Unit 7 – reaction rates

Rates of Reaction [203] The rate of a reaction is defined as: Increase in concentration of products or decrease in concentration of reactants per unit time Units of rate are generally Ms-1

Rates of Reaction [203-204] On the following graph: Monitoring reactants or products? Explain the shape of the curve. Measure the rate from the curve. Initial rate of reaction.

Rates of Reaction [203-204]

Rates of Reaction [202] Do not confuse measuring rate and simply measuring time of reaction. Shortest time = fastest rate

Rates of Reaction [203]

Compare rates of appearance and dissapearance A + 2 B → 3 C Rates of Reaction [203] Compare rates of appearance and dissapearance A + 2 B → 3 C

Collision Theory [208-211] In order for a reaction to occur, reactant particles must: Collide Collide with correct orientation Collide with enough energy to break bonds in reactants Called the activation energy (Ea)

Collision Theory [208-211]

Collision Theory [208-211] Any factor that increases the frequency or energy of collisions will increase the reaction rate

Rates of Reaction [211-214] The rate at which a reaction occurs can be sped up by: adding a catalyst increasing surface area of reactant Or decreasing particle size increasing concentration of reactants increasing temperature

Collision theory and factors that influence rate: Catalysts [213] Catalysts work by providing an alternative route with lower activation energy. a catalyst is the only factor that changes the value of the activation energy

Collision theory and factors that influence rate: Surface Area [213] When a solid reagent is broken into smaller pieces the surface area of the solid increases. This increases the frequency of collisions. Example: HCl(aq) + MgCO3(s) → MgCl2(aq) + CO2(g) + H2O(l) Note: both conditions produce same Amount of product

Collision theory and factors that influence rate: Concentration[212-213] An increase in concentration of a reagent increases the number of particles in a given volume, and so increases the frequency of collisions. An increase in pressure of a gaseous reaction is the same as increase of concentration of aqueous reaction.

Collision theory and factors that influence rate: Concentration[212-213] Example: sketch graph of these situations: 100 mL of 1 M HCl + excess MgCO3 50 mL of 1 M HCl + excess MgCO3 100 mL of 5 M HCl + excess MgCO3 500 mL of 1 M HCl + excess MgCO3

Collision theory and factors that influence rate: Temperature[212] Two reasons: As temperature is increased, kinetic energy increases and increase in number of particles with sufficient energy to overcome activation energy. (the important reason) As temperature is increased, kinetic energy increases and particle move faster increasing frequency of collisions.

What is the only factor that changes Ea of a reaction? Name two things that will increase the rate of a reaction. Name two things that must happen in order for a reaction to occur. Name two ways to increase the frequency of collisions. Name one way to increase the amount of energy a particle possesses. What is a common unit for the rate of a reaction? Does increasing the pressure of a gaseous reaction increase or decrease the rate?

Collision theory and factors that influence rate: Temperature[212] The Maxwell-Boltzmann distribution is used to show how temperature affects rate of reaction.

Collision theory and factors that influence rate: temperature[212]

Maxwell-Boltzmann and catalyst [212]

Which quantities in the enthalpy level diagram are altered by the use of a catalyst? A. I and II only B. I and III only C. II and III only D. I, II and III

Excess magnesium, was added to a beaker of aqueous hydrochloric acid Excess magnesium, was added to a beaker of aqueous hydrochloric acid. A graph of the mass of the beaker and contents was plotted against time (line 1). What change in the experiment could give line 2? A. The same mass of magnesium in smaller pieces B. The same volume of a more concentrated solution of hydrochloric acid C. A lower temperature D. A more accurate instrument to measure the time

Which curves best represent the evolution of carbon dioxide Equal masses of powdered calcium carbonate were added to separate solutions of hydrochloric acid. The calcium carbonate was in excess. The volume of carbon dioxide produced was measured at regular intervals. Which curves best represent the evolution of carbon dioxide against time for the acid solutions shown in the table below.   25 cm3 of 2 mol dm–3 HCl 50 cm3 of 1 mol dm–3 HCl 25 cm3 of 1 mol dm–3 HCl A. I III IV B. C. II D.

Measuring Reaction Rates [204-207] Properties that differ between reactants and products can be used to measure the rate of a reaction 1. Absorption of colored light Products are different color than reactants Intensity of increase or decrease in color can be measured Precipitate will decrease light transmittance 2. Electrical conductivity Presence of ions 3. Production of gas Collect gas or measure gas pressure Should not be water soluble if collected over water

Measuring Reaction Rates [204-207] 4. measurement of mass Total mass of mixture will differ is gas is evolved Gas should have large molar mass Gas should not be soluble in reaction mixture 5. Clock techniques Determine time required to see sudden color change 6. titration Remove samples from reaction over time and titrate to determine concentration

Measuring Reaction Rates [204-207]

Measuring Reaction Rates [204-207] Describe how to determine the rate of the following reactions. Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g) Na2S2O3(aq) + 2HCl(aq) → 2NaCl(aq) + SO2(aq) + H2O(l) + S(s) CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l) 2MnO4-(aq) + 5C2O42-(aq) + 16H+(aq) → 2Mn2+(aq) + 10CO2(g) + 8H2O(l) NH4Cl(aq) + NaNO2(aq) → N2(g) + 2H2O(l) + NaCl(aq) 2HI(g) → H2(g) + I2(g) Mg(s)+ 2 HCl(aq) → MgCl2(aq)+ H2(g) 2 Fe(s) + 3 CuCl2(aq) → 2 FeCl3(aq) + 3 Cu(s) PCl3(aq) + 3 H2O(l) → H2PO3-(aq) + 4 H+(aq) + 3 Cl-(aq)

questions Page 208 #1-3 Page 211 # 5-6 Pages 214-215 # 7-10