Chapter 9 Chemical Reactions

Reactions and Equations 9.1 A chemical reaction is another name for chemical change. How to tell if a chemical reaction has taken place Is there evidence? (Temperature change, color change, odor, gas bubbles, general appearance) Equations are used to represent chemical reactions Show reactants: the starting substances Show Products: resulting or ending substances Do not show numerical values so arrow is used instead of an equals sign Show indicator of chemical state

Types of Equations Three types of equations Word equations Skeleton Chemical Word equations Used to indicate the reactants and products of chemical reactions Uses the actual names of the substances involved Setup: Reactant 1 + Reactant 2  Product 1 Ex: Iron(s) + Chlorine(g)  Iron(III) Chloride(s) Skeleton Equations Uses chemical formulas to represent the substances unlike the word equation Chemical formulas take the place of the words Iron(s) + Chlorine(g)  Iron(III) Chloride(s) would become: Fe(s) + Cl2(g)  Fe Cl3(s)

Balancing Chemical Equations Uses chemical formulas like the skeleton equation Shows matter is conserved during a reaction (Law of Conservation of Mass) Chemical eqation: Uses identities and relative amounts of the substances involved in a chemical reaction Everything is balanced in a chemical equation Fe(s) + Cl2(g)  Fe Cl3(s) would become: 2Fe(s) + 3Cl2(g)  2Fe Cl3(s) Balancing Chemical Equations Finding the correct coefficients for chemical formulas in the skeleton equation is very important Coefficient is usually a whole number and 1 is understood

Steps to Balance a Chemical Equation 1: Write the skeleton equation 2: Count the atoms of elements in the reactants 3: Count the atoms of elements in the products 4:Change the coefficients to make both sides of the equation equal 5:Write the coefficient(s) in the lowest ratio possible 6:Check your work Ex: H2 (g) + O2(g)  H2O(g) 4H2 (g) + 2O2(g)  4H2O(g) 2H2 (g) + O2(g)  2H2O(g)

Classifying Chemical Reactions 9.2 There are five types of chemical reactions. Synthesis Combustion Decomposition Single-Replacement Double-Replacement Synthesis Reaction Chemical reaction in which two or more substances combine to form one single product. A + B  AB Ex: 2Na(s) + Cl2(g)  2NaCl(s)

Decomposition reactions Combustion Reaction Oxygen combined with a substance releasing energy in the form of light and heat Many combustion reactions are also synthesis reactions Ex: 2H2(g) + O2  2H2O(g) Decomposition reactions Only one compound Breaks down into 2 or more compounds or elements Setup: AB A + B Often requires an energy source (i.e. heat, light, electricity) Ex: NH4NO3(s)  N2O(g) + 2H2O(g) The above reaction occurs when the reactant is heated Replacement Reactions Unlike the other reactions, replacement reactions involve the replacement of an element in a a compound 2 kinds of replacement reactions: Single Double

Single Replacement reactions When atoms of one element replace the atoms of another element in a compound Setup: A+ BX  AX + B Whether or not a metal will replace another metal or any other substance depends on the reactivity it has Double Replacement reactions Invovles the exchange of ions between 2 compounds Setup: AX + BY  AY + BX If a solid is produced during the reaction it is called a precipitate

Reactions in Aqueous Solutions 9.3 Vocab Aqueous solution- Solution in which the solvent is water Solvent- The water in the solution Solute- The compound dissolved in the water Precipitate- Solid formed in a chemical reaction Spectator ion- Ions that do not participate Complete ionic equation- Shows all particles as they really exist Net ionic equation- Only includes particles that participate (No spectator ions)

Aqueous Solutions When two aqueous solutions react it is always a double replacement reaction. The water doesn’t react There are three possible products when aqueous solutions react, they are; precipitates, water, or gases. Spectator ions do not participate in the reaction and are usually not included in the ionic equation.

Writing Ionic Equations Ionic and chemical equations differ in that substances that are ions in the solution are written as ions in the equation. To write an ionic equation you must show the reactants and the product as ions. A complete ionic equation shows all of the particles in the solution as they realistically exist. A net ionic equation is a simplified version of the complete ionic equation. It only shows particles that participate in the reaction. To write a net ionic equation from a complete ionic equation cross out all spectators.

Ionic equations What type of reaction is this? AgNO3(aq) + NaCl(aq)  AgCl(s) + NaNO3(aq) What type of reaction is this? A complete ionic equation shows dissolved ionic compounds as their free ions. Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq)  AgCl(s) + Na+(aq) + NO3-(aq)

Ionic Equations Ag+(aq) + NO3-(aq) + Na+(aq) + Cl-(aq)  AgCl(s) + Na+(aq) + NO3-(aq) Ions that are not directly involved in a reaction are called spectator ions. Rewriting the equation, leaving out the spectator ions, gives us the net ionic equation… Ag+(aq) + Cl-(aq)  AgCl(s)

Ionic Equations You also have to make sure that the net charge is balanced. For example… Pb(s) + 2Cl-(aq)  PbCl2(s)

Examples HCl(aq) + ZnS(aq)  H2S(g) + ZnCl2(aq) Cl2(g) + NaBr(aq)  Br2(l) + NaCl(aq)