Chapter 7: Chemical

Equations and Reactions

7.1 Chemical Equation

7.2 Balancing Chemical Equations

7.3 Information Obtained From a Balanced Equation

7.4 Types of Chemical Reactions

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7.1 Chemical Equation • Is a shorthand expression for a chemical

change or reaction.• Reactants – the substance entering the

reaction.• Products – is the substance formed. • During a chemical reaction, atoms, molecules,

or ions interact and rearrange themselves to form the products.

• During the process chemical bonds are broken and new bonds are formed.

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7.2 Balancing Chemical Equations

• Balanced Equation – contains the same number of each kind of atom on each side of the equation.

• Obeys the Law of Conservation of Mass

• Example :

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Pointers in balancing chemical equations • Set up the equation which

shows the correct formulas of all reactants and products. (diatomic elements can stand alone – H2,O2,N2,F2,Cl2,Br2 &I2)

• Start balancing elements that appear only once in each side of the equation but in an unequal numbers of atom. Balanced each elements, one at a time by placing whole number coefficients in front of the formulas containing the unbalanced element. A coefficients multiplies every atom in the formula by that number.

• Ex. 2H3PO4

• Balance polyatomic ions as a group when they appear unchanged on both sides of the equation.

• Use the smallest possible set of coefficients that will give the same number of atoms of the elements on both sides of the equations.

• A subscript should never be changed while balancing an equation.

• Convert a fractional coefficient into a whole number by multiplying the entire equation by the appropriate integer.

• Check each element or polyatomic ion is balanced.

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• Example 7.1 Write the balanced equation for the reaction that takes place when magnesium metal is burned in air to produce magnesium oxide.

Write the formula equation Mg + O2 MgO

Start balancing the O atoms since there are two O atoms on the left side and one on the right side.

Mg + O2 2MgO Mg is not balanced. 2Mg + O2 2MgO Check: Each side has two Mg and two O atoms.

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• Example 7.2 Calcium chloride and iron (III) sulfate solutions react to form a precipitate of calcium sulfate and iron (III) chloride solution. Write the balanced equation for this reaction.

Write the formula equation.CaCl2 + Fe2(SO4)3 CaSO4 + FeCl3

3CaCl2 + Fe2(SO4)3 CaSO4 + 2FeCl3 3CaCl2 + Fe2(SO4)3 3CaSO4 + 2FeCl3 Check the equation

Reactant Product 3 Ca 36 Cl 62 Fe 23 S 312 O 12

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• Example 7.3 When butane gas undergoes complete combustion, it reacts with oxygen to form carbon dioxide and water. Write the balanced equation for this reaction.

• C4H10 + O2 CO2 + H2O• C4H10 + O2 4 CO2 + H2O• C4H10 + O2 4 CO2 + 5 H2O• C4H10 + 6 ½ O2 4 CO2 + 5 H2O

• C4H10 + 13 O2 4 CO2 + 5 H2OReactant Product

8 C 820 H 2026 O 26

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Exercise 7.1 Balancing Chemical Equations A. Formulate and balance the following word equations

1. Tetraphosphorus decoxide + water phosphoric acid

P4O10 + H2O H3PO4

2. Zinc sulfate heptahydrate zinc sulfate + water

ZnSO4 7H2O ZnSO4 + H2O

3. Silver nitrate + sodium chromate silver chromate + sodium nitrate

AgNO3 + Na2CrO 4 Ag2CrO4 + NaNO3

4. Aluminum + copper (II) nitrate aluminum nitrate + copper

Al + Cu(NO3)2 Al(NO3)3 + Cu 5. Lead (II) oxide + ammonia lead +

nitrogen + water PbO + NH3 Pb + N2 + H2O

6 4

7

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33 22

323 3

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B. Balance the following chemical equations1. Mg + HCl MgCl2 + H2

Mg + 2HCl MgCl2 + H2

2. NaOH + H2SO4 Na2SO4 + H2O 2NaOH + H2SO4 Na2SO4 + 2H2O

3. Na2SO3 + S Na2S2O3

balanced already

4. NaCl Na + Cl2

2NaCl 2Na + Cl2

5. C + S2Cl2 CCl4 + SC + 2S2Cl2 CCl4 + 4S

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7.3 Information Obtained From a Balanced Equation

• Consider the equation2 Na (s) + Cl2 (g) 2 NaCl (s)

2 Na Cl2 2 NaCl2 atoms 1 molecule 2 formula units 2 moles 1 mole 2 moles 45.98 g 70.9 g 116.88 g

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7.4 Types of Chemical Reactions

• COMBINATION REACTION

In a combination reaction, two reactants combine to give a single products. The general form of the equation is

A + B AB

* where A and B are either elements or compounds and AB is a compound that fall into this category are the following:

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• Metal + Oxygen Metal Oxide ex. Al + 3O2 2Al2O3

• Nonmetal + Oxygen Nonmetal Oxide ex. C + O2 CO2

• Metal + Nonmetal Salt ex. 2Fe + 3Br2 2FeBr3

• Metal Oxide + Water Base ex. Li2O + H2O 2LiOH

• Nonmetal Oxide + H2O Oxyacid ex. P2O5 + 3H2O 2H3PO4

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• Let’s try this!

• 1. Predict the product formed from the following combination reaction:

a. Si + N2 ___________b. Al + Br2 ________________c. MgO + H2O ____________d. Cs + Br2 _______________e. Cu + O2 ______________

SiN2 AlBr3Mg(OH)2

CsBrCu2O

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•DECOMPOSITION REACTION

In a decomposition reaction a single reactant is decomposed or broken down to give two or more different products. The general form of the equation is …

AB A + B

in which AB is a compound and A and B are either elements or compounds. Many compounds undergo decomposition reactions when heated; some, when passed through electric current (electrolysis).

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The following are decomposed by heating

• metal oxides metal + oxygen ex. 2HgO 2Hg + O2

• Most carbonates (except 1A) oxide + carbon dioxide ex. CaCO3 CaO + CO2

• Group 1A bicarbonates carbonate + carbon dioxide + water ex. 2NaHCO3 Na2CO3 + CO2 + H2O• Other bicarbonates oxide + carbon dioxide + water ex. Mg(HCO3)2 MgO + 2CO2 + H2O• Hydrates anhydrous salt + water ex. CuSO45H2O CuSO4 + 5H2O

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• Chlorates chloride + Oxygen ex. 2KClO3 2KCl + 3O2

• Nitrates nitrite + oxygen ex. 2NaNO3 2NaNO2 + O2

• Peroxides oxide + oxygen ex. 2Na2O2 2Na2O + O2

Water and some salts are compounds that are decomposed by electrolysis

2H2O 2H2 + O2

2NaCl 2Na + Cl

electrolysis

electrolysis

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• Let’s try this!

• Predict the products formed from the following decomposition reactions.

a. SiH4 ________________b. H2O2 _________________c. BaCO3 _______________d. CaSO4 2H2O _____________e. KHCO3 __________________

Si + H2

H2O + O2

BaO + CO2

CaSO4 + H2O

K2CO3 + CO2 + H2O

2

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2

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• DISPLACEMENT REACTION

• In a displacement reaction one free element reacts with a compound to replace one of the elements of that compound. A different element and a different compound are formed. Most displacement reactions fit into one of three subcategories:

» Metal displacement » Hydrogen displacement » Halogen displacement

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A. Metal Displacement • In a metal displacement reaction, a more reactive

metal replaces a less reactive metal from its compound. The general form equation is :

A + BC B + ACIn which A and B are both metals. A will displace B

from BC, providing A is a more reactive metal than B.

The activity series for selected metals and their degree of reactivity are shown in Table 7.1. We describe the metals as highly reactive, slightly reactive, and unreactive based on their ability to displace hydrogen gas from an acid and water.

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B. Hydrogen displacement• the metals, depending on their reactivity, will

displace hydrogen gas from acids and water. The metals reacts as follow:

All metals above H (from Li to Pb) in the activity series

will displace hydrogen from acid. Mg + 2HCl H2 + MgCl2

The highly reactive metals Li, Ba, Ca, and Na will displace hydrogen from water even at room temperature.

2Na + 2H2O(l) H2 + 2NaOH

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The moderately reactive metals Mg, Al, Zn, Cr, and Fe will displace hydrogen only from steam or hot water. 2Al + 3H2O(g) 3H2 + Al2O3

The slightly reactive metals Cd, Co, Ni, Sn and Pb will displace hydrogen from an acid but not water.

The unreactive metals Cu, Hg, Ag, Pt and Au will not displace H2 gas from an acid or water.

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C. Halogen Displacement• For the halogens, the most reactive is fluorine,

followed by chlorine, bromine, and iodine in decreasing order. As in metal displacement, a more reactive halogen will displace a less reactive one from its compound. The general from of equation is:

A + BC C + BAin which A and C are both halogens. A will

displace C from BC, providing A is a more reactive halogen from C.

Example: Cl2 + 2NaBr Br2 + 2NaClBr2 + 2KI I2 + 2KBr

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Let’s Try This!

Predict the products formed, if any, from the following displacement reactions:

1. Au + Ag2O ______________2. Zn + Fe(NO3)3 ______________3. KF + Cl2 ________________4. PbO2 + Ca ______________5. Al + SnCl2 ______________

No reaction

Fe + Zn(NO3)3

No reaction

CaO2 + PbSn + AlCl2

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• DOUBLE DISPLACEMENT

In a double displacement reaction, two compounds exchange component ions with each other to produce two new compounds. The general form of equation is

AB + CD AD + CB

this reaction may be described as an exchange of positive and negative components, where A combines with D and C combines with B. Reaction occurs when one of the following products are formed:

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1. Water H2SO4 + 2NaOH Na2SO4 + H2O(l)

2. A precipitate Na2SO4 + BaCl2 BaSO4(s) + 2NaCl3. A gas MgS + 2HCl MgCl2 + H2S(g)

4. A weak electrolyte, such as H2CO3 and NH4OH. These products, however, are unstable enough to decompose further into CO2 + H2O, SO2 + H2O and NH3 + H2O, respectively. To illustrate,

K2CO3 + H2SO4 K2SO4 + H2CO3 K2SO4 + CO2(g) + H2O Na2SO3 + 2HNO3 2NaNO3 + H2SO3 2NaNO3 + SO2(g) + H2O NaOH + NH4Cl NaCl + NH4OH NaCl + NH3(g) + H2O

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Let’s Try This!

Predict the products formed from the following double displacement reactions:

1. 3HCl + 3Al(OH)3 _______________________2. 2BaCl2 + (NH4)2SO4 _______________________3. Mg(NO3)2 + K3PO4 ________________________4. Li2CO3 + CaCl2 ____________________________5. K2SO3 + HCl ______________________________

3H2O + 3AlCl

Mg(NO3)2 + K3PO4

2LiCl + CaCO3KCl + H2SO4 KCl + SO4 + H2O

2Ba2SO4 + 4NH4Cl

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Exercise 7.3 Types of Chemical Reactions • Classify the following into combination,

decomposition, displacement or double displacement

1. CO + 2H2 CH3OH ___________________2. 2Al + 3H2O Al2O3 + 3H2 _______________3. Al(OH)3 + 3HCl AlCl3 + 6H2O ______________4. Br2 + 2KI 2KBr +I2 ___________________5. 2NaClO3 2NaCl + 3O2 __________________6. 6Li + N2 2Li3N ______________________7. MgSO4 7H2O MgSO4 + 7H2O ______________8. 2NO N2 + O2 ___________________9. 2Ag + PtCl2 2AgCl + Pt ________________10.2HClO4 + Mg(OH)2 Mg(ClO4)2 + 2H2O __________

Combination Displacement (hydrogen)Double

DisplacementDisplacement (halogen)Decomposition

Combination Decomposition

DecompositionDisplacement (Metal)

Double Displacement

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…..for listening and for your cooperation