Chapter 10 Chemical Reactions

Reactions and Equations A chemical reaction is the process by which the atoms of one or more substances are rearranged to form different substances. Chemists use statements called equations to represent chemical reactions. Their equations show the reactants (starting substance) and the products (final substances). Reactant 1 + reactant 2  product 1 + product 2

Law of Conservation of Mass Dalton’s atomic theory proves the law of conservation of mass ▫ Mass is not created or destroyed; it’s just moved around

Law of Conservation of Mass This law basically says that whatever you start out with you have to end up with! So, if the reactants’ total mass is 50.0 grams, then the products’ total mass also has to equal 50.0 grams. 2HCl  H 2 + Cl g = 20.0 g g

Practice Problems If you have 25.0 grams of Na and 45.0 grams of Cl, how many grams of NaCl should be produced? 2Na + Cl 2 = 2NaCl 25.0 g 45.0 g = ?? = 70.0 grams

If you had 30.0 grams of K and you produced 55.0 grams of KCl, how much Cl 2 was used in the reaction? 2K + Cl 2 = 2KCl 30.0 g ?? 55.0 g 30.o g g = 55.0 g

Labels within a chemical equation Coefficient: the big numbers written in front of a reactant or product. Subscripts: the small numbers written slightly below element symbols. (s): solid (l): liquid (g): gas (aq): aqueous 2Fe (s) + 3Cl 2 (g)  2FeCl 3 (s)

Diatomic molecules When writing equations, some elements are always written as diatomic molecules (two atoms). So, any of the super 7 elements written by themselves will be written with a subscript of 2. H 2 N 2 O 2 F 2 Cl 2 Br 2 I 2

Writing equations from word equations A word equation is an equation with the words of the elements/compounds written out instead of using their chemical symbols. You can write a normal chemical equation using a word equation. Ex: Iron (s) + chlorine** (g)  iron (III) chloride (s) Fe (s) + Cl 2 (g)  FeCl 3 (s) **Notice chlorine is diatomic, so it’s written as Cl 2 in the equation.

Practice writing equations Hydrogen (g) +bromine (g)  hydrogen bromide (g) Carbon monoxide (g) + oxygen (g)  carbon dioxide (g) Potassium chlorate (s)  potassium chloride (s) + oxygen (g)

Tricks for balancing equations Always balance hydrogen and oxygen last. If you try to balance and nothing matches up, try putting a 2 in front of a compound with an odd number of a certain element. ▫Ex: AgI + Fe2(CO3)3  FeI3 + Ag2CO3 ▫The I’s don’t balance, so try a 2 in front of the FeI3 since it has an odd number of I’s. If you have 2 of something on one side and 3 of something on the other, the coefficients will be opposite (3 and 2 to make 6 of each) ▫Ex: Fe (s) + Cl2 (g)  FeCl3 (s) ▫There are two chlorines on the left and three on the right, so the coefficients will be opposite. ▫2Fe (s) + 3Cl2 (g)  2FeCl3 (s)

Classifying Chemical Reactions Synthesis reactions: reaction in which two or more substances react to produce a single product. ▫A + B  AB ▫2Fe + 3Cl 2  2FeCl 3

Decomposition Reactions Reaction in which a single compound breaks down into two or more elements or new compounds. AB  A + B NH 4 NO 3  N 2 O + 2H 2 O

Combustion reactions Oxygen combines with a substance and releases energy in the form of heat and light. 2H 2 (g) + O 2 (g)  2H 2 O (g) C (s) + O 2 (g)  CO 2 (g) CH 4 (g) +2O 2 (g)  CO 2 (g) +2H 2 O (g)

Single replacement reactions Reaction in which the atoms of one element replace the atoms of another element in a compound. A + BX  AX + B Cu (s) + 2AgNO 3 (aq)  2Ag (s) + Cu(NO 3 ) 2 (aq)

Double-replacement reactions Reaction involving an exchange of ions between two compounds. AX + BY  AY + BX Ca(OH) 2 (aq) + 2HCl (aq)  CaCl 2 (aq) + 2H 2 O (l)

Evidence of Chemical Reactions The process by which the atoms of one or more substances are rearranged to form different substances is called a chemical reaction. Indications of a chemical reaction: ▫Temperature change ▫Color change ▫Odor change ▫Gas bubbles ▫Appearance of a solid

Reactions in Aqueous Solutions In solutions, the solute is what dissolves and the solvent is the substance that dissolves the solute. ▫Example: Solute – sugar solvent – water Water is the most common solvent and is called the universal solvent. When the solvent is water, the solution is called an aqueous solution.

Aqueous Solution Reactions HCl (g)  H + (aq) + Cl - (aq) ▫This shows the reaction of hydrochloric acid dissolving in water; it forms two ions. Some reactions that occur in aqueous solutions produce precipitates (solids). ▫2NaOH (aq) +CuCl 2 (aq)  2NaCl (aq) + Cu(OH) 2 (s)

Ionic Equations Ionic equations show substances that are ions in solution by writing them as ions in the equation. Only substances that are aqueous (aq) can break up into their ions, so if a substance is a solid (s), liquid (l), or gas (g) it stays the same in the equation. A complete ionic equation shows all of the particles in a solution as they really exist.

Ionic Equations Ions that do not participate in a reaction are called spectator ions. Any ions that are exactly the same on both sides of the equation are spectator ions and they are cancelled out. A net ionic equation does not show the spectator ions.

Ionic Equations 2Na + (aq) + 2OH - (aq) +Cu 2+ (aq) + 2Cl - (aq)  2Na + (aq) + 2Cl - (aq) + Cu(OH) 2 (s) ▫Complete ionic equation 2OH - (aq) + Cu 2+ (aq)  Cu(OH) 2 (s) ▫Net ionic equation (spectator ions not included)

Determining if a reaction will occur A metal will not always replace another metal in a compound dissolved in water because metals have different reactivities, which is their ability to react with another substance. The most active metals are those that will replace the metal in another compound.

Reactions Some common metals are listed on an activity series chart showing which metals will replace other ones. Any metal on the activity series list can replace any metal that is below it on the list. Metals cannot replace other metals that are listed higher on the list. The letters NR (no reaction) are commonly used to indicate that a reaction will not occur. The same thing can happen with reactions involving halogens.

Activity Series