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Chemical Reactions

Rate of Reaction

Molecules in areas with higher concentrations have a greater chance of combining.

The rate of a reaction is the speed at which a chemical reaction happens. If a reaction has a low rate, that means the molecules combine at a slower speed than a reaction with a high rate. Some reactions take hundreds, maybe even thousands, of years while others can happen in less than one second. If you want to think of a very slow reaction, think about how long it takes plants and ancient fish to become fossils (carbonization). The rate of reaction also depends on the type of molecules that are combining. If there are low concentrations of an essential element or compound, the reaction will be slower.

There is another big idea for rates of reaction called collision theory. The collision theory says that as more collisions in a system occur, there will be more combinations of molecules bouncing into each other. If you have more possible combinations there is a higher chance that the molecules will complete the reaction. The reaction will happen faster which means the rate of that reaction will increase.

Think about how slowly molecules move in honey when compared to your soda even though they are both liquids. There are a lower number of collisions in the honey because of stronger intermolecular forces (forces between molecules). The greater forces mean that honey has a higher viscosity than the soda water.

Factors That Affect Rate

As concentration, pressure, or temperature increase, reactions rates will increase. Reactions happen - no matter what. Chemicals are always combining or breaking down. The reactions happen over and over, but not always at the same speed. A few things affect the overall speed of the reaction and the number of collisions that can occur.

Temperature: When you raise the temperature of a system, the molecules bounce around a lot more. They have more energy. When they bounce around more, they are more likely to collide. That fact means they are also more likely to combine. When you lower the temperature, the molecules are slower and collide less. That temperature drop lowers the rate of the reaction. To the chemistry lab! Sometimes you will mix solutions in ice so that the temperature of the system stays cold and the rate of reaction is slower.

As pressure increases, the gas molecules can have more collisions. Concentration: If there is more of a substance in a system, there is a greater chance that molecules will collide and speed up the rate of the reaction. If there is less of something, there will be fewer collisions and the reaction will probably happen at a slower speed. Sometimes, when you are in a chemistry lab, you will add one solution to another. When you want the rate of reaction to be slower, you will add only a few drops at a time instead of the entire beaker.

Pressure: Pressure: Pressure affects the rate of reaction, especially when you look at gases. When you increase the pressure, the molecules have less space in which they can move. That greater density of molecules increases the number of collisions. When you decrease the pressure, molecules don't hit each other as often and the rate of reaction decreases.

Pressure is also related to concentration and volume. By decreasing the volume available to the molecules of gas, you are increasing the concentration of molecules in a specific space. You should also remember that changing the pressure of a system only works well for gases. Generally, reaction rates for solids and liquids remain unaffected by increases in pressure.

Next Stop On Chem4Kids Tour
Next Page on Reactions

- Overview
> Rates
- Measuring
- Stoichiometry
- Thermodynamics
- Equilibrium I
- Equilibrium II
- Catalysts and Inhibitors
- Acids/Bases I
- Acids/Bases II


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Keywords to Review

Kinetic Energy: The energy of an object that is related to the motion of the object. On a simple level, an object that is not moving has no kinetic energy. An object that is moving has some amount of kinetic energy. The faster an object moves, the more kinetic energy it has. An object gains its kinetic energy as it accelerates and increases its velocity. For example, as you increase the temperature of a gas, the molecules become more energetic and move faster. The system has an overall increase in kinetic energy.

Exothermic: A chemical reaction that releases energy. The energy is usually released as heat, but can it can also be light or sound. On a small scale, a burning candle releases light and heat because of exothermic reactions as the wax burns. On a large scale, an explosion might occur when enough hydrogen (H2) and oxygen (O2) combine to form water (H2O).

Activation Energy: The least amount of energy needed for a chemical reaction to occur. Reactions often require some amount energy to get moving. For example, placing hydrogen and oxygen gases in a container will not give you water. There is a certain amount of energy required to get the first reaction going. Catalysts are substances that help to lower activation energies so that reactions can proceed.

Viscosity: A term used to measure the fluidity of a liquid. As the attractions between the molecules increase, viscosity increases. Fluids with high viscosities don’t flow easily. Some substances such as honey or sap are very slow moving and have high viscosities. Other fluids such as water or mercury (HG) have very low viscosities.

Useful Reference Materials (Kinetics):
Wikipedia (Carbonization):
Encyclopædia Britannica:

- Chem4Kids: Measuring Reactions
- Chem4Kids: Enzymes
- Chem4Kids: Metabolism
- Biology4Kids: Scientific Method
- Biology4Kids: Homeostasis
- Geography4Kids: Biogeochemical Cycles
- Geography4Kids: Temperature Scales
- Physics4Kids: Thermodynamics
- Physics4Kids: Thermodynamic Laws

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