Presentation on theme: "Kinetics: Reaction Rates and Potential Energy Diagrams"— Presentation transcript:
1 Kinetics: Reaction Rates and Potential Energy Diagrams Honors ChemistryUnit 9- Chapter 17
2 Collision Theory of Reactions A successful reaction depends upon the effective collisions between molecules.Reactant molecules must collide with enough energy to break bondsNot every collision will result in a reaction.The minimum energy required to break the bonds of the reactants and start a reaction is called the Activation Energy
3 Examples of Activation Energy Scraping a match on a rough surface to light it.Using that lighted match to light a candle.Lightening strike causes oxygen (O2) to form ozone (O3)
4 Reaction RatesThe rate of a reaction can be increased, in general, by increasing the chances for effective collisions between molecules.Likewise, you can slow a reaction down by reducing the chances of collisions.There are four main factors that affect reaction rates.
5 1. TemperatureHigher temperature molecules move faster Molecules will have more energyThere will be more effective collisionsMolecules collide with the minimum energy (activation energy) more often.
6 2. Concentration Higher Concentration = More particles More particles = more possible collisionsMore collisions = faster reaction!For a gas - Increase Pressure Volume decreases concentration increases.
7 3. Surface Area Particle size determines surface area Smaller particles more surface area.More surface area means more area for collisions to occurMORE COLLISIONS = FASTER REACTION!
8 4. CatalystsA catalyst is a substance that increases the rate of a reaction without being used up in the reaction.It increases the rate of the reaction by lowering the activation energy for a reactionLow activation energy means = faster reactionsThere is a greater chance for an effective collision if the activation energy is lower.
9 Potential Energy Diagrams In a reaction mixture the reactants and products contain potential energy.This potential energy is also known as enthalpy (symbol H). During a chemical reaction the enthalpy (or Potential Energy ) of the reactants changes as the reactants form new products. The enthalpy change (∆H) for a reaction can be calculated from a potential energy diagram. ∆H = H(products) - H(reactants)∆H is measured in kilojoules per mole (kJ/ mol)
10 Exothermic ReactionsAn exothermic reaction releases heat, we can feel the heat given off to the surroundings as the reaction happens.**Energy required to break reactant bonds is less than the energy released when product bonds form.A + B C + D + Energy/heatThe enthalpy change for an exothermic reaction is always negative. (-ΔH)
11 Endothermic Reactions An endothermic reaction absorbs heat, we can feel a beaker become colder as a reaction proceeds as heat is taken in from the surroundings.**Energy required to break reactant bonds is more than the energy released when product bonds formA + B + Energy/heat C + DThe enthalpy change for an endothermic reaction is always positive. (+ΔH)
12 ΔHrxn = Heat energy released/absorbed in reaction Reverse reactionActivation energyΔHrxn = Heat energy released/absorbed in reaction
13 Potential Energy Diagram with Catalyst With a catalyst – The Activation Barrier is LoweredThis Increases Reaction RateNote - Enthalpy , ΔH Remains the same!