GASES
Lorena Fuentes
Química y Bióloga
Universidad Nacional
PREINGENIERIA
Tabla de contenido
Comenzaremos la unidad con un análisis de las sustancias que existen como gases y sus propiedades, las unidades adecuadas para expresar las variables de los gases y las diferentes leyes.
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SUSTANCIAS QUE EXISTEN COMO GASES
Atmósfera | |
Gas | Composición porcentual |
N2 | 78% |
O2 | 21% |
Otros | 1% |
NaCl | |
P. Fusión | P. Ebullición |
801°C | 1000°C |
¿Porqué no existen compuestos iónicos gaseosos a 25°C y 1 atm?
PREINGENIERIA
Unidad 1. Gases
¿Es lo mismo “gas” y “vapor”?
Nota
Cuanto más fuertes sean las atracciones intermoleculares, menor es la posibilidad de que un compuesto exista como gas a la temperatura ordinaria.
Características físicas
NO2
I2
Cl2
2. VARIABLES DE LOS GASES
6
Un gas queda definido por cuatro variables:
ESCALAS DE TEMPERATURA
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¿Tengo fiebre a 101 °F?
PRESIÓN DE UN GAS
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¿Cómo funciona un pitillo?
Al succionar el aire del pitillo se reduce la presión en su interior, el vacío creado se llena de líquido que es empujado hacia su parte superior debido a la mayor presión atmosférica.
76 cm
PRESIÓN vs ALTURA
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¿Por qué varia el punto de ebullición con la altura?
¿Por qué no se puede jugar un mundial en Bolivia?
VOLUMEN DE UN GAS
1dm3
1x106 cm3
1 ml
Pase 50000 dm3 a Km3
LEYES DE LOS GASES
Ley de Boyle y Mariotte
El volumen de un gas es inversamente proporcional a la presión que soporta (a temperatura y cantidad de materia constantes).
Transformación isotérmica
Ley de Charles
El volumen de un gas es directamente proporcional a la temperatura absoluta (a presión y cantidad de materia constantes).
Transformación isobárica
Ley Gay-Lussac
P (atm)
T (K)
La presión de un gas es directamente proporcional a la temperatura absoluta (a volumen y cantidad de materia constantes).
Transformación isocórica
Ley de Avogadro
El volumen de un gas es directamente proporcional a la cantidad de materia (número de moles), a presión y temperatura constantes.
V (L)
n
La adición de más partículas provoca un aumento de los choques contra las paredes, lo que conduce a un aumento de presión, que desplaza el émbolo hasta que se iguala con la presión externa
Ley combinada de los gases
Vi.Pi.To= Vo.Po.Ti
i: inicial
o: final
Ejemplo: A la presión de 3 atm y 20 ºC, una cierta masa gaseosa ocupa un volumen de 30 litros. Calcula el volumen que ocuparía en condiciones normales.
3 atm · 30 L· 273 K �V2 =
1 atm · 293 K
V1=
P1=
T1=
30L
3 atm
20°C +273= 293K
V2=
P2=
T2=
?
1 atm
273K
V1.P1.T2= V2.P2.T1
V1.P1.T2 = V2
P2.T1
= 83,86 L
Mismo lado de la igualdad:
Inversamente proporcional
Distinto lado de la igualdad:
Directamente proporcional
LEY DE LOS GASES IDEALES
P.V = n. R. T
R se calcula para:
n = 1 mol
P = 1 atm
V = 22,4 L
T = 273 K
R = 0,082 atm.L/ mol.K
atm
L
moles
K
LEY DE LOS GASES IDEALES
Masa molar
Ejemplo: Calcula la masa molecular de un gas, sabiendo que 32,7 g del mismo ocupan a 50ºC y 3040 mm de Hg de presión un volumen de 6765 ml
Como
m m� n =— ⇒ P.V = R .T .� M M
Despejando M queda:
m.R.T 32,7 g ·0,082 atm ·L ·323 K
M = =� P.V mol ·K· 6,765 L ·4 atm
= 32,0 g/mol
3040/760= 4 atm
50+273= 323K
EJERCICIO 1: Una botella de aire cerrada herméticamente a 27°C tiene un volumen de 30 cm3 de aire a presión atmosférica 1 atm. Si se calienta la botella a 200°C, ¿Cuál es la presión dentro de la botella?
1 atm. 473 K �P2 = =
300 K
V1.P1.T2= V2.P2.T1
P1.T2 = P2
T1
1,57 atm
V1=
P1=
T1=
30 cm3
1 atm
27°C +273= 300K
V2=
P2=
T2=
30 cm3
?
200°C+273= 473K
EJERCICIO 2: 5 moles de NH3 ocupan un volumen de 3500 cm3 a una temperatura de 77°C. ¿A qué presión se halla sometido este gas?
n=
V=
T=
P=
5 moles
3500 cm3
77°C +273= 350K
?
P= 5n. 0,082 .350K
3,5L
= 41 atm
P= n.R.T
V
PV= n.R.T
Mismo lado = Inversa
Distinto lado = Directa
EJERCICIO UNAL: De las siguientes gráficas la que mejor expresa la relación entre el número de moles del gas en función de la temperatura en condiciones P y V constantes es:
n
T
A
n
T
B
n
T
C
n
T
D
EJERCICIO UNAL: Un gas ideal ocupa un volumen V, a cierta presión P, y temperatura T. El volumen que ocupará al triplicar la presión y reducir la temperatura a la mitad, será:
3PV= n R T
2
EJERCICIO UNAL: La gráfica que representa mejor la densidad d, del aire verus la temperatura, T, es:
d
T
A
d
T
B
d
T
C
d
T
D
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Is the color of gold, butter and ripe lemons. In the spectrum of visible light, yellow is found between green and orange.
Blue
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Red
Is the color of blood, and because of this it has historically been associated with sacrifice, danger and courage.
Yellow
Is the color of gold, butter and ripe lemons. In the spectrum of visible light, yellow is found between green and orange.
Blue
Is the colour of the clear sky and the deep sea. It is located between violet and green on the optical spectrum.
Red
Is the color of blood, and because of this it has historically been associated with sacrifice, danger and courage.
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