Science 9: Unit B – Matter and Chemical Change Topic 7: Writing Chemical Equations Background All compounds are created, changed, or broken apart through chemical reactions. Signs that a chemical change have occurred include a color change, bubbling, a PRECIPITATE (solid) forming in a liquid, and heat and light being produced or absorbed. In a chemical reaction, the BONDS between atoms are broken and new bonds are formed so that atoms are rearranged to form new compounds. Eg. hydrogen + oxygen = water 2H2 + O2 → 2H2O or: H-H + H-H + O-O = H-O-H + H-O-H Reactants and Products In chemical reactions, the chemicals that undergo the reaction are called the reactants. These are your original materials. In the above example hydrogen and oxygen are your reactants. The chemicals that come out of a chemical reaction are called the products. In the above example, two water molecules are the product. Note that when we write down a chemical reaction, we are writing down the lowest RATIO of molecules that will react together. In reality, billions of molecules are reacting with each other. Signs of a Chemical Reaction Products are new chemicals with different chemical properties than the reactants that make them. Water is a liquid at room temperature; hydrogen and oxygen are gases at room temperature. There are signs that indicate a chemical reaction has taken place: color change, bubbles forming, heat absorbed or given off, and a new material formed all point towards a chemical reaction. Writing Chemical Reactions To describe what occurs during a chemical reaction, scientists use chemical equations. Chemical equations involve the following parts: The Parts of a Chemical Reaction 1. The reactants on the left side, using the symbols of elements for each atom. These symbols are the same ones from the periodic table. For each molecule indicate in brackets behind it whether it is a solid (s), liquid (l), or gas (g). An ‘+’ is used to separate the different reactants. 2. An arrow pointing to the right separating the reactants from the products. 3. The products on the right side of the equation are labeled in the same manner as the reactants. Eg. CH4(g) + O2(g) → CO2(g) + H2O(l) Exothermic and Endothermic Reactions Exothermic reactions RELEASE energy when reacting. Reactants → Products + Energy Endothermic reactions ABSORB energy from the surrounding environment. Reactants + Energy → Products An exothermic example is the starting of a barbecue involves the igniting of propane and oxygen. This releases a large amount of energy. C3H8(l) + 5O2(g) → 3CO2(g) + 4H2O(g) + Energy An endothermic reaction example is the splitting up of water into hydrogen and oxygen molecules using an electric current. 2H2O(l) + Energy → 2H2(g) + O2(g) An Important Note about Endothermic and Exothermic Reactions An important note is that if a reaction is exothermic then the products are MORE stable than the reactants. If the reaction is endothermic the products are LESS stable than the reactants. Since all elements are trying to become more stable exothermic reactions are more natural while endothermic reactions need an outside energy source.