1. CINETICA QUIMICA Prof. Ulises Urzúa Prog. Biología Celular y Molecular ICBM – Facultad de Medicina, Universidad de Chile Química General TM Clases 13-14, 8 Abr 2009
2. Cinética – ¿que tan rápido o lento ocurre una reacción? Termodinámica – ¿ocurre una reacción? ¿en que dirección?
3. La velocidad de reacción (v) es el cambio en la concentración de reactante o de producto en función del tiempo. Cambio en la concentración de A durante el periodo de tiempo t Cambio en la concentración de B durante el periodo de tiempo t Habitualmente se usan unidades M ó mM ó M / seg ó min A B v = - [A] t v = [B] t
6. Br 2 ( ac ) + HCOOH ( ac ) 2Br - ( ac ) + 2H + ( ac ) + CO 2 ( g ) t 13.1 Oxidacion de ác. Fórmico con Br 2 [Br 2 ] absorbancia a 393 nm 393 nm Br 2 ( ac )
7. ¿Como se determina la velocidad de reacción? v promedio = - [Br 2 ] t = - [Br 2 ] final – [Br 2 ] inicial t final - t inicial v instantánea = v promedio calculada en un intervalo muy corto 13.1
8. Velocidades de reacción - Br 2 y HCOOH a 25 C v [Br 2 ] v = k [Br 2 ] k = v [Br 2 ] k , constante de velocidad = 3.50 x 10 -3 s -1 a 25 C 1.04 1.23 1.48 1.75 2.09 2.49 2.96 3.52 4.20 v x 10 -5 (M/s) 3.48 x 10 -3 0.0035 350 0.0030 0.0042 0.0050 0.0060 0.0071 0.0085 0.0101 0.0120 [Br2] (M) 3.51 x 10 -3 400 3.52 x 10 -3 300 3.50 x 10 -3 250 3.51 x 10 -3 200 3.51 x 10 -3 150 3.50 x 10 -3 100 3.49 x 10 -3 50 3.50 x 10 -3 0 k = v (s -1 ) [Br 2 ] Tiempo (s)
9. Por cada mol de B que se forma desaparecen 2 moles de A v = v = - Velocidades y estequiometría H 2 ( g ) + I 2 ( g ) 2 HI ( g ) [HI] t v = - [H 2 ] t = La [HI] aumenta al doble en relación a la disminución de [H 2 ] Ejemplo: Forma general 2 A B [B] t [A] t 1 2 a A + b B c C + d D v = - [A] t 1 a = - [B] t 1 b = [C] t 1 c = [D] t 1 d 1 2
10. CH 4 ( g ) + 2O 2 ( g ) CO 2 ( g ) + 2H 2 O ( g ) Ejercicio 1 Escribir las expresiones de velocidad para las siguientes reacciones: 3O 2 ( g ) 2O 3 ( g ) v = - [CH 4 ] t = - [O 2 ] t 1 2 = [H 2 O] t 1 2 = [CO 2 ] t [O 3 ] t v = - [O 2 ] t 1 3 = 1 2 5 Br - ( ac ) + BrO 3 - ( ac ) + 6 H + ( ac ) 3 Br 2 ( ac ) + 2 H 2 O ( l )
11. Expresa la velocidad ( v ) de una reacción en función de la constante de velocidad ( k ) y las concentraciones de reactantes elevadas a alguna potencia . v = k [A] x [B] y x es el orden con respecto a A y es el orden con respecto a B El órden total de la reacción es ( x + y ) La ecuación (o ley) de velocidad a A + b B c C + d D
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15. = k [A] [A] t = concentracion de A a tiempo t [A] 0 = concentracion de A a tiempo t =0 Reacción de primer orden ¿Cuanto de A va quedando a cualquier tiempo t ?. Debemos integrar: A producto v = - [A] t
16. Reacción de primer orden (cont) y = m x + b pendiente= - k = ln [A] t CH 3 NC CH 3 CN
17. La reacción 2A B es de primer orden y su k = 2.8 x 10 -2 s -1 a 80 C. ¿En cuanto tiempo A disminuirá desde 0.88 M a 0.14 M ? ln[A] = ln[A] 0 – k t kt = ln[A] 0 – ln[A] t = = 66 s [A] 0 = 0.88 M [A] = 0.14 M Ejercicio 3 ln[A] 0 – ln[A] k ln [A] 0 [A] k = ln 0.88 M 0.14 M 2.8 x 10 -2 s -1 =
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19. b) ¿Cúal es la k de la reacción de descomposición de N 2 O 5 si su t½ = 20 min ? Ejercicio 4 a) El etano (C 2 H 6 ) se descompone en radicales metilo. Esta reacción es de primer orden y su k = 5.36 x 10 -4 s -1 a 700 C. Calcule la t½ en minutos. C 2 H 6 ( g ) 2 CH 3 • ( g ) t ½ 2 N 2 O 5 ( g ) 4 NO 2 ( g ) + O 2 ( g ) 0.693 k =
20. # de vidas medias [A] 1 2 3 4 [A] 0 /2 [A] 0 /4 [A] 0 /8 [A] 0 /16 Tiempo de vida media - reacción de primer orden
21. Datación con 14 C - reacción de primer orden 5570 75 1 t½ 11140 38 2 t½ 0 150 Contemporánea Edad (años) 14 C (cps) Muestra
35. La ecuación de Arrhenius A, factor de frecuencia y = mx + b ln k = - E a R 1 T + ln A
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39. Generalmente la molecularidad de la etapa lenta conicide con el orden de la reacción v = k [NO 2 ] 2 NO 2 ( g ) + CO ( g ) NO ( g ) + CO 2 ( g ) Pasos elementales NO 2 + NO 2 NO 3 + NO (lento) NO 3 + CO NO 2 + CO 2 (rápido) Molecularidad 2 2 2
42. Pasos elementales: H 2 O 2 + I – H 2 O + IO – (lento) H 2 O 2 + IO – H 2 O + O 2 + I – (rápido) En este caso: k = k cat Descomposición de H 2 O 2 2 H 2 O 2 ( ac ) 2H 2 O ( l ) + O 2 ( g ) En presencia de ion I – , la reacción: v = k [H 2 O 2 ][I – ] ? v = k cat [H 2 O 2 ][I – ]
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44. CO + HC no combustionados + O 2 CO 2 + H 2 O 2NO + 2NO 2 2N 2 + 3O 2 Convertidores catalíticos
First, a light source generates light at a specific wavelength or wavelengths. Normally, a UV/Vis spectrophotometers utilize two light sources: a deuterium arc lamp for consistent intensity in the UV range (190 to 380 nm) and a tungsten- halogen lamp for consistent intensity in the visible spectrum (380 to about 800 nm). Some spectrophotometers use xenon flash lamps, which offer decent intensity over the UV and visible regions. The source light is then directed to a dispersion device that causes different wavelengths of light to be dispersed at different angles. Two common dispersion devices used in UV/Vis spectrophotometers are prisms and holographic gratings. The angle of dispersion with a prism, however, can be nonlinear and sensitive to changes in temperature. Holographic gratings are glass blanks with narrow ruled grooves. The grating itself is usually coated with aluminum to create a reflecting source. Holographic gratings eliminate nonlinear dispersion and are not temperature sensitive. They do require filters, though, since light is reflected in different orders with overlapping wavelengths. Once the light has been passed through the dispersion device and the sample of interest it reaches a detector. Detectors in UV/Vis spectrophotometers come in a variety of shapes and sizes. Photomultiplier tubes are common; they provide good sensitivity throughout the UV/Visible spectral range and are highly sensitive at low light levels. Photodiodes have seen increasing use as detectors in spectrophotometers, bringing to the table a wider dynamic range. A photodiode is generally made up of a semiconductor and a capacitor to charge the semiconductor. As light hits the semiconductor, electrons flow through it, thereby lowering the charge on the capacitor. The intensity of light of the sample is proportional to the amount of charge needed to recharge the capacitor at predetermined intervals. As opposed to having single photodiodes, some spectrophotometers are composed of a photodiode array. Here, several photodiode detectors are arranged on a silicon crystal. The advantage of an array is the ability to do side-by-side readings, thus increasing speed. The entrance slit, dispersion device, and exit slit are referred to as the monochromator. Light passing through the monochromator exits as a band. The width of this band of light at half the maximum intensity is the spectral bandwidth. Bandwidth comes in to play with regard to accuracy, since the accuracy of any absorbance measurement is dependent on the ratio of the spectral bandwidth to the natural bandwidth of the substance being measured. The natural bandwidth is the width of the absorption band of the sample at half the absorption maximum. As a rule, a ratio between spectral bandwidth and natural bandwidth of 0.1 or less will generate absorbance measurements 99.5 percent accurate or better. In a conventional spectrophotometer, polychromatic light from the monochromator is transmitted through the sample, and the sample absorbance is determined by comparing the intensity of the light hitting the detector with just a sample blank with the intensity of light hitting the detector with the sample in place. With a diode array spectrophotometer, polychromatic light passes through the sample and is focused on the entrance slit of the polychromator (the entrance slit and dispersion device). The light is then dispersed onto the diode array with each diode measuring a portion of the spectrum.
the concentration of HI increases 2 times as fast as the concentration of H2 decreases
S2O8 es peroxidisulfato
ISOMERIZATION of Methyl Isonitrile to Acetonitrile
- En una reaccion quimica, se rompen enlaces en los reactantes, lo cual da lugar a la formacion de nuevos enlaces en los productos. - Esto ocurre sólo si hay colisiones entre moleculas. - Aún más, las moleculas deben colisionar con una orientación correcta y con una energía mínima para que ocurra ruptura y formación de enlaces.
Intermediates are species that appear in a reaction mechanism but not in the overall balanced equation.
CO is necessary for this reaction to occur, but the rate of the reaction does not depend on its concentration. This suggests the reaction occurs in two steps.
