CHEMICAL REACTIONS Describing Chemical Reactions Formulas show chemistry at a standstill. Equations show chemistry in action. Writing Chemical Equations To write a chemical equation, you have to know: What chemicals you are starting with What new chemicals you get during the reaction. Then you can use symbols to stand for the elements and compounds involved. Symbols Most elements are represented by a one- letter or two-letter symbol. Chemical Formulas A compound is represented by a word, called a chemical formula, which shows the ratio of elements in the compound. Subscripts Look at H 2 O Notice that the number 2 is smaller than the letters. This Subscript shows the number of atoms of an element in a molecule. If a symbol does not have a subscript, the number 1 is understood to be there. Structure of an Equation Shows: the reactants which enter into a reaction. the products which are formed by the reaction. the amounts of each substance used and each substance produced. Reactant + Reactant --> Product All those who believe in psychokinesis, raise my hand. Coefficients A coefficient is a number in front of a chemical formula in the equation. It tells you how many molecules or atoms of each reactant and product take part in the reaction. If the coefficient is 1, you dont need to write it. Conservation of Mass Matter is neither created nor destroyed, according to the law of conservation of mass. conservation of mass The number of atoms before and after a reaction does not change. Atoms are not lost. For my birthday I got a humidifier and a de-humidifier. I put them in the same room and let them fight it out. (Still conserving that mass!) Consider the following reaction : Hydrogen Gas + Oxygen Gas Water The formula for each molecule above is correct. However, it seems that we have gained an extra oxygen atom. This is not possible. Something is wrong. I saw a bank that said "24 Hour Banking", but I don't have that much time. Balancing Chemical Equations Balancing this equation will satisfy the law of conservation of mass. See the solution above: Hydrogen Gas + Oxygen Gas Water I almost had a psychic girlfriend but she left me before we met. An example of equation balancing: Let's say I ask you the following thing on a test: "Balance the equation that takes place when sodium hydroxide reacts with sulfuric acid to form sodium sulfate and water." How do we solve this? I went to a general store. They wouldn't let me buy anything specifically. One method: Draw boxes around all the chemical formulas. You're drawing those boxes so that you'll be sure not to mess around with the formulas to balance the equation. I watched the Indy 500, and I was thinking that if they left earlier they wouldn't have to go so fast. Make an element inventory In this inventory, your job is to figure out how many atoms of each element you have on the left and right sides of the equation. I went to a restaurant that serves "breakfast at any time". So I ordered French Toast during the Renaissance. Balancing Equations Add coefficients to equations as needed. Do not change the formulas inside the boxes. That means count, but do not change subscripts. Final count of elements in reactants (before) must balance elements in product (after). The sign said "eight items or less". So I changed my name to Les. Balance this! Magnesium + Oxygen --> Magnesium Oxide Mg + O 2 --> MgO ? 2Mg + O 2 --> 2MgO Balance This! Nitrogen + Hydrogen --> ammonia N 2 + H 2 --> NH 3 ? N 2 + 3H 2 --> 2NH 3 Balance This! NaCl + BeF 2 --> NaF + BeCl 2 ? 2 NaCl + BeF 2 --> 2 NaF + BeCl 2 Balance This! CH 4 + O 2 --> CO 2 + H 2 O ? CH 4 + 2O 2 --> CO 2 + 2H 2 O Balance This! Mg + Mn 2 O 3 --> MgO + Mn ? 3Mg + Mn 2 O 3 --> 3MgO + 2Mn To PowerPoint Index