Chemical Reactions

Chemistry

Reactions and Equations Objectives

1. Recognize evidence of chemical change.2. Represent chemical reactions with equations. 3. Balance chemical equations.4. Classify chemical reactions.5. Identify the characteristics of different classes of chemical reactions.6. Describe aqueous solutions.7. Write complete ionic and net ionic equations for chemical reactions in aqueous solutions.8. Predict whether reactions in aqueous solutions will produce a precipitate, water, or a gas.

Reactions and Equations

chemical reaction: process by which one or more substances are rearranged to form different substances; also called a chemical change

-reactants: the starting substances

-products: resulting, new substances

The law of conservation of mass states that the total mass of reactants must equal the total mass of the products.

Evidence of Chemical Reactions

While some reactions are hard to detect, most provide evidence they have occurred.

1. temperature change

-exothermic-heat released

-endothermic-heat absorbed

2. color change

-by itself it doesn’t necessarily indicate a chemical

change since it is also an indication of a physical

change

3. production of a gas

-may also produce odor

4. formation of a precipitate, a solid formed as a

result of a chemical reaction in solution and that

separates from the solution

There are several ways to represent chemical reactions:

1. word equations

2. skeleton equations

3. chemical equations

No matter the method of representation, there is a universal set of symbols all scientists use.

-must always state the physical state of each reactant and product

-arrow always points to the products; most often reactants are written on the left, products on the right

Symbol Meaning

+ plus; separates 2

  or more reactants

  or products

→ produces, yields,

  or forms

(s) solid

(l) liquid

(g) gas

(aq) aqueous; dissolved

  in water

Word Equations

Word equation uses words to represent chemical reactions.

“Iron and chlorine react to produce iron (III) chloride”

iron + chlorine iron (III) chloride

iron is a solid

chlorine is a gas

iron (III) chloride is a solid

iron (s) + chlorine (g) iron (III) chloride (s)

Problem: cumbersome and lack important information

Skeleton Equations

Skeleton equations uses chemical formulas rather than words to identify the reactants and products.

iron (s) + chlorine (g) iron (III) chloride (s)

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

You must remember the rules for naming and writing formulas for ionic and covalent compounds.

Problem: lack important information, such as how much of each reactant and product we have (law of conservation of mass)

Word and Skeleton Equations Practice 1

Write word and skeleton equations for:

1. Hydrogen reacts with bromine to produce hydrogen bromide gas.

2. Carbon dioxide is produced from the reaction of carbon monoxide with oxygen.

3. Solid potassium chlorate decomposes to form solid potassium chloride and oxygen.

Reactions Review 1

1. What is a chemical reaction?

2. Describe the law of conservation of mass.

3. Give the four indications of a chemical reaction.

4. What is a precipitate?

5. What are the three ways to represent chemical reactions.

6. What is a word equation? What is a problem with them?

7. What are the symbols for “produces”, “solid”, “liquid”, “gas”, and “aqueous”?

Chemical Equations

chemical equation: uses chemical formulas to show the identities and relative amounts of reactants and products in a chemical reaction.

-must show mass being conserved

# of reactant atoms = # of product atoms

-accomplished by balancing equations

Chemical equations are the representation chemists use to describe chemical reactions.

Balancing Chemical Equations

coefficient: number written in front of a reactant or product that states the ratio of amounts for each substance

-usually a whole number

-number ‘1’ is assumed and not written

Steps for balancing equations:

1. Write the skeleton equation for the reaction

2. Count the atoms of elements in the reactants/products

3. Place coefficients in front of each substance; change until the equation is balanced

4. Reduce coefficients to smallest possible ratio

5. Check your work.

Note: Never change the subscript in the formula because that changes the identity of the substance.

Example: Hydrogen chloride is formed during the

reaction between hydrogen and chlorine.

1. Write the skeleton equation for the reaction.

hydrogen + chlorine hydrogen chloride

H2 (g) + Cl2 (g) HCl (g)

2. Count the atoms of elements in the reactants/products

___H2 (g) + ___Cl2 (g) ___HCl (g)

H

Cl

3. Place coefficients in front of each substance; change until the equation is balanced

___H2 (g) + ___Cl2 (g) ___HCl (g)

H

Cl

4. Reduce coefficients to smallest possible ratio

H2 (g) + Cl2 (g) 2HCl (g)

5. Check your work.

Chemical Equations Practice 2

Write chemical equations and balance each:

1. In water, iron (III) chloride reacts with sodium hydroxide, producing solid iron (III) hydroxide and sodium chloride.

2. Liquid carbon disulfide reacts with oxygen, producing carbon dioxide and sulfur dioxide.

3. Solid zinc and aqueous hydrogen sulfate react to produce hydrogen gas and aqueous zinc sulfate.

Reactions Review 2

1. Why do we not change subscripts when balancing equations?

2. How do we ensure conservation of mass?

3. Why are chemical equations better representations than word equations?

4. List the 7 diatomic gases.

5. What is a precipitate?

6. What is a chemical equation?

7. What is the purpose of coefficients?

Chemical Equations Practice 3

Write skeleton equations and then balance the following:

1. Hydrogen iodide gas decomposes to form hydrogen gas and iodine gas.

2. Aluminum reacts with solid iodine to form solid aluminum iodide.

3. Iron (II) oxide solid reacts with oxygen to form solid iron (III) oxide.

Classifying Chemical Reactions

It is more useful to classify reactions according to their similarities and regularities than trying to remember the equations for so

many different reactions.

There are 4 basic types of reactions:

1. synthesis reaction- 2 or more simple substances combine to form a new, more complex substance

-general formula: A + X AX

where A and X can be elements or compounds

-examples: 2Na + Cl2 2NaCl

CaO + H2O Ca(OH)2

-there are several types of synthesis reactions:

a. combustion reaction- reaction in which a

substance combines with oxygen, releasing a

large amount of energy in the form of heat and

light

-ex: 2CH4 + 4O2 2CO2 + 4H2O + energy

b. polymerization reaction: reaction in which

monomer units (a small, simple organic molecule)

are bonded together to form a polymer, a large

molecule consisting of many repeating structural

units (monomers)

b. polymerization reaction

-addition polymerization: all of the atoms

present in the monomers are present in the

polymer product

~ex: ethene (ethylene) polyethylene

H2C=CH2 -CH2-CH2-CH2-CH2-

~can occur by breaking the unsaturated double

bonds and adding more identical molecules

~see table 23-4, p 763 for examples and uses

such as PVC, saran, etc.

-condensation polymerization: when monomers

containing at least 2 functional groups combine

with the loss of a small by-product, usually water

~example p 23-22, p 764

~see page 763, Table 23-4 for more examples

and uses

-polymers are easy to synthesize and the starting

materials used are relatively inexpensive; they

can be softer than silk, as strong as steel (but not

rust like steel), drawn or molded into different

shapes

2. decomposition reaction - a complex substance breaks down into at least 2 or more simpler substances

-they are opposite of synthesis reactions

-many require energy to occur

-general formula: AX A + X

-examples:

a) H2CO3 H2O + CO2

b) electrolysis: decomposition of a compound by

electric current

3. single-displacement reaction - an uncombined element replaces a similar element that is part of a compound

-more reactive element displaces the less reactive one (use activity series; metals will replace H) -general formula: A + BX AX + B ~A and B are single elements (or diatomic molecules); BX and AX are compounds

-example: 2Na + 2H20 2NaOH + H2

4. double-displacement reaction- different atoms (usually ions) in 2 different compounds exchange places to form new compounds

-general formula: AX + BY AY + BX ~A, X, B, and Y in the reactants represent ions while AY and BX represent ionic or molecular compounds ~one of the compounds formed is usually a precipitate, gas, or liquid -use solubility rules to determine if a precipitate forms

-example: MgCO3 + 2HCl MgCl2 + H2CO3

Classifying Reactions Practice 1

1. Heating solid sodium hydrogen carbonate produces aqueous sodium carbonate, carbon dioxide gas and water.

2. Water and dinitrogen pentoxide gas react to produce aqueous hydrogen nitrate.

3. Solid potassium and aqueous zinc (II) chloride react to form solid zinc metal and potassium chloride.

4. Aqueous lithium iodide and aqueous silver nitrate react to produce solid silver iodide and lithium nitrate.

5. Predict the product(s) of the reaction between chlorine gas and aqueous hydrogen fluoride.

6. Predict the product(s) of the reaction between aqueous barium chloride and aqueous potassium carbonate.

Classifying Reactions Practice 2

1. Iron metal reacts with copper (II) sulfate to produce…2. Liquid bromine reacts with aqueous magnesium

chloride to produce…3. Magnesium reacts with aqueous aluminum chloride to

produce…4. Iron reacts with aqueous sodium phosphate to

produce…5. Aqueous calcium reacts with hydrochloric acid to

produce…6. Aqueous potassium cyanide reacts with hydrobromic

acid to produce…

Reactions Review 3

1. What is the difference between synthesis and decomposition reactions?

2. What is the difference between single and double displacement reactions?

3. Describe combustion reactions. What are the products of all combustion reactions?

4. What are the products of double displacement reactions?

5. Why does reaction a occur, but reaction b does not?

a. 2KBr(aq) + Cl2 (g) 2KCl (aq) + Br2 (g)

b. 2KBr(aq) + I2 (g) 2KI (aq) + Br2 (g)

6. What type of reactions are the examples in #5?

Reactions in Aqueous Solutions

solute: substance being dissolved

solvent: substance doing the dissolving

aqueous solution: when water is the solvent

Some aqueous solutions contain molecules (such as sugar or ethanol).

Others contain ionic compounds (or acids) that break apart, or dissociate, in water to form ions.

-when two aqueous solutions that contain ions react,

a double displacement reaction occurs

-these are what we will discuss here

Aqueous SolutionsRemember: the products of a double displacement reaction is

either a precipitate, liquid or gas.

-Use your solubility rules to determine if a precipitate occurs.

Chemists use ionic equations to show the details of an ionic equation:

-complete ionic equation: shows all particles in a

solution

-net ionic equation: shows only the particles that

participate in the reaction (forms a solid, liquid, gas)

-spectator ions: ions that do not participate; they

just ‘watch’

Net Ionic EquationsExample:

2NaOH (aq) + CuCl2(aq) 2NaCl (aq) + Cu(OH)2 (s)

2Na+(aq) + 2OH-(aq) + Cu+2(aq) + 2Cl-(aq) 2Na+(aq) + 2Cl-(aq) + Cu(OH)2 (s)

(notice the solid precipitate is not split up into its ions-this is true for gases and liquids produced as well)

2Na+(aq) + 2OH-(aq) + Cu+2(aq) + 2Cl-(aq) 2Na+(aq) + 2Cl-(aq) + Cu(OH)2 (s)

2OH-(aq) + Cu+2(aq) Cu(OH)2 (s)

Na+ and Cl- are the spectator ions since they are not part of the formation of Cu(OH)2 (s)

Net Ionic EquationsYou try:

Write the chemical, complete ionic & net ionic equations for the reaction between solutions of barium nitrate and sodium carbonate that forms solid barium carbonate. (Hint: You must determine the second product.)

Net Ionic Equations Review & Practice

p 294 # 33-35,

p 296 # 38-40,

p 299 # 44-50