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Reactants and Products

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Did you ever wonder what happens to a candle when it burns? A candle burning is a chemical change in matter. In a chemical change, one type of matter changes into a different type of matter, with different chemical properties. Chemical changes occur because of chemical reactions. You can see more examples of chemical changes at this URL: http://www.youtube.com/watch?v=66kuhJkQCVM .

From Reactants to Products

All chemical reactions—including a candle burning—involve reactants and products.

  • Reactants are substances that start a chemical reaction.
  • Products are substances that are produced in the reaction.

When a candle burns, the reactants are fuel (the candlewick and wax) and oxygen (in the air). The products are carbon dioxide gas and water vapor.

Relating Reactants and Products

The relationship between reactants and products in a chemical reaction can be represented by a chemical equation that has this general form:

Reactants → Products

The arrow (→) shows the direction in which the reaction occurs. In many reactions, the reaction also occurs in the opposite direction. This is represented with another arrow pointing in the opposite direction (←).

Q: Write a general chemical equation for the reaction that occurs when a fuel such as candle wax burns.

A: The burning of fuel is a combustion reaction. The general equation for this type of reaction is:

Fuel + O 2 → CO 2 + H 2 O

Q: How do the reactants in a chemical reaction turn into the products?

A: Bonds break in the reactants, and new bonds form in the products.

Breaking and Making Chemical Bonds

The reactants and products in a chemical reaction contain the same atoms, but they are rearranged during the reaction. As a result, the atoms end up in different combinations in the products. This makes the products new substances that are chemically different from the reactants.

Consider the example of water forming from hydrogen and oxygen. Both hydrogen and oxygen gases exist as diatomic (“two-atom”) molecules. These molecules are the reactants in the reaction. The Figure below shows that bonds must break to separate the atoms in the hydrogen and oxygen molecules. Then new bonds must form between hydrogen and oxygen atoms to form water molecules. The water molecules are the products of the reaction.

Q: Watch the animation of a similar chemical reaction at the following URL. Can you identify the reactants and the product in the reaction? http://www.youtube.com/watch?v=SErRUqJ_x30

A: The reactants are hydrogen (H 2 ) and fluorine (F 2 ), and the product is hydrogen fluoride (HF).

Summary

  • All chemical reactions involve both reactants and products. Reactants are substances that start a chemical reaction, and products are substances that are produced in the reaction.
  • A chemical reaction can be represented by the general chemical formula: \text{Reactants} \rightarrow \text{Products} .
  • Bonds break and reform during chemical reactions. Reactants and products contain the same atoms, but they are rearranged during the reaction, so reactants and products are different substances.

Vocabulary

  • product : Substance produced in a chemical reaction.
  • reactant : Substance that starts a chemical reaction.

Practice

Do the activities at the following URL for practice with reactants and products.

http://phet.colorado.edu/en/simulation/reactants-products-and-leftovers

Review

  1. Identify the reactants and products in the following chemical reaction:
CH 4 + 2O 2 → CO 2 + 2H 2 O
  1. How do reactants change into products during a chemical reaction?

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