MiniUnit 12 Reaction Rates and Equilibrium Introduction to

Mini-Unit 12: Reaction Rates and Equilibrium Introduction to Rates of Reaction

Reaction Rates Rxn speed: Examples Very fast Mg and hydrochloric acid, sodium and water, burning a match, combustion of fuel, etc. Moderate Food spoiling, rusting of iron, baking a cake, food digestion, cooking, etc. Very slow Forming diamonds, weathering of rocks, setting concrete, silver tarnishing, etc.

Rates A rate measures changes that occur within an interval of time.

In chemistry, the rate of a reaction is expressed as the amount of reactant changing per unit time. (Example: mol/sec)

Collision Theory Collision theory: atoms, ions, and molecules can react to form products when they collide with one another, provided they have enough kinetic energy. • If they do not have enough kinetic energy they will collide then bounce apart. • Similarly, bonds holding the molecules together can break apart if supplied with enough energy.

Activation Energy (Ea) The minimum energy colliding particles must have in order to react. • During a reaction there is an inbetween state called the activated complex or transition state which is the unstable arrangement of atoms at the peak of the activation energy barrier. (Exists for a very short amount of time, ~10 -13 seconds. )

Energy Changes in a Reaction Energy Ea of forward rxn Ea Energy of reactants Energy of products Enthalpy (ΔH) of reaction Time

Factors That Affect Reaction Rates The rate of a chemical reaction depends on: 1. Temperature 2. Concentration 3. Particle size 4. Adding a catalyst

Temperature Increasing the temperature raises the frequency of collisions and the number of particles that have enough kinetic energy to get over the energy barrier.

Concentration Increasing the concentration causes an increase in the frequency of collisions.

Particle Size The smaller the particle size, the larger the surface area for a given mass of particles.

Adding a Catalyst The addition of a catalyst lowers the energy barrier of the activated complex. Example: Pt 2 H 2(g) + O 2(g) 2 H 2 O(l)

Catalyst: Increases the rate of a reaction by lowering the Ea. Inhibitor: A substance that decreases the rate of a reaction by increasing Ea.