Sodium reacts violently with water. That’s what is happening in the picture above. Why does sodium have such explosive reactions? It’s because the reactions occur so quickly.
How Fast Does It Go?
How fast a chemical reaction occurs is called the reaction rate. Several factors affect the rate of a given chemical reaction. They include the:
- temperature of reactants.
- concentration of reactants.
- surface area of reactants.
- presence of a catalyst.
At the following URL, you can see animations showing how these factors affect the rate of chemical reactions.
When the temperature of reactants is higher, the rate of the reaction is faster. 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 the Figure below). This is why we store foods in the refrigerator or freezer (like the bread on the right in the Figure below). The lower temperature slows the rate of spoilage.
Left image: Bread after 1 month on a warm countertop. Right image: Bread after 1 month in a cold refrigerator.
Concentration of Reactants
Concentration is 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 below? 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.
Q: It is dangerous to smoke or use open flames when oxygen is in use. Can you explain why?
A: Because of the higher-than-normal concentration of oxygen, the flame of a match, lighter, or cigarette could spread quickly to other materials or even cause an explosion.
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.
Presence of a Catalyst
Some reactions need extra help to occur quickly. They need another substance called a catalyst. A catalyst is a substance that increases the rate of a chemical reaction. A catalyst isn’t a reactant, so it isn’t changed or used up in the reaction. Therefore, it can catalyze many other reactions.
- How fast a chemical reaction occurs is called the reaction rate.
- Several factors affect the rate of a chemical reaction, including the temperature, concentration, and surface area of reactants, and the presence of a catalyst.
Watch the video about reaction rate at the following URL, and then answer the questions below.
- What is collision theory?
- How does collision theory relate to factors that affect reaction rate?
- Define reaction rate.
- List factors that influence the rate of a chemical reaction.
- Choose one of the factors you listed in your answer to question 2, and explain how it affects reaction rate.