Illustrated · 3 min
Factors That Affect Reaction Rate
What factors affect the rate of reactions?
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Increased Concentration.
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Increase in Temperature.
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Increased Surface Area.
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Increase in Pressure.
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Addition of a Catalyst.
The reaction rate is the speed at which a chemical reaction takes place.
How Concentration affects the reaction rate:
Increasing the concentration of the reactants speeds up the reaction rate. Imagine a synthesis reaction between two molecules. The generic balanced chemical equation would be . Let’s say we increased the concentration of reactant A. That means there are more molecules of reactant A per volume. So, reactant B is more likely to collide with reactant A and produce a reaction. More collisions equal faster reactions.
Image source: Caroline Monahan
How Temperature affects the reaction rate
Increasing the temperature also increases the reaction rate. Remember, temperature measures a system's average kinetic energy. So, raising the temperature increases the kinetic energy of the molecules. The molecules move faster inside their container. What's the result? More collisions! And more collisions create a faster reaction! But, there's more. For a reaction to occur, the collision needs to overcome the activation energy. With more kinetic energy, they are more likely to overcome the barrier. So, the reaction rate increases.
Image source: Caroline Monahan
How pressure affects the reaction rate
At this point, we sound like a broken record. Increasing the pressure causes the molecules to collide more frequently. And more collisions increase the reaction rate!
Image source: Caroline Monahan
How surface area affects the reaction rate
Let's imagine an ionic compound. We know they form a crystal lattice. So, you have repeating unit cells of ions, that build the overall structure. But, that means some of the ions are buried inside the crystal. Only the ions on the surface can react. Let's imagine we took an ionic crystal and smashed it with a hammer. That would be a lot of fun! But, we would expose some of the ions in the center of the crystal. Now they can react.
Increasing a reactant's surface area exposes reactive particles. So, the reactants are more likely to successfully collide! When a reactant has a small surface area, it’s harder for the reactants to interact. There are fewer successful collisions. So, increasing a reactant's surface area increases the reaction rate.
Image source: Caroline Monahan
How catalysts affect the reaction rate
A catalyst helps increase the rate of the reaction. But, there's a trick. It doesn't get consumed during the reaction. If you put a catalyst in the reaction, you get the same catalyst out of the reaction. It's not considered a reactant because it doesn't undergo a permanent chemical change. It just speeds up the reaction! How do catalysts work? They create an alternative reaction mechanism. The new mechanism has a lower activation energy. Remember, that's the energy required for the reaction to occur. So, more molecules will have sufficient energy to react.
There's a significant variety of catalytic mechanisms. It's a major subset of physical chemistry and chemical engineering. But, the general mechanism is that the reactants adsorb onto the catalyst's surface. Adsorption is the adhesion of an ion, atom, or molecule to a surface. There are two types of adsorption. In physisorption, the molecule-surface interaction is caused by Van der Waal's forces. In chemisorption, the molecule forms a covalent bond with the surface. Both types of adsorption are exothermic processes. They lower the potential energy of the reactants. But, the reactant-catalyst interaction does not inhibit the reaction. Instead, the reaction proceeds on the catalyst's surface. We still create a transition state. But now, the transition state is also interacting with the catalyst. The transition state-catalyst complex is more stable than the non-catalyzed one. So, we have a lower activation energy. How is the catalyst regenerated? As the products form, they desorb from the catalyst.
