Chemistry Chapter 8 & 10 Chemical Reactions and Energy World of Chemistry Zumdahl Last revision Fall 2009

Most reactions take place in water. Water as a Solvent and Electrolytes. The reason water is such a great solvent is because it is a small polar molecule capable of disrupting the intermolecular forces that hold crystals together

Chemical Change- Changes that are the result of chemical reactions. The bonds of the reactants are broken, the atoms rearrange and new bonds are formed creating new substances. Remember

Signs of a chemical change 1) Definite color change. 2) A gas is released (not boiling). 3) Energy is exchanged (released or absorbed in the form of heat, light and sound). 4) A precipitate (solid) is formed.  a clear solution turns cloudy.

Chemical Equations are used as a shorthand to describe a reaction instead of words. They use formulas and symbols to represent the reaction. CH 4 (g) + O 2 (g)  CO 2 (g) + H 2 O (l) For example: Methane gas reacts with oxygen gas to produce carbon dioxide and water.

Symbols for chemical equations – Solid: (s) or (cr) for crystalline (ionic) solid – Liquid: (l) – Gas: (g) or (v) vapor – Dissolved in water: (aq) for aqueous – Yields, produces, makes, forms: – Catalyst: written above the yield arrow. A catalyst is something that speeds up the rate of the reaction but is not consumed by the reaction. A substance that is usually a liquid at room temperature

Reactants (the beginning substances) are always written on the left of the equation. Bonds are broken over the yield sign. Products (the ending substances) are always written on the right of the equation. REACTANTS  PRODUCTS

Conservation of Mass The Law of Conservation of Mass – A French scientist, Antoine Lavoisier established that the total mass of the reactants equals the total mass of the products. In a balanced chemical equation, the number of reactant atoms must equal the number of product atoms. What you get out of a reaction is what you put into it. Matter is neither created nor destroyed, only rearranged

Steps to balancing equations 1. Tally up the number of atoms on each side of the reaction by type. CH 4 (g) + O 2 (g)  CO 2 (g) + H 2 O (l) REACTANTS PRODUCTS C______________________ H______________________ O______________________

2. Write whole number coefficients in front of molecules. __CH 4 (g) + __O 2 (g)  __CO 2 (g) + __H 2 O (l) REACTANTS PRODUCTS C______________________ H______________________ O______________________ Coefficient: Multiplies all the atoms in the formula it preceeds

Classifying Reactions Additive or Synthesis Reactions In an additive reaction two or more substances combine to form a new compound. A + B  AB Example: Fe (s) + O 2 (g)  Fe 2 O 3 (s) Making bonds releases energy. These are usually exothermic reactions. They release energy and have it written as a product + Energy

Combustion Reactions This is a type of additive or synthesis reaction and happens when a substance combines with oxygen. It releasing a large amount of energy in the form of heat and light. Carbon Compound + O 2 (g)  CO 2 (g) + 2H 2 O Example: CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O (v) + Energy

Decomposition Reactions In a decomposition reaction a single compound is broken down to produce two or more smaller compounds. AB  A + B Example: 2H 2 O (l)  2H 2 (g) + O 2 (g) Breaking bonds requires energy. These are usually endothermic reactions. They absorb energy and have it written as a reactant Energy +

Single-Displacement Reactions A single element replaces a similar element in a compound. Cationic: positive ions (metals) trade places. A + BC  AC + B Example: Fe (s) + CuCl 2 (aq)  FeCl 2 (aq) + Cu (s)  Anionic: negative ions (halogens) trade places.

Activity Series Not all single displacement reactions will actually occur. It depends on how active the metal or halogen is that is being displaced. Metals below the list cannot displace metals above them. This means metals high on the list are very reactive and want to be part of a compound. This works the same for the halogens below.

Q: Which of these will react? Fe+ CuSO 4  Ni+ NaCl  Li+ ZnCO 3  Al+ CuCl 2  Cu + Fe 2 (SO 4 ) 3 Yes, Fe is above Cu NR (no reaction) No, Ni is below Na Zn + Li 2 CO 3 Cu + AlCl 3 Yes, Al is above Cu Li K Na Ca Mg Al Zn Fe Ni Sn Pb H Cu Hg Ag Au Yes, Li is above Zn

Double-displacement Reactions Ions from two compounds in solution exchange places to produce two new compounds. AB + CD  AD + CB Example: PbNO 3 (aq) + 2KI (aq)  PbI 2 (s) + 2KNO 3 (aq) The product is often a precipitate (solid), gas that bubbles out or water.

Solubility rules

Most reactions take place in solution 1.Molecular equation (reactants and products shown as compounds) AgNO 3(aq) + NaCl (aq)  AgCl (s) + NaNO 3(aq) 2.Complete ionic equation (all ionic substances dissolved are shown as ions) Ag + (aq) + NO 3 - (aq) + Na + (aq) + Cl - (aq)  AgCl (s) + Na + (aq) + NO 3 - (aq)

Describing Reactions in Solution (continued) 3. Net ionic equation (show only components that actually react) Start AgNO 3(aq) + NaCl (aq)  AgCl (s) + NaNO 3(aq) Next Ag + (aq) + NO 3 - (aq) + Na + (aq) + Cl - (aq)  AgCl (s) + Na + (aq) + NO 3 - (aq) Finally Ag + (aq) + Cl - (aq)  AgCl (s) spectator ions are : NO 3 - and Na +.