CONSTANTE DE AVOGADRO: IMPORTÂNCIA E METODOLOGIAS DE DETERMINAÇÃO ATRAVÉS DO TEMPO
Data
2022-06-22
Autores
Muniz, Giuliana Assunção 
Orientadores
Giongo, Carolina Vautier Teixeira
Tipo
Trabalho de conclusão de curso
Título da Revista
ISSN da Revista
Título de Volume
Resumo
A Constante de Avogadro (NA) é uma das constantes fundamentais mais importantes, em especial para as Ciências Químicas, já que além de fornecer um balanço detalhado das reações, estabelece a importante conexão entre constantes macroscópicas e propriedades microscópicas da matéria. Apesar da incontestável relevância e de sua utilização rotineira e extensiva pelos químicos, os cursos de graduação em Química não têm dado atenção às diferentes estratégias experimentais empregadas na obtenção desta constante, bem como às evoluções na precisão de sua determinação. Dentro deste contexto, o presente Trabalho apresenta, em uma espiral do tempo, os diferentes métodos por meio dos quais a constante de Avogadro vem sendo determinada, desde a sua primeira estimativa, feita em 1865 por Loschmidt, usando conceitos da Teoria Cinética dos Gases, às determinações mais recentes e altamente precisas, empregando interferometria de raios-X de monocristais esféricos de alta pureza, feitos com o isótopo de silício 28, discutindo cada um dos conceitos e abordando a importância da constante de Avogadro no contexto histórico para definição do mol, bem como sua importância atual na nova definição do quilograma, com base na Constante de Planck (h), que entrou em vigor em maio de 2019. Foram revisitados em laboratório métodos distintos para a obtenção da constante de Avogadro: (i) determinação a partir de processos eletroquímicos (eletrólise e eletrodeposição) e (ii) determinação a partir da formação de filmes monomoleculares anfifílicos na superfície da água. Adicionalmente, os dados originais referentes ao estudo do equilíbrio de sedimentação em dispersões coloidais e ao monitoramento do movimento browniano de partículas esféricas dispersas em água foram aqui tratados e empregados no recálculo da Constante de Avogadro, seguindo os métodos de tratamento proposto e publicado em 1909 por Jean Perrin. Os valores de NA obtidos neste trabalho são da ordem de grandeza do valor esperado para esta constante.
Avogadro's Constant (NA) is one of the most important fundamental constants, especially for the Chemical Sciences. In addition to providing a detailed balance of reactions, it establishes the connection between macroscopic constants and microscopic properties of matter. Despite its relevance and extensive use by chemists, the Chemistry courses have not paid attention to the different experimental strategies used to obtain this constant, as well as the evolutions in the precision of its determination. In this context, this work presents, in a timeline, the different methods by which the Avogadro constant has been determined, since its first estimate, in 1865 by Loschmidt, using concepts from the Kinetic Theory of Gases, until the most recent determination, highly accurate, using X-ray interferometry of high purity spherical single crystals of silicon isotope 28, discussing each concepts and addressing the importance of the Avogadro constant in the historical context for the definition of the mole, as well as its current importance in the new definition of the kilogram, based on the Planck Constant (h), that started in May 2019. Three different methods were revisited in the laboratory to obtain the Avogadro constant: (i) determination from electrochemical processes (electrolysis and electrodeposition) and (ii) determination from the formation of an amphiphilic monomolecular film on the water surface. Additionally, the original data obtained by the study of the sedimentation of colloidal dispersions and that of the monitoring of the Brownian motion of spherical particles dispersed in water were here treated and used in the recalculation of the Avogadro Constant, following the treating methods proposed and published by Jean Perri in 1909. The NA values obtained in this work are of the order of magnitude of the expected value for this constant.
Avogadro's Constant (NA) is one of the most important fundamental constants, especially for the Chemical Sciences. In addition to providing a detailed balance of reactions, it establishes the connection between macroscopic constants and microscopic properties of matter. Despite its relevance and extensive use by chemists, the Chemistry courses have not paid attention to the different experimental strategies used to obtain this constant, as well as the evolutions in the precision of its determination. In this context, this work presents, in a timeline, the different methods by which the Avogadro constant has been determined, since its first estimate, in 1865 by Loschmidt, using concepts from the Kinetic Theory of Gases, until the most recent determination, highly accurate, using X-ray interferometry of high purity spherical single crystals of silicon isotope 28, discussing each concepts and addressing the importance of the Avogadro constant in the historical context for the definition of the mole, as well as its current importance in the new definition of the kilogram, based on the Planck Constant (h), that started in May 2019. Three different methods were revisited in the laboratory to obtain the Avogadro constant: (i) determination from electrochemical processes (electrolysis and electrodeposition) and (ii) determination from the formation of an amphiphilic monomolecular film on the water surface. Additionally, the original data obtained by the study of the sedimentation of colloidal dispersions and that of the monitoring of the Brownian motion of spherical particles dispersed in water were here treated and used in the recalculation of the Avogadro Constant, following the treating methods proposed and published by Jean Perri in 1909. The NA values obtained in this work are of the order of magnitude of the expected value for this constant.