Persamaan Laju Reaksi

Teacher Training and Educational Studies Sebelas Maret University

M. Masykuri_Physical Chemistry IVSep, 2010

9/28/2010 1

METODE PENENTUAN TETAPAN LAJU & ORDE REAKSI

Dr. M. Masykuri, M.Si.Chemical Education Study ProgramTeacher Training and Educational Studies Sebelas Maret University (UNS)Website: http://masykuri.staf.fkip.uns.ac.id, email: [email protected]

Solo, Sep 2010

Kimia Fisika IV

Modul 1



Rate of ReactionConsider a chemical reaction having the overall stoichiometry:

aA + bB → cC + dD The Rate of Reaction is defined as

Experimentally we find that k = rate coefficient Ci = Concentration of Reactant “i” or [i]γi = Order of reaction with respect to reactant “i”



Macam Metode

Metode Penentuan Tetapan Laju:

1. Metode Diferensial

2. Metode Integral

3. Metode Waktu Paruh

4. Metode Lain

Metode relaksasi, metode analisis guegenheim



Metode Diferensial

• Data dikumpulkan sebagai laju perubahan konsentrasi/waktu terhadap konsentrasi reaktan.

• Contoh dasar persamaan bagi reaksi dengan 2 reaktan:aA + bB → produk

qp BAkdt

Adr ][][

][

]ln[]ln[ln][

lnln BqApkdt

Adr

• Dibuat kurva ln r terhadap ln [A] atau ln [B]



Metode Diferensial

]ln[]ln[ln][

lnln BqApkdt

Adr

• Kurva ln r terhadap ln [A]

]ln['lnln Apkr

Orde reaksi terhadap A = p, diperoleh dari slope = tg Tetapan laju k, diperoleh dari intersep grafik

Slope = tg = p

]ln[ln'ln BqkkIntersep



Metode Diferensial

Data konsentrasi zat A pada reaksi,

yang dikatalisis suatu katalis C sebagai fungsi waktu diketahui sbb:A → produk

Contoh:

t (menit) A (M) t (menit) A (M)

0 0,800 8 0,382

2 0,647 10 0,329

4 0,534 12 0,286

6 0,448 14 0,252

a. Tentukan orde reaksi terhadap A sampai kelipatan ½ terdekat

b. Tentukan harga tetapan laju reaksi k dengan satuannya yang tepat



Metode Integral

• Bersifat trial and error (empiris), yaitu dengan memasukkan data pada persamaan laju bentuk integral.

• Jika harga k relatif tetap (konstan), maka orde reaksinya sesuai.

• Penentuan empiris orde reaksi terhadap reaktan:– Orde nol– Orde pertama– Orde kedua tipe I– Orde kedua tipe II– Prde ketiga



Metode Integral

• The rate of the reaction is independent of the concentration of the reacting substance

Zero Order Reactions

A → products

dtkAd

kdt

Ad

kdt

Ad

A

A

][

][ 0

][

][

][

[A]t - [A]0 = - kt

The reaction half-time t1/2 is the time required for the concentration to decrease to one-half of its initial value.



Metode IntegralZero Order ReactionsGrafik [A] versus t,

Persamaan grafik,

Slope = - kIntersep = [A]0

Waktu paruh,



Metode IntegralFirst Order ReactionsThe rate of the reaction is proportional to the first power of the concentration of the reacting substance (order = 1)

A → products

dtkA

Ad

kdtA

Ad

Akdt

Ad

A

A

][

][ 0][

][

][

][

][][

Examples: • decay of radioactive nuclei fluorescence • decay of electronically excited molecules • isomerization of cyclobutene to butadiene • milk turns sour when left out overnight



Metode IntegralFirst Order ReactionsGrafik ln [A] versus t,

Persamaan grafik,

Buktikan bahwa,



Metode IntegralSecond Order ReactionsThe rate of the reaction is proportional to the second power of the concentration of one reactant or to the first power of the concentrations of two reactants (order = 2)

a) Second order in one component

A → products

b) First order in each of two components

A +B → products



Metode IntegralSecond Order Reactionsa) Second order in one component

A → products




Metode IntegralSecond Order Reactions

A +B → products

Special cases: i) [A]o = [B]o

ii) [B]o <<[A]o




Metode IntegralSecond Order Reactions

Special cases (cont.): i) [A]o = [B]o ii) [B]o <<[A]o

This is like 2nd order in one component

Where,

This is called a pseudo 1st order reaction



Metode IntegralThird Order ReactionsSeveral possible cases. Could be:

(a) three identical chemical molecules interact simultaneously (unusual)

(b) two molecules of one species interact with one of another

(c) three different chemical species interact with each other



Metode IntegralThird Order Reactions(b) two molecules of one species interact with one of another

Example: recombination of homonuclear molecules such as iodine

Mechanism:



Metode IntegralThird Order Reactions(b) two molecules of one species interact with one of another

Example: recombination of homonuclear molecules such as iodine



Metode IntegralThird Order Reactions(c) three different chemical species interact with each other

Very important example: formation of ozone in the stratospheric ozone layer,

(M = air)

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Persamaan Laju Reaksi