The concentration of reactants can affect reaction Re=对A同 的s322eeh色e代saeo g n describes how temper ature 2-2 Acids and Bases:Electrophiles and Nucleophiles Acdand base are measured by :aproton dono wy ase:a proton a In water: Water itself is both an acid and a base R,=l.0°0-1=10x0 Thended pll=-logll,O' nions the ph is less than The behavior of a general acid,HA,in water can be described: k,-k,-k小 thanthemr 22 Collisions supply the energy to get past the activation-energy barrier. The average kinetic energy of molecules at room temperature is about 0.6 kcal mol-1. The kinetic energies of individual molecules can be plotted as a Boltzmann distribution curve: As can be seen from the curves, there are more molecules having large kinetic energies at high temperature than at low temperature. Since the energy required to reach the transition state in a chemical reaction comes from molecular collisions, the rate of chemical reactions always increases with rising temperatures. The concentration of reactants can affect reaction rates. The rate of a chemical reaction can be expressed as a rate law. The rate law must be experimentally determined; it cannot be derived directly from the balanced chemical equation. First Order Rate = k[A] Second Reaction: A B, Reaction: A + B C + Order D, Rate = k[A][B] → → The Arrhenius equation describes how temperature affects reaction rates. The rate constant, k, depends upon temperature according to the Arrhenius equation: / Ea RT k Ae− = In general, raising the reaction temperature by 10 oC will increase the rate constant by a factor of 2 or 3. 2-2 Acids and Bases; Electrophiles and Nucleophiles Acid and base strengths are measured by equilibrium constants. Brønsted-Lowry acid: a proton donor. Brønsted-Lowry base: a proton acceptor. In water: Water itself is both an acid and a base: + 14 W 3 + 22 3 o K = [H O ][OH ] = 1.0 x 10 H O + H O H O + OH at 25 C − − − ƒ + 3 pH = log[H O ] − The pH of a solution is defined: In pure water, the pH is 7. In acidic solutions the pH is less than 7 and in basic solutions the pH is above 7. The behavior of a general acid, HA, in water can be described: + 2 + 3 A 3 A A [H O ][A ] K = , pK = log(K ) [HA HA + H O H O ] + OH − − − ƒ The pKA is the pH at which the acid is 50% dissociated. If the pKA is less than 1, the acid is termed strong; if greater than 4, weak