Chemical Equations

Review A + B  AB Reactant SideProduct side

Definition 1. Chemical reaction: a change in composition. This means that one or more substances changes into a new a different substance 2. Ex: vinegar and baking sode  water and carbon dioxide

Properties of a Chemical Reaction 1. Reactions are usually irreversible 2. Physical and chemical properties are changed. 3. New products are formed.

Five Types of Reactions 1. Synthesis Means to put together. A. Two substances combine to form one new substance B. Sample equation i. A + X  AX

2. Decomposition A. One substance decomposes into two or more new substances. B. Sample equation i. AX  A + X

3. Single Replacement A. A single element reacts with a compound creating a new compound and a new element. B. Sample equation i. A + BX  B + AX

C. If the lone element (A on the reactant side) is a cation, then it can only switch with another cation (B) if cation A has a higher activity according to the activity series of metals.

Activity Series of Metals Name Symbol LithiumLi Potassium K CalciumCa SodiumNa Magnesium Mg Aluminum Al Zinc Zn Iron Fe (Hydrogen) (H) Copper Cu MercuryHg Silver Ag

d. Predict what would happen in the following equation: Li (s) + NaCl (aq)  e. Predict what would happen in the following equation when solid silver is mixed with copper(II) nitrate. Ag (s) + Cu(NO 3 ) 2(aq)  Why is there no change? LiCl (aq) + Na (s) No Change

f. Anions also follow a reactivity rule. Anions higher on the periodic table will replace anions lower on the periodic table

4. Double displacement A. Two compounds react and trade partners to form two new compounds. B. Sample Equation: i. AX + BY  AY + BX

5. Combustion A. A substance that reacts with oxygen gas to form water and carbon dioxide OR an oxide (rust).

Sample Problems Identify the type of reaction. 2Mg + FeO 2  Fe + 2MgO NaCl + AgNO 3  NaNO 3 + AgCl CaCO 3  CaO + CO 2 2 C 6 H O 2  6 H 2 O + 12 CO 2 F 2 + 2NaCl  2NaF + Cl 2 2 C 5 H 5 + Fe  Fe(C 5 H 5 ) 2

Chemical Equation 1. Defined as the amount of compounds and elements taking part in a reaction. A. Show with an arrow separating the reactants from the products. B. Remember: reactants are found on the left side in the chemical equation while products are found on the right.

Phases of Chemicals in Equations SymbolPhaseDefinition (s)SolidMolecules touching and organized. Has definite shape and volume.

SymbolPhaseDefinition (l)liquidMolecules touching but not organized. Has a definite volume but takes the shape of its container.

SymbolPhaseDefinition (g)GasSpace between molecules. Have no definite shape and no volume.

SymbolPhaseDefinition (aq)AqueousMolecules are dissolved in water. Has a definite volume but takes the shape of its container.

Common Gases and Acids 1. Certain gases have a special formula because they are diatomic. a. These gases include: Hydrogen (H 2 ) Nitrogen (N 2 ) Oxygen (O 2 ) Fluorine (F 2 ) Chlorine (Cl 2 ) Bromine (Br 2 ) Iodine (I 2 )

2. Also, be familiar with certain acids, though you will be given their formula on the test. A. These acids include Hydrochloric acid (HCl) Phosphoric acid (H 3 PO 4 ) Sulfuric acid (H 2 SO 4 ) Nitric acid (HNO 3 ) Carbonic acid (H 2 CO 3 )

Numbers and Formulas 2Mg + O 2  2MgO Coefficient Coefficients tell the relative amount of a substance in an equation Subscripts Tell the number of atoms you have.

Side lesson Why write 2MgO? Why not just write it as Mg 2 O 2 ? 2MgO tells us we have 2 separate MgO compounds Mg-O

Mg 2 O 2 tells us we have 1 compound made of 2 magnesium atoms and 2 oxygen atoms Mg-O-O-Mg This is actually impossible because Mg does not have 8 valence electrons around it

Naming Rules 1. Remember: gases of certain elements must be written in diatomic form. A. Example: oxygen gas = O 2 ; not O 2. Names of acids and how they are written in chemical equations can be found on your periodic table. Acids normally begin with a hydrogen atom. A. Example: hydrochloric acid = HCl

Law of Conservation in Action 1. All chemical equations must be balanced. 2. Only coefficients can be changed when balancing equations. 3. Ex: Magnesium reacts with oxygen gas to form magnesium oxide. What is the chemical equation? Mg  +O2O2 MgO

A. Notice that the number of oxygen atoms on the reactant side is different than the number of oxygen atoms on the product side B. This is impossible due to the law of conservation of mass, which states that matter cannot be created nor destroyed.

4. The equation needs to be balanced

Balancing Chemical Equations 1. All chemical equations must be balanced. 2. Start by writing the proper formulas for the reactant side and product side 3. Pick an element to start (usually leave oxygen and hydrogen for last). 4. Count the number for each type of atom on the reactant side and be sure they match the same number of atoms for each type of atom on the product side. A. For example: if there are 3 oxygen atoms on reactant side, then there must be 3 oxygens on the product side.

5. If the number of atoms are different, then only the coefficients can be changed. Mg + O 2  MgO Number of oxygen atoms are different Mg + O 2  2MgO Number of magnesium atoms are different 2Mg + O 2  2MgO The number of Mg and O atoms are the same on both side

Practice Problems Al 2 O 3  Al + O 2 Al O

____As + ____I 2  ____AsI 2AsI

____CH 4 + ____O 2  ____CO 2 + ____H 2 OCHO

Exothermic and Endothermic Reactions in Equations 1. Exothermic reactions: reactions that release energy. In the chemical equation the energy is written in the product side. Ex: combustions reactions have exothermic reactions 2. Endothermic reactions: reactions that absorb energy. These reactions feel cold. In the chemical equation the energy is written in the reactant side.

3. The change in energy can be written as ΔH A. The triangle (delta) stands for “change in” and the H stands for enthalpy or heat. B. If ΔH is positive, then the reaction is endothermic C. If ΔH is negative, then the reaction is exothermic.

Practice Determine if the reaction is exothermic or endothermic. State what side of the reaction the energy should be written on.

2HCl (g)  H 2(g) + Cl 2(g) ΔH = +185 kJ/mol Endothermic. Energy written on the reactant side H 2(g) + Cl 2(g)  2HCl (g) ΔH = -185 kJ/mol Exothermic. Energy written on the product side.