Kinetics

How do chemical reactions occur?

• For a chemical reaction to occur the reactants have to collide with sufficient energy and the correct orientation

• The minimum energy for reaction to take place is the activation energy

• Not all collisions will be successful

• Some will not have sufficient energy

• Some will not have the correct orientation

What affects the rate of chemical reactions?

• Increasing temperature

  • A higher temperature means the particles have a higher kinetic energy

  • This means they will collide more frequently

  • It also means a greater proportion of collisions will have enough energy to react

• Increasing concentration (or pressure for gas phase reactions)

  • Higher concentration means more reactants per unit volume and thus more frequent collisions

• Increased surface area (for solid reactants)

  • The higher the surface area, the more particles available for collision (on the surface) and thus more frequent collisions

• Catalysts

  • These change the rate of a reaction without being used up

  • Provides an alternate reaction pathway with a lower activation energy, does not lower activation energy

Measuring rates of reaction

How can we measure the rate of a chemical reaction?

• Spectroscopy (UV-Vis, IR, NMR)

• Colorimetry

• Titration

• Chromatographic techniques (HPLC, GC)

• Stopped flow

• Flash photolysis

• Laser techniques

• Basically if anything measurable changes during the reaction then it can be used to obtain suitable data

Maxwell-Boltzmann Distribution

• On the diagram, the line never goes through the origin

• The line approaches the X axis but never touches it other than at the origin - it is asymptotic

• The most probable energy is at the highest point of the curve

• The average energy will not be the same as the most probable energy

• Higher temperatures:

  • The new curve goes through the origin

  • It will have a maximum that is lower in height but higher in energy

  • The higher temp line will only cross the lower temp line once