Activation energy and temperature change in reactions
Energy changes • Exothermic and endothermic reactions
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Definition of activation energy
Activation energy is the minimum energy required for reactant particles to form an activated complex and proceed to products. Activation energy corresponds to the energy difference between reactants and the top of the energy barrier on an energy profile diagram. A reaction does not occur unless at least some colliding particles have kinetic energy equal to or greater than the activation energy.
Energy profile diagrams and activation energy
An energy profile diagram shows reactant energy, product energy and the energy barrier for the reaction. Activation energy appears as the vertical energy gap from the reactants to the highest point (transition state) on the curve. The height of the barrier determines how many particles can overcome it at a given temperature and therefore determines the reaction rate.
Effect of temperature on particles and reaction rate
Temperature measures the average kinetic energy of particles. Increasing temperature shifts the kinetic energy distribution so a larger fraction of particles have energies greater than the activation energy. Greater fraction of energetic collisions produces more successful collisions per unit time and increases reaction rate. Lowering temperature reduces the fraction of particles above the activation energy and slows the reaction.
Role of catalysts and limiting factors
A catalyst lowers the activation energy by providing an alternative reaction pathway with a smaller energy barrier. Lower activation energy increases the fraction of collisions that succeed at the same temperature and increases the reaction rate without changing overall energy change. Concentration, pressure (for gases), surface area and temperature remain limiting factors that affect collision frequency and the proportion of collisions above the activation energy.
Temperature change of the surroundings: exothermic vs endothermic
Exothermic reactions release net energy to the surroundings because products have lower chemical energy than reactants. Energy release causes the temperature of the surroundings to increase. Endothermic reactions absorb net energy from the surroundings because products have higher chemical energy than reactants. Energy absorption causes the temperature of the surroundings to decrease. Measurement of a temperature rise or fall in the surroundings distinguishes the two types of reaction.
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