Describing Chemical Reactions A chemical equation is a short, easy way to show a chemical reaction. All chemical equations use formulas to represent the substances involved in a reaction. H + O H 2 O Na + O 2 Na 2 O

Describing Chemical Reactions Reactants are substances at the beginning of an equation. Products are new substances when the reaction is complete. Reactants -> Products

Describing Chemical Reactions Conservation of mass states that during a chemical reaction, matter is not created or destroyed. All the atoms present at the start of the reaction are present at the end. Antoine Lavoisier

Describing Chemical Reactions In an open system, matter can enter from or escape to the surroundings. A burning match is an example of an open system.

Describing Chemical Reactions In a closed system, matter is not allowed to enter or leave. Decaying pear under a glass dome is an example of a closed system.

Describing Chemical Reactions To describe a reaction accurately, a chemical equation must show the same number of each type of atom on both sides of the equation. SnO2 + H2 --> Sn + H2O SnO2 + 2 H2 --> Sn + 2 H2O Write the unbalanced equation Balance the equation

Describing Chemical Reactions A coefficient is a number placed in front of a chemical formula in an equation. To balance the equation, use coefficients. If the coefficient is 1, dont write it. C2H5OH + O2 2CO2 + 3H2O

Describing Chemical Reactions Many chemical reactions can be classified in one of three categories: synthesis, decomposition, and replacement.

Describing Chemical Reactions When two or more elements or compounds combine to make a more complex substance, the process is called synthesis. Hydrogen + Oxygen = Water

Describing Chemical Reactions Decomposition breaks down compounds into simpler products. Hydrogen peroxide decomposes into water and oxygen over time.

Describing Chemical Reactions When one element replaces another in a compound, or when two elements in different compounds trade places, the process is called replacement. Single Replacement Reaction Double Replacement Reaction