21.1 Chemical Reactions A chemical reaction is the process of breaking of chemical bonds in one or more substances, and the reforming of new bonds to create new substances. When you make pizza, which changes are physical and which are chemical changes?

21.1 Chemical Reactions The process of making pizza involves some physical changes (like chopping vegetables). The processes used by yeast in the dough or by the gas stove to bake the pizza are chemical changes.

21.1 Evidence of chemical change Four indicators of chemical change are: Formation of new gas Formation of new solid Release of energy (heat or light) Color change

21.1 Reactants and products In chemical reactions, you start with reactants that are combined to make products. The reactants are the starting substances. The products are the new substances which result from the chemical reaction.

21.1 Reactants and products In the reaction, methane (a natural gas) is burned or combusted. Some energy is added to get the reaction started.

21.1 Reaction symbols The small symbols in the parentheses (s, l, g, aq) next to each chemical formula indicate the phase of each substance in the reaction.

21.1 Law of conservation of mass Antoine Laurent Lavoisier, established an important principal based on his experiments with chemical reactions. He stated that the total mass of the products of a reaction is equal to the total mass of the reactants. The law of conservation of mass holds true for even a burning mass of wood.

21.1 Law of conservation of mass The combined mass of the burning wood and oxygen is converted into carbon dioxide and water.

21.1 Conservation of mass Lavoisier showed that a closed system must be used when studying chemical reactions. When chemicals are reacted in a closed container, you can show that the mass before and after the reaction is the same.

21.1 How are reactions written? When a chemical reaction is written using chemical formulas and symbols, it is called a chemical equation.

14.1 Numbers in equations

21.2 Chemical equations An arrow is always included between reactants and products. It means “to produce” or “to yield.” to produce Reactants Products “Methane combines with oxygen gas to produce carbon dioxide gas and water vapor.”

21.2 Balancing equations The law conservation of mass is applied by balancing the number and type of atoms on either side of the equation.

21.2 Balancing equations Counting atoms is necessary to balance an equation. How many carbon atoms? How many hydrogen atoms? How many oxygen atoms?

21.2 Balancing chemical equations A balanced chemical equation has the same number of each type of atom on the product side and the reactant side. To balance the equation, we add another water molecule to the product side and add another oxygen molecule to the reactant side. We can practice balancing equations using CPO periodic table tiles and pencil and paper.

Solving Problems In this reaction, chalcocite (a mineral) reacts with oxygen in the presence of heat. The products are a type of copper oxide and sulfur dioxide. Balance this equation: Cu2S + O2 → Cu2O + SO2

Looking for: Given …the coefficients for each molecule Solving Problems Looking for: …the coefficients for each molecule Given … chemical formulas which show types and no. of atoms

Solving Problems Relationships Coefficients can be added in front of any chemical formula in a chemical equation. When a coefficient is added in front of a chemical formula, all atoms in that formula are multiplied by that number. Use common denominators to help choose coefficients to try.

Solution- Trial and error Solving Problems Solution- Trial and error

21.3 Synthesis reactions In a synthesis reaction, two or more substances combine to form a new compound.

Fe (s) + O2 (g) -----> Fe2O3 (s) 21.3 Synthesis reaction A + B -----> AB Fe (s) + O2 (g) -----> Fe2O3 (s) Remember to balance! Generally these synthesis reactions are: two elements combine to produce a single compound OR two compounds (of which one is usually water or oxygen) combine to form a single compound When iron combines slowly over time with oxygen: Fe + O2  Fe2O3 For simplicity sake, textbook shows the UNBALANCED reaction using chemical symbols. 4Fe + 3O2  2 Fe2O3 Set up Demonstrations (account for enough materials for one per class) CPO video series or teacher can film the reactions ahead of time The result is a rusty nail. Use a nail or other piece of iron metal that has been spritzed with water several days head of time and then dried to pass around. Mention that other metals when combined with oxygen become weakened or corroded, such as Al or Mg or Ag on exposure to air over time. These are very slow chemical reactions and are “oxidations” Also elemental zinc can combine with iodine: Zn + I2  ZnI2 Extension demo for advanced students***: materials: match, superfine steel wool (size 0000), ring stand, metal jawed test tube clamp, alum pie pan, balance, goggles Find the mass of a steel wool pad (w/o soap) . The pad can be ignited with a single match if clipped with metal jawed tube clamp to a ring stand. (CAUTION: the pad will ignite quickly!) If the product is collected on an aluminum pie pan whose mass is also known before hand, the mass of the product and the pan can be measured. When the pan’s mass is subtracted, observers will notice that the mass of the product is greater than that of the starting mass of the steel wool pad. Fe + O2 -----> Fe2O3 4 3 2

21.3 Synthesis reactions The process of creating large molecules from small ones is called polymerization.

21.3 Decomposition reactions A chemical reaction in which a single compound is broken down to produce two or more smaller compounds is called a decomposition reaction.

21.3 Decomposition reaction AB -energy-> A + B 2 Hg (s) 2 HgO3 (s) -heat-> + O2 (g) Generally these reactions are: a single binary compound decomposing to form two elements OR a singe ternary compound decomposing to form an element and a compound or two compounds Most require the input of energy in the form of heat, light or electricity. 2 H2O (l) -electricity-> 2 H2 (g) + O2 (g)

21.3 Single Displacement In a single-displacement reaction, one element replaces a similar element in a compound.

21.3 Single Displacement A A + X X B B -----> + Fe Fe + Cu Cu Cl2 This reaction shows: -one element combining with one compound to form a new compound with a different element precipitated Generally they are performed in “aqueous solutions” (dissolved in a water medium to facilitate the reaction) Other metals when combined with oxygen become weakened or corroded, such as Al or Mg metals with exposure to air over time. * Remember, for iron to replace copper, copper must be higher on the activity series of metals, copper is Cu2+ and iron Fe2+ -----> +

21.3 Double Displacement In a double-displacement reaction, ions from two compounds in solution exchange places to produce two new compounds. One of the compounds formed is usually a precipitate that settles out of the solution, a gas that bubbles out of the solution, or a molecular compound such as water.

21.3 Double Displacement AB + CD ---> AC + BD Pb(NO3)2 + KI ---> PbI2 + KNO3 2 2 Generally these reactions are: two ionic compounds combining to produce a two new compounds Generally they are performed in “aqueous solutions” (dissolved in a water medium to facilitate the reaction) When aqueous lead (II) nitrate combines with aqueous potassium iodide it yields lead iodide and potassium nitrate. The solution changes from white to fluorescent yellow. Other metals when combined with oxygen become weakened or corroded, such as Al or Mg metals on exposure to air over time.

21.3 Double displacement reactions A precipitate is a new solid product that comes out of solution in a chemical reaction. The formation of a cloudy precipitate is evidence that a double-displacement reaction has occurred.