reactants If it is endothermic forward, increasing the temp favors the products Only affects gases Imagine volume has been decreased, increasing the pressure Immediate effect is increase in concentration of both product & reactant According to principle, system will adjust to decrease the pressure A pressure increase favors the reaction that produces fewer molecules (stoichiometry) 2NOCl 2 NO + Cl2 H2O + CO H2 + CO2 Acids – sour taste, conduct electricity well, react with many metals, generate hydronium ions (H3O+), turn litmus paper red Bases – bitter taste, slippery feel, varying solubility, generate hydroxide ions (OH-), turn litmus paper blue Strong acids & bases COMPLETELY dissociate or ionize in water (one way reaction) ◦ HNO3 + H2O H3O+ + NO3◦ NaOH Na+ + OHWeak acids & bases only partially dissociate (reversible reaction) ◦ HOCl + H2O H3O+ + ClO◦ NH3 + H2O NH4+ + OH- Acid – ionizes to form an H3O+ ion when added to water Base – generate OH- when dissolved in water Acid – donates a proton (H+) to another substance Base – accepts a proton (H+) NH3 + H2O NH4+ + OHH2O is the Bronsted-Lowry acid & NH3 is the Bronsted-Lowry base Always reactants Conjugate Acid – Formed when a base gains a proton (H+) Conjugate Base – Formed when an acid loses a proton (H+) NH3 + H2O NH4+ + OH NH4+ is the conjugate acid & OHis the conjugate base Always products Can act as an acid or a base depending on what it is combined with Can act as a Bronsted-Lowry acid or base H2O + H2O H3O+ + OH Called the self-ionization of water Results in equal concentrations of H3O+ and OH- in pure water [H3O+] = [OH-] = 1.00 x 10-7 M [H3O+] x [OH-] = 1.00 x 10-7 x 1.00 x 10-7 = 1.00 x 10-14 Found to be true for other aqueous solutions at equilibrium [H3O+] x [OH-] = 1.00 x 10-14 Also abbreviated as Kw Have proportional amounts of H3O+ & OH [H3O+] x [OH-] = 1.00 x 10-14 H3 O+ OH- ACID H3 O+ OH- NEUTRAL H3O+ BASE OH- [H3O+] x [OH-] = 1.00 x 10-14 If [H3O+] = 1.00 x 10-2, what is [OH-]? [OH-] = 1.00 x 10-12 If [H3O+] = 1.00 x 10-5, what is [OH-]? [OH-] = 1.00 x 10-9 1909 – Soren Sorenson – negative exponents are annoying… So let’s just look at the exponents! Logarithm – power to which 10 must be raised to equal that number log 100 = 2 because 100 = 102 log 0.001 = -3 because 0.001 = 10-3 log log log log log 10,000 = 0.01 = 10 = 0.000001 = 1= Represents the “power” of “Hydrogen” pH = - log [H3O+] What is the pH of a 0.00010 M solution of HNO3? pH = - log [1.0 x 10-4] = -(-4) =4 What is the pH of a 0.2 M solution of a strong acid? pH = - log [.2] pH = 0.70 [H3O+] pH x [OH-] = 1.00 x 10-14 + pOH = 14 You can calculate [H3O+] by 1.00 x 10-14 / [OH-] Then you can calculate pH What is the pH of a 0.0136 M solution of KOH, a strong base? [H3O+] [H3O+] = 1.00 x 10-14 / 0.0136 = 7.35 x 10-13 pH = -log [H3O+] pH = - log [7.35 x 10-13] pH = 12.13 Lemonade has a hydronium ion concentration of 0.0050 moles/L. What is it’s pH? pH = -log [H3O+] pH = 2.3 What is it’s pOH? Reaction of H3O+ & OH- to form water molecules and often a salt H3O+ & OH- 2H2O ◦ Neutral means [H3O+] = [OH-] HCl + NaOH H2O + NaCl Common way to deal with acid & base spills Baking soda = NaHCO3,Ammonia = NH3 Change color at a certain pH level Red cabbage juice – changes to blue between 3 & 4 and to green at 8/9 Litmus paper – red or blue Phenolphthalein – turns bright pink in the presence of a base Used to determine the unknown concentration of a known reactant Uses an indicator to show the equivalence point For strong acid/strong base… Equivalence point is where [H3O+] = [OH-] or where moles of acid = moles of base Often uses phenolphthalein Remember that electronegativity is a measure of how tightly atoms hold on to their electrons Atoms with large electronegativity differences form ionic bonds by electron transfers 2Na + Cl2 2NaCl Can be written as 2Na + Cl2 2Na+Cl- Oxidation = ◦ Na Na+ Reduction Loss of electrons = Gain of electrons ◦ Cl2 2 Cl These 2 reactions happen together Oxidation-Reduction or REDOX OIL RIG Use “oxidation” numbers The number of electrons that must be added or removed to convert the atom to elemental or neutral form In other words, it’s the charge the atom would have if it were an ion 1. Look at the equation 2. Assign known oxidation numbers 3. Calculate unknowns & verify - Sum of all atoms in a molecule is zero - Sum of all atoms in a polyatomic is equal to the charge on that ion Uncombined = 0 O2 Monatomic ion = ion charge Zn 2+ Flourine = -1 (most electronegative) Group 1 = +1 K Group 2 = +2 Ca Binary compounds – most electronegative element = ion charge CaCl2 Hydrogen usually = +1 ◦ If combo with metal, H = -1 Oxygen usually = -2 ◦ If combo with Flourine, O = +2 ◦ Can also be -1 in peroxides like H2O2 Transition metals have multiple oxidation states so save them for last S2O72- Oxygen = -2 so O7 = -14 Entire molecule must = 2 So S2 + (-14) = 2 S2 = +12 S = +6 Ca (OH)2 Ca = +2 The entire molecule must = 0 So (+2) + (OH)2 = 0 (OH)2 = -2 O = -2 so O2 = -4 So -4 + H2 = -2 H2 = +2 so H = +1 From the given, balanced formulas, assign oxidation numbers 2H3O+ + Zn H2 + 2H2O + Zn2+ Since Zn changes from 0 to +2 and some of the H changes from +1 to 0, it is a redox reaction If the oxidation number goes UP during a reaction, it is oxidized 2H3O+ + Zn H2 + 2H2O + Zn2+ If the oxidation number goes DOWN during a reaction, it is a reduction 1s2 2s2 2p3 – 5 valence electrons, -3 oxidation number 1s2 2s2 2p6 3s1 – 1 valence electron, +1 oxidation number 1s2 2s2 2p5 1s2 2s2 2p6 3s2 3p1