IB Chem Unit 7 (Kinetics)

When Atoms are Reacting

• They must have collisions that lead to successful reactions. 

• Collision theory tells us the conditions required for successful collisions.

Collision Theory

• The rate of a reaction is proportional to the rate of reactant collisions

• The reactants must collide at a certain angle that allows contact between the atoms so that they bon.

• The collision must happen with enough activation energy so that the electrons can rearrange and form new bonds and products.

Kinetics and Reaction Rates

• Study of reaction rates

  • Change in concentration per unit time

  • NOT how long it takes for a reaction to finish.

• Measured experimentally by…

  • Loss of reactants over time

    • Decreases with time

  • Production of products over time

    • Increases with time.

Changing Reaction Rates affects:

• Temperature

• Concentration

• Surface Area

• Based on effective collisions

• Sufficient Energy

  • Activation energy

• Proper Orientation

  • Correct angle.

Rate of Reaction on Temperature and Concentration

• Temperature

  • As the temperature increases, particles move faster, increases more collisions, which leads to faster rate of reaction.

• Concentration

  • Amount of solute dissolved in solvent

  • Increased concentration = particles closer = more collisions, = faster rate of reaction.

Rate of Reaction on Surface Area

Increased surface area of solid reactants means increasing more collisions, which leads to faster rate of reaction

Rate of Reaction on Catalysts

• Catalyst increases the rate of reaction by lowering the minimum activation energy required to start a reaction to form products.

Catalyst need to be:

• chemically unchanged at the end of the reaction (regenerated)

• needs to change the mechanism of reaction

• Decrease activation energy

Makes collisions more effective

Exothermic Reactions

• Exothermic reactions release energy/heat to surroundings

  • Surroundings get warmer (feels hot to you)

  • ΔH < 0  (negative)

• Products are more stable than reactants

• Bonds made are more stable than bonds broken

• Products have less energy than reactants

  • CH₄  +  2O_{2 (g)}  🡪  CO_{2 (g)}  +  2H₂O _(l) + energy

(exothermic) Enthalpy of products is:

• Enthalpy of products is less than reactants.

Endothermic Reactions

• Endothermic reactions absorb energy/heat from surrounding

  • Surroundings get colder (feels cold to you)

  • H > 0   (positive)

• Bonds made are less stable than bonds broken

• Products have more energy than reactants

  • NaHCO₃  + HCl  + energy 🡪  NaCl +  CO₂ + H₂O

(Endothermic) Enthalpy of products is:

Enthalpy of products is greater than reactants

Reaction Pathways

• Both diagrams start with an absorption of energy (activation energy)

  • Activation energy is the energy it takes to break existing bonds

  • Breaking bonds absorbs energy

• Both diagrams end with a release of energy

  • Making new bonds releases energy

    

Chemical Reactions & Energy

• Exothermic Reactions

  • Bonds in the products are stronger than bonds in the reactants

• Endothermic Reactions

  • Bonds in the reactants are stronger than bonds in the products