Skills to develop
- Explain the term chemical kinetics, and describe factors that influence
rate of chemical reactions
- Define and use proper units for chemical reaction rates.
- Give the conditions for homogeneous and heterogeneous reactions.
Chemical kinetics is the study and discussion of chemical reactions
with respect to reaction rates, effect of various variables, re-arrangement
of atoms, formation of intermediates etc. There are many topics
to be discussed, and each of these topics is a tool for the study of
chemical reactions. By the way, the study of motion is called kinetics,
from Greek kinesis, meaning movement.
At the macroscopic level, we are interested in amounts reacted, formed,
and the rates of their formation. At the molecular or microscopic level,
the following considerations must also be made in the discusion of
chemical reaction mechanism.
- Molecules or atoms of reactants must collide with each other in chemical
- The molecules must have sufficient energy (discussed in terms of
activation energy) to initiate the reaction.
- In some cases, the orientation of the molecules during the collision
must also be considered.
Chemical reaction rates are the rates of change in concentrations or
amounts of either reactants or products. For changes in amounts, the units
can be one of mol/s, g/s, lb/s, kg/day etc. For changes in concentrations,
the units can be one of mol/(L s), g/(L s), %/s etc.
With respect to reaction rates, we may deal with average rates,
instantaneous rates, or initial rates depending on the experimental
Thermodynamics and kinetics are two factors that affect reaction
rates. The study of energy gained or released in chemical reactions is called
thermodynamics, and such energy data are called
However, thermodynamic data have no direct correlation with
reaction rates, for which the kinetic factor is perhaps more
important. For example, at room temperature (a wide range of temperatures),
thermodynamic data indicates that diamond shall convert to graphite,
but in reality, the conversion rate is so slow that most people think
that diamond is forever.
Factors Influence Reaction Rates
Many factors influence rates of chemical reactions, and these are summarized
below. Much more extensive discussion will be given in other pages.
The detailed explanation at the molecular level how a reaction proceeds is
called reaction mechanism. The explanation is given in some
elementary steps. Devising reaction mechanisms requires a broad understanding
of properties of reactants and products, and this is a skill for matured
chemists. However, first year chemistry students are often given a
mechanism, and be asked to derive the rate law from the proposed mechanism.
The steady-state approximations is a
technique for deriving a rate law from the proposed mechanism.
- Nature of Reactants
Acid-base reactions, formation of salts, and exchange of ions are fast
reactions. Reactions in which large molecules are formed or break apart
are usually slow. Reactions breaking strong covalent bonds are also slow.
Usually, the higher the temperature, the faster the reaction. The temperature
effect is discussed in terms of
- Concentration Effect
The dependences of reaction rates on concentrations are called
Rate laws are expressions of rates in terms of concentrations of reactants.
Keep in mind that rate laws can be in differential forms or integrated forms.
They are called differential rate laws and integrated rate laws.
The following is a brief summary of topics regarding rate laws.
Rate laws apply to homogeneous reactions in which all reactants and
products are in one phase (solution).
- Heterogeneous reactions: reactants are present in more than one
For heterogeneous reactions, the rates are affected by surface areas.
- Catalysts: substances used to facilitate reactions
By the nature of the term, catalysts play important roles in chemical
Confidence Building Questions
What drives chemical reactions?
b. activation energy
d. physical conditions
Recognize energy as the driving force of all changes, and explain what
are thermodynamic data, and activation energies.
Which one of the following cannot be a unit for reaction rate:
mol/(L s), g/(L s), %/s g/s, mol/s and mol/L?
Analyze dimensions of units.
The enthalpies of formation of Al2O3 and
B2O3 are -1676 and 1274 kJ / mol respectively.
Do these data indicate the rate of oxidation of Al and B?
Describe thermodynamic data.
What is the discipline that studies chemical reactions with respect to
reaction rates, effect of various variables, re-arrangement
of atoms, formation of intermediates etc.
Remember a term and use it to communicate efficiently.
Which one of the following reactions reacts the most rapidly at room
a. 2 H2 + O2 -> 2 H2O
b. H+ + OH- -> H2O (neutralization)
c. C12H22O11 (sugar) -> 12 C + 11 H2O
d. H + OH -> H2O (radical reaction)
Discuss how reaction rates depends on nature of reactants.
A pressure cooker has a heavy weight on a small opening, so that the pressure
in contact with the water is higher. In this case, would you expect the
boiling point of water to be higher or lower than the standard
boiling point of 373 K?
Recall the phase diagram of water, and explain this effect.
Which one takes the least period of time to cook a potato of 10 cm in diameter?
a. boil in a pressure cooker
b. boil on top of a 6000-m mountain
c. bath in a steam
d. using a microwave oven
Explain the temperature effect with a twist.
You store food in a fridge to prevent spoilage. Which factor are you
applying to slow the rate of reaction?
a. nature of reactants
b. isolation of reactants
c. avoid catalysts
Lowering temperature slows down the reaction rate.
Which one of the following burns easily and why?
a. a bar of steel
b. steel wool
c. steel sheet
d. steel pipe
Describe heterogeneous reaction, and discuss surface area effect.