What makes reaction occur

A catalyst increases the reaction rate by providing an alternative pathway or mechanism for the reaction to follow Figure 3. Catalysis will be discussed in greater detail later in this chapter as it relates to mechanisms of reactions. Chemical reactions occur when molecules collide with each other and undergo a chemical transformation.

Before physically performing a reaction in a laboratory, scientists can use molecular modeling simulations to predict how the parameters discussed earlier will influence the rate of a reaction. The rate of a chemical reaction is affected by several parameters. Reactions involving two phases proceed more rapidly when there is greater surface area contact. If temperature or reactant concentration is increased, the rate of a given reaction generally increases as well.

A catalyst can increase the rate of a reaction by providing an alternative pathway that causes the activation energy of the reaction to decrease. Observe the reaction. Is the rate of reaction fast or slow? This time, increase the initial temperature until, on the graph, the total average energy line is completely above the potential energy curve. Describe what happens to the reaction.

Higher molarity increases the rate of the reaction. Higher temperature increases the rate of the reaction. Smaller pieces of magnesium metal will react more rapidly than larger pieces because more reactive surface exists. The amount of reactants decreases, and the amount of products increases. Skip to content Chapter Learning Objectives By the end of this section, you will be able to:. Describe the effects of chemical nature, physical state, temperature, concentration, and catalysis on reaction rates.

Chemistry End of Chapter Exercises Describe the effect of each of the following on the rate of the reaction of magnesium metal with a solution of hydrochloric acid: the molarity of the hydrochloric acid, the temperature of the solution, and the size of the pieces of magnesium.

Consider the heterogeneous reaction between a solid phase and either a liquid or gaseous phase. Compared with the reaction rate for large solid particles, the rate for smaller particles will be greater because the surface area in contact with the other reactant phase is greater.

For example, large pieces of iron react more slowly with acids than they do with finely divided iron powder Figure 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. Gas burners, hot plates, and ovens are often used 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 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.

Phosphorus burns rapidly in air, but it will burn even more rapidly if the concentration of oxygen is higher. Activities which provide problems to be explored and challenge existing ideas are useful in encouraging students to seek new explanations for things they observe. Students should investigate a number of changes and ask questions similar to those above. In all of these students should be encouraged to observe the changes that take place and to identify what products are formed. Discussion can also centre on how these are different from the starting materials.

Some examples could include:. Other activities can involve chocolate making. There are many other similar chemical changes that can be investigated - further cooking activities can include: making a chocolate cake, melting and browning cheese, making honeycomb, baking bread, poaching eggs and making toast.

Other changes can include the setting of two component glues like Araldite and mixing steel wool and a solution of copper sulfate available from plant nurseries.

Oxygen is a very important reactant in many chemical reactions and students can investigate changes involving this component of air. It is important at this stage to clarify and consolidate what students have observed and to focus on what happens in a chemical reaction which is different from melting, boiling and freezing.

To achieve this students could be asked in groups to make mini posters which show the changes that take place in the one or more of the reactions they have seen, particularly comparing the products with the starting materials and demonstrating how they are different.

Students then present their posters to the class. Resulting class discussion should bring out student ideas, examine alternatives and move to more accepted scientific views about chemical reactions. Activities should be carried out which test the usefulness of the chemical reactions model and further consolidate student ideas about what constitutes a chemical reaction.

Students can be further encouraged to compare the products with the starting materials. To further develop students' appreciation of the role of chemical change in their lives, they could research the production of metals from ores such as aluminium and steel or the production of plastics and synthetic fibres.

The emphasis in this exploration is on the importance of chemical change in producing the materials we use every day. Our website uses a free tool to translate into other languages. This tool is a guide and may not be accurate. For more, see: Information in your language.

You may be trying to access this site from a secured browser on the server.