Chemistry · MU Pleven

TEMPERATURE DEPENDENCE OF REACTION RATE. ACTIVATION ENERGY. ARRHENIUS EQUATION

Explains how reaction rates rise with temperature, using the van't Hoff rule of thumb and a collision-theory account of why faster, more energetic and correctly oriented collisions matter. It then covers the Maxwell-Boltzmann distribution, activation energy, transition-state theory, and how exothermic and endothermic reactions differ in their activation energies. The note finishes with the Arrhenius equation, its logarithmic linear form, and how catalysts lower activation energy, with a few diagrams and formulas along the way.

Inside: 6 sections · 10 labelled pictures · 12 formulas · 25 flashcards · 12 min

Topic 12 of 47 in the MU Pleven syllabus for Chemistry (Medicine, Year 1). The free AceMed note covers it in 6 sections, about 1,773 words (12 min of reading), with 25 flashcards.

In this note

  1. TEMPERATURE DEPENDENCE OF REACTION RATE. ACTIVATION ENERGY. ARRHENIUS EQUATION
  2. I. Temperature and rate
  3. II. Relationship between temperature and rate. Activation energy.
  4. III. Exothermic and endothermic reactions
  5. IV. Arrhenius equation
  6. V. Catalysts reduce activation energy

I. Temperature and rate

Rates of most reactions are very sensitive to temperature. Most increase rapidly with increasing temperature. Some reaction rates are very sensitive to temperature changes, whereas others are only slightly affected. For example, an increase of 20oC can lead to a doubling of the reaction rate. According to a rule formulated by Vant Hoff, a 10oC temperature raise increases the reaction rate in a homogeneous system of 2 to 4 times (for the interval 0 - 100оС):

The reason for this is not hard to understand. It can be suggested that as the temperature rises, the relative speeds of molecules with respect to one another increase, leading to an increase in collission frequency. As the temperature rises, molecules move faster and collide more vigorously, greatly increasing the likelihood of bond cleavages and rearrangements.

Whether it is through the collision theory, transitionstate theory, or just common sense, chemical reactions are typically expected to proceedfaster at higher temperatures and slower at lower temperatures.

Collision Theory

Collision theory provides a qualitative explanation of chemical reactions and the rates at which they occur, appealing to the principle that molecules must collide in order to react. Consider the elementary gaseous bimolecular reaction: A + B → products

If the two molecules A and B are to react, they must collide with sufficient force so that chemical bonds break. The frequency of collisions between A and B will be proportional to the concentration of each gas. If the concentration of A is doubled, the frequency of A-B collisions will double, and doubling the concentration of B will have the same effect. Therefore, according to collision theory, the rate at which molecules collide will have an impact on the overall reaction rate.

In order for a collision to be successful by resulting in a chemical reaction, A and B must collide with sufficient energy to break chemical bonds. Therefore, in order to effectively initiate a reaction, the reactants must be moving fast enough (with enough kinetic energy) so that they collide with sufficient force for bonds to break. This minimum energy with which molecules must be moving in order for a collision to result in a chemical reaction is known as activation energy.

Continue reading free

The whole note, all 6 sections, is free with an AceMed account, with its 25 flashcards. Premium adds flashcard recall, highlights and Ask AI.

Start freeAlready a member? Open the note