Chemical reactions

CHEMICAL REACTIONSDr Sharipah Ruzaina Syed Aris

Empirical and Molecular FormulaBalancing Chemical Reactions

Types of Chemical Reactions

Empirical and Molecular Formulas

Empirical Formula -

Molecular Formula -

The simplest formula for a compound that agrees with the elemental analysis and gives rise to the smallest set of whole numbers of atoms.

The formula of the compound as it exists, it may be a multiple of the empirical formula.

The empirical formula mass of a compound refers to the sum of the atomic masses of the elements present in the empirical formula.

The Molecular Mass (formula mass, formula weight or molecular weight) of a compound is a multiple of the empirical formula mass.     MM = n x empirical formula mass

Empirical Formula can be calculated from the percentage (or percent) composition of a compound.

mass(g) of each element

Sample Problem 3.4 Determining the Empirical Formula from Masses of Elements

PROBLEM:

PLAN:

SOLUTION:

amount(mol) of each element

empirical formula

Elemental analysis of a sample of an ionic compound showed 2.82 g of Na, 4.35 g of Cl, and 7.83 g of O. What are the empirical formula and name of the compound?

preliminary formula

change to integer subscripts

use # of moles as subscripts

divide by M(g/mol)

Once we find the relative number of moles of each element, we can divide by the lowest mol amount to find the relative mol ratios (empirical formula).

2.82 g Namol Na

22.99 g Na= 0.123 mol Na

4.35 g Clmol Cl

35.45 g Cl= 0.123 mol Cl

7.83 g Omol O

16.00 g O= 0.489 mol O

Na1 Cl1 O3.98 NaClO4Na1 Cl1 O3.98 NaClO4

NaClO4 is sodium perchlorate.

assume 100g lactic acid and find the mass of each element

Sample Problem 3.5 Determining a Molecular Formula from Elemental Analysis and Molar Mass

PROBLEM:

PLAN:

amount(mol) of each element

During physical activity. lactic acid (M=90.08 g/mol) forms in muscle tissue and is responsible for muscle soreness. Elemental anaylsis shows that this compound contains 40.0 mass% C, 6.71 mass% H, and 53.3 mass% O.

(a) Determine the empirical formula of lactic acid.

(b) Determine the molecular formula.

preliminary formula

empirical formula

divide each mass by mol mass(M)

molecular formulause # mols as

subscripts

convert to integer subscripts

divide mol mass by mass of empirical formula to get a

multiplier

Sample Problem 3.5 Determining a Molecular Formula from Elemental Analysis and Molar Mass

continued

SOLUTION: Assuming there are 100. g of lactic acid, the constituents are

40.0 g C 6.71 g H 53.3 g Omol C

12.01g C

mol H

1.008 g H

mol O

16.00 g O

3.33 mol C 6.66 mol H 3.33 mol O

C3.33 H6.66 O3.33

3.33 3.33 3.33CH2O empirical formula

mass of CH2O

molar mass of lactate 90.08 g

30.03 g3 C3H6O3 is the

molecular formula

EXERCISES

What is the empirical formula and molecular formula of a compound with a molar mass of 245.8g The composition is 19.53 C 2.44 H 13.02 O and 65.01 Br?

Calculate the simplest formulas for the compounds whose compositions are listed:

a) carbon, 15.8%; sulfur, 84.2%b) silver,70.1%; nitrogen,9.1%;

oxygen,20.8%c) K, 26.6%; Cr, 35.4%, O, 38.0%

The simplest formula for glucose is CH2O and its molar mass is 180 g/mol. What is its molecular formula?

translate the statement

balance the atoms

specify states of matter

adjust the coefficients

check the atom balance

SOME PRACTICE PROBLEMS:

1. __NaCl + __BeF2 → __NaF + __BeCl2

2. __FeCl3 + __Be3(PO4)2 → __BeCl2 +__FePO4

3. __AgNO3 + __LiOH → __AgOH + __LiNO3

4. __CH4 + __O2 → __CO2 + __H2O

5. __Mg + __Mn2O3 → __MgO + __Mn

TYPES OF CHEMICAL REACTIONS

DECOMPOSITION REACTIONS

In a decomposition reaction a single substance is broken down to form two or more simpler substrances.

reactant -------> product + product

Exercise:

CaCO3 (s) →

COMBINATION REACTIONS

Also called synthesis or addition reactions. Two or more substances react to form a

single substance. Exp: 2Mg (s) + O2 (g) → 2MgO (s) Exercises:

SO3 (g) + H2O (l) →

PCl3(l) + Cl2(g) →

2Cu + O2 →

DOUBLE REPLACEMENT REACTIONSTwo compounds switch places to form two new

compounds. Two reactants yield two products.

For example when silver nitrate combines with sodium chloride, two new compounds--silver chloride and sodium nitrate are formed because the sodium and silver switched places.

Exercise:

HCl (aq) + NaOH (aq) →

COMBUSTION REACTIONS

Combustion reactions are different in that they are characterized by the fact that one of the reactants is always oxygen.

A combustion reaction is a reaction of a substance with oxygen, usually with the rapid release of heat to produce a flame.

Organic compounds burn in oxygen to produce carbon dioxide and water vapor.

Exp: butane burning in air to produce carbon dioxide and water vapor.

2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(g)

FERMENTATION REACTION

Ethanol fermentation is a biological process in which sugars such as glucose, fructose, and sucrose are converted into cellular energy and thereby produce ethanol and carbon dioxide as metabolic waste products.

The chemical equation below summarizes the fermentation of glucose. One glucose molecule is converted into two ethanol molecules and two carbon dioxide molecules:

C6H12O6 → 2C2H5OH + 2CO2

all metal oxides soluble in water gives alkalies(bases)

CaO (s) + H2O (l) ↔ Ca(OH)2

Na2O + H2O → 2 NaOH

(MgO + H2O → Mg(OH)2),

All nonmetals form covalent oxides with oxygen, which react with water to form acids

N2O5 + H2O → 2HNO3.

Metal oxide reaction with water

Non-metal oxide reaction with water