Chemical ReactionsAnd Equations

Chemical Change - objectives

1. List the common indicators of a chemical change and use them to identify when a chemical reaction has probably occurred. They are: color change, change of state (gas bubbles, precipitate, etc), temperature change, pressure change in a closed container, and change in odor. Note the emphasis on change

Indications of a Chemical ReactionThere are some easily observed changes that usually indicate a chemical reaction1.Color change2.Change of state (gas bubbles, precipitate, etc), 3.Temperature change4.Pressure change in a closed container5.Change in odor

1. Color change

2. Change of state: gas bubbles

Release of gas bubbles when two substances mixed is often evidence of chemical reaction

Antacid in water

2. Change of state: formation of precipitate

Many reactions happen in solution

If solid appears after two solutions mixed, a reaction has probably happened

Example: Pb(NO3)2 + KI

A bright yellow solid PbI2 appears as the proof that the reaction has happened

2. Change of state: formation of precipitate The solid that appears

is called precipitate Precipitate a solid

that is made as a result of a chemical reaction in solution and that separates from the solution

PbI2 precipitate

3. Temperature change

Note: Heat by itself NOT necessarily sign of chemical change

Reaction between vinegar and baking soda is accompanied by drop of temperature

Reaction between glycerin C3H5(OH)3 and potassium permanganate KMnO4 produces extensive heat (burning)

4. Pressure change in a closed containerIf reaction is going in the gas phase and new gases are formed, the pressure in the closed container will change

5. Odor change

Odor Changes It only takes one experience with a rotten egg to learn that they smell different that fresh eggs. When eggs and food spoil, they undergo a chemical change. The change in odor is a clue to the chemical change

Understanding Check

Is it a chemical or physical change?1. “Dry ice”, solid carbon dioxide, is sublimated into

gaseous CO2 at room temperature

2. Salt is dissolved in water3. Iron rusts in a damp environment4. Gasoline burns in the presence of oxygen5. Liquid hydrogen peroxide (H2O2) decomposes to water

and oxygen6. Rotting of eggs

Chemical Equations - objective2. Given a word equation, write a chemical equation (this requires good formula writing skills)

Chemical reaction is a process by which one or more substances are changed into one or more different substances

Chemical equations represent chemical reaction with symbols and formulas. Chemical equations show the identities, physical phases and relative amounts of the reactants and products in a chemical reaction

Reactants (state) → Products (state) Reactants are compounds being brought together to react Products are the new compounds that are made in the reaction

Chemical Reactions and Equations

Physical Phase

•Solid (s)•Liquid (l)•Gas (g)•Aqueous solution (aq) – solution in water

•Examples: Na (s); O2 (g); HCl (l); Ca(NO3)2 (aq)

Word Equations First step in writing chemical equation is to identify

facts to be represented Word equation “equation” in which reactants and

products are represented by words Does not give quantities of reactants/products

Reactants ==> Productschemicals that react ==> chemicals that are produced

hydrogen + oxygen ==> waterhydrogen peroxide ==> water + oxygenmagnesium + oxygen ==> magnesium oxidesodium chloride ==> sodium + chlorine

Formula Equations

Word equation: hydrogen (gas) + oxygen (gas) water (liquid)Formula equation: H2 (g) + O2 (g) H2O (l)

magnesium (solid) + oxygen (gas) magnesium oxide (solid)

Mg (s) + O2 (g) MgO (s)

Characteristics of Chemical Equations1. The equation must contain correct formulas for the

reactants and products.2. The Law of Conservation of Mass must be satisfied:

mass is neither created nor destroyed in any ordinary chemical reaction. • The quantity of substances produced (products)

by a chemical reaction is always equal to the quantity of the reacting substances (reactants).

• The “quantity” here means mass or the number of atoms for each element participating in reaction

Balancing a Chemical Equation1. Write a formula equation

H2O(l) H2(g) + O2(g)

2. Create a table showing the number of atoms for each element: This equation is not balanced!The number

of atomsReactants Products

H 2 2

O 1 2

Balancing Equations3. Correct the number of O by placing coefficient 2 in front of H2O2H2O(l) H2(g) + O2(g)

4. Correct the table:

Oxygen is now balance with 2 on left and 2 on right. H is not!5. Now balance hydrogen – 4 on left, 2 on right. Add coefficient 2 to H2

2H2O(l) 2H2(g) + O2(g) balanced!

The number of atoms

Reactants Products

H 4 2

O 2 2

The number of atoms

Reactants Products

H 4 4

O 2 2

Guidelines for Balancing Equations Create a table of elements for reactant and product

side of the equation Balance the different types of atoms one at a time. First balance the atoms of elements that are

combined and that appear only once on each side of the equation.

Balance polyatomic ions that appear on both sides as single units.

Balance lone atoms, especially H and O atoms (or any), last.

DO NOT!!!.....

• DO NOT WRITE INCORRECT FORMULAS, THIS WILL MESS UP YOUR BALANCING

• DO NOT CHANGE SUBSCRIPTS IN FORMULAS TO BALANCE THE EQUATION!

H2O(l) H2(g) + O2(g) H2O(l) H2(g) + O(g)

DO!.....

• When you think you have balanced the equation, COUNT THE NUMBERS OF EACH TYPE OF ATOM ON EITHER SIDE OF THE EQUATION

2H2O(l) 2H2(g) + O2(g)

Reactant side Product sideH 4 4O 2 2

Sample Problem 1Write balanced chemical equations for the following reaction: Solid sodium combines with chlorine gas to produce solid sodium chloride.•Word reaction:Sodium (solid) + chlorine (gas) sodium chloride (s)•Formula reaction:Na(s) + Cl2(g) → NaCl(s)

•Balanced reaction2Na(s) + Cl2(g) → 2NaCl(s)

Balance ReactionNa(s) + Cl2(g) → NaCl(s)

1.Count atoms:1. Na (in compound): 1 on the left, 1 on the right - balanced2. Cl (lone element): 2 on the left, 1 on the right – not balanced!

2.Adjust Cl: Na(s) + Cl2(g) → 2 NaCl(s)

Count atoms:1. Na: 1 on the left, 2 on the right – not balanced!2. Cl: 2 on the left, 2 on the right – balanced

3.Adjust Na: 2 Na(s) + Cl2(g) → 2 NaCl(s)

Count atoms – all balanced!1. Na: 2 on the left, 2 on the right2. Cl: 2 on the left, 2 on the right

Sample Problem 2Write word, formula, and balanced chemical equations for magnesium and hydrochloric acid (HCl) react to produce magnesium chloride and hydrogen.Word equation: Magnesium (solid) + hydrochloric acid (liquid) magnesium chloride (solid) + hydrogen (gas)Formula equation: Mg (s) + HCl (l) MgCl2 (s) + H2 (g)Count atoms:

1. Mg (in compound): 1 on the left, 1 on the right2. Cl (in compound): 1 on the left, 2 on the right3. H (lone element): 1 on the left, 2 on the right

Adjust HCl: Mg + 2HCl MgCl2 + H2

Recount the atoms – all balanced!

Word equation: Methane (gas) + oxygen (gas) carbon dioxide (gas) + water (gas)

Formula equation: CH4(g) + O2(g) CO2(g) + H2O(g)

Sample Problem 3

Reactants Products Balanced?

1. C (in compounds in both sides) 1 1 Yes

2. H (in compound) 4 2 No

3. O (in compound and a lone element)

2 3 No

• Adjust H: CH4(g) + O2(g) CO2(g) + 2H2O(g)

• Adjust O: CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

Sample Problem 3 (cont.)

Reactants Products Balanced?

1. C (in compounds in both sides) 1 1 Yes

2. H (in compound) 4 4 Yes

3. O (in compound and a lone element)

4 4 Yes

Reactants Products Balanced?

1. C (in compounds in both sides) 1 1 Yes

2. H (in compound) 4 4 Yes

3. O (in compound and a lone element)

2 4 No

Practice Problems

Fe + Cl2 = FeCl3

2Fe + 3Cl2 = 2FeCl3

Zn + HCl ZnCl2 + H2

Zn + 2HCl ZnCl2 + H2

Al + O2 Al2O3

4Al + 3O2 2Al2O3