REACTION RATES•Chemical Kinetics is the study of the rates of reaction & the factors that influence these rates.•Crosses over into many other areas of science & engineering•Examples:

- rates of metabolic reaction - progress of reactions with growth

& bone regeneration- decrease rate of rusting in cars- reactions involved in spoiling of

food

• The speed of any activity (e.g. running, reading, cooking hamburgers, etc.) involves quantifying how much you accomplish in a specific amount of time. We can quantify, or measure, the speed of a chemical reaction(also known as its reaction rate).

• Based on the above definition: reaction kinetics describes how fast or slow a reactant disappears or a product forms.

• Ex. Fast reactions have a short reaction time, slow reactions take a long time.

Reaction rate is described as a change in an observable property over time.

The observable property should be selected based upon what can be measured in the lab.

This could be a colour change, a temperature change, a pressure change or the appearance of a new substance.

What does rate of reaction mean?

The speed of different chemical reactions varies hugely. Some reactions are very fast and others are very slow.

What is the rate of these reactions?The speed of a reaction is called the rate of the reaction.

rusting baking explosion

slow fast very fast

Variables to Measure for Rate of Reaction

Pressure: A manometer can be used to measure a change in pressure when a reaction results in a change in the number of moles of gas.

  pH: You can use a pH meter to measure the change in acidity over

time. This data can then be used to determine the concentration of hydrogen ion over time.

  Colour: A spectrometer can be used to measure the concentration of

a reactant or product that absorbs (or give off) light of a narrow range of wavelengths.

  Temperature: A change in temperature can be used to measure

reaction rates.  Conductivity: Electrodes are placed in the reaction mixture and the

increase/decrease in conductivity of the products can be used to measure reaction rate.

Measuring Reaction Rates A. Consider this heterogeneous reaction:

Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

  In general, to get an idea of how “fast” the reaction

is going, we measure the rate at which some product is produced or the rate at which some reactant is used up. It is measuring the change in amount of reactant or product over a time interval.

  Note: The idea that the rate of a reaction as a

whole can be described by the rate of appearance or disappearance of a single species.

 

Example: If 0.20 moles of Mg are used up in 20. Seconds, we could specify the rate as:

  Rate = moles Mg used up/ # of seconds 

0.20/20 = 0.010 or 1.0 x 10-2 moles/s =R  The idea of “kinetics” is to get a

mathematical description of how “fast” a reaction is going.

B. Homogeneous Reactions  For reactions involving solutions of known

concentrations, reaction rates are expressed in terms of a change in concentration of one of the participants per unit of time. 

  R = C C = change in

concentration t t = change in time

  Units are moles/L/s OR M/s (molar per

second) 

Changes in Concentration and Reaction Rates as a Reaction Progresses

The concentrations of reactants and products vary with time in this general fashion:

  As more products are made, the amount of

reactants decrease.  The rate of reaction may be found by measuring

the slope of a concentration vs. time graph.  The initial rate of reaction is high and the rate

becomes slower as the reaction progresses. 

The changes in concentration, which provide information about the reaction rate, may be measured in a number of ways.- directly – weighing, volumetric analysis- changes in colour- changes in density- changes in electrical conductivity- changes in gas pressure

  Other factors can affect the rate or reactions

besides concentration. They are surface area, temperature, catalysts and the nature of reactants.

http://www.chm.davidson.edu/vce/kinetics/ReactionRates.html