The platinum electrode is made of a small square of platinum foil which is platinized (known as platinum black). Hydrogen gas, at a pressure of 1 atmosphere, is bubbled around the platinum electrode. The platinum black serves as a large surface area for the reaction to take place, and the stream of hydrogen keeps the solution saturated at the electrode site with respect to the gas. It is interesting to note that even though the SHE is the universal reference standard, it exists only as a theoretical electrode which scientists use as the definition of an arbitrary reference electrode with a half-cell potential of 0.00 volts.

(Because half-cell potentials cannot be measured, this is the perfect electrode to allow scientists to perform theoretical research calculations.) The reason this electrode cannot be manufactured is due to the fact that no solution can be prepared that yields a hydrogen ion activity of 1.00M.

Hydrogen electrode is made by adding platinum black to platinum wire or a platinum plate. It is immersed in the test solution and an electric charge is applied to the solution and platinum black with hydrogen gas. The hydrogen-electrode method is a standard among the various methods for measuring pH. The values derived using other methods become trustworthy only when they match those measured using hydrogen electrode method. However, this method is not appropriate for daily use because of the effort and expense involved, with the inconvenience of handling hydrogen gas and great influence of highly oxidizing or reducing substances in the test solution.

E⁰ (H₂/H⁺) = 0 = E⁰ (H⁺/H₂)

Now the other half cell can be divided into two categories:

Þ            one which will act as anode

Þ            the others which will acts as cathode

Each type of cell arrangement will give an EMF value which will be actually the EMF value of unknown electrodes as EMF value of SHE is ‘0’ volts.

Example

Cu electrode (half cell) acts as cathode with SHE i.e., as Cu²⁺/Cu [Cu²⁺ (1.0 M) + 2e --> Cu]. The experimental measurement of EMF value for this cell arrangement give 0.34 volts. Since Cu electrode shows reduction with SHE, the given value of EMF represents the reduction potential of Cu half-cell.

E⁰ (Cu²⁺/Cu) = + 0.34 volts

The oxidation potential of Cu half-cell is just the negative of this value.

E⁰ (Cu/Cu²⁺) = – 0.34 volts

Þ      simple calculation based on mole concept (Gravimetric Analysis)

Þ      complex calculation based on volume of solutions and their concentrations (Volumetric analysis)

ü        While studying gravimetric analysis, we have three types of relationships in a chemical reaction.

Þ      Calculations involving mass – mass relations

Þ      Calculation involving mass – volume relations

Þ      Calculation involving volume – volume relations

Mass – Mass Relationship

ü        Following steps are followed while making necessary calculations

Þ      Write down balanced molecular equation for the chemical change

Þ      Write down the number of moles below the formula of each of the reactant and product

Þ      Write down the relative masses of the reactants and the products with the help of formula below the respective formula. These are the theoretical amounts of reactants and products.

Þ      By the applications of unitary method, mole concept or proportionality method, the unknown factor or factors can be determined.

Illustrations

1. How many gms of oxygen are required to burn completely 570 gm of octane.

Now balancing the chemical equation, we get

Then let us understand what this reaction means.

Does it mean,

Well, actually, the reaction does not mean either of the two. The first interpretation is wrong because the numbers (stoichiometric coefficients) appearing before substances in a reaction are not the number of atoms or molecules. If it were so, what do you think would be the interpretation of this reaction.

Limiting Reagent

ü        The reagent that gives the least no of moles of the product is the limiting reagent.

Illustrations

1.      1g of Mg is burnt in a closed vessel which contain 0.5g of O₂.

i)    Which reactant is left in excess?

ii)      Find the mass of excess reactant?

Percent Yield

Generally when a reaction is carried out in the laboratory we do not obtain the theoretical amount of the product. The amount of the product that is actually obtained is called the actual yield. Knowing the actual yield and theoretical yield the percentage yield can be calculated as

Volumetric analysis is done with the help of titrations. Suppose, we have a solution of unknown strength of a strong acid. We know that strong acids react with strong bases to give salts. We can prepare a standard solution (i.e. a solution of known strength) of base. Now a fix volume of solution is taken and base is slowly added to acid, in presence on an indicator (Phenolphthalein). After addition of a specific amount of base we find that pink colour appears in the reaction mixture which indicates that solution is completely neutralised.

In terms of m-equivalents same no. of (m-eq.) of reactants react and same no. of m-equivalent of product are formed. This is basic principle involved in volumetric analysis.

For the above reaction

Important

Þ    Acidic salts react with acid as well as base.

Þ      Neutral salts react with neither acid nor base.

Þ    Salts of strong acid and strong base do not react  with base.

Þ    Metal Oxide normally reacts with acid & non metal oxide reacts with base.

Þ    Metal normally reacts with acid and not with base at normal temperature.

or

When oxidation number of an atom increases in a reaction, it is said to be oxidised.

Oxidation Number / Oxidation State

The real or hypothetical charge present over an element is called oxidation number. Whereas oxidation state defines charge on one atom but oxidation number refers to charge present on all atoms of one element in a compound.

Certain features of oxidation number

• The pure oxidation number is always an integer, but the mathematical average may be in fraction.
• Oxidation number may be positive as well as negative.
• Oxidation number of I(A) group elements is +1, II(A) group element +2, and in III(A) group Al & B have +3 oxidation number and rest are variable.
• The molecules which exist in free state, always have zero oxidation number (NH₃, H₂O etc).
• Oxidation state of hydrogen is always +1, but when hydrogen is directly attached to metal (metal hydride) then its oxidation no. is always –1.
• The oxidation No. of oxygen is –2 but in peroxide compounds its oxidation No. is –1.
• Oxidation No. of oxygen is +ve when it is directly attached to fluorine.
• ^· In superoxide compounds oxidation number of oxygen  –¹/₂.

• The sum of oxidation No. in neutral species is zero.

• In halogens oxidation number of F is ¬–1 because F has maximum electronegative value in periodic table.

Let us now focus our attention on the calculation of the oxidation number

• Break a molecule into its atoms

• Now valencies of all the atom’s are added

• This is than equated to the total formal charge on the molecule

• The total formal charge on a neutral molecule is taken as zero. For a charged molecule, total formal charge is taken equal to charge on cation/anion.

Important
For finding out formal charge on an atom, hypothetically break all bonds to that atom. The e-pair of bond goes to more electronegative atom. After this exercise total charge left on central atom would be called the formal charge on that atom.

Illustrations

]]> http://smartlearnwebtv.com/iitjee/IITJEE/oxidation/feed/ 0 http://smartlearnwebtv.com/iitjee/IITJEE/mole-concepts-and-chemical-arithmetics/ http://smartlearnwebtv.com/iitjee/IITJEE/mole-concepts-and-chemical-arithmetics/#comments Wed, 21 Oct 2009 04:58:46 +0000 janani http://smartlearnwebtv.com/iitjee/?p=314 Introduction:

The branch of chemistry which deals with mass relationship in chemical reactions is called stoichiometry. Stoichiometry is the quantitative analysis of various types of chemical reactions. Most of these calculations are done on the basis of mole concept. The term ‘mole’ was first introduced by ‘Ostwald’. It is a Latin version of the term ‘heap’ or ‘pile’ or ‘weight’, which refers the amount of a substance containing a fixed number of its elementary particles equivalent to the Avogadro’s number                (6.023 × 10²³). In modern practice, it is easy to express the mole of substance in terms of its weight or its volume. The analysis based on weight is called Gravimetric analysis whereas the analysis based on volume is known as volumetric analysis.
Core Concepts
Equivalent weight
The minimum weight of any chemical species, which reacts (completely) or liberates 1 g hydrogen (11.2 litre), 8 g Oxygen (5.6 litre), 35.5 g Chlorine (11.2 litre), 80 g Bromine (11.2 litre), 127 g Iodine (11.2 litre) is called Equivalent weight of that particular chemical species.

The above definition for equivalent weight is not sufficient. For example if an acid is given then the equivalent weight of an acid is defined as the ratio of Molecular weight of acid to its basicity.
Basicity means number of acidic hydrogens present in the molecule.
Acidic hydrogen means hydrogen atoms directly attached to electronegative element.