Warm Up #? What is a rate? A ratio of two different units Give some examples of rates. What do you think might change the rate of a reaction? A ratio of two different units Temperature, concentration, a catalyst, an inhibitor, or particle size (surface area)

Rate of a Reaction

Reaction Rate Reactant → products A B

*Because [A] decreases with time, D[A] is negative. rate = - D[A] Dt rate = D[B] Dt D[A] = change in concentration of A over time period Dt *Because [A] decreases with time, D[A] is negative. D[B] = change in concentration of B over time period Dt

Average Instantaneous Rate averaged over a specific period of time Rate for a specific instance in time Rate = Δ concentration Δ time Rate = — [A]final – [A]initial tfinal - tinitial Units: M or mol/L s s

Example Hydrogen reacts with chlorine to produce hydrochloric acid. Calculate the average reaction rate for all reactants. Write the balanced equation.

Example H2 + Cl2 → 2HCl average reaction rate expressed in moles H2 consumed per liter per second average reaction rate expressed in moles Cl2 consumed per liter per second Rate = -[H2] = -[0.025 – 0.060] t 5.00 – 0.00 = 0.0070 M/s, rate will always be positive Rate = -[Br2] = 0.035 – 0.070 t 5.00 – 0.00 = 0.0070 M/s, rate will always be positive

Br2 (aq) + HCOOH (aq) 2Br- (aq) + 2H+ (aq) + CO2 (g) Example Determine the average rate for the consumption of molecular bromine for the following time intervals. a. first 50 sec b. first 100 sec red-brown Br2 (aq) + HCOOH (aq) 2Br- (aq) + 2H+ (aq) + CO2 (g) time

Slope of the tangent is the instantaneous rate

Avg. Reaction Rate and Stoichiometry 2A B Two moles of A disappear for each mole of B that is formed. rate = - D[A] Dt 1 2 rate = D[B] Dt aA + bB cC + dD rate = - D[A] Dt 1 a = - D[B] Dt 1 b = D[C] Dt 1 c = D[D] Dt 1 d

Review of Concepts Write a balanced chemical equation for a gas-phase reaction whose rate is given by

Collision Theory rate  number of collisions sec Summary: 1) reacting substances must collide 2) must collide with the correct orientation 3) must collide with sufficient energy, Ea

A + B AB C + D + Exothermic Reaction Endothermic Reaction The activation energy (Ea ) is the minimum amount of energy required to initiate a chemical reaction.

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