Comunidade do segundo ano
Autor: Simone  | Publicado: 2013-08-10T16:15:00-07:00

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Fatores que alteram o equilíbrio químico



Quando se aplica uma força em um sistema em equilíbrio, ele tende a se
reajustar no sentido de diminuir os efeitos dessa força. 

Esse princípio foi dado por Le Chatelier em 1884 e é a partir dele que se
aplicam os fatores que afetam as condições de equilíbrio de um sistema:
concentração, pressão e temperatura. 

Concentração 

Considere o equilíbrio químico: 

C(s) + CO2
(g) 
 
 
  
  
  
  
  
  
  
  
  
  
  
  
 
 
 

 
&nbsp;2 CO (g)


Se adicionarmos mais dióxido carbônico ao equilíbrio, este se desloca para a
direita, vejamos a razão! 
Aumento de CO2 (g) acarreta em maior número de choques com C (s) e
consequente aumento da velocidade da reação, o que favorece a formação de
monóxido de carbono – CO (g). Dizemos então que a concentração do reagente
interferiu sobre o equilíbrio. 

Pressão 

Se considerarmos o equilíbrio como sendo gasoso a uma temperatura constante,
teremos um deslocamento do equilíbrio no sentido de diminuir o aumento da
pressão. Exemplo: 

2 SO2 (g)
+ O2 (g) 
 
&nbsp;2 SO3
(g) 
&nbsp;&nbsp;&nbsp; 2
mol&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
1
mol&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
2 mol 

Repare que a soma da quantidade de mols para os reagentes (3 mol) é maior do
que o produto (2 mol). 

Um aumento de pressão no sistema favorece a formação de SO3 (g) porque
nesse sentido há uma diminuição do número de mol do gás. O equilíbrio tende a
deslocar para o lado de menor volume (menor número de mol) e assim a pressão
também diminui. 

Se diminuirmos a pressão haverá uma expansão de volume dos reagentes e com isso
o equilíbrio desloca-se para a esquerda (maior número de mol). 

Temperatura 

Considerando que em um equilíbrio ocorre tanto reações endotérmicas como
exotérmicas, conforme a equação:
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;exotérmica 
N2 (g)
+ 3 H2 (g) 
 
&nbsp;2 NH3
(g)
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; endotérmica 
Temos que: 
Aumento de temperatura do sistema: o equilíbrio se desloca para a esquerda
(Reação endotérmica), para que o calor seja absorvido e não afete o equilíbrio.


Diminuição de temperatura do sistema: o equilíbrio é deslocado para a direita.
(Reação exotérmica) para compensar o calor retirado do equilíbrio.



Por Líria Alves
Graduada em Química
Equipe Brasil Escola 



Fonte:&nbsp; Brasil escola