DEFINITION
History of chemistry
Early potters found beautiful glazes to decorate and preserve their wares. Herdsmen, brewers and vintners used fermentation techniques to make cheese, beer and wine. Housewives leached the lye from wood ash to make soap. Smiths learned to combine copper and tin to make bronze. Crafters learned to make glass; leatherworkers tanned hides.
Modern chemistry
After the discovery by Ernest Rutherford and Niels Bohr of the atomic structure in 1912, and by Marie and Pierre Curie of radioactivity, scientists had to change their viewpoint on the nature of matter. The experience acquired by chemists was no longer pertinent to the study of the whole nature of matter but only to aspects related to the electron cloud surrounding the atomic nuclei and the movement of the latter in the electric field induced by the former (see Born-Oppenheimer approximation).
Importance of chemistry
The real importance of chemistry is that it serves as the interface to practically all of the other sciences, as well as to many other areas of human endeavour. For this reason, chemistry is often said (at least by chemists!) to be the "central science".
Chemistry can be "central" in a much more personal way, with a solid background in chemistry, you will find it far easier to migrate into other fields as your interests develop.
Chemistry can be "central" in a much more personal way, with a solid background in chemistry, you will find it far easier to migrate into other fields as your interests develop.
History of some scientists
Greatest scientists have not contributed to the world by demystifying only but by shaping also how we live in it with their ingenious inventions. From Sir Isaac Newton to Charles Darwin to Albert Einstein and many more brilliant minds, here is a group of famous scientists who have made major advances in the field of science. Antoine Lavoisier was the important scientist in the field of chemistry who invented various laws.
Define an Element
An element is a species of an atom which has the same number of protons in the atomic nuclei.
Examples: Any atom which has 3 protons in its atomic nuclei is Lithium (element).
There are 118 elements that have been identified. First 94 elements occur naturally on Earth while the remaining 24 are synthetic elements.
Examples: Any atom which has 3 protons in its atomic nuclei is Lithium (element).
There are 118 elements that have been identified. First 94 elements occur naturally on Earth while the remaining 24 are synthetic elements.
Units
The base units of length and volume are linked in the metric system. By definition, a liter is equal to the volume of a cube exactly 10 cm tall, 10 cm long, and 10 cm wide. Because the volume of this cube is 1000 cubic centimeters and a liter contains 1000 milliliters, 1 milliliter is equivalent to 1 cubic centimeter.
S.I units
International System of Units (SI) is the unit system adopted by the General Conference on Weights and Measures in 1960 and recommended for use in all scientific and technical fields. It consists of seven base units (meter, kilogram, second, ampere, kelvin, mole, candela) plus derived units and prefixes.
Length and Mass using NPL system
National Physical Laboratory of India (NPL), is the measurement standards laboratory of India. It maintains standards of SI units in India. Each modernized country, including India has a National Metrological Institute, which maintains the standards of measurements. This responsibility has been given to the National Physical Laboratory in India.
Kelvin, Celsius and Fahrenheit scale
Relation between Kelvin, Celsius and Fahrenheit scale is
Where C= temperature in Celsius scale.
F =temperature in Fahrenheit scale.
K =temperature in Kelvin scale.
let the temperature in celsius scale is 40.
Then temperature will be 104 in Fahrenheit scale and 313 in kelvin scale.
Where C= temperature in Celsius scale.
F =temperature in Fahrenheit scale.
K =temperature in Kelvin scale.
let the temperature in celsius scale is 40.
Then temperature will be 104 in Fahrenheit scale and 313 in kelvin scale.
Subsidiary unit
In most systems of units, a single unit is defined as a base unit for the description of a specified quantity. Other units are then defined as fractions and multiples of this base unit. These are called subsidiary units, e.g., the unit of area is , which is derived from the unit of length.
Rules for determining the number of significant figures
- All nonzero digits are significant.
- Zeros are also significant with two exceptions:
- zeros preceding the decimal point.
- zeros following the decimal point and preceding the first nonzero digit.
- Terminal zeros preceding the decimal point in amounts greater than one is an ambiguous case.
Accuracy
Accuracy refers to the closeness of a measured value to a standard or known value. For example, if in lab you obtain a weight measurement of 3.2 kg for a given substance, but the actual or known weight is 10 kg, then your measurement is not accurate. In this case your measurement is not close to the known value.
Precision
Precision refers to the closeness of two or more measurements to each other. Using the example above, if you weigh a given substance five times, and get 3.2 kg each time, then your measurement is very precise. Precision is independent of accuracy. You can be very precise but inaccurate, as described above. You can also be accurate but imprecise.
For example, if on average, your measurements for a given substance are close to the known value, but the measurements are far from each other, then you have accuracy without precision.
For example, if on average, your measurements for a given substance are close to the known value, but the measurements are far from each other, then you have accuracy without precision.
Law of conservation of mass
the law of conservation of mass states that mass in an isolated system is neither created nor destroyed by chemical reactions or physical transformations. According to the law of conservation of mass, the mass of the products in a chemical reaction must equal the mass of the reactants.
Law of constant proportions
The law of constant composition says that, in any particular chemical compound, all samples of that compound will be made up of the same elements in the same proportion or ratio. For example, any water molecule is always made up of two hydrogen atoms and one oxygen atom in a 2 : 1 ratio.
Use of Gay Lussac's Law
Q. Find the temperature in Celsius needed to change the pressure of liters of a gas that has a pressure of at to standard pressure. Standard pressure is .
Sol : First, we have to convert to Kelvin
From Gay Lussac's law, we know that
( at constant volume )
............. (1)
Here,
Putting the above values in equation (1), we will get,
In order to get temperature in celsius, we will have to subtract by ,
Sol : First, we have to convert to Kelvin
From Gay Lussac's law, we know that
( at constant volume )
............. (1)
Here,
Putting the above values in equation (1), we will get,
In order to get temperature in celsius, we will have to subtract by ,
Numerical on multiple proportions
For carbon dioxide, 2.66g of oxygen/1.00g of carbon 2.66. For carbon monoxide, 1.33 g of oxygen/1.00 g of carbon1.33. The two ratio are in the proportion 2.66/1.33 2:1.There for the ratio of masses of oxygen that combine with the same mass of carbon is 2:1 i.e. a simple ratio.
Avogadro's Law
Under the same conditions of temperature and pressure, equal volumes of all the gases contain the same number of molecules.
e.g If we assume that 1 litre of oxygen gas contains 'n' molecules of the gas then by Avogadro's law:
1. 1 litre of hydrogen will contain 'n' molecules of hydrogen.
2. 1 litre of nitrogen will contain 'n' molecules of nitrogen.
3. 1 litre of any gas will contain 'n' molecules of any gas.
e.g If we assume that 1 litre of oxygen gas contains 'n' molecules of the gas then by Avogadro's law:
1. 1 litre of hydrogen will contain 'n' molecules of hydrogen.
2. 1 litre of nitrogen will contain 'n' molecules of nitrogen.
3. 1 litre of any gas will contain 'n' molecules of any gas.
Dalton's atomic theory
According to Daltons atomic theory, all matter, whether an element, a compound or a mixture is composed of small particles called atoms. The postulates of this theory may be stated as follows:
(i) All matter is made of very tiny particles called atoms.
(ii) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
(iii) Atoms of a given element are identical in mass and chemical properties.
(iv) Atoms of different elements have different masses and chemical properties.
(v) Atoms combine in the ratio of small whole numbers to form compounds.
(vi) The relative number and kinds of atoms are constant in a given compound.
Limitations of Dalton's atomic theory :
(i) All matter is made of very tiny particles called atoms.
(ii) Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
(iii) Atoms of a given element are identical in mass and chemical properties.
(iv) Atoms of different elements have different masses and chemical properties.
(v) Atoms combine in the ratio of small whole numbers to form compounds.
(vi) The relative number and kinds of atoms are constant in a given compound.
Limitations of Dalton's atomic theory :
- Atoms of the same or different types have a strong tendency to combine together to form a new group of atoms. For example, hydrogen, nitrogen, oxygen gases exist in nature as group of two atoms. This indicates that the smallest unit capable of independent existence is not an atom, but a group of atoms.
- With the discovery of sub-atomic particles, e.g.,electrons, neutrons and protons, the atom can no longer be considered indivisible.
Mole
A mole of a substance is defined as the mass of a substance containing the same number of fundamental units as there are atoms in exactly 12 g of .
Mole in terms of volume:
One mole of all gaseous substances at 273 K and 1 atm pressure occupies a volume equal to 22.4 liter or 22,400 ml. The unit of molar volume is liter per mole or milliliter per mole.
Avogadro Number :
The number of particles present in 1 mole of any substance is fixed with a value of 6.023×10^23 This is known as Avogadro number or constant represented by No.
Mole in terms of volume:
One mole of all gaseous substances at 273 K and 1 atm pressure occupies a volume equal to 22.4 liter or 22,400 ml. The unit of molar volume is liter per mole or milliliter per mole.
Avogadro Number :
The number of particles present in 1 mole of any substance is fixed with a value of 6.023×10^23 This is known as Avogadro number or constant represented by No.
Steps to write empirical formula
1. Calculate the moles of an element by dividing the mass percent by the atomic weight.
2. Divide the moles obtained in the 1st step by the smallest quotient or the least value from among the values obtained for each element. This gives the simplest molar ratio.
3. Write the symbols of the various elements side by side. Insert the numerical value of the simplest whole number ratio of each element as obtained in step 3. at the lower right hand corner of each symbol.
2. Divide the moles obtained in the 1st step by the smallest quotient or the least value from among the values obtained for each element. This gives the simplest molar ratio.
3. Write the symbols of the various elements side by side. Insert the numerical value of the simplest whole number ratio of each element as obtained in step 3. at the lower right hand corner of each symbol.
Steps to write molecular formula
1. Identify the symbol of the cation (first part of the name) and the anion
2. Identify the valence or charge of each symbol and place it in parenthesis just above the symbol
3. Balance the total positive and negative charge on the cation and anion. You ask yourself do the total positive charge and total negative charge add up to zero. If the answer is no then we ask how many of each ion must we have in order to balance charge. We must have the same number of positive charges as we do of negative charges. Another way of saying that is that they must add up to zero.
4. Once you have determined the number of units of the cation and anion those become the subscripts which are placed right after the respective symbol.
2. Identify the valence or charge of each symbol and place it in parenthesis just above the symbol
3. Balance the total positive and negative charge on the cation and anion. You ask yourself do the total positive charge and total negative charge add up to zero. If the answer is no then we ask how many of each ion must we have in order to balance charge. We must have the same number of positive charges as we do of negative charges. Another way of saying that is that they must add up to zero.
4. Once you have determined the number of units of the cation and anion those become the subscripts which are placed right after the respective symbol.
Determine molecular formula of a compound
If empirical formula is given and the molecular mass of the compound is provided, then calculate the empirical mass of the given compound. Divide the molecular mass with the empirical mass. A number is obtained and multiply this number by the empirical formula. This gives the molecular formula.
Stoichiometry of a chemical reaction
One molecule of methane reacts with two molecules of oxygen gas to yield one molecule of carbon dioxide and two molecules of water. Stoichiometry measures quantitative relationships, and is used to determine the amount of products/reactants that are produced/needed in a given reaction.
Stoichiometric coefficients
In a balanced reaction, both sides of the equation have the same number of elements. The stoichiometric coefficient is the number written in front of atoms, ion and molecules in a chemical reaction to balance the number of each element on both the reactant and product sides of the equation.
Stoichiometry of reaction in solution
On occasion, a liquid reactant may be used and the mass is not given. Instead, the volume of the liquid is given as the starting quantity. Be careful with this as 22.414 L/mol cannot be used since that is only useful for gases. If lucky, the density of the liquid will be given in the problem. If not, then it must be found in literature. Using the density formula, the mass of the substance can be found (mass equals volume multiplied by density) and from there, the moles of the substance can be found. One last reminder: densities are given in g/mL.
Mass percent or weight percent
To calculate percent by mass, you need to determine two things: the mass of just the element, and the molar mass of the whole compound. Then, you take the molar mass of just the element and divide it by the molar mass of the whole compound, and multiply by 100%.
Limiting reagent
The limiting reagent is the reactant that is completely used up in a reaction, and thus determines when the reaction stops. From the reaction stoichiometry, the exact amount of reactant needed to react with another element can be calculated. If the reactants are not mixed in the correct stoichiometric proportions (as indicated by the balanced chemical equation), then one of the reactants will be entirely consumed while another will be left over. The limiting reagent is the one that is totally consumed; it limits the reaction from continuing because there is none left to react with the in-excess reactant.
0 Comments