Activation energy Activation energy and energy profile Background (Scientific context)

Activation energy and energy profile Two basic requirements for an effective collision the particles collide in the right orientation. the colliding particles possess sufficient kinetic energy. Activation energy, Ea Activation energy is the minimum energy required for a chemical reaction to occur. It is an energy barrier that the reactant particles have to overcome before a reaction can occur.

Only those reactant particles with energy equal to or greater than the activation energy can react to form the product. Fast reactions have low activation energy; slow reactions have high activation energy.

Energy profile The energy change of chemical species during the course of a reaction can be represented in an energy profile. activated complex Potential energy Ea reactants H products Reaction coordinate An energy profile of a single-step exothermic reaction.

Activated complex is the species that exists at the point of maximum energy; forms in the transition state that lies between the reactants and products of a chemical reaction; contains partially broken and partially formed bonds; is highly unstable and can never be isolated. The difference in energy between the reactants and the activated complex is the activation energy, Ea.

A reaction is exothermic if the energy of the products is less than the energy of the reactants. Potential energy Ea reactants H (negative) products Reaction coordinate Energy profile of an exothermic reaction showing Ea and H (negative).

A reaction is endothermic when the energy of the products is greater than the energy of the reactants. Potential energy Ea products H (positive) reactants Reaction coordinate Energy profile of an endothermic reaction showing Ea and H (positive).

Step 1: CD  intermediate Step 2: intermediate + E  CE + D activated complex 1 activated complex 2 Ea1 Ea2 Potential energy intermediate + E CD + E CE + D Reaction coordinate Energy profile of a two-step reaction between C–D and E. Step 1: CD  intermediate Step 2: intermediate + E  CE + D ______________________________________________________ Overall reaction: CD + E  CE + D

Ea1 > Ea2 The energy barrier for the first step is larger, so fewer particles possess sufficient energy to react. The first step is slower. It limits the rate of the overall reaction. The first step is regarded as the rate-determining step.

Rates of reaction

Measuring rates of reaction Calcium carbonate + hydrochloric acid calcium chloride + water + carbon dioxide Example

Factors that affect the rate of reaction surface area of a solid reactant. concentration or pressure of a reactant. temperature. nature of the reactants. presence/absence of a catalyst.

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Lab. Report Request Form due 14/10 (Wed) You should finish your research question, background, materials list and methods (IV, DV, CV) by 14/10

Lab. Report Day 1: Do the experiment (29/10) Day 2: Report Due (30/10)