What affects the rate of a chemical reaction?

The rate at which a chemical reaction occurs is determined by a number of factors. Specifically:

-- Chemical Reactivity

First and foremost, it depends on the specific properties of the chemicals involved. Some chemicals react violently and very quickly, and others very very slowly. Usually, if you have a specific reaction in mind, you don't get to control this part (except by using a catalyst -- see below), but you might be able to find a way to make the same products by a different method that has an inherently faster of slower rate. The chemical reactivity is determined by the activation energy of the reaction, or the transition state energy. This is the point of highest energy reached in the reaction, and most reaction rates are determined by the reactants ability to get over this barrier. The size of the barrier is simply determined by the inherent properties of the reactants.

-- Temperature

Another very important factor, and usually the easiest to control is the temperature at which the reaction takes place. Increasing the temperature of the reacts will almost always make the reaction go faster. Decreasing the temperature will slow the reaction rate. This isn't always the case, but it is true for the very large majority of reactions. Increasing the temperature will help the reactants get reach the activation energy, and get over the barrier, as discussed above.

-- Surface Area and Mixing

Equally important as the temperature is the exposed surface area of the reactants. When a chemical reaction occurs, the atoms of one substance must interact with the atoms of another substance. To do that, the atoms must be in physical contact with each other! That is why chemical reactions are commonly performed in solvents, because in a liquid, dissolved chemicals are easily able to come in direct contact with other dissolved chemicals in the solution and react together. The gas phase also allows good mixing to two chemicals because individual atoms/molecules are free to move about and bump into each other. In contrast, solids are the least reactive. This is because the atoms inside a solid aren't exposed at all to other chemicals -- they just "see" the rest of the solid they are inside! If you need to react two solids together, you want to increase their surface area as much as possible for better mixing. You want the solid to be as fine a powder as possible. It is also possible to have reactions between a liquid and a solid, a gas and a solid, or a liquid and a gas, but these will generally be slower because of the limited interaction between the two different phases. In all cases, vigorous stirring will help mix things up!

-- Reactant Concentration

The rate of a reaction is also determined by the concentration of the reactants. In most cases, the higher the concentration of the reactants, the faster the reaction (again, this is almost always true, but there are exceptions, such as radioactive decay).

-- pH

The rate of certain reactions is affected by the pH of the solution. If the reaction involves a proton, or H⁺, than a more acidic solution will have a faster rate. This is really the same effect as in the previous section, "Reactant Concentration" because the pH is really just a measure of the H⁺ concentration. So if H⁺ is a reactant, than a low pH will have a higher concentration of that reactant, making for a faster reaction. Similarly, if OH^- is a reactant, than a high pH will make the reaction go faster because at a high pH, the concentration of OH^- is large (and the concentration of H⁺ is small -- they are inversely proportional).

-- Catalysts, etc.

The rate of certain special reactions can also be increased with other things, such as shining light on it, or adding a catalyst, among other tricks! A catalyst (or enzyme for biological reactions) is a way to reduce the activation energy of the reaction, and lower the transition state energy. By reducing the energy of this barrier, the reaction can proceed much faster.

COLLISION ORIENTATION

Two species can only react together if they come into contact with each other. They first have to collide, and then they may react. It isn't enough for the two species to collide - they have to collide the right way around, and they have to collide with enough energy for bonds to break. The chances of all this happening if the reaction needed a collision involving more than 2 particles are remote. All the particles would have to arrive at exactly the same point in space at the same time, with everything lined up exactly right, and having enough energy to react. That's not likely to happen very often.

Of the collisions shown in the diagram, only collision 1 may possibly lead on to a reaction.

One may wonder why collision 2 won't work as well. The double bond has a high concentration of negative charge around it due to the electrons in the bonds. The approaching chlorine atom is also slightly negative because it is more electronegative than hydrogen. The repulsion simply causes the molecules to bounce off each other. In any collision involving unsymmetrical species, you would expect that the way they hit each other will be important in deciding whether or not a reaction happens.

First answer by ID1413754422. Last edit by ItsMeexD. Contributor trust: 2 [recommend contributor]. Question popularity: 73 [recommend question]