What do chemical reactions affect

Except for substances in the gaseous state or in solution, reactions occur at the boundary, or interface, between two phases. Hence, the rate of a reaction between two phases depends to a great extent on the surface contact between them. A finely divided solid has more surface area available for reaction than does one large piece of the same substance. Thus a liquid will react more rapidly with a finely divided solid than with a large piece of the same solid.

For example, large pieces of iron react slowly with acids; finely divided iron reacts much more rapidly Figure 1. Large pieces of wood smolder, smaller pieces burn rapidly, and saw dust burns explosively. Watch this video to see the reaction of cesium with water in slow motion and a discussion of how the state of reactants and particle size affect reaction rates.

Chemical reactions typically occur faster at higher temperatures. Food can spoil quickly when left on the kitchen counter. However, the lower temperature inside of a refrigerator slows that process so that the same food remains fresh for days. We use a burner or a hot plate in the laboratory to increase the speed of reactions that proceed slowly at ordinary temperatures. The rates of many reactions depend on the concentrations of the reactants. Rates usually increase when the concentration of one or more of the reactants increases.

For example, calcium carbonate CaCO 3 deteriorates as a result of its reaction with the pollutant sulfur dioxide. The rate of this reaction depends on the amount of sulfur dioxide in the air Figure 2. An acidic oxide, sulfur dioxide combines with water vapor in the air to produce sulfurous acid in the following reaction:. In a polluted atmosphere where the concentration of sulfur dioxide is high, calcium carbonate deteriorates more rapidly than in less polluted air.

Phosphorous burns rapidly in air, but it will burn even more rapidly if the concentration of oxygen in is higher. Watch this video to see an example. However, in the absence of these catalysts for example, in the bottle in the medicine cabinet complete decomposition can take months. Some reactions can proceed at explosively fast rates like the detonation of fireworks Figure The chemical reaction in fireworks happens at an explosive rate.

The rusting of barbed wire occurs over many years. To understand the kinetics of chemical reactions, and the factors that affect kinetics, we should first examine what happens during a reaction on the molecular level.

This visualization shows an ineffective and effective collision based on molecular orientation. During a molecular collision, molecules must also possess a minimum amount of kinetic energy for an effective collision to occur. This energy varies for each reaction, and is known as the activation energy E a Figure The rate of reaction therefore depends on the activation energy; a higher activation energy means that fewer molecules will have sufficient energy to undergo an effective collision.

This potential energy diagram shows the activation energy of a hypothetical reaction. Reactant concentration. Increasing the concentration of one or more reactants will often increase the rate of reaction.

We will start with a general explanation of how chemists study the rates of reactions. Chemical reactions require varying lengths of time for completion, depending upon the characteristics of the reactants and products and the conditions under which the reaction is taking place. Chemical Kinetics is the study of reaction rates, how reaction rates change under varying conditions and by which mechanism the reaction proceeds. How does temperature affect the rate of a chemical reaction? For two chemicals react, their molecules have to collide with each other with sufficient energy and in the correct orientation for the reaction to take place.

The two molecules will only react if they have enough energy. By heating the mixture, you are raising the energy levels of the molecules involved in the reaction. Increasing temperature also means the molecules are moving around faster and will therefore "bump" into each other more often. More collisions afford more opportunities for reaction. How do catalysts affect the rate of a reaction?

Catalysts speed up chemical reactions. Only very minute quantities of the catalyst are required to produce a dramatic change in the rate of the reaction.

This is really because the reaction proceeds by a different pathway when the catalyst is present essentially lowering the activation energy required for the reaction to take place. How does concentration affect the rate of a reaction? Increasing the concentration of the reactants will increase the frequency of collisions between the two reactants. When collisions occur, they do not always result in a reaction atoms misaligned or insufficient energy, etc. Higher concentrations mean more collisions and more opportunities for reaction.

What affect does pressure have on the reaction between two gasses? You should already know that the atoms or molecules in a gas are very spread out. For the two chemicals to react, there must be collisions between their molecules.