Chapter 8 Chemical Reactions. Physical Change In a physical change, The identity and composition of...

Chapter 8

Chemical Reactions

Physical Change

In a physical change,

• The identity and composition of the substance do not change

• The state can change or the material can be torn into smaller pieces

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Chemical Change

In a chemical change,

• Reacting substances form new substances with different compositions and properties

• A chemical reaction takes place

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Chemical Reaction

In a chemical reaction,

• Old bonds are broken and new bonds are formed

• Atoms in the reactants are rearranged to form one or more different substances

• Fe and O2 form rust (Fe2O3)

Chemical Reaction

In a chemical reaction,

• A chemical change produces one or more new substances

• There is a change in the composition of one or more substances

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

Evidence of a Chemical Reaction

• Changes that can be seen are evidence of a chemical reaction.

Writing a Chemical Reaction

• Chemists use a shorthand approach when writing the specifics of a chemical reaction. This approach is called the chemical equation.

Reactants -----> Products

Chemical Equations

A chemical equation,

• Gives the chemical formulas of the reactants on the left of an arrow and the products on the right

Reactants Product

C(s)

O2 (g)CO2 (g)

Symbols Used in Equations

Symbols used in chemical

equations show:

• The states of the reactants

• The states of the products

• The reaction conditions

Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings

Chemical Equations Are Balanced

In a balanced chemical reaction,

• Atoms are not gained or lost

Chemical Equations Are Balanced

In a balanced chemical reaction,

• The number of reactant atoms are equal to the number of product atoms

Chemical Equations•Chemical equations: symbolic descriptions of chemical reactions.•Two parts to an equation:

•reactants and products

H2 + O2 H2OA Chemical Equation must also be balanced.

2H2 + O2 --> 2H2O

Balanced Chemical Equations

• Chemical Equations must be balanced– There must be equal numbers of atoms of

each element on both sides of the equation (both sides of the arrow)

1. Write the correct symbols and formulas for all of the reactants and products.

2. Count the number of each type of atom on BOTH sides of the equation.

3. Insert coefficients until there are the equal numbers of each kind of atom on both sides of the equation.

A Balanced Chemical Equation

Al + S Al2S3 Not Balanced

How many atoms of Al and S are on each side of the equation?

coefficients

2Al + 3S Al2S3 Balanced

2 Al = 2 Al

3 S = 3 S

Learning Check

State the number of atoms of each element on thereactant and on the product sides of the equations:

P4(s) + 6 Br2(l) → 4 PBr3(g)

Learning Check

State the number of atoms of each element on thereactant and on the product sides of the equations:

2Al(s) + Fe2O3(s) → 2Fe(s) + Al2O3(s)

Check the balance of atoms in the following:

Fe3O4(s) + 4H2(g) 3Fe(s) + 4H2O(l)

1. Number of _____atoms in reactants.

2. Number of ____ atoms in reactants.

3. Number of ____ atoms in reactants.

Learning Check

Learning CheckDetermine if each equation is balanced or not.

Na(s) + N2(g) → NaN3(s)

C2H4(g) + H2O(l) → C2H5OH(l)

Balancing Equations

• Methane (CH4) reacts with oxygen (combustion reaction) to form carbon dioxide and water.

Write a properly balanced chemical equation

1. Write out chemical formulas

CH4 + O2 CO2 + H2O

2. Use coefficients to balance the equation

The Numbers in Chemical Equations

More Practice: Balancing Reactions

C2H6 + O2 CO2 + H2O

C3H6 + O2 CO2 + H2O

NH3 + O2 NO + H2O

And more practice….

B2H6 + H2O H3BO3 + H2

C3H8O + O2 CO2 + H2O

Balancing with Polyatomic Ions

MgCl2(aq) + Na3(PO4)(aq) NaCl(aq) + Mg3(PO4)2(s)

• HINT: Balance PO43- as a unit.

Set of # coef?

Type of Reactions

Chemical reactions can be classified as

• Combination reactions.

• Decomposition reactions.

• Single Replacement reactions.

• Double Replacement reactions.

• Combustion reactions.

Combination In a combination reaction,

• Two or more elements form one product.

• Or simple compounds combine to form one product.

+

2Mg(s) + O2(g) 2MgO(s)

2Na(s) + Cl2(g) 2NaCl(s)

SO3(g) + H2O(l) H2SO4(aq)

A B A B

DecompositionIn a decomposition reaction,

• One substance splits into two or more simpler substances.

2HgO(s) 2Hg(l) + O2(g)

2KClO3(s) 2KCl(s) + 3O2(g)

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Learning Check

Classify the following reactions as

A) combination or B) decomposition:

1.

2.

3.

Single ReplacementIn a single replacement reaction,

• One element takes the place of a different element in a reacting compound.

Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)

Fe(s) + CuSO4(aq) FeSO4(aq) + Cu(s)

Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings

Double ReplacementIn a double replacement reaction,• Two elements in the reactants exchange places.

AgNO3(aq) + NaCl(aq) AgCl(s) + NaNO3(aq)

ZnS(s) + 2HCl(aq) ZnCl2(aq) + H2S(g)

Copyright © 2008 by Pearson Education, Inc. Publishing as Benjamin Cummings

Learning Check Classify the following reactions as

A) single replacement B) double replacement

1.

2.

3.

In a combustion reaction,

• A compound such as carbon reacts with oxygen, O2.

C(s) + O2(g) CO2(g)

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

C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)

• Fuels burned in oxygen produce CO2, H2O, and energy.

Combustion

Learning Check

Balance the combustion equation

___ + ___ ___ + ___

Summary of Reaction Types

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Heat of Reaction

The heat of reaction,• Is the amount of heat absorbed or released

during a reaction at constant pressure.• Is the difference in the energy of the

reactants and the products.• Is shown as the symbol ΔH. ΔH = Hproducts − Hreactants

Endothermic Reactions

In an endothermic reaction,

• Heat is absorbed.

• The sign of ΔH is +.

• The energy of the products is greater than the energy of the reactants.

• Heat is a reactant.

N2(g) + O2 (g) + 181 kJ 2NO(g)

ΔH = +181 kJ (heat added)

Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings

Exothermic Reactions

In an exothermic reaction,• Heat is released.• The sign of ΔH is -.• The energy of the products is less

than the energy of the reactants.

• Heat is a product.

C(s) + 2H2(g) CH4(g) + 75 kJ

ΔH = - 75 kJ (heat given off) Copyright © 2008 by Pearson Education, Inc.Publishing as Benjamin Cummings

Learning Check

Identify each reaction as

1) exothermic or 2) endothermic.

A. N2 + 3H2 2NH3 + 92 kJ

B. CaCO3 + 556 kJ CaO + CO2

C. 2SO2 + O2 2SO3 + heat