Chemical Reactions

Reactants & Products During a chemical equation, reactants (the stuff you start with) are changed into products (the stuff you finish with) The products actually are composed of the same stuff you started with, but the molecules have been rearranged by chemical bonding to produce completely new compounds with unique physical properties A basic chemical equation looks like this: REACTANTS PRODUCTS

What you put in (before) = reactant A chemical reaction is when atoms interact with each other to form new substances What you put in (before) = reactant What comes out (after) = product Mass of reactants = mass of products MASS IS CONSERVED

Conservation of Mass A chemical equation is a chemical reaction in which reactants and products (separated by an arrow) are expressed as formulas The Law of Conservation of Mass states that mass is neither created nor destroyed during a chemical reaction

Balancing Equations/Manipulating Coefficients In order to show that mass is conserved during a reaction, a chemical equation must be balanced A coefficient is the number that appears BEFORE a formula Recall that subscripts are used to balance a formula Coefficients are used to balance the equation

Steps to balancing a chemical equation: 1. Write out the chemical equation 2. Balance the most unusual element or compound first, use subscripts as a guide 3. Continue with other elements, ending with the most common Example: Balance the chemical equation C2H6 + O2 → H2O + CO2 2 C2H6 + 7 O2 → 6 H2O + 4 CO2

Moles & Molar Mass a mole is the SI unit that measures the amount of substance A mole contains 6.02 x 1023 particles (molecules, atoms, Formula units) of a substance Molar mass is the mass of one mole Molar mass is calculated by adding up the atomic masses of each element (multiplied by the subscript) Example: CO2 Carbon = 12 * 1(subscript) = 12 Oxygen = 16* 2 = 32 + The molar mass of CO2 is 44 g

Types of Reactions Synthesis Decomposition Single displacement Double displacement Combustion reactions

Synthesis or Combination Reactions synthesis reactions occur when two substances combine to form a new more complex substance general form A + B → AB Example: 2H2 + O2 → 2H2O

Decomposition Decomposition occurs when one large substance breaks down to two or more smaller substances general form AB → A + B Example: 2NaClO3 → 2NaCl + 3O2

Single Displacement Single displacement reactions occur when atoms of one element take the place of an element in a compound general form XA + B → A + XB example 3CuCl2 + 2Al → 2AlCl3 + 3Cu

Double Displacement Double displacement reactions occur when two compounds exchange their elements general form AX + BY → AY + BX example : AgNO3 + HCl → AgCl + HNO3

Combustion combustion reactions occur when a compound and oxygen burn With enough oxygen the products are carbon dioxide and water without enough the products are carbon monoxide and water general form A + O2 → H2O + CO2 example 2CH4 + 4O2 → 4H2O + 2CO2

Energy Changes & Chemical Reactions Indications of a chemical reaction include color change, gas formation, and temperature change Atoms are just rearranged as bonds are broken and new ones formed Breaking bonds requires energy Forming bonds releases energy

chemical energy is the energy stored in the chemical bonds of a substance Chemical reactions involve the breaking of chemical bonds in the reactants & the formation of chemical bonds in the products Q: Does breaking chemical bonds require energy or release energy?

Exothermic & Endothermic Reactions that release energy are exothermic Reactions that absorb energy are endothermic ENERGY IS CONSERVED. Example: for an exothermic reaction energy stored in bonds (chemical energy) = energy released during reaction

Conservation of Energy Law of Conservation of Energy: during a chemical reaction, energy is neither created nor destroyed In exothermic reactions, the chemical energy of the reactants is converted to heat plus the chemical energy of the products