Understanding Chemical Changes and Thermodynamics

Posted on Dec 7, 2024 in Chemistry

Understanding Chemical Changes

One change is the transformation of a system over time. Type: Physical change: No change of the material (mechanical, electricity, magnetism, chemical, …). Change: This involves a modified form and has associated energy changes. He studied chemistry and thermodynamics (thermochemistry). The matters that become known as the reagents and obtained products. The change is the process or chemical reaction.

Chemical Laws

Chemical rearrangement of atoms = Change = link restructuring laws of chemistry:

  • Law of Conservation of Mass or Lavoisier (1785): He established the law of conservation of mass: In any chemical change, the mass is conserved.
  • Law of Definite Proportions or Proust (1801): When two elements combine to give a compound, they do so in a fixed mass ratio.
  • Law of Multiple Proportions or Dalton (1803): If one or more chemicals react with a fixed amount of another substance, they do so in a simple ratio of integers.
  • Gay-Lussac’s Law (1808): The volumes of gaseous substances involved in chemical changes (measured under the same conditions of pressure and temperature) are in a ratio of integers.
  • Avogadro’s Hypothesis: Under the same conditions of temperature and pressure, equal volumes of any gas have the same number of molecules.

Types of Reactions

  • Synthesis Reactions: The combination of various reagents to give rise to a product.
  • Decomposition Reactions: From a single reagent and energy supply, it transforms into two or more substances.
  • Displacement or Substitution Reactions: A reaction between a compound and an element, where the element integrates into the compound and releases another item that was part of the initial compound.
  • Double Decomposition Reactions: Consists of the reaction between two compounds with dual exchange between cations and anions.
  • Regrouping Internal Reactions: Abundant, based on the change of a compound to another which has the same atoms but different structure.
  • Combustion Reactions of Organic Products: The organic products that are only C and H (hydrocarbons) or C, H, and O always yield carbon dioxide and water upon combustion.

Chemical Equations

A chemical change or chemical reaction is expressed by a chemical equation: reagents – products. The reagents are placed on the left and the products of the reaction on the right, separated by an arrow. If these chemicals are in aqueous solution, the reaction is symbolized by the subscript (aq). If in a liquid state or in solution, and as a result of the reaction, a solid precipitate is produced, it is symbolized in the chemical equation with a downward arrow. If the reagents are solid, liquid, or in solution and a gas product is obtained, it can be symbolized with an upward arrow. If the chemical equation is written as a whole or in ionic form, you must indicate the charge on each ion.

Equalization of Chemical Equations

An equation is balanced when the number of atoms of each element in the products equals the number of atoms in the reactants. To balance chemical equations, it is recommended to follow these steps:

  • Start with the atoms that are not the same, typically H or O.
  • Finally, balance the atoms of O.
  • For each substance, you can assign any number that is appropriate according to the reaction, keeping the numbers as small as possible.
  • If there are no numbers to assign to a substance, it means there is 1.
  • In the macroscopic world (moles), fractions can be used; in the microscopic world (molecules), only integers are allowed.

Stoichiometry

Stoichiometry is the quantitative study of chemical reactions.

Method Overview

Originally, reading the sentence is the most important part in solving a problem. This reading should allow us to distinguish two basic parts: what data we are given and what questions we are asking.

Limiting Reactant

The limiting reactant is the substance that is consumed first and determines the order of the reaction. The fuel is almost always the limiting reagent, as oxygen is abundant in the atmosphere.

Purity of Reagents

Most commercial reagents are not pure and are not 100% in the product.

Yield of the Reaction

In laboratory or industrial experimentation, theoretical calculations are carried out on the expected amount of product for the reaction.

Empirical and Molecular Formulas

  • Empirical Formula: It is the simplest relationship between the atoms of a molecular compound.
  • Molecular Formula: It is the actual number of atoms that form a compound.

Percent Composition

The percent composition of a compound is the number of grams of each element or group of items per 100 grams of the compound.

Isomerism

Isomerism is the phenomenon whereby, although the chemicals have the same molecular formula, they are different and have different properties.

  • Structural Isomerism: Compounds that have identical molecular formulas but differ in the order in which the atoms are linked.
  • Geometric Isomerism: Compounds that have the same formula but differ in the arrangement of atoms.
  • Positional Isomerism: Compounds that have the same formula but differ in the position of a functional group.

Thermodynamic Systems

Studied the changes of matter from an energy point of view. The concept of system: A certain amount of material isolated from the rest of the universe in order to study it. The changes that occur are called a system process.

Types of Systems

  • Open System: Enables the exchange of matter and energy with its surroundings.
  • Closed System: Does not allow the exchange of matter, but energy.
  • Isolated System: No matter nor energy exchanged with the environment.

Thermodynamic Variables

If we study any thermodynamic process, which involves changes of matter and energy, we need some figures that define the conditions given in an instant process. Types of variables include mass, refractive index, density, viscosity, volume, temperature, pressure, and concentration. State variables can locate exactly the thermodynamic state of the system.

Functions of State

The functions of state variables are studied in thermodynamic transformations (chemical or physical). The variation of a function of thermodynamic state in a process only depends on the values of this function at the beginning and end of the process, not the path followed.

Main Functions of State

  • Volume (V)
  • Pressure (P)
  • Temperature (T)
  • Internal Energy (U)
  • Enthalpy (H)
  • Entropy (S)
  • Gibbs Free Energy (G)

Heat and Work

In any chemical process, in a non-isolated system, there is an energy exchange with the external environment. This energy can be presented in different ways: mechanical, heat, electric, etc. The difference between them is evident in the conditions of transfer:

  • If there is a temperature difference between the system and the environment, energy can be transferred as heat (Q).
  • If there is no difference in the state of motion, the transfer may be in the form of work (W).

Heat

Heat depends on the temperature variation and mass.

Work

Work is the force applied to a body to perform a certain movement. In thermodynamics, this work is compression or expansion, defined as the energy transmitted to a system by applying an outside force.

Internal Energy and the First Principle of Thermodynamics

In all chemical reactions, the law of conservation of mass is upheld. Along with this law, the first principle of thermodynamics, which is the law of conservation of energy, is also upheld: the energy of a system is conserved. The energy of a system is the sum of all energies (kinetic, potential, vibrational, etc.) that the molecules comprise, known as internal energy (U). This energy is transferred as heat and work. It is not possible to measure the absolute value of internal energy; only its change can be measured (in joules (J) and kilojoules (KJ)).

Convention Signs

According to the IUPAC convention:

  • If the environment performs work on the system, it is positive since that energy is transferred to the system.
  • If the system performs work on the environment, it is negative since energy is transferred by the system.
  • If the system absorbs heat from the environment, this energy is transmitted to the system, and therefore, the system will have a positive sign.
  • If the system releases heat, it takes a negative sign since energy is transferred by the system.

Exothermic and Endothermic Processes

Processes that release heat are called exothermic. Processes that absorb heat are called endothermic.

Enthalpy of Formation

The variation of enthalpy of a reaction depends on the physical state of reactants and products, as well as the temperature and pressure conditions under which it occurs. To compare multiple variations of different reactions, reference conditions have been defined, known as standard conditions. The standard enthalpy of the elements in their usual stable state is defined as zero. The enthalpy change that occurs in the reaction of formation of one mole of a compound from the elements that constitute it in their usual stable state at standard temperature and pressure is called the standard enthalpy of formation.

Hess’s Law

Hess’s law states that if a chemical reaction can be expressed as an algebraic sum (sum or difference) of other partial chemical reactions, the enthalpy of that reaction, being a function of state, is equal to the algebraic sum of the enthalpies of the partial reactions.

Entropy

The universe tends toward a state of minimum energy. With this trend, chemical processes also occur: the spontaneity of a chemical reaction depends largely on the state of minimum energy of the system composed of reactants and products. In any reaction involving a different order, entropy (S) measures the degree of disorder of a system.

The Second Principle of Thermodynamics

Spontaneous changes in the universe tend toward a state of maximum disorder, i.e., maximal entropy.

The Third Principle of Thermodynamics

All ordered crystalline solids at absolute zero temperature have an entropy (S) of zero (S = 0).

Spontaneity of Reactions: Gibbs Free Energy

Some reactions occur spontaneously, while others require an energy supply to proceed. Gibbs found a mathematical relationship between entropy and enthalpy. Thus, for any process at constant pressure, the concept of free enthalpy or Gibbs free energy is defined as: G = H – TS, where S is the entropy of the system, T is the temperature in K, and H is the enthalpy of the system.

Free Enthalpy of Formation

The change in Gibbs free energy necessary to form a mole of a compound from its constituent elements in their standard state at standard pressure and temperature is called the free enthalpy of formation.

Chemical Kinetics

Reaction Rates: Determines the speed at which reactants are transformed into products. If the components of the reaction are gaseous, we refer to partial pressures rather than concentrations. The reaction rate is derived from the concentration of any reagent or product divided by its stoichiometric coefficient and converted into a positive number. The reaction is usually studied from the variation of concentrations of reagents.

Theories of Chemical Reactions

To explain how chemical reactions occur, scientists have developed two theories: the theory of collisions and the theory of transition states, which complement each other.

The Theory of Collisions

This theory assumes that all molecules have kinetic energy and move at enormous speeds, continuously colliding with each other. The effectiveness of a collision depends on two factors:

  • The energy of the molecules.
  • The orientation of the collisions.

Energy Distribution

The molecules of the reactants at a given temperature do not all have the same kinetic energy. At each temperature, we can make a distribution of the number of molecules that have a certain energy. At a given temperature, only those with sufficient energy will collide effectively, breaking bonds and leading to products. The required minimum energy is called activation energy.