# week 5 notes.docx

7/28/2019 # Week 5 Notes.docx

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FACTORS THAT AFFECT REACTION RATES

Although a balanced chemical equation describes the quantitative relationships between the reactants and

the products, it gives us no information about whether and how fast a given reaction will occur. This

information is obtained by studying the chemical kinetics of a reaction, which depend on various factors:

Higher concentration of reactants more frequent collisions between reactants higher reaction rateo Reaction rates usually decrease with time as reactants are used up (their concentration decreases)

Higher temperature particles move faster more frequent collisions higher reaction rateo In systems where more than one reaction is possible, the same reactants can produce different

products under different reaction conditions

Homogeneous reactions, when all reactants are in the same phase (liquid or gas), have a higherreaction rate than heterogeneous reactions

o The reaction rate of a heterogeneous reaction depends on the surface area of the more condensedphase

Some reactions occur faster in some solvents. Also, the lower the viscosity of the solvent, the faster thereaction will happen

Reactions normally do not occur in one step, but instead in a number of intermediate steps. Theslowest intermediate reaction determines the overall reaction rate.

Even though the overall products may be at a lower energy level than the reactants, the intermediariesmay be at a higher level. In order for the reaction to occur, this barrier, called activation energy, must

be overcome. For similar reactions under comparable conditions, the one with the lowest activation

energy will occur most rapidly.

Catalysts accelerate reactions. Sometimes catalysts lower the activation energy. Carefully selectedcatalysts will accelerate only one of possible reactions.

RATE LAWS

Reaction rates are usually expressed as the change in concentration of any reactant consumed or the

change in concentration of any product formed per unit time (moles per litre () per second):

ACTIVATION ENERGY


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Activation energy can be calculated from the Arrhenius equation:

( )

(

)

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ZEROTH ORDER REACTIONS

A zeroth order reaction is one whose rate is constant and independent of concentration:


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FIRST ORDER REACTIONS


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SECOND ORDER REACTIONS


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REACTION ORDER SUMMARY


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In general, reaction order is the slope of the line obtained from plotting the log of reaction rate against the

log of varying concentrations.


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If there is more than one reactant, one must calculate the reaction order coefficients separately for each

reactant to obtain the overall reaction order. Doubling the concentration of one reactant while keeping all

others constant and observing the reaction rate, we can calculate the reaction order coefficient for the

varying reactant. After doing the same for the other reactants, the overall reaction order is the sum of all

coefficients.

# week 5 notes.docx

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