READ: Reaction Rate
READ: Reaction Rate
How fast a chemical reaction occurs is called the reaction rate (a measure of how fast products are made in a chemical reaction). The rate of a reaction can be obtained by graphing how the concentration of a reactant or product changes over time. The slope of the line is the rate of the reaction. A steeper slope, means the reaction is going faster. Consider the following example.
In this reaction, the reactant (A) is being used to make the product, and its concentration is going down as it is being used. The concentration of the product (B) is going up, as more and more of it is made over time.
Several factors affect the rate of a given chemical reaction. They include the following:
- Temperature of reactants
- Concentration of reactants
- Surface area of reactants
- Presence of a catalyst
Temperature of Reactants
When the temperature of reactants increases, the rate of the reaction increases. At higher temperatures, particles of reactants have more energy, so they move faster. As a result, they are more likely to bump into one another and to collide with greater force. For example, food spoils because of chemical reactions, and these reactions occur faster at higher temperatures (see the bread on the left in theFigure below). This is why we store foods in the refrigerator or freezer (like the bread on the right below in the Figure below). The lower temperature slows the rate of spoilage.
There are two major effects due to increasing the temperature: more frequent collisions, and more energetic collisions. Thus, more particles will collide, and more of those collisions will have enough energy for a reaction to occur. In other words, more particles will have the energy needed in order to react.
Concentration of Reactants
The rate of a reaction also depends on concentration (the number of particles of a substance in a given volume). When the concentration of reactants is higher, the reaction rate is faster. At higher concentrations, particles of reactants are crowded closer together, so they are more likely to collide and react. Did you ever see a sign like the one in the figure? You might see it where someone is using a tank of pure oxygen for a breathing problem. Combustion, or burning, is a chemical reaction in which oxygen is a reactant. A greater concentration of oxygen in the air makes combustion more rapid if a fire starts burning.
If you had one red ball and one green ball flying around randomly in an enclosed space and undergoing perfectly elastic collisions with the walls and with each other, in a given amount of time, the balls would collide with each other a certain number of times as determined by probability. If you now put two red balls and one green ball in the room under the same conditions, the probability of a collision between a red ball and the green ball would exactly double. The green ball would have twice the chance of encountering a red ball in the same amount of time.
The rate of reaction is proportional to the number of collisions in a certain amount of time, so increasing the concentration of either reactant increases the number of collisions, the number of successful collisions, and the reaction rate.
Surface Area of Reactants
When a solid substance is involved in a chemical reaction, only the matter at the surface of the solid is exposed to other reactants. If a solid has more surface area, more of it is exposed and able to react. Therefore, increasing the surface area of solid reactants increases the reaction rate. Look at the hammer and nails pictured in the Figure below. Both are made of iron and will rust when the iron combines with oxygen in the air. However, the nails have a greater surface area, so they will rust faster.
CC-BY-NC http://www.ck12.org/user%3Adxrhac1vzxitzg9llnbhcnruzxjay2sxmi5vcmc./book/Chemistry/section/6.2/
After you have completed this part of the lesson, you can check the associated box on the main course page to mark it as complete 