Summary Factors Affecting Rate
Nature of the Reactant • Reactions involving the breaking of bonds tend to be slower at room temperature • Reactions of complicated species (polyatomic ions) tend to be slower than simple species (monatomic ions). 5C2O42- + 2MnO4- +16H+ 10CO2 + 2Mn2+ + 8H2O 5Fe2+ + MnO4- + 8H+ 5Fe3+ + Mn2+ + 4H2O Which one will be faster? Why?
Nature of the Reactant (cont.) • More reactive elements will often result in a faster reaction 2NO + O2 2NO2 2CO + O2 2CO2 Which one will be faster? 2Na + 2H2O 2NaOH + H2 2Fe + 3H2O Fe2O3 (rust) + 3H2 Which one will be faster?
Concentration • Raising the concentration of the reactant(s) will increase the rate of the reaction • ionic reactions will take place faster than reactions involving more than one phase
Temperature • Increasing the temperature causes an increase in the kinetic energy of the particles. • If particles have more KE, they are more likely to have enough energy to overcome the activation energy (Ea) and the reaction will proceed. • An increase in temperature results in an increase in reaction rate. • Reactions with a lower Ea will be faster at room temperature.
Catalysts • Catalysts lower the activation energy required for a reaction. Therefore more particles will have the sufficient KE to overcome the new, lowered Ea, and the rate will increase. • Catalysts are not consumed in the reaction.
Other factors • States of matter and mixtures: - Two solid reactants will often have a slower reaction rate than two liquids or two gases. Why? - What about two separate phases? • Nature of the reaction • Analyze each reaction for clues. • ie. Combustion reactions will not take place until a spark is added (Ea is achieved).
Activated Complex • If the reactants have sufficient KE to overcome the Ea, they will collide and form a temporary structure called the activated complex (if the collision geometry is correct). • The activated complex may continue in the reaction to rearrange and form products, or it may break apart back into the original reactants.
Heat of Reaction, ΔH • The ΔH is the difference in potential energy between the products and reactants ΔH = Eproducts – Ereactants The ΔH for an exothermic reaction is negative. The ΔH for an endothermic reaction is positive.