Chemical Reactions

Chemical Reactions The words grow, ripen, decay, and burn describe chemical reactions that take place every day. Many different chemical reactions take place inside your body all of the time.

Chemical Reactions Chemical reactions occur when substances go through chemical changes to form new substances. Often, you can tell when a chemical reaction is occurring because you can see the changes. (formation of a gas, formation of a solid, release of energy)

Chemical reactions Chemical reactions rearrange atoms. A reactant is a substance that participates in a chemical reaction. A product is a substance that forms in a chemical reaction.

Chemical Reactions The products and reactants of a chemical reaction contain the same types of atoms. The reaction does NOT create atoms of the products or destroy the atoms of the reactants. The reaction rearranges the bonds between atoms and the mass stays the same.

Energy and Reactions Chemical reactions always involve changes in energy. The process of breaking bonds uses energy. Many forms of energy can be used such as heat, electricity, sound, or light. When molecules collide and enough energy is transferred to separate the atoms, bonds can break.

Energy and Reactions When new bonds form, chemical energy is released. Energy is conserved in chemical reactions. Energy that is stored in the form of chemical bonds is called chemical energy.

Energy and Reactions The total amount of energy of the reactants must always equal the total amount of energy of the products and their surroundings. Energy in a chemical reaction can change form, energy is never created or destroyed.

Energy and Reactions Reactions that release energy as heat to the surroundings are called exothermic reactions. The released energy comes from the chemical energy of the reactants.

Energy and Reactions Reactions that require energy input are called endothermic reactions. In endothermic reactions, more energy is needed to break the bonds in the reactants than is given off by forming bonds in the products. Sometimes we can see this as a drop in temperature.

Chemical Equations A chemical equation uses symbols to represent a chemical reaction and shows the relationship between the reactants and the products of a reaction.

Chemical Equations In a chemical equation, the reactants are on the left side, and the products are on the right. The arrow in the middle means “yields.”

Chemical Equations When the number of atoms of each element on the right side of the equation matches the number of atoms of each element on the left side, the chemical equation is said to be balanced. A balanced chemical equation follows the law of conservation of mass.

Chemical Equations A chemical equation is balanced by adding coefficients in front of one or more of the formulas. A coefficient is a number that shows the relative amount of a compound in a reaction.

Chemical Equations A balanced equation tells you the mole ratio, or proportion of reactants and products, in a chemical reaction. We can then take to mole ratio to tell the relative amounts of reactants and products because mole ratios can be converted to mass.

Types of Chemical Reactions Even though there are millions of unique substances and millions of possible reactions, there are only a few general types of reactions. A synthesis reaction is a reaction in which multiple substances combine to form a new compound.

Synthesis Reactions Synthesis reaction = A + B  AB Example: 2Na + Cl2  2NaCl Because a synthesis reaction joins substances, the product is a compound that is more complex than the reactants.

Decomposition reactions Decomposition reactions are reactions in which substances are broken apart. Decomposition reaction = AB  A + B Example: 2H2O  2H2 + O2

Combustion reaction Oxygen is a reactant in every combustion reaction, so at lease one product will contain atoms of oxygen. Example: CH4 + 2O2  CO2 + 2H2O

Single-displacement reactions When atoms of one element appear to move into a compound and atoms of another element appear to move out, a single-displacement reaction occurs. Single-displacement = AX + B  BX + A Example: 3CuCl2 + 2Al  2AlCl3 + 3Cu

Double displacement reactions A double-displacement reaction is a reaction in which two compounds appear to exchange ions. Double-displacement = AX + BY  AY + BX Example: Pb(NO3)2 + K2CrO4  PbCrO4 + 2KNO3

Oxidation-Reduction Reactions Substances that accept electrons are said to be reduced. Substances that give up electrons are said to be oxidized. Reactions in which one substance loses electrons and another gains electrons are called oxidation-reduction reactions.

Free radicals A free radical is an atom or a group of atoms that has one unpaired electron. Free-radical reactions and redox reactions can be understood as changes in the numbers of electrons that atoms have.

Reaction rates and Equilibrium Anything that increases contact between particles will increase the rate of a reaction. Things that will speed up a reaction: Higher temperatures Large surface area Higher concentration of reactants Presence of a catalyst

Reaction rates and Equilibrium A catalyst speeds up or slows down a reaction, but is not changed by the reaction. Enzymes are catalysts in living things.

Reaction rates and Equilibrium Chemical equilibrium is a state in which a reversible chemical reaction is proceeding in both directions equally. No net change happens. Example: CaCO3  CaO + CO2

Reaction rates and Equilibrium Le Chátelier’s principle – If a change is made to a system in chemical equilibrium, the equilibrium shifts to oppose the change until a new equilibrium is reached.

Analogy Activity In your group, you will come up with an analogy for the 4 main types of chemical reactions: synthesis, decomposition, single- displacement, double-displacement. An analogy is a comparison between two things, typically for the purpose of explanation or clarification. Here are some examples.