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LECTURE 3

SCES1200

Dr Zaharah Aiyub

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1.3 WRITING AND BALANCING

CHEMICAL EQUATIONS

Objectives

Write and balance chemical equations

Determine stoichiometric equivalence

Calculating reactants and products fora reaction

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The formation of HF gas on the macroscopic and molecular levels

Fig 2.1

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A three-level view of the chemical reaction in a flashbulbFig 2.2

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BALANCE MOLECULAR

REACTIONS

H2(g) + F2(g) 2HF(g)

Mg(s) + O2(g) MgO2(s)

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

2NaOH (aq) + 2Al (s) + 2H2O(l) 3H2(g) +

2NaAlO2(aq)

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BALANCE REDOX REACTIONS

4Na(s) + O2(g) 2N2O(s)

2HCl(aq) + Fe(s) FeCl2(aq) + H2(g)

2MnO4-(aq) + 10 Fe2+(aq) + 16 H+(aq)

permanganate ion ferrous ion

10Fe3+(aq) + 2Mn2+(aq) + 8H2O(l)

ferric ion manganous ion

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BALANCE IONIC REACTIONS

Ba2+(aq) + SO42-(aq) 4(s)

sulphate ion

n

2S2O32-(aq) + I2(aq) S4O6

2- (aq) + 2I-(aq)

thiosulfate ion tetrathionate ion

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QUIZ

Balance the following equations:

(a) CO(g) + O2(g) CO2(g)

(b) CH4(g) + Cl2(g) CCl4(l) + HCl(g)

(c) Ca(OH)2(aq) + H3PO4(aq) Ca3(PO4)2(s) +H2O(l)

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translate the statement

Sample Problem 2.1 Balancing Chemical Equations

PROBLEM:

PLAN: SOLUTION:

balance the atoms

specify states of matter

Within the cylinders of a cars engine, the hydrocarbon octane

(C8H18), one of many components of gasoline, mixes with oxygenfrom the air and burns to form carbon dioxide and water vapor.Write a balanced equation for this reaction.

adjust the coefficients

check the atom balance

C8H18 + O2 CO2 + H2O

C8H18 + O2 CO2 + H2O825/2 9

2C8H18 + 25O2 16CO2 + 18H2O

2C8H18 + 25O2 16CO2 + 18H2O

2C8H18(l) + 25O2 (g) 16CO2 (g) + 18H2O (g)

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1.4 CALCULATING THE AMOUNTSOF REACTANT AND PRODUCTS

To be able to calculate the numberof moles of substances in a balancechemical reaction if you know the

number of moles of one substance.

Objectives

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STOICHIOMETRIC EQUIVALENCEAND REACTION STOICHIOMETRY

Chemical equations provide relationships betweensubstances that have been involved in a reaction. Thesestoichiometric relationships are used to predict the quantityof a product formed, the quantity of a reactant needed, or the

quantities of substances that will combine.

(g moles

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Stoichiometric equivalence

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

1 mol of C3H

8is stoichiometrically equivalent to 5 mol of O

21 mol of C3H8 is stoichiometrically equivalent to 3mol of CO21 mol of C3H8 is stoichiometrically equivalent to 4 mol of H2O

or

3 mol of CO2 is stoichiometrically equivalent to 4 mol of H2O

5 mol of O2 is stoichiometrically equivalent to 3 mol of CO2

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Consider the equation

4NH3(g) + 5O2(g) 4NO(g) + 6H2O(l)

This equation means that 4 mol NH3 (g) and 5 mol(g) react to form 4mol NO(g) and 6 mol H2O (l)

An amount of 6.48 mol of NH3(g) requires how many moles of O2(g)for complete reaction?

6.48 mol NH3(g) x 5 mol O2 = 8.10 mol O2(g)

4 mol NH3 A mass of 5.44 g O2(g) is sufficient to produce what mass of H2O(l)?

5.44 g O2(g) x 1 mol O2 x 6 mol H2O x 18.02 g H2O

32.00 g O2 5 mol O2 1 mol H2O

= 3.68 g H2O(l)

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QUIZ

Determine the stoichiometric equivalence of reactants andproducts in the following reactions:

1. 4Na(s) + O2(g) 2Na2O(s)

2. 2HCl(aq) + Fe(s) FeCl2(aq) + H2(g)

3. 2MnO4-(aq) + 10 Fe2+(aq) + 16 H+(aq)

permanganate ion ferrous ion

10Fe3+(aq) + 2Mn2+(aq) + 8H2O(l)

ferric ion manganous ion

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Sample Problem 2.2 Calculating Amounts of Reactants and Products

PROBLEM: In a lifetime, the average American uses 1750lb(794g) of copperin coins, plumbing, and wiring. Copper is obtained from sulfide

ores, such as chalcocite, or copper(I) sulfide, by a multistageprocess. After an initial grinding step, the first stage is to roast

the ore (heat it strongly with oxygen gas) to form powderedcopper(I) oxide and gaseous sulfur dioxide.

(a) How many moles of oxygen are required to roast 10.0mol of

copper(I) sulfide?(b) How many grams of sulfur dioxide are formed when 10.0molof copper(I) sulfide is roasted?

(c) How many kilograms of oxygen are required to form 2.86Kgof copper(I) oxide?

PLAN: write and balance equation

find mols O2 find mols SO2

find g SO2

find mols Cu2O

find mols O2 find kg O2

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SOLUTION:

Sample Problem 2.3 Calculating Amounts of Reactants and Products

continued

2Cu2S(s) + 3O2(g) 2Cu2O(s) + 2SO2(g)

3mol O2

2mol Cu2S

10.0 mol Cu2S x = 15.0 mol O2

10.0 mol Cu2S x

2 mol SO2

X2 mol Cu2S

x 64.07g SO2

mol SO2

= 641 g SO2

2.86 kg Cu2O x103 g Cu2O x

kg Cu2O

= 0.960 kg O2kg O2

103 g O2

mol Cu2O

143.10 g Cu2O= 20.0 mol Cu2O

20.0 mol Cu2O x3 mol O2 X

2 mol Cu2O

32.00 g O2 X

mol O2

(a)

(b)

(c)

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Sample Problem 2.4 Calculating Amounts of Reactants and Products in

a Reaction Sequence

PROBLEM: Roasting is the first step in extracting copper from chalcocite,

the ore used in the previous problem. In the next step, copper(I)oxide reacts with powdered carbon to yield copper metal andcarbon monoxide gas. Write a balanced overall equation for thetwo-step process.

PLAN:

write balanced equations for each step

cancel reactants and products common

to both sides of the equations

sum the equations

SOLUTION:

2Cu2S(s) + 3O2(g) 2Cu2O(s) + 2SO2(g)

Cu2O(s) + C(s) 2Cu(s) + CO(g)

2Cu2O(s) + 2C(s) 4Cu(s) + 2CO(g)

2Cu2S(s)+3O2(g)+2C(s) 4Cu(s)+2SO2(g)+2CO(g)

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Figure 2.3

Summary of the Mass-Mole-Number

Relationships in a Chemical Reaction

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QUIZ

If 10 grams of sodium chloride, NaCl is treatedwith excess silver nitrate, AgNO3, how muchsilver chloride is precipitated?(NaCl = 55.44)