Reaction Kinetics & Potential Energy Diagrams

Reactions that Release Energy These reactions are Exothermic the system loses heat The surroundings heat up

Reactions that Absorb Energy These reactions are Endothermic system gains heat the surroundings cool down

Graphing Heat in Reactions We’ve already looked at one graphical representation… q = mHv q = mCΔT Temperature q = mHf Heat (Joules) or Time

What Heating Curves Tell Us Heating Curves tell us how temperature changes when energy is absorbed or released. For Example: Energy Added  Kinetic Energy  ΔTemp Energy Added  Potential Energy  Δ Phase Heating Curves do NOT tell us how much energy was needed to get the rxn started. Heating Curves do NOT tell us how the energy used to start the reaction compares to the energy produced by the reaction.

However, only a small fraction of collisions produces a reaction. Why? We know that molecules must collide to react. However, only a small fraction of collisions produces a reaction. Why? Particles must collide with the correct orientation and the right amount of energy Particles lacking the correct site or necessary kinetic energy to react bounce apart unchanged when they collide.

Activation energy is the amount of energy needed to cause a reaction. How Reactions Happen Activation energy is the amount of energy needed to cause a reaction. Collisions must equal or exceed the activation energy

Why are we learning this? Energy diagrams show how a reaction works. Energy diagrams display The energy of reactants and products The energy needed for a reaction to occur The energy absorbed or released due to reaction

Energy Diagram Parts Potential Energy of the reactants Potential Energy of the products Activation Energy E – the amount of energy required to reach the transition state or activated complex from the reactant Transition State, Ts (Activated Complex) – when bonds break and form and reactants become products Enthalpy, DH – science word for heat, Energy of products – energy of reactants

Types of Energy Diagrams Endothermic – the reaction absorbs heat so DH is positive The reactants have less potential energy than the products A - - - B AB DH > 0 A + B

Types of Energy Diagrams Exothermic – the reaction releases heat so DH is negative The reactants have more potential energy than the products A - - - B A + B DH < 0 AB

A POTENTIAL ENERGY DIAGRAM: C + O2 → CO2 + 395 KJ Products have lower energy than reactants Energy released Exothermic ΔH = -395kJ (Heat of Reaction) C + O2 Energy 395kJ CO2 Reactants ® Products

A POTENTIAL ENERGY DIAGRAM: CaCO3 + 176kJ → CaO + CO2 Products have higher energy than reactants Energy absorbed Endothermic ΔH = +176kJ (Heat of Reaction) CaO + CO2 Energy 176kJ CaCO3 Reactants ® Products

Potential Energy Diagrams One thing that is not represented in these simple examples… Activation energy Minimum amount of energy required to start a reaction

Endothermic with Activation Energy However, adding 150kJ won’t make the reaction happen 100 kJ 250 kJ Products have 350kJ of PE Activation Energy ΔH = Heat of Reaction 150 kJ Reactants have 200kJ of PE

Endothermic with Activation Energy Activated Complex – temporary, unstable, intermediate union of reactants 100 kJ 250 kJ 150 kJ 450 kJ ΔH = 150kJ Activation Energy

Exothermic with Activation Energy Activation energy = 10kJ Activation energy Reactants 40kJ ΔH = 25kJ ΔH Products15kJ

Catalyst: something that reduces the amount of activation energy needed to start a reaction – speeds up a reaction Activation energy with a catalyst

Endothermic Reaction with a Catalyst

Exothermic Reaction with a Catalyst

Five Factors Affecting Reaction Rate Nature of the Reactants: what the reactants are (solids, liquids or gases) Temperature: Increasing temperature always increases the rate of a reaction. Surface Area: Increasing surface area increases the rate of a reaction

Five Factors Affecting Reaction Rate Concentration: Increasing concentration USUALLY increases the rate of a reaction Presence of Catalysts Catalyst: A substance that speeds up a reaction, without being consumed itself in the reaction