Transcript
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
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