Chemical Equations Chapter 9

Objectives Interpret Information in Chemical Equations Balance chemical equations and relate this process to law of conservation of mass Classify chemical rxns Predict Products of Single and Double displacement

Word Equations When chemical changes take place, substances change into new substances Reactants- substances exist before a chemical change (left) Products- new substances that are formed during chemical changes (right) Word Equations- describes chemical change using names of reactants and products

What is the word equation when methane gas and oxygen form carbon dioxide and water? Methane + Oxygen carbon dioxide + water

Determining Whether an equation is balanced. Law of Conservation of Mass Balanced Equation- the # of atoms of each element as a reactant= # of atoms of that element as a product

Is this Balanced? 2Na + H2O  2NaOH + H2 2Na 2Na 2H 2H +2H=4H 1O 2O If it is wrong, how could we fix it so reactants=products?

Balancing Chemical Equations Step 1: Write word equation for rxn Step 2: Write correct formula for all reactants and products Remember diatomic elements! Step 3: Determine coefficients that make equation balanced

Practice Problem Write a balanced equation for chlorine and sodium bromide that reacts to form bromine and sodium chloride.

Step 1 Chlorine + sodium bromide  bromine + sodium chloride

Step 2 Cl2 + NaBr  Br2 + NaCl

Step 3 Cl2 + NaBr  Br2 + NaCl 2Cl 1Cl 1Na 1Na 1Br 2Br How can we balance the equation?

Check your answer! Cl2 + 2NaBr  Br2 + 2NaCl 2Cl 2Br 2Na 2Na 2Br 2Cl BALANCED!

Showing Energy Changes in Equations Chemists can show in an equation what kind of reaction is taking place by adding an energy term.

Endothermic Rxn 2H2O + energy 2H2 + O2 Energy is absorbed in the reaction when it is on the left hand side of the equation.

Exothermic Rxn 2H2 +O2  2H2O + energy Energy is given off when it is located in the product side of the reaction

Show Phases in Chemical Equations Physical phases- solid, liquid, gas, aqueous Chemists provide this information in the chemical equation l (liquid phase) s (solid phase) g (gas phase) aq (aqueous)

Examples Cl2 (g) H2O (l) NaCl (s) NaCl (aq)

Classifying Chemical Rxns Synthesis Decomposition Single Replacement Double Displacement

Synthesis or Direct Combination In this type of reaction, two or more substances combine to produce a single, more-complex substance. A +B  AB A or B could be an element or compound Example 2CO (g) + O2 (g)2CO2 (g)

Decomposition or Analysis In this type of reaction a single substance is broken down into two or more simpler substances. AB  A + B AB= compound; A, B are elements Example: 2NaCl (l)  2Na(l) + Cl2(g)

Single Replacement Reactions A reaction in which an element replaces a less active element in a compound, setting the replaced element free A + BC  AC + B Example Zn (s) + H2SO4 (aq)  ZnSO4 (aq) + H2 (g) Zinc and Hydrogen switch places!

Double Replacement Reaction A reaction in which two ionic compounds exchange ions to form two different ionic compounds. AB + CD  AD + CB Example AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq)