CHEMISTRY 12: KINETICS

1. Measuring reaction rates
2. Factors affecting the rate of reaction
3. Collision Theory
4. Activation Energy
5. Reaction Mechanisms

 
1. Measuring reaction rates
• Reaction rates measure how quickly or slowly a reaction occurs over a period of time
• Any observable change can be used to measure reaction rate:
• mass change/time    e.g. g/s
• volume change/time e.g. L/min
• pressure change/time  e.g. Pa/s
• temperature change/time  e.g. °C/s
• colour intensity change/time (not used a lot)
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2. Factors affecting the rate of reaction
• Temperature:
• reaction rate increases as temperature increases
• the time taken for the reaction is shorter
• approximately: the rate doubles for each 10°C increase
• Concentration:
• as the concentration increases the rate increases
• Pressure/Volume:
• as the volume decreases the pressure increases
• as the pressure increases the reaction rate increases
• as the volume decreases the reaction rate increases
• The nature of reactants:
• term describing the chemical properties of a substance
• number of bonds which are broken and made during the reaction
• the more bonds which have to be broken the slower the reaction
• Ability of reactants to meet:
• phase and surface area
• homogeneous reactions - two gases, two miscible liquids, or two aqueous solutions will mix completely so surface area does not affect them.
• heterogeneous - solid/solid, solid/liquid, solid/gas, liquid/gas, or 2 immiscible liquids have a limited surface on which the reaction occurs.  Surface area affects the rate of reaction!!
• the larger the surface area the faster the reaction rate
• for solids: the smaller the particle size the faster the reaction rate
• ability of the particles to mix freely is important
• aqueous ions mix fastest and so the reaction rate is fastest
• solid particles do not mix freely so they have much slower reaction rates
 

FASTEST >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> SLOWEST

Aqueous                >>              Gases         >          Liquids                 >> Solids

(memorize this chart!!)

• Catalysts and inhibitors:
• catalyst - increases the rate of a reaction without being consumed in the reaction
• inhibitor - slows the rate of a reaction by bonding to a catalyst or one of the reactants so that the reaction cannot occur efficiently
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3.   Collision Theory:
• particles must collide for a reaction to occur







• factors affecting collision include:
• the energy of the collision: not all collisions are successful because energy is required to overcome the repulsion of the electron clouds surrounding the particles
• as temperature increases the kinetic energy of the particles increases.  The particles move faster which causes collisions to occur more frequently and have more energy

• the orientation of the particles: atoms and molecules have reactive sites where reaction can occur, these active sites must be aligned or the collision will not be successful
• the concentration of the particles: the more particles there are the more collisions there will be which increases the probability of having successful collisions
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4. Activation Energy
• Activation energy is the energy required for a reaction to occur.
The progress of a reaction is frequently described using a Potential Energy Diagram:

• Exothermic reactions tend to have low activation energy, so once enough energy is provided to overcome the activation energy the reaction proceeds spontaneously since energy is produced to maintain the reaction:
• for example: a spark will start a piece of wood burning... once the fire starts the combustion continues until there is no fuel left
• Endothermic reactions tend to have high activation energy.  An endothermic reaction CONSUMES energy so the reaction stops if the energy source is removed
• for example: using baking powder (decomposition of NaHCO₃) to cause dough to rise... the dough only rises while it is in the oven (heat is provided).  The dough stops rising when removed from the heat source... baking soda only decomposes to CO₂ when heated
• Catalysts:  a catalyst provides a different mechanism with a LOWER activation energy so the reaction is faster
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5. Reaction Mechanisms

• reactions occur as a series of 2 particle collisions - called elementary processes
• reactions with more than 2 particles will rarely happen as a single step... the probability of correct geometry and energy is so small that the reaction wouldn't happen as a single step in an observable amount of time
• energy curves tend to have TWO or MORE hills or activation energies, each hill corresponds to a step in the reaction

• the largest hill is the slowest step... called the rate determining step because the reaction cannot be any faster than the slowest step
• the largest hill is the activation energy because it is the largest barrier between the reactants and products