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CSEC Chemistry: Rates of Reaction

The rate at which something occurs is how quickly it happens over time. As you know, in chemical reactions, reactants are converted into products. So, the concentration of reactants decreases while the concentration of products increases. Knowing this, the rate of a reaction is the change in concentration of reactant or product over time at a stated temperature.


Rate of reaction is calculated like this:

decrease (change) in concentration of reactants/time taken for change

or

increase (change) in concentration of products/time taken for change


You may be wondering how exactly you would be able to measure a change in concentration of either a reactant or product. Well, that would be very difficult, so you often don't have to measure it directly. Instead, if one product is in a different physical state than the reactants, it becomes far easier to discern this change. So, we can measure the change in a volume of gas, the rate of the formation of a precipitate, or a change in the pH (where pH differs between the products and reactants).


The rate of a certain reaction is affected by several factors:

  • concentration (or gas pressure in reactions involving gas)

  • temperature

  • surface area (particle size)

  • presence or absence of a catalyst

  • Intensity of light (for specific reactions only)


Concentration and the Rate of Reaction

When the concentration increases, there are more reactant molecules in a given volume. This increases the chances of more frequent effective collisions, causing a faster rate of reaction.


For reactions involving gases, increasing the pressure of the gases is the same thing as increasing concentration. This is because, according to Boyle's Law, the volume of a gas decreases as pressure increases and vice versa (given that temperature is constant). So, increased pressure means the same number of molecules in a smaller volume, which results in higher concentration.


For example, in a reaction between a magnesium strip and excess hydrochloric acid (reactants), we can discern the rate of reaction based on the volume of hydrogen gas (H2) liberated as a product. In order to understand the effect of concentration on the rate of reaction, we must keep all other variables constant:

  1. quantity of magnesium

  2. temperature (27°C)

  3. the surface area of the strip of magnesium

We only change the concentration of hydrochloric acid (HCl), our manipulated (independent) variable.

The data showing the rate of reaction (responding/dependent variable) can be represented on a graph of concentration (volume of hydrogen gas) vs time.



Temperature and the Rate of Reaction

As the temperature of the reactants increase, the rate of reaction increases as well. This is because particles move faster (have more kinetic energy) with greater temperatures. Hence, particles collide more frequently, and the collisions are more effective since more particles have energy equal or more than the activation energy (energy necessary for a reaction to occur).

In certain chemical reactions, the reaction rate increases two-times for every 10°C rise in temperature.

We actually manipulate temperature and apply its effect on rate of reaction in our every day lives. We put meats and other products in the freezer to reduce the rate of spoilage reactions. Pressure cookers are used because water boils at 120°C (in the high pressure environment of the pressure cooker) causing the food to cook faster.


Catalysts and the Rate of Reaction

In a previous biology article, we discussed enzymes, which are biological catalysts. A catalyst is basically a substance which speeds up a reaction (by reducing the activation energy) and isn't used up or changed in the reaction. They are not written as a part of the chemical equation (though they may be written as a condition under or over the arrow).

Essentially, whenever catalysts are present, they reduce the energy necessary for reactions to occur and cause it to occur faster. They provide an easier way for the reactants to form products.

Some catalysts in the solid state catalyse reactions involving gases or liquids by providing a surface for the reactants to react on. Other catalysts in the same state as the reactant often provide an easier route for the reaction to occur.


Surface Area and Rate of Reaction

The rate at which a reaction occurs can be increased by increasing the surface are of the solid. If you decrease the surface area of the solid, the reaction will occur slower.

This is because these reactions involve collisions between moving molecules and solid reactants. When the solid is broken down into smaller particles, there is a greater surface area available for collisions.

The application of this is best seen in the human digestive system, where the teeth break down solid food into smaller pieces so that the enzymes can work more effectively on them.


Light and Rate of Reaction

Most chemical reactions are unaffected by the presence of light. However, some very important reactions, like photosynthesis, rely on light to occur. Such reactions that are initiated or sped up by light are called photochemical reactions.

Traditional early photography was based on light sensitive reactions. Photographic film and paper are made up of photosensitive grains of silver and halides. These grains react with light to create a latent image that isinvisible to the eye. In the darkroom, the alkaline, or basic, developing solution reduces the silver halide molecules to atomic metal silver. This creates the dark areas that constitute the visible image. An acidic stop bath is used to halt the developing process and a fixing solution to preserve the image by dissolving the leftover silver halides that could still react with light.


(If you are interested in traditional film development, there is a great USC thesis on the Chemistry of Photography here)