Chemical Reactions

When a chemical undergoes a chemical change, it changes its identity. Wood burns to ashes Dynamite explodes into gaseous compounds. During these changes chemical bonds are broken and new ones are formed, atoms are rearranged, this shuffling of atoms is called a chemical reaction.

The Chemical Equation Reactants – the materials about to react. Products - the newly formed materials. Reactants → Products In a chemical equation, + means to combine, and → means yields, or turns into. Numbers are placed in front of the chemical formulas to show the ratio in which reactants combine and products form. These numbers are known as coefficients.

Examples of Chemical Equations One atom of Carbon reacts with one molecule of Oxygen to form one molecule of Carbon Dioxide. 1C + 1O 2 → 1CO 2 reactants product How many atoms of Carbon are there? 1 How many atoms of Oxygen are there? 2

Chemical Equations Con’t Two Hydrogen molecules, react with one Oxygen molecule, to produce two molecules of water. 2H 2 + 1O 2 → 2H 2 O How many atoms of oxygen are there? 2 How many atoms of Hydrogen are there? 4

Conservation of Mass One of the most important principles in Chemistry is the principle of the conservation of mass, which states that … matter is neither created nor destroyed in a chemical reaction. A chemical equation must then, be balanced that is the, number of atoms of a reactant must equal the number of atoms of a product. They could be rearranged, but the total number of atoms must be equal.

Energy and Chemical Reactions During chemical reactions chemical bonds are broken and atoms rearrange themselves to form new chemical bonds. This process involves changes in energy. To pull atoms apart requires energy to break the bonds. When atoms combine there is an energy output.

Endothermic and Exothermic Reactions that absorb more energy than they release are called endothermic, these reactions feel cool. –Like a cold pack. Reactions that release more energy than they absorb are called exothermic, these reactions feel hot. –Like striking a match.

Types of Reactions SYNTHESIS REACTION In a synthesis reaction two or more simple substances combine to form a more complex substance. Two or more reactants yielding one product is another way to identify a synthesis reaction. For example, simple hydrogen gas combined with simple oxygen gas can produce a more complex substance----- water! The chemical equation for this synthesis reaction looks like:

DECOMPOSITION REACTION In a decomposition reaction a more complex substance breaks down into its more simple parts. One reactant yields 2 or more products. Basically, synthesis and decomposition reactions are opposites. For example, water can be broken down into hydrogen gas and oxygen gas. The chemical equation for this decomposition reaction looks like: reactant > product + product

SINGLE REPLACEMENT REACTION In a single replacement reaction a single uncombined element replaces another in a compound. Two reactants yield two products. For example when zinc combines with hydrochloric acid, the zinc replaces hydrogen. The chemical equation for this single replacement reaction looks like:

DOUBLE REPLACEMENT REACTION In a double replacement reaction parts of two compounds switch places to form two new compounds. Two reactants yield two products. For example when silver nitrate combines with sodium chloride, two new compounds- -silver chloride and sodium nitrate are formed because the sodium and silver switched places. The chemical equation for this double replacement reaction looks like: