Chemistry: Chemical Reactions

Observing Chemical Reactions

A chemical reaction produces materials that have different properties than the starting materials had. Each reaction either absorbs or releases energy.

Color change, production of a gas or a precipitate, a change in temperature, or a change in the properties of a substance are all clues that a chemical reaction has taken place.

Chemical reactions occur when chemical bonds are formed or broken.

• Precipitate - A solid that forms from a solution during a chemical reaction.
• Exothermic Reaction - A reaction that releases energy in the form of heat.
• Endothermic Reaction - A reaction that absorbs energy in the form of heat.

Writing Chemical Equations

A chemical equation uses symbols to show the reactants and products of a chemical reaction.

Matter is neither created nor destroyed during a chemical reaction.

Chemical reactions may be classified by the types of changes in reactants and products:

• Synthesis - A chemical reaction in which two or more simple substances combine to form a new, more complex substance.
• Decomposition - A chemical reaction that breaks down a compound into simpler products.
• Replacement reaction - A reaction in which one element replaces another in a compound, or in which two elements in different compounds trade places.
  

• Chemical equation - A short, easy way to show a chemical reaction, using symbols instead of words.
• Subscript - A number in a chemical formula that tells the number of atoms in a molecule or the ratio of elements in a compound.
• Reactant - A substance that enters into a chemical reaction
• Product - A substance formed as a result of a chemical reaction.
• Conservation of mass - The principle stating that matter is not created or destroyed during a chemical reaction.
• Coefficient - A number placed in front of a chemical formula in an equation that indicates how many atoms or molecules of each reactant and product are involved in a reaction.

Controlling Chemical Reactions

Every chemical reaction needs activation energy to get started. Endothermic reactions need energy to continue.

The rate of chemical reaction can be controlled by such factors as concentration, surface area, temperature, and use of a catalyst or inhibitor.

• Activation energy - The minimum amount of energy needed to start a chemical reaction.
• Catalyst - A material that increases the rate of a chemical reaction by lowering the activation energy.
• Inhibitor - 
• Concentration - The amount of one material dissolved in a given amount of another material.
• Enzyme - A biological catalyst that lowers the activation energy of reactions in cells.