CHEMICAL REACTIONS Chapter 8

Chemical Equations: Pretend that the element symbols are the alphabet of chemistry Molecular and ionic compounds would be the words of chemistry Chemical equations are the sentences of chemistry. In a chemical equation, there are numerous pieces of information that are hidden. You can also think of a chemical equation as being a recipe…..

A recipe contains: ingredients--what kind and how much of each? products--how many dozen chocolate chip cookies are you going to make? and how do you get from ingredients to products?--cook at 350 o for 12 minutes

Chemical equations have ingredients also, we just call them REACTANTS Chemical equations have products also, we call them PRODUCTS Chemical equations tell us how to get from reactants to products--just like a recipe

This is a balanced chemical equation: 2NaOH + H 2 SO 4  Na 2 SO 4 + 2H 2 O The two chemicals on the left of the arrow are the reactants (NaOH and H 2 SO 4 ). The two chemicals on the right are the products (Na 2 SO 4 and H 2 O)

The arrow in the middle tells us how to get from one side to the other side. Since no information is given, we can assume that this reaction will go from the reactants to the products as soon as the reactants are mixed together (no baking required)

Additional Signs…

Elements that Normally Exist as Diatomic Molecules…

Characteristics of a Chemical Equation…. The following requirements will aid you in writing and reading chemical equations correctly. 1. The equation must represent known facts. 2. The equation must contain the correct formulas for the reactants and products. 3. The law of conservation of mass must be satisfied.

CHEMICAL REACTIONS

Chemical reactions occur when substances undergo chemical changes to form new substances. Chemical reactions rearrange atoms –Law of Conservation of Mass

Energy Energy is conserved in chemical reactions. Chemical energy is the energy released when a chemical compound reacts to produce new compounds. The total energy that exists before the reaction is equal to the total energy of the products and their surroundings. An exothermic reaction is a chemical reaction in which heat is released to the surroundings. An endothermic reaction is a chemical reaction that absorbs heat.

Exothermic vs Endothermic Reactions

Is This an Exothermic or Endothermic Reaction? Paper burning with bright flame Plastics becoming brittle after being left in the sun Firecracker exploding

Chemical Reactions Evidence of a Chemical Reaction Rearrangement of atoms in molecules Evidence Color Changes Formation of a Solid Formation of a Gas Heat Change Absorption Emission Light emission Sound emission

Evidence of a Chemical Reaction Color Change

Evidence of a Chemical Reaction Formation of a Solid

Evidence of a Chemical Reaction Formation of a Gas

Evidence of a Chemical Reaction Emission of Light

Evidence of a Chemical Reaction Emission or Absorption of Heat

Review What is a substance that undergoes a chemical change? What is the substance produced by a chemical change called? What happens to the atoms of a substance that undergoes a chemical reaction? What are the two kinds of chemical reactions with respect to energy? What are evidences of chemical reaction?

Balancing Equations

Chemical Reactions 1.When the number of atoms of reactants matches the number of atoms of products, then the chemical equation is said to be balanced. Balancing equations follows the law of conservation of mass. 2.You cannot balance chemical equations by changing chemical formulas themselves, because that would change the substances involved. 3.To balance chemical equations, numbers called coefficients must be placed in front of the chemical formulas.

Balancing Equations:

1.Mg(s) + O 2 (g) → MgO 2.H 2 (g) + N 2 (g)  NH 3 (g) Balance The Following Equations:

Types of Reactions

A synthesis reaction is a reaction in which two or more substances combine to form a new compound. Synthesis reactions have the following general form: A + B → AB Example: In the following synthesis reaction, the metal sodium reacts with chlorine gas to form sodium chloride, or table salt. 2Na + Cl 2 → 2NaCl

Types of Reactions A decomposition reaction is a reaction in a single compound breaks down to form two or more simpler substances. Decomposition reactions have the following general form: AB → A + B Example: The following shows the decomposition of water. 2H 2 O → 2H 2 + O 2 Electrolysis is the process in which an electric current is used to produce a chemical reaction, such as the decomposition of water.

Types of Reactions A single-displacement reaction is a reaction in which one element or radical takes the place of another element or radical in the compound. Single-displacement reactions have the following general form: AX + B → BX + A Example: The single-displacement reaction between copper(II) chloride and aluminum is shown as follows. 3CuCl 2 + 2Al → 2AlCl 3 + 3Cu

Single-displacement Reaction Consult the Activity Series Table (pg.286) to predict if single- displacement reaction will take place…

ACTIVITY SERIES TABLE

Types of Reactions A double-displacement reaction is a reaction in which a gas, a solid precipitate, or a molecular compound forms from the apparent exchange of atoms or ions between two compounds. Double-displacement reactions have the following general form: AX + BY → AY + BX Example: The double-displacement reaction that forms lead chromate is as follows. Pb(NO 3 ) 2 + K 2 CrO 4 → PbCrO 4 + 2KNO 3

Double Displacement Reactions Precipitation Reactions Potassium iodide mixed with Lead(II) nitrate Bright yellow precipitate Write the balanced reaction. 2KI + Pb(NO 3 ) 2 2KNO 3 + PbI 2 How do you know which product is the solid?

Double-displacement Reaction Consult the Solubility Table (pg.860) to predict if double- displacement reaction will take place…

Solubility Chart

Types of Reactions A combustion reaction is the oxidation reaction of an organic compound (a hydrocarbon), in which heat is released. Combustion reactions use oxygen as a reactant. Water is a common product of combustion reactions. In combustion the products depend on the amount of oxygen available for the reaction Combustion of a hydrocarbon… CH 4 + O 2 ⟶ CO 2 + H 2 O CH 4 + O 2 ⟶ CO + H 2 O

Additional Types of Reactions An oxidation-reduction reaction is any chemical change in which one species gains electrons and another species loses electrons. Oxidation-reduction reactions are often called redox reactions for short. Substances that accept electrons in a redox reaction are said to be reduced. (Comparing oxidation numbers) Substances that give up electrons in a redox reaction are said to be oxidized. (Comparing oxidation numbers) 2Na + Cl 2  2 NaCl 2KNO 3  2KNO 2 + O 2

Oxidation and Reduction: Tools for remembering definitions… Oxidation is Loss of Electrons (O.I.L.) Reduction is Gain of Electrons (R.I.G.) More in Chapter 19

Remember the patterns…

Types of Reactions

Identifying Reactions & Predicting Products…