Understanding Points
Reactivity 2.2.1—The rate of reaction is expressed as the change in concentration of a particular
reactant/product per unit time.
Reactivity 2.2.2—Species react as a result of collisions of sufficient energy and proper orientation.
Reactivity 2.2.3—Factors that influence the rate of a reaction include pressure, concentration,
surface area, temperature and the presence of a catalyst.
Reactivity 2.2.4—Activation energy, Ea, is the minimum energy that colliding particles need for a
successful collision leading to a reaction.
Reactivity 2.2.5—Catalysts increase the rate of reaction by providing an alternative reaction
pathway with lower Ea.
Collision theory
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Rate of reaction = Δ[reactant or product]/time = ↓[reactant] or ↑[product]/time
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The greater the probability that molecules will collide with sufficient energy and proper orientation, the higher the rate of reaction.
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Criterias for successful collision:
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Correct geometry/orientation of collision
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Sufficient energy
Rate measurement
Technique | Condition | Method |
Volume of gas produced | Product = gas
* can’t use if the no. of moles of gas are equal on both sides of the reaction | Gas syringe |
Decrease in mass
(due to gas given off) | When the reaction produces a heavy gas
(anything but hydrogen) | Digital balance |
Colorimetry | If reaction changes colour then you can measure the absorbance/time | Colorimetry (pass in light of selected λ through soln/measure intensity of visible light transmitted)
*high [conc] absorbs more light |
Electrical conductivity | Change in the no. of moles of ions from reactants to products
*more conducted if reaction produces more ions | Conductivity meter (electrodes) |
pH | If a reaction produces/uses up H+/OH- then you can measure the change in pH over time | pH probe |
Clock method | Any distinguishable change over time
(e.g. time it takes for X to dissolve) | Example: iodine clock |
Maxwell-Boltzmann distribution
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Activation energy (Ea): the minimum energy that colliding molecules need for successful collisions leading to a reaction
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Curve shows the distribution of kinetic energy molecules have for a reaction
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Only a small proportion of particles at a particular temp. have kinetic energy greater than the required Ea to successfully react
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Area under the curve represents the total no. of particles
Factors that affect rate of reaction