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Course lecturer :

Jasmin Šutković

18th March 2015

Chemistry - SPRING 2015

Lecture 3:Chemical Reactions

Contents International University of Sarajevo

1. What is a Chemical Reaction ? 1.2 The mol and molar masses

2. Determining Empirical and Molecular formulas3. Chemical equations 4. Mass relationships in chemical equations5. Types of Chemical equations 6. Earths atmosphere

1. Chemical reactions

A process where a substance is converted to one or more other substances with different compositions and properties.

The MOLE

To analyze the transformations that occur between individual atoms or molecules in a chemical reaction, it is therefore absolutely essential for chemists to know how many atoms or molecules are contained in a measurable quantity in the laboratory.

The unit that provides this link is the mole (mol)!!

The MOLE cont...

Abbreviated - molThe mole is a unit of measurement used in chemistry to

express amounts of a chemical substance, defined as the amount of any substance that contains as many elementary entities (e.g., atoms, molecules, ions, electrons) as there are atoms in 12 grams of pure carbon-12 (12C), the isotope of carbon with atomic weight 12.

In 12g of C there is 6.022×1023 atoms of carbon,

This corresponds to "Avogadro constant" (6.022×1023 ) elementary entities of the substance.

1.2 Atomic, molecular and Molar masses

Atomic mass The mass number is approximately or relativeley equal to

the numerical value of the atomic mass (Ar = relative atomic mass )..check PSE !

Exp of atomic mass (Ar) : for H it is 1 or for O it is 16

Molecular mass Molecular mass or relative molecular mass represents the

sum of atomic masses in a compuond (the mass of the molecule).

2.1 Molecular mass (Mr)

The molecular mass is the mass of a molecule, sometimes shown as Mr

It is calculated by adding together the atomic masses of the elements in the substance, each multiplied by its subscript (written or implied) in the molecular formula. Expample 1 : Mr of H2O= 2(1)+ (16) = 18

Mr of a compound is the number that represents how much the mass of this coumpound is higher than the atomic mass unit (amu) =(1/12 mass of C12 isotope)

SO the unit for Mr is amu or u .

The procedure for calculating molecular masses is illustrated in Example 2

Find the molecular mass of table sugar (sucrose), which has a molecular formula C12H22O11

EXAMPLE 2

molecular mass(Mr) of C12H22O11

= 12(12.01) + 22(1.008) + 11(16.00) = 342.30

Empirical formula

Unlike molecules, which have covalent bonds, ionic compounds do not have a readily identifiable molecular unit.

So for ionic compounds we use the formula masses (also called the empirical formula masses) of the compound rather than the molecular mass.

The formula mass is the sum of the atomic masses of all the elements in the empirical formula, each multiplied by its subscript (written or implied).

It is directly analogous to the molecular mass of a covalent

compound.

Molar Mass

Molar mass is measured in grams/mole, and is the mass of one mole of any substance.

In each case, the number of grams in 1 mol is the same as the number of atomic mass units that describe the atomic mass, the molecular mass, or the formula mass, respectively.

Exp: Relative atomic mass if Natrium is 23 (Ar) and Molar mass of Natrium is 23 g/mol

Example ...

The molar mass of ethanol is the mass of ethanol (C2H5OH) that contains 6.022 x 1023 ethanol molecules. Using the procesure in Example 1 and 2 , the molecular mass (M) of ethanol would be 46.069 g/mol.

• How ..?

Because 1 mol of ethanol contains 2 mol of carbon atoms (2 × 12.011 g), 6 mol of hydrogen atoms (6 × 1.0079 g), and 1 mol of oxygen atoms (1 × 15.9994 g), its molar mass is 46.069 g/mol.

Flowchart for convertingbetween units.

2. Determining molecular and empirical formulas

When a new chemical compound, such as a potential new pharmaceutical, is synthesized in the laboratory or isolated from a natural source, chemists determine its elemental composition, its empirical formula, and its structure to understand its properties.

In this section, we focus on how to determine the empirical formula of a compound .

Laws and order in Chemical binding

Dalton's Atomic Theory - Dalton Law of Conservation of Mass - Lavosier Law of Definite Proportion – J.Proust Law of Multiple Proportion - Dalton's

Law

http://sciencepark.etacude.com/chemistry/law.php

Calculating mass percanteges

The law of definite proportions states that a chemical compound always contains the same proportion of elements by mass; that is, the percent composition — the percentage of each element present in a pure substance — is constant (although we now know there are exceptions to this law).

Calculating mass percanteges cont...

For example, today we know that sucrose (cane sugar) is 42.11% carbon, 6.48%hydrogen, and 51.41% oxygen by mass. This means that 100.00 g of sucrose always contains 42.11 g of carbon, 6.48 g of hydrogen, and 51.41 g of oxygen.

How to prove this ?

First we will use the molecular formula of sucrose (C12H22O11) to calculate the mass percentage of the component elements; then we will show how mass percentages can be used to determine an empirical formula.

According to its molecular formula, each molecule of sucrose contains 12 carbon atoms, 22 hydrogen atoms, and 11 oxygen

atoms.

We can then use these masses to calculate the percent composition of sucrose. To three decimal places, the calculations are the following:

Sucrose example cont...

Sum = 342.297g = 342.297 g/mol = Molar Mass (M)

The mass percentage of each element in sucrose is the mass of the element present in 1 mol of sucrose divided by the molar mass of sucrose, multiplied by 100 to give a percentage. The result is shown to two decimal places:

Sucrose example cont...

You can check your work by verifying that the sum of the percentages of all the elements in the compound is 100%:

42.12% + 6.48% + 51.41% = 100.01%

If the sum is not 100%, you have made an error in your calculations. (Rounding to the correct number of decimal places can, however, cause the total to be slightly di erent from 100%.) ff

Thus 100.00g of sucrose contains 42.12 g of carbon, 6.48g of hydrogen, and 51.41 g of oxygen; to two decimal places, the percent composition of sucrose is indeed 42.12% carbon, 6.48% hydrogen, and 51.41% oxygen.

Percentage = Compositions

Empirical formula of Penicilin

Antibiotics are chemical compounds that selectively kill microorganisms, many of which cause diseases.

Although we may take antibiotics for granted today, penicillin was discovered only about 80 years ago. The discovery of penicillin is a historical detective story in which the use of mass percentages to determine empirical formulas played a key role.

In 1928, Alexander Fleming discovered penicilin…

Story of Penicilin ….

It was Alexander Fleming at St Mary’s Hospital Medical School who noticed a mould growing on a culture plate of bacteria one day in September 1928. Around the Penicillium notatum mould was a clear area where the colonies of bacteria appeared to have been dissolved.

Fleming set about establishing the identity of the substance that was killing the bacteria, giving it the name ‘penicillin’

Penicillum

Penicilin empirical formula cont...

Although Fleming was unable to isolate penicillin in pure form, the medical importance of his discovery stimulated researchers in other laboratories.

Finally, in 1940, two chemists at Oxford University, Howard Florey and Ernst Chain isolated penicilin and its use greatly increased the survival rate of wounded soldiers in World War II.

As a result of their work, Fleming, Florey, and Chain shared the Nobel Prize in Medicine in 1945

They discovered that a typical sample of penicillin G contains 53.9% carbon, 4.8% hydrogen, 7.9% nitrogen, 9.0% sulfur, and 6.5% sodium by mass. The sum of these numbers is only 82.1%, rather than 100.0%, which implies that there must be one or more additional elements.

A reasonable candidate is oxygen, which is a common component of compounds that contain carbon and hydrogen! Hovewre for techniqual reasons it is difficult to analyze the oxygen directly. If we assume that all the missing mass is due to oxygen, then penicillin G contains (100.0% - 82.1%) = 17.9% oxygen. From these mass percentages, the empirical formula and eventually the molecular formula of the compound can be determined.

Penicilin empirical formula cont...

The Way...

In order to determine the empirical formula we need to convert the mass percentages to relative number of atoms.

We assume that we deal with 100g of compounds ...

100-82.1 = 17,9

The way...cont ....

But ..moles are not whole number ration that we need for empirical formula ...

The empirical formula expresses the relative numbers of atoms in the smallest whole numbers possible. To obtain whole numbers, we do it as follows :

C16H17N2NaO4S

Penicilin Empirical formula

Combustion analysis

Analytical procedure where we determine the composition of an unknown hydrocarbon . This is done in an OXYGEN atmosphere by one of several possible methods.

STEPS IN OBTAINING AND EMPIRICAL FORMULA VIA COMBUSTION ANALYSIS

Book page number 156

Determine the empirical formula of naphthalene ?

Complete combustion of a 20.10 mg sample of naphthalene in oxygen yielded 69.00 mg of CO2 and 11.30 mg of HO.

Given: mass of sample and mass of combustion products

Asked for: empirical formula

Example shown in page 157 !!!

From Empirical to Molecular Formula

Empirical formula = smallest possible ratio –shortes representation of a chemical formula

For covalent compound we usually use Molecular formula, giving the reall number of atoms in a compound.

Without additional info it is not easy to detemine wether a compound is shown via empirical or molecular formula .

Exp :Pinicilin C16H17N2NaO4S or C32H34N4Na2O8S2

Example with Glucose ...

Combustion analysis report that Glucose contains 39.68% carbon and 6.58% hydrogen ..where is O ? Combustion analysis occurs in Oxygen atmosphere ..it can not calculate O % !!

If we assume that the remaining percentage is due to oxygen, then glucose would contain 53.79% oxygen

A 100.0 g sample of glucose would therefore contain 39.68 g of carbon, 6.58 g of hydrogen, and 53.79 g of oxygen. To calculate the number of moles of each element in the 100.0 g sample, we divide the mass of each element by its molar mass (M)

Example with Glucose cont....

The oxygen : carbon ratio is 1.018, or approximately 1, and the hydrogen : carbon ratio is approximately 2. The empirical formula of glucose is therefore CHO2, but what is its molecular formula?

Example with Glucose cont....

Each glucose contains 6 CH2O formula units, which gives a molecular formula for glucose of (CH2O) x 6, which is more commonly written as

C6H12O6

3.Chemical equations

We can describe chemical reaction as chemical equations, an expression that gives the identities and quantities of the substances in a chemical reaction.

Chemical formulas and other symbols are used to indicate the starting material(s), or reactant(s), which by convention are written on the left side of the equation, and the final compound(s), or product(s), which are written on the right. An arrow points from the reactant to the products:

Example book page 165

How to interpret a chemical equation ?

Ammonium Dichromate Volcano

Balancing simple chemical equations

When a chemist faces a new chemical reaction it is not balanced not he/she knows whether what is a reactant or product!

Exp: combustion of n-heptane (C7H16)

Equation is not balanced!!!

Steps in Balancing a C.Equation

The following method is used , called “inspection,” or better known as trial and error.

n-heptane balancing sample

4.Mass relationships in chemical equations Stoichiometry is a collective term for the quantitative relationships

between the masses, the numbers of moles, and the numbers of particles (atoms, molecules, and ions) of the reactants and the products in a balanced chemical equation.

A stoichiometric quantity is the amount of product or reactant specified by the coe cients in affi balanced chemical equation

Why Stoichiometry? TO ANSWER problems such as:

How much oxygen is needed to ensure complete combustion of a given amount of isooctane?

How many grams of pure gold can be obtained from a ton of lowgrade gold ore? If an industrial plant must produce a certain number of tons of sulfuric acid per

week, how much elemental sulfur must arrive by rail each week?

Example ...again Glucose ...

Limiting reactants problem

The reactant that restricts the amount of product obtained is called the limiting reactant. Example:

5. Clasifying Chemical reactions

Oxidation-Reduction reactions

• The term oxidation was first used to describe reactions in which metals react with oxygen in air to produce metal oxides.

– Metal acquires a positive charge by transferring electrons to the neutral oxygen atoms of an oxygen molecule.

– Oxygen atoms acquire a negative charge and form oxide ions (O2-).

– Metals lose electrons to oxygen and have been

oxidized—oxidation is the loss of electrons.

– Oxygen atoms have gained electrons and have been reduced—reduction is the gain of electrons.

· Oxidation and reduction reactions are now characterized by a change in the oxidation states of one or more elements in the reactants.

· Oxidation states of each atom in a compound is the charge that atom would have if all of its bonding electrons were transferred to the atom with the greater attraction for electrons. Atoms in their elemental form are assigned an oxidation state of zero.

· Oxidation-reduction reactions are called redox reactions, in which there is a net transfer of electrons from one reactant to another. The total number of electrons lost must equal the total number of electrons gained.

Oxidation-Reduction reactions cont..

Oxidation-Reduction reactions cont..

Rules for assigning oxidation states

1. The sum of the oxidation states of all the atoms in a neutral molecule or ion must equal the charge on the molecule or ion.

2. The oxidation state of an atom in any pure element, whether monatomic, diatomic, or polyatomic, is zero.

3. The oxidation state of a monatomic ion is the same as its charge.

Oxidants and Reductants

Oxidants – Compounds that are capable of accepting electrons are called

oxidants, or oxidizing reagents, because they can oxidize other compounds.

– An oxidant is reduced in the process of accepting electrons.

Reductants – Compounds that are capable of donating electrons are called

reductants, or reducing agents, because they can cause the reduction of another compound.

– A reductant is oxidized in the process of donating electrons.

Oxidation-Reduction reactions cont..

Oxidants and Reductants

Relationships of oxidants and reductants are summarized in the following equation:

oxidant + reductant oxidation – reduction

gains e- loses e- redox reaction

is reduced is oxidized

Oxidation-Reduction reactions cont..

OXIDATION STATE (number)

In chapter 2 several examples are shown how chemical bonds are formed (how electrons are shared) like MgO,CaCl2 etc...

There are some rule that we may memorize to know the basic elements oxidation states such as Oxigen, Hydrigen, Chlor .

EXAMPLE (page 193)

In the following example we can apply those rules: reduction of copper I oxide

Condensation reactions

Condensation reactions have the general form: A + B AB

Some condensation reactions are redox reactions.

Reaction of an amine (-NH2) with a carboxylic acid(COOH) is a variant of a condensation reaction, in which –OH from the carboxylic acid group and –H from the amine group are eliminated as water. The reaction forms an amide or peptide bond, the essential structural unit of proteins and many polymers.

Example

The reaction of bromine with ethylene to give 1,2-dibromoethane, which is used in agriculture to kill nematodes in soil, is as follows:

This is a condensation reaction because it has the general form A + B => AB This reaction, however, can also be viewed as an oxidation–reduction reaction, in which electrons are transferred from carbon (-2 => -1) to bromine (0 => -1)

Is this a REDOX reaction ?

Catalysts

A substance that speeds up a chemical reaction, but is not consumed by the reaction

Catalysts are classified as 1. homogeneous — uniformly dispersed throughout the

reaction mixture to form a solution(for exp : Sulfuric acid)

2. heterogeneous — in a different physical state from the reactants (iron oxides)

6. The Earths atmosphere

The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity.

The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night.

Layers of the atmosphere

Troposphere

– Lowest layer of the atmosphere extending from Earth’s surface to an altitude of 11–13 km (7–8 miles)

– Temperature of troposphere decreases steadily with increasing altitude. Hot air rises, so this temperature gradient leads to continuous mixing of the upper and lower regions within the layer, which produces fluctuations in temperature and precipitation, called weather.

Stratosphere

– Lies above the troposphere and extends from an altitude of 13 km (8 miles) to 44 km (27 miles)

– In the stratosphere, ultraviolet light reacts with O2 molecules to form atomic oxygen, which then reacts with an O2 molecule to produce ozone, O3

–Ozone molecules make up the ozone layer, which acts as a protective screen that absorbs ultraviolet light that

would otherwise reach the surface of the earth

– Protects the earth’s surface from the sun’s harmful effects

Layers of the atmosphere cont...

Ozone hole

Organic compounds that contain chlorine and fluorine (chlorofluorocarbons, CFCs) are capable of reaching the stratosphere, where they react with ultraviolet light to generate chlorine atoms and other chlorine-containing species that catalyze the conversion of ozone to O2, thereby decreasing the amount of ozone in the stratosphere.

Chlorofluorocarbon gases (CFCs) eat the ozone layer alive where we use it everyday for industrial processes although the ozone layer can repair Itself.

Replacing chlorofluoro-carbons with hydrochlorofluoro-carbons (HCFCs) or with hydrofluorocarbons (HFCs) has been developed to minimize further damage to the earth’s ozone layer.

The Ozone hole sample...

Readings....

Book pages :135 -215 ...

Good luck ...

Dont forget tutorials at Friday