Introduction to chemistry
1. Chemistry: definitions
Chemistry is the discipline that studies the composition, structure and transformations of matter .
His method of investigation is experimental or inductive . this method can be divided into the following phases:
- Observation of the phenomenon.
- Collection of experimental data.
- Hypothesis formulation.
- Critical processing of experimental data.
- Wording of the law.
- Application of the law and experimental verification of its validity.
2 . Definition of model in the scientific field
By model we mean the visualization of an invisible reality, therefore unknown, through one or more concrete images taken from everyday experience.
The model formulated must not be considered a miniature reproduction of a macroscopic reality ( eg a model of a ship ), but rather a conceptual tool used to interpret the behavior of things.
2.1 – Gas model:
A gas can be conceived according to the following model:
- Formed by very small corpuscles or particles , detached from each other and dispersed in space.
- The particles are equipped with movement without any relationship of mutual dependence; they frequently collide with each other and with the walls of the container.
- The forces between the particles are very modest.
- The actual volume of the particles is negligible compared to that of the container that contains the gas.
2.2 – Liquid model:
Liquids, intermediate between gases and solids, can be represented according to this model:
- Consisting of very small corpuscles or particles very close to each other but which do not occupy fixed positions; between one particle and another there are small gaps.
- Among the particles there are attractive forces of medium intensity which, while forcing them to remain in contact, allow them to slide over each other.
2.3 – Solid model:
A solid can be represented by the following model:
- Formed by very small corpuscles or particles that are very close to each other.
- Among the particles there are very intense attractive forces which, in addition to keeping them in close contact with each other, prevent any appreciable movement; however, it is admitted that they can vibrate around their equilibrium positions while occupying an almost fixed position.
- The particles can be distributed in space with regularity ( ordered or crystalline solids ) or without any rules ( disordered or amorphous solids ).
2.2 – Kinetic and corpuscular theory of matter:
According to this theory, all matter is made up of particles (atoms, molecules or ions); they are in constant motion and all collisions are perfectly elastic (elastic collision: there is no change in the total kinetic energy ).
3. The matter
3.1 – Definitions:
Matter is everything that is in the universe and has its own mass . A limited portion of matter is called the body .
3.2 – The phenomena:
Any event or transformation that affects a body is called a phenomenon. The phenomena can be divided into:
- Physical phenomena : those that affect the physical properties of the body such as mass, volume, color, state of aggregation, melting point, electrical conductivity.
- Chemical phenomena : those that lead to a change in the chemical composition of a body ( chemical reactions ).
3.2 – Simple substances:
Simple substances, commonly called elements , are chemical species that can no longer be broken down into even simpler substances. They are made up of atoms of the same type.
3.3 – The compounds:
They are chemical species made up of several elements and, therefore, of atoms of different types.
3.4 – The phases:
A body or a limited set of bodies subject to chemical-physical investigations is called a system . A portion of a system limited by physically defined surfaces and with a series of constant physical properties at each point is called a phase. A system is defined physically homogeneous when it is made up of a single phase ( eg air, wine, solutions ). A system is said to be physically heterogeneous when it consists of several phases separated from each other by well-defined surfaces, observable with the naked eye or under the microscope. A heterogeneous mixture between a solid and a liquid is called a suspension ( eg water – sand mixture). A heterogeneous mixture between two immiscible liquids is called an emulsion ( eg milk, water – oil mixture ).
4. The quantities
Physicists define magnitude as any being, introduced to describe a physical phenomenon, which is, in some way, measurable. The quantities are described with a number followed by their unit of measurement.
The quantities can be classified into:
- Fundamental quantities: those whose units of measurement are defined and fixed, by convention, by the International System ( SI ) are seven (see section ” Tables “).
- Derived quantities: they derive from the previous ones and are obtained from them through algebraic combinations, products or divisions. The most important, of interest to chemists and physicists, are listed in the ” Tables “ section
SI units of measurement are very often either too large or too small, so it is necessary to use their submultiples or their multiples , defined by multiplier prefixes. In some cases, units of measurement outside the SI are also used, often different from country to country.
Physical quantities, fundamental and derived, therefore define the physical properties of matter.
5. Properties of matter
5.1 – General:
The properties of matter and, therefore of substances, can be classified into:
Physical properties: these are the physical quantities already seen and depend on the substance itself. They can be divided into:
- Extensive properties : they depend on the extension of the sample, i.e. on the quantity of matter of the same (e.g. volume, mass, length).
- Intensive properties : they do not depend on the quantity, but on the type of matter (e.g. melting point, density).
- Chemical properties : mainly depend on the influence of other substances on the test substance (eg reactivity with oxygen, reactivity with water, reactivity with acids).
- Organoleptic properties : they can be perceived and evaluated by the sense organs (eg taste, smell, color).
5.2 – Length:
Fundamental quantity corresponding to the largest horizontal dimension. The SI unit of measurement is the meter ( m ).
5.3 – Volume:
It is the portion of space that a body occupies; varies according to temperature and pressure. This variation is not significant in solids and liquids since, even if they may undergo modest dilatations, they are to be considered incompressible. In gases, on the other hand, it is decisive as they can undergo considerable expansion or compression due to the effect of pressure and temperature.
Volume is a quantity derived from length and the SI unit of measurement is the cubic meter ( m 3 ). This unit is, however, too large for the chemist who commonly uses its submultiples. Among these the most used is the cubic decimeter ( dm 3 ) which corresponds to the volume occupied by one kg of H 2 O distilled at a temperature of 4 ° C.
In the laboratory, the cubic centimeter ( cm 3 ) is used more frequently .
When volumes of fluids (liquids and gases) have to be measured, non-SI units of measurement called capacity are commonly used ; the most important are the liter ( L ) which corresponds to 1 dm 3 and the milliliter ( mL ) which corresponds to 1 cm 3 . Therefore:
1 dm 3 = 1000 cm 3 = 1 L = 1000 mL; 1 cm 3 = 1 mL.
5.4 – Mass:
Mass is defined as the quantity of matter that makes up a body. It is a constant extensive property, in fact it does not vary as the position of the body in space varies and is independent of temperature and pressure. It is measured by comparison with a quantity of matter taken as a sample. The SI unit of measurement is the kilogram ( kg ). In the laboratory, its submultiples are commonly used, such as, for example, the gram (g), corresponding to 10 -3 kg and the milligram ( mg ), equal to 10 -6 kg.
5.5 – Strength:
The force ( F ) is a vector quantity that can be defined as the physical agent capable of modifying the state of motion or rest of a body; it is always applied from one material body to another and is characterized by an intensity and a direction along which it acts.
The SI unit for force is the newton ( N ); 1 N corresponds to the force capable of imparting an acceleration of 1 m / s 2 to a body of mass = 1 kg .
5.6 – Weight:
By weight ( P ) we mean the force with which a body is attracted to the center of the earth. Weight is, therefore, a force and is directly proportional to the mass of the body; the constant of proportionality is the gravitational attraction force ( g ) which, in a given place, is the same for all bodies. For a body located at sea level the value of g is 9.8 m / s 2 . The relationship to derive the weight is: P = m × g .
The unit of measurement of weight, as a force, is the newton ( N ). In practice, the kilogram-weight ( kg p ) is often used , that is the weight of a body having mass = 1 kg, placed at 45 ° latitude and at sea level; it follows that 1kg p = kg 9.8 m / s 2 = 9.8 m / s 2 and, consequently, 1kg p = 9.8N .
To illustrate, the weight of an object brought to the moon is reduced to 1/6 of the weight it had on the earth since the gravitational pull on the moon is 1/6 of that of the earth; obviously the mass of the object remains unchanged.
5.7 – Absolute density:
It is an intensive property of matter, ie independent of the extension of the sample, and expresses the mass of the unit of volume of a homogeneous body. In other words, it is given by the ratio between mass and volume. The density is indicated, in Italy, with the symbol d, even if the SI recommends the use of the Greek letter r ( rh o); the SI unit of measurement is the kilogram per cubic meter ( kg / m 3 ); the gram per cubic centimeter ( g / cm 3 ) is also commonly used .
The expression of the density is: d = m / v , from which V = m / d and m = v · d derives .
5.8 – Relative density:
It represents the ratio between the mass of a sample and the mass of an equal volume of distilled water at a temperature of 4 ° C. It is a dimensionless quantity, expressed, that is, by a pure number that represents the ratio between two quantities defined by the same unit of measurement. For example, the density of gold is = 19.3, i.e. gold has a density 19.3 times greater than that of distilled water at 4 ° C.
5.9 – Pressure:
The pressure ( P ) is defined as the force exerted on the unit of surface: P = force / surface .
The SI unit of pressure is the pascal ( Pa ) corresponding to 1N / m 2 , that is to say a force of 1 newton acting on an area of 1 m 2 . The pascal is a very small unit, so its multiples, the hectopascal (1ePa = 100 Pa) and the kilopascal (1kPa = 1000 Pa) are commonly used . Much used, especially in meteorology, is the bar which corresponds to 100000 Pa and the millibar ( mbar ), which corresponds to 100 Pa and, therefore, synonymous with hectopascals.
Traditionally different non-SI units of measurement are used in many fields, such as, for example, the atmosphere ( atm ), equal to the pressure exerted on 1cm 2 of surface, at sea level and on a clear day, by a column of air as high as the atmosphere. This value also corresponds to the pressure exerted on 1cm2 of surface by a column of mercury 760 mm high, at a temperature of 0 ° C.
It follows that 1atm = 760 mm Hg . 1atm then corresponds to 101325 Pa and, consequently, to 1013.25 ePa or mbar.
In honor of Evangelista Torricelli , the mm of mercury was called torr : 1mm Hg = 1 torr = 133.32 Pa .
Finally, remembering that 1Kg p = 9.8N , we have the relation:
5.10 – Energy:
Energy is the attitude of a body to do a job. It can be presented in six aspects: mechanical, thermal, radiant, electromagnetic, chemical, nuclear.
- Mechanical energy: it is a particular type of kinetic energy; ex. energy of a turbine, of a propeller.
- Thermal energy: it is that accumulated by bodies when they are heated without changes in the state of aggregation.
- Radiant energy: it is the energy associated with light or other electromagnetic radiation. It comes in the form of visible light, infrared or ultraviolet light, X-rays, microwaves, etc.
- Electromagnetic energy : energy that comes from the flow of electric current resulting from unbalanced electric forces.
- Chemical energy : energy contained in chemicals; chemical reactions allow this energy to be released and converted into other forms of energy, eg. light or heat.
- Nuclear energy : energy produced by nuclear fission or fusion reactions.
The energy is presented in three basic forms: kinetic energy, potential energy and mass energy .
Kinetic energy: it is the energy associated with the movement of bodies. A body of mass m moving with velocity v has a kinetic energy E c = 1/2 mv 2 .
Potential energy (E p ): it is, for example, that associated with the position of a body on which particular forces due to other bodies constantly act. Such forces are, for example, the gravitational force and the forces of an electrical nature. A particular type of ep is the chemical potential energy which represents the contribution to the total energy content due to interactions between particles.
Mass energy : it is the energy associated with the inertial mass of a body; it is expressed by Einstein’s equation E = mc 2 where c is the speed of light in vacuum ( ~ 3 × 10 8 m / s ) and m represents mass.
The SI unit of energy is the joule ( J ) which corresponds to the work done by the force of 1N when its point of application moves by 1m in the direction and in the direction of the force itself
( 1J = 1N × m ).
In practice using other units of measure, such as the calorie ( cal ), the kilocalorie ( kcal ) and
l ‘ electron volts ( eV ).
The conversion factors of the energy units of measurement are reported below:
|joule||6.242 × 10 18||0.2389||2.389 × 10 -4|
|electron volts||1.602 × 10 -19||3.827 × 10 -20||3,827 × 10 -23|
|calorie||4,186||2.613 × 10 19||0.001|
|kilocalorie||4186||2.613 × 10 22||1000|
By calorie we mean the amount of heat necessary to raise the temperature of one gram of distilled water from 14.5 to 15.5 ° C, at a pressure of 1 atm.
5.11 – Heat:
By heat we mean the thermal energy that passes between two bodies as a result of a temperature difference. If a body at a higher temperature and one at a lower temperature are placed in contact, the second receives heat given off by the first. This step is a transfer of thermal energy that was found in the hottest body in the form of potential energy .
In other words, heat always spreads from warmer areas or bodies to colder areas or bodies.
Unit of measurement of heat in the SI is the joule ( J ); as already known, calorie ( cal ) and kilocalorie ( kcal ) are often used .
Temperature is the measure of the intensity of heat.
The unit of measurement of temperature in the SI is the Kelvin ( K ) and its scale is called the absolute scale; the centigrade degree ( ° C ) of the centigrade scale is commonly used . Both units of measurement are equivalent ( 1K = 1 ° C ), but the zero of the absolute scale is set at -273.15 ° C and is called absolute zero. It follows that the zero of the centigrade scale is set at 273.15 K. The absolute temperatures are indicated with T while the centigrade ones with t .
If with the heat transfer there is a change of state, the heat transferred is called latent heat; depending on the change of state we have:
Latent heat of fusion: amount of heat necessary to make 1g of substance pass from solid to liquid state. For example, the latent heat of melting of water is equal to 80 cal / g:
Latent heat of vaporization or boiling: amount of heat necessary to make 1g of substance pass from the liquid to the vapor state. Eg. the latent heat of boiling water is 540 cal / g.
The amount of heat required by a given amount of substance during the transition is given by the relation:
Q = latent heat × mass in g of the substance.
By specific heat ( c ) of a substance we mean the amount of heat necessary to increase 1g of substance by 1 ° C and is expressed, in SI, in J / g ° C ; the cal / g ° C is also used .
By heat capacity ( C ) of a body we mean the amount of heat necessary to raise its temperature by 1 ° C; the expression is given by the product of the specific heat for the mass of the body ( C = c × m ) and is expressed with the same units of measurement of the specific heat.
In physical transformations where chemical reactions or changes of state do not take place, a body absorbs and releases heat according to the relationship Q = m × c × D t ( D t = final t – initial t ).
For example, the amount of heat needed to heat 500 g of iron ( average c = 0.115 cal / g ° C) from 50 ° C to 120 ° C is:
Q = 500 g × 0.115 cal / g ° C × 70 ° C
Q = 4025 cal.