Chapter 9 – Introduction Chapter 9 – Introduction to Equationsto Equations

Writing and Balancing Writing and Balancing Equations Equations Section 9.1Section 9.1

Chemical ReactionsChemical Reactions

One or more substances are rearranged to One or more substances are rearranged to form different substancesform different substances

Example: C8H18 + O2 CO2 + H2O + ENERGY

Indicators of Chemical ReactionsIndicators of Chemical Reactions

Animation of Experiment (GIF Format, 321 k)

Temperature changeTemperature change Color changeColor change Light is producedLight is produced Gas is produced (bubbles)Gas is produced (bubbles) Formation of a solid - Formation of a solid - precipitateprecipitate

Representing Chemical ReactionsRepresenting Chemical Reactions

Statements called Statements called equationsequations are used to are used to describe what happens describe what happens during a chemical during a chemical reactionreaction

ReactantsReactants are the are the starting substances in a starting substances in a reaction. The arrow reaction. The arrow points away from points away from reactants.reactants.

ProductsProducts are the are the substances formed substances formed during a reaction. The during a reaction. The arrow points toward the arrow points toward the products.products.

SymbSymbolol

MeaningMeaning

++ Separates two or Separates two or more reactants or more reactants or productsproducts

Separates Separates reactants from reactants from productsproducts

(s)(s) Solid stateSolid state

(l)(l) Liquid stateLiquid state

(g)(g) Gaseous stateGaseous state

(aq)(aq) Aqueous/water Aqueous/water solution (solid solution (solid dissolved in dissolved in water)water)

What is a Chemical Equation?What is a Chemical Equation?

A chemical equation is like a recipe in that you have the ingredients and you make your product.

In a chemical equation we call the ingredients reactants and what we form are called the products.

The reactants and products come in various physical states. For instance, you can add

 melted butter to a recipe or solid butter.

You can add solid, liquid, or gas reactants and you

can make solid, liquid, or gaseous products.

Can you match the physical Can you match the physical state of the compounds on the state of the compounds on the left with the correct symbol it left with the correct symbol it

represents on the right?represents on the right?

_____ solid A. (aq)_____ liquid B. (g)_____ gas C. (s)_____ aqueous solution D. (l)

Can you list all of the physical Can you list all of the physical states of each compound in the states of each compound in the

chemical reaction below?chemical reaction below?

NH3(g) + H2O(l) NH4+(aq) + OH-(aq)

NH3(g) _______________

H2O(l) ________________

NH4+(aq) _____________

OH-(aq) ______________

Can you label the Can you label the reactantsreactants and and the the productsproducts??

 NH3(g) + H2O(l) NH4+(aq) + OH-(aq)

These are the ______ These are the ______

Word Equations and Skeleton Word Equations and Skeleton EquationsEquations

Word EquationsWord Equations

-Statements that indicate -Statements that indicate the reactants and the reactants and products in a rxnproducts in a rxn

-Describe chemical -Describe chemical reactionsreactions

-Lack important -Lack important informationinformation

Iron + chlorine iron (III) chloride

Skeleton Skeleton EquationsEquations

-Use chemical -Use chemical formulas to identify formulas to identify reactants and reactants and productsproducts

Fe (s) + Cl2 (g) FeCl3 (s)

Practice ProblemsPractice ProblemsWrite skeleton equations for the following Write skeleton equations for the following

word equations:word equations:1. hydrogen 1. hydrogen (g)(g) + bromine + bromine (g)(g) hydrogen bromide hydrogen bromide (g)(g)

HH22 (g)(g) + Br + Br22 (g)(g) HBr HBr (g)(g)

2. carbon monoxide 2. carbon monoxide (g)(g) + oxygen + oxygen (g)(g) carbon dioxide carbon dioxide (g)(g)

CO CO (g)(g) + O + O22 (g)(g) CO CO22 (g)(g)

3. 3. potassium chloratepotassium chlorate(s)(s) potassium chloride potassium chloride(s) (s) + oxygen+ oxygen(g)(g)

KClOKClO33 (s)(s) KCl KCl (s)(s) + O + O22 (g)(g)

Balancing EquationsBalancing Equations Mass/matter cannot be created nor destroyedMass/matter cannot be created nor destroyed Atoms you start with (reactants) must still be Atoms you start with (reactants) must still be

there when you end (products)there when you end (products) This is called balancing the equation This is called balancing the equation Mass of Reactants = Mass of ProductsMass of Reactants = Mass of Products

Chemical Equations also must obey the Law of Conservation of Mass. The number of atoms that are used in the reactants must equal the number of atoms found in the products. Matter can neither be created nor destroyed. A chemical equation that follows the law of conservation of mass is called a balanced chemical equation

A Chemical EquationH2 + O2 -> H20

A Balanced Chemical EquationCoefficient… -> 2H2 + O2 -> 2H2O

What happens in every What happens in every chemical reaction?chemical reaction?

Balancing EquationsBalancing Equations

Mg + 2HCl Mg + 2HCl MgCl MgCl22 + H + H22

Big numbers = Big numbers = CoefficientsCoefficients- can changecan change

Small numbers = Small numbers = SubscriptsSubscripts- DO NOT CHANGE, if you change you no DO NOT CHANGE, if you change you no

longer have the same substancelonger have the same substance

Balancing Chemical EquationsBalancing Chemical Equations There is a procedure for balancing chemical There is a procedure for balancing chemical

equations:equations:1.1. Write formulas for all compounds (sometimes given) Write formulas for all compounds (sometimes given)

(remember your diatomic molecules)

2.2. Separate reactants and products by a lineSeparate reactants and products by a line

3.3. Count how many atoms of each element are present on the Count how many atoms of each element are present on the reactant and the product side (element inventory)reactant and the product side (element inventory)

4.4. Add or adjust the coefficients so that element inventory is Add or adjust the coefficients so that element inventory is balanced balanced (subscripts cannot be changed)

5.5. Reduce coefficients to lowest possible terms (simplify)Reduce coefficients to lowest possible terms (simplify)

6.6. Check your work Check your work

Hydrogen + Chlorine Yields Hydrochloric AcidHydrogen + Chlorine Yields Hydrochloric Acid

HH22 + Cl + Cl22 -> HCl -> HCl

Which of the compounds in the equation is one of the 7 diatomic molecules? ______

CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

List the other diatomic molecules (remember they make a 7 on the periodic table)_____________________ __________________________________________ __________________________________________ ______________________

What are the physical states of the compounds in the reaction? _________________

Notice those little numbers in front of the O2(g) and the H2O(g)? Those are called coefficients.Coefficients help to balance out the atoms on both sides of the chemical reaction.

Balancing Equations PracticeBalancing Equations Practice1. Al + O1. Al + O22 -> Al -> Al22OO

4Al + O4Al + O22 -> 2Al -> 2Al22OO33

2. C2. C33HH8 8 + O + O22 -> CO -> CO22 + H + H22OO

CC33HH88 + 5O + 5O22 -> 3CO -> 3CO22 + 4H + 4H22OO

3. KNO3. KNO33 -> KNO -> KNO22 + O + O22

2KNO2KNO33 -> 2KNO -> 2KNO22 + O + O22

4. O4. O22 + CS + CS22 -> CO -> CO22 + SO + SO22

3O3O2 2 + CS + CS22 -> CO -> CO22 + 2SO + 2SO22