12.2 Factors Affecting Rate of a Reaction
Factors Affecting the rate of a Reaction
Collision Theory
To react with one another, molecules must not only collide, they must collide with the correct atoms striking one another, and with enough energy to change their intermolecular bonds.
Figure 12.8. Collision Visualizations
Figure 12.8 shows an ineffective collision in the top row. This collision is ineffective because molecule A needs to strike molecule X to react. BX’s X side needs to face A for the collision to produce change.
To increase the number of effective collisions we can, increase the reactant concentration, because more molecules means more collisions. We can increase the temperature because molecules that move faster collide both more often and more forecefully. More collisions and more energetic collisions means a faster reaction.
Let’s take a closer look at a typical reaction:
A + B → P
To react, A and B must approach closely enough to disrupt some of their existing bonds and create new ones that form P. Collisions between A and B are proportional to the concentration of each. If we double either reactant in this example, we can double the reaction rate.
rate = k[A][B]
We can also often boost the reaction rate by raising the system’s temperature. This makes the molecules move more quickly, and also increases the collision rate.
Collisions Need Sufficient Energy
In a gas at room temperature and normal atmospheric pressure, there are over a thousand collisins within each cubic centimeter every second, yet there are almost no reactions. The gas molecules bounce off one another and float away.
This is also the most likely outcome if the reaction between two substances requires a significant disruption or rearrangement of the bonds between their atoms. For molecules to react, their collisions must have enough kinetic energy to start the first step of the reaction.
In other words, effective molecular collisions also require enough energy to overcome the energy of activation, Ea. The more activation energy a reaction requires, the fewer molecular collisions will be effective. You can watch a short video on activation energy.
Figure 12.9. Potential Energy and Activation Energy
We can think of activation energy as a speed bump. The reaction in Figure 12.9 is exothermic, but colliding molecules don’t have enough energy to get it started. Usually we heat chemnicals to supply this energy, just as the students in this video heat a mixture of sugar and baking soda to make a sugar snake.
Factors that Effect the Rate of Reaction
- Physical state of the reactants — extra intermolecular bonds to break.
- Reactant concentrations — more chemical means more collisions.
- Reaction temperature — heat adds energy.
- Presence of a catalyst — alters the reaction’s molecular mechanism.
Physical State of the Reactant
Reactants that are the same phase of matter have lower activation energies. Think ionic species dissolved in water.
Increasing surface are also increase also lowers activation energy. A fine powder has lower Ea than a chunky solid.
Concentration of Reactants
While unprotected, iron-rich metal takes days to weeks to rust outdoors, adding hydrogen peroxide and salt, increases the oxygen molecules available, and rusts steel wool in seconds. Though it’s a bit more complicated than this, more reactants generally mean a faster reaction and possibly a lower activation energy, due to more molecular collisions.
Temperature
We know from unhappy observation, that milk curdles more easily at higher temperatures. Heat (and sometimes light or electricity) increases molecules’ kinetic energy. Faster molecules collide more strongly and more often. Heat also adds energy to get over the energy of activation hump. For a more extensive discussion of thermal energy’s effect on activation energy see below.
Catalysts
Catalysts change the behind-the-scenes reaction mechanism and thus lower Ea. In living things, enzymes are catalysts that let chemical reactions occur at temperatures well below temperatures where proteins denature.