CONCURRENT ENROLLMENT CHAPTER 5 CONCURRENT ENROLLMENT

Chemical Reactions Evidence that a chemical reaction may have occurred: Change in temperature Change in color Odor Gas bubbles Appearance of a solid

CHEMICAL EQUATIONS This table summarizes the symbols used in Reactants - are the starting substances. Products - are the substances formed in the reaction. This table summarizes the symbols used in chemical equations.

Law of conservation of matter Balanced equation An equation in which the number of atoms of each element in the reactants is the same as the number of atoms of that same element in the products

MASS RELATIONS IN REACTIONS Writing a balanced equations In a balanced equation you must conserve mass (atoms) First you write a “skeleton” equation CS2 + O2 ---> CO2 + SO2 Indicate the physical state CS2(l) + O2(g) ---> CO2(g) + SO2(g) Balance the equation CS2(l) + 3O2(g) ---> CO2(g) + 2SO2(g)

THREE STEPS IN BALANCING Determine the type of reaction. What are the reactants, products and physical states involved Write a unbalanced equation Balance equation, start with the most complicated molecule. Find the right coefficient to give the same number of each type of atom of both sides. DO NOT CHANGE THE IDENTITIES (NH4)2 Cr2O7(s)----> Cr2O3(s) + N2(g) + H2O(g)

TYPES OF REACTIONS SYNTHESIS When two elements react, the reaction is always a synthesis reaction.

DECOMPOSITION A decomposition reaction is one in which a single compound breaks down into two or more elements or new compounds. Decomposition reactions often require an energy source, such as heat, light, or electricity, to occur. CaCO3 (s) –Δ CaO(s) + CO2(g)

SINGLE REPLACEMENT A metal will not always replace a metal in a compound dissolved in water because of differing reactivities. An activity series can be used to predict if reactions will occur. Halogens also have different reactivities and do not always replace each other. Zn(s) + CuSO4(aq)  Cu(s) + ZnSO4(aq)

DOUBLE REPLACEMENT occur when ions exchange between two compounds Ca(OH)2(aq) + HCl(aq)  CaCl2(aq) + H2O(l) balance this The solid product produced during a chemical reaction in a solution is called a precipitate. All double replacement reactions produce either water, a precipitate, or a gas.

CONTINUED COMBUSTION oxygen combines with a substance and releases energy in the form of heat and light.

RULES FOR OXIDATION NUMBERS Elements found in nature is always zero Cu, O2 Monoatomic ions number is the same as its charge Cl-, Mg2+ Fluorine always 1- Oxygen 2- Except with fluorine 2+ Peroxide 1-, H2O2 Superoxide ½ -, KO2 Hydrogen 1+ With group one elements then it is a hydride 1- Sum of the oxidation numbers of ALL atoms in a neutral compound is zero The most electronegative element in a binary compound oxidation number is the same as the ion charge Sum of oxidation numbers of ALL atoms in a polyatomic ion is same as its charge

REDOX The concept of oxidation numbers explain oxidation and reduction Oxidation increases the oxidation number (loss electrons) Reduction decreases the oxidation number (gains electrons)

OXIDATION-REDUCTION REACTIONS An oxidation-reduction reaction involves transfer of electrons Addition of O2 or H2 What species goes through oxidation? Species that lose electrons What species goes through reduction? Species that gain electrons LEO goes GER Cl-  Cl2 Cu+ + Al  Cu + Al3+ Oxidizing agent The element accepts that the electron Reducing agent The element that gives the electron

ENERGY AND REACTION EXOTHERMIC REACTIONS 2H2(g) + O2(g)  2H2O(g) + heat ENDOTHERMIC REACTIONS 2H2O(g) + heat  2H2(g) + O2(g)

CONTINUED Balancing redox equations Half-equation method Split the equation into two half reactions One for reduction and the other for oxidation Balance one of the half-reactions with respect to both atoms and charge , use water to add oxygen’s, add hydrogen ions (in an acidic solution you will have both), add electrons to balance charge Balance the half-reactions Combine the two in such away as to eliminate all the electrons

Aqueous Solutions An aqueous solution contains one or more dissolved substances (called solutes) in water. The solvent is the most plentiful substance in a solution. Ionic compounds can also be solutes in aqueous solutions When ionic compounds dissolve in water, their ions separate in a process called dissociation When two solutions that contain ions as solutes are combined, the ions might react.

If they react, it is always a double replacement reaction. Three products can form: precipitates, water, or gases Aqueous solutions of sodium hydroxide and copper(II) chloride react to form the precipitate copper(II) hydroxide. 2NaOH(aq) + CuCl2(aq) → 2NaCl(aq) + Cu(OH)2(s) Ionic equations that show all of the particles in a solution as they actually exist are called complete ionic equations. 2Na+(aq) + 2OH–(aq) + Cu2+ (aq)+ 2Cl–(aq) → 2Na+(aq) + 2Cl–(aq) + Cu(OH)2(s)

CONTINUED Ions that do not participate in a reaction are called spectator ions and are not usually written in ionic equations. Formulas that include only the particles that participate in reactions are called net ionic equations. 2OH–(aq) + Cu2+(aq) → Cu(OH)2(s)

MOLE TO MOLE Reactions involving masses in grams of reactants and products can be achieved by converting grams into moles

LIMITING REACTANTS AND THEORETICAL YIELD When a reaction goes to completion and there is no loss of product, then the amount produced is called the theoretical yield Many times there is one reactant that is expensive so an excess of the other reactant is used, this ensures that all the expensive reactant is used. When the limiting reactant is used up the reaction stops. Method to determine limiting reactant Calculate the amount of product that would be formed if the first reactant was consumed Repeat step 1 for second reactant