Chemical Reactions

Different than a “Physical Change” (like ice to liquid water) When a chemical undergoes a chemical change, it changes its identity.

Chemical Reactions 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.

Chemical Equations Chemical equations show chemical reactions in written format. Show the reactants and products in a chemical reaction Chemical reactions represent changes in bonding and in energy

2 KClO 3 (s) + energy  2 KCl (s) + 3 O 2 (g) 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.

2 KClO 3 (s) + energy  2 KCl (s) + 3 O 2 (g) Subscripts (in Formulas) indicate the number of atoms WITHIN a formula KClO 3 : There are 3 oxygen atoms in potassium chlorate Coefficients (in equations) are placed in front of the chemical formulas to show the ratio in which reactants combine and products form. 2 KCl : There are 2 potassium chlorides

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

2 H O 2 → 2 H 2 O Two Hydrogen molecules, react with one Oxygen molecule, to produce two molecules of water. How many atoms of oxygen are there on each side of the equation? 2 How many atoms of Hydrogen are there on each side of the equation? 4

Conservation of Mass An important principle in chemical reactions is the principle of the conservation of mass, which states that … Matter (mass) is neither created nor destroyed in a chemical reaction.

Conservation of Mass A chemical equation must be balanced That is, the number of atoms of a reactant must equal the number of atoms of a product. They will be rearranged, but the total number of atoms must be equal.

Energy & Chemical Reactions During chemical reactions chemical bonds are broken and atoms rearrange themselves to form new chemical bonds. This process involves changes in energy. Pulling atoms apart requires energy to break the bonds. When atoms combine, energy is given off or released.

Endothermic Reactions Reactions that absorb more energy than they release are called endothermic These reactions feel cool. e.g. Like a cold pack. Notice that energy is a REACTANT 2 KClO 3 (s) + energy  2 KCl (s) + 3 O 2 (g)

Exothermic Reactions Reactions that release more energy than they absorb are called exothermic These reactions feel warm/hot. e.g. Like striking a match. Notice that energy is a PRODUCT 2 H 2 (g) + O 2 (g)  H 2 O (g) + energy

TYPES OF CHEMICAL REACTIONS There are FIVE types of chemical reactions: Synthesis Decomposition Single replacement Double replacement Combustion

SYNTHESIS 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! 2 H 2 (g) + O 2 (g)  H 2 O (g) + energy

Decomposition 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. reactant  product + product

Single Replacement 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.

Double Replacement 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.

Combustion CH 4 (g) + 2 O 2 (g)  CO 2 (g) + H 2 O hydrocarbon + oxygen  carbon dioxide + water When a compound or molecule burns in air, it is actually reacting with the oxygen in the air.