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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).
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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.
