Unit 1 – Reaction Kinetics Chemistry 12 1.4 – Experimental Measurement of Reaction Rates

[Reactant] is initially high As reaction occurs, [reactant] decreases Therefore, reaction rate is fastest As reaction occurs, [reactant] decreases causing a slower reaction rate Exact rate is given by the slope of the line tangent to the curve at that time

Do p.11 #18-19

Unit 1 – Reaction Kinetics Chemistry 12 1.5 – Reaction Rates and Collision Theory

What causes reactions to occur? Collision theory (or Kinetic Molecular theory) states that molecules act as small, hard spheres which bounce off each other and transfer energy among themselves during their collisions … This means all reactions require collisions!

What does this mean? Effect of Concentration Higher concentration = More molecules able to collide Therefore, reaction rate INCREASES

What does this mean? Effect of Temperature Higher temperature = higher Kinetic Energy (KE) Therefore, more movement, greater speed, and reaction rate INCREASES Kinetic Energy – The energy which a system possesses because of movement within the system *Note: There are other reasons for increase in reaction rate…. later

Do p. 12 #20-22

Chemistry 12 Unit 1 – Reaction Kinetics 1.6 – Enthalpy Changes in Chemical Reactions

Bond Energies Potential Energy – Energy existing as a result of an object’s position in space, as well as the sum of all the attractive and repulsive forces existing among the particles which make up the object. PE is directly related to the energy of the electrons in the chemical bonds, as well as the number and types of atoms in the molecules

Potential Energy Negatively charged electrons hold positively charged nuclei together  Form chemical bond Electrons repel each other Nuclei repel each other *Therefore, the push and pull of all these forces together result in potential energy

a) Bond Energies Kinetic Energy - The energy which a system possesses because of movement within the system By moving the entire system or moving the molecules in the system (or individual atoms within the molecules)

How are these used together? Bond Energy – The amount of energy required to break a bond between two atoms To break a bond: Energy added must equal bond energy Cl2(g) + 243 kJ  2 Cl(g)

To form a bond: Bond energy must equal released energy 2Cl(g)  Cl2(g) + 243 kJ

High initial PE, then it releases energy to form a bond High initial PE, then it releases energy to form a bond. This allows it to be at a more stable level of energy H + H H2 Potential energy Reaction Proceeds Potential Energy Diagram Remember: Atoms are more stable with a full octet. So, bonds completes this octet

Low initial PE, then energy is added to break a bond Low initial PE, then energy is added to break a bond. This allows it to be at a more stable level of energy H + H H2 Potential energy Reaction Proceeds Potential Energy Diagram

b) Reaction Heats Enthalpy (H) – The total kinetic and potential energy which exists in a system when at constant pressure ΔH = HPROD – HREACT ΔH = The change in enthalpy during the course of the reaction HPROD = Combined enthalpies of all products HREACT = Combined enthalpies of all Reactants

HREACT HPROD ΔH = HPROD – HREACT Enthalpy Diagram Enthalpy (H) Reaction Proceeds Enthalpy Diagram ΔH We only need Change in enthalpy value. Will not use H prod or Hreact

c) The sign of ΔH Endothermic  Products have MORE energy than reactants HPROD > HREACT Therefore, ΔH = HPROD – HREACT ΔH > 0

2N2O 2N2 + O2 Endothermic Reaction 2 N2 + O2 + 164kJ  2 N2O 2 N2 + O2  2 N2O ; ΔH = +164kJ Heat included in REACTANTS of the equation Enthalpy is POSITIVE 2N2O 2N2 + O2 Enthalpy (H) Reaction Proceeds Enthalpy Diagram ΔH = +164kJ Note: Arrow up means the reactants GAIN energy to become products

ENdo = ENter  Heat ENters the system Endothermic Reaction ENdo = ENter  Heat ENters the system Heat (Energy) is being absorbed into the system *System = Reactants in reaction *Surroundings = Container reaction takes place in or surroundings around reaction Surroundings System System becomes warmer  Surroundings become cooler

Exothermic Reaction Exothermic  Products have LESS energy than reactants HPROD < HREACT Therefore, ΔH = HPROD – HREACT ΔH < 0

Exothermic Reaction H2 + Cl2 2HCl H2 + Cl2  2 HCl + 184 kJ Heat included in PRODUCTS of the equation H2 + Cl2  2 HCl + 184 kJ H2 + Cl2  2 HCl ; ΔH = -184kJ Enthalpy is NEGATIVE 2HCl H2 + Cl2 Enthalpy (H) Reaction Proceeds Enthalpy Diagram ΔH = -184 kJ Note: Arrow down means the reactants LOSE energy to become products

EXo = EXit  Heat EXits the system Exothermic Reaction EXo = EXit  Heat EXits the system Heat (Energy) is being released into the surroundings *System = Reactants in reaction *Surroundings = Container reaction takes place in or surroundings around reaction Surroundings System System becomes cooler  Surroundings become warmer

Homework Questions 1.4 – p.11 #18-19 1.5 – p.12 #20-22 1.6 – p. 13-16 #23-28