# Chemical Kinetics

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

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 discussion of chemical reaction mechanism.

• Molecules or atoms of reactants must collide with each other in chemical reactions.
• 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.

### Reaction Rates

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 conditions.

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 thermodynamic data. 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.

1. 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.
2. Temperature
Usually, the higher the temperature, the faster the reaction. The temperature effect is discussed in terms of activation energy.
3. Concentration Effect
The dependences of reaction rates on concentrations are called rate laws. 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).
4. Heterogeneous reactions: reactants are present in more than one phase
For heterogeneous reactions, the rates are affected by surface areas.
5. Catalysts: substances used to facilitate reactions
By the nature of the term, catalysts play important roles in chemical reactions.

### Reaction Mechanisms

The detailed explanation at the molecular level of 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 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.

## Confidence Building Questions

1. What drives chemical reactions?
1. energy
2. activation energy
3. electrons
4. physical conditions

Hint: a. energy

Skill -
Recognize energy as the driving force of all changes, and explain what thermodynamic data and activation energies are.

2. Which one of the following cannot be a unit for reaction rate: mol/(L s), g/(L s), %/s g/s, mol/s or mol/L?

Hint: mol/L cannot be units for rates because it lacks the time factor.

Skill -
Analyze dimensions of units.

3. The enthalpies of formation of $$\ce{Al2O3}$$ and $$\ce{B2O3}$$ are -1676 and 1274 kJ / mol respectively. Do these data indicate the rate of oxidation of $$\ce{Al}$$ and $$\ce{B}$$?

Hint: No, thermodynamic data are unrelated to the rate of reaction.

Skill -
Describe thermodynamic data.

4. 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.?

Hint: chemical kinetics.

Skill -
Remember a term and use it to communicate efficiently.

5. Which one of the following reactions reacts the most rapidly at room temperature?
1. $$\ce{2 H2 + O2 \rightarrow 2 H2O}$$
2. $$\ce{H+ + OH- \rightarrow H2O\:(neutralization)}$$
3. $$\ce{C12H22O11\: (sugar) \rightarrow 12 C + 11 H2O}$$
4. $$\ce{H + OH \rightarrow H2O\:(radical\: reaction)}$$

Hint: b. because reactants are ions.

Skill -
Discuss how reaction rates depend on nature of reactants.

6. 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?

Hint: higher than 373 K

Skill -
Recall the phase diagram of water, and explain this effect.

7. Which one takes the least period of time to cook a potato of 10 cm in diameter?
1. boil in a pressure cooker
2. boil on top of a 6000-m mountain
3. bath in a steam
4. using a microwave oven

Hint: d. But microwave works differently than heat.

Skill -
Explain the temperature effect with a twist.

8. You store food in a fridge to prevent spoilage. Which factor are you applying to slow the rate of reaction?
1. nature of reactants
2. isolation of reactants
3. avoid catalysts
4. temperature

Hint: d. temperature

Skill -
Lowering temperature slows down the reaction rate.

9. Which one of the following burns easily and why?
1. a bar of steel
2. steel wool
3. steel sheet
4. steel pipe

Hint: b. Steel wool can be ignited with a lighter.

Skill -
Describe heterogeneous reaction, and discuss surface area effect.