Categories

Relative Molecular Masses In Solutions Topic 6: Energetics ~ Chemistry Form 5 Chemical Kinetics

TOPIC 2: CHEMICAL KINETICS ~ CHEMISTRY FORM 6

TOPIC 2: CHEMICAL KINETICS ~ CHEMISTRY FORM 6

Chemical kinetics is the study of speed or rate of chemical reaction under different conditions and mechanisms of the reaction.

The rate of reaction

Rate of reaction is the change in concentration of reactants and products per unit time.

Consider the following hypothetical reaction:

Word Image 1412

Rate of reaction can be expressed in terms of disappearance of A or B and rate of formation of C or D. But these rate are not the same i.e. B disappears 3 times as fast as A. Therefore rate of disappearance of B is equal to 3 times rate of disappearance of A.

Rate of disappearance of  B= 3 times rate of disappearance of A

Similarity;

Rate of formation of  C = 2 times rate of disappearance of A

Rate of formation of  D = 2 times rate of disappearance of A

Rate of formation of  D = times rate of disappearance of B

Word Image 1413

The rates of disappearance are negative (since concentration decreases with time) and rate of formation are positive (concentration increase with time). But all result in single positive expression for the rate of reaction.

To remove the effect of stoichiometry, the rate for each species is divided by the co-efficient of the molecules in the balanced equation

Word Image 1414

Word Image 1415

Word Image 1416

Word Image 1417

Example: 01

In the above hypothetical reaction, the initial concentration of A is 1.0M and 1minute later was found to be 0.9982M.

i)   What is rate of reaction in moles per litre per second?

ii)   What is rate of formation of C in?

Word Image 1418

Solution;

i)

Word Image 1419

Word Image 1420

Word Image 1421

Word Image 1422

ii)

Word Image 1423

Word Image 1424

Word Image 1425

Example: 2

In the reaction;

Word Image 1426

Reactant A is found to be disappearing at a rate of

Word Image 1427

a)      What is the rate of reaction?

b)      What is the rate of formation of D?

Solution;

a)

Word Image 1428

Word Image 1429

Word Image 1430

b)

Word Image 1431

Chemical Kinetic

Chemical Kinetic

RATE LAW

The law states that “The rate of reaction is direct proportional to the concentration of the reactants each raised to the power of equal to the order of the reaction”

Word Image 1434

Word Image 1435

Where K = Rate constant (Velocity constant)

By definition;

Rate constant is a constant of proportionality in the rate equation which is the measure of the speed with which a reaction is taking place at the given temperature

Read Also  THE PRACTICES OF HUMAN RIGHTS

The units of K depends on the order of reaction

Order of reaction this is the number which shows the manner with which the rate of reaction depends on the concentration of reactants.

Order of reaction can only be obtained experimentally and cannot be deduced from overall balanced equation. OR order of reaction is the sum of power of concentration of the reactants in

the rate law or rate equation.

Consider the example of the reaction;

Word Image 1436

The rate law or rate equation is given by;

Chemical Kinetic

The reaction order with respect to [A] is a and [B] is b and the overall order of reaction is () and that can be a fraction

Word Image 1438

1.      Zero order reaction

Is the reaction in which the rate of the reaction is independent on the concentration of the reactants.

Consider the reaction;

Word Image 1439

Word Image 1440

                           Word Image 1441

The rate is constant and independent of the concentration of A

Units of rate constant, K

Word Image 1442

But

Chemical Kinetic

Chemical Kinetic

Chemical Kinetic

Chemical Kinetic

Assignment: Find any two examples of zero order reactions

Graph of rate against concentration A

Chemical Kinetic

2.   First order reaction

Is reaction in which the rate of reaction is directly proportional to the first power of the concentration of the single reaction reactant.

Consider the reaction;

Chemical Kinetic

Chemical Kinetic

Chemical Kinetic

The units of rate constant, K

Chemical Kinetic

Chemical Kinetic

Word Image 1452

Word Image 1453

Graph of rate against concentration A

Chemical Kinetic

Assignment: Find any 2 reaction of first order reaction

3.      Second order reaction

Is the reaction in which rate of reaction is proportional to the second power of concentration of a single reactant or first power of concentration of two reactants (Although rate of reaction

increases with increase in concentration of the reactant, the relationship is not direct)

s

Word Image 1454

Chemical Kinetic

Or

Word Image 1456

Chemical Kinetic

Units of rate constant, K

Chemical Kinetic

Chemical Kinetic

Word Image 1460

Chemical Kinetic

Graph of rate against concentration A

Chemical Kinetic

DETERMINATION OF ORDER OF REACTION

This can only be found experimentally.

The order of the reaction can be found by keeping the concentration of one reactant constant and changing the concentration of other at a given temperature.

Read Also  Notes For Primary School, Ordinary Level (Form 1 - 4) And Advanced Level (Form 5 and 6) All Subjects

Example:

1. The following data was obtained for the reaction

Chemical Kinetic

Word Image 1463
Word Image 1464
Word Image 1465
Word Image 1466
Word Image 1467
Word Image 1468
Word Image 1469
Word Image 1470
Word Image 1471
Word Image 1472
Word Image 1473
Word Image 1474
Word Image 1475
Word Image 1476
Word Image 1477
Word Image 1478

a)      What is the order with respect to;

i)    Reactant A

ii)   Reactant B

iii)  Reactant A and B

b)      Calculate the velocity constant and the rate of reaction with concentration A and B are 0.01M and 0.1M respectively.

Solution:

Word Image 1479

a)   i)    The value of a is found when the concentration of B must be kept constant so experiment 1 and 2 are considered;

Chemical Kinetic

Word Image 1481

Divide equation (1) by (2)

Word Image 1482

Word Image 1483

Word Image 1484

The reaction is first order with respect to reactant A

ii) The value of b is found when the concentration of A must be kept constant, so experiment 1 and 2 are considered;

Word Image 1485

Chemical Kinetic

Divide equation (1) by (2)

Word Image 1487

Word Image 1488

Word Image 1489

The reaction is second order with respect to reactant B

iii)  The overall order of reaction;

The overall order of the reaction is third order

Word Image 1490

 Word Image 1491
 Word Image 1492

b)      The velocity constant is the average because there are some errors.

Experiment 1:-

Chemical Kinetic

Word Image 1494

Experiment 2:-

Word Image 1495

Word Image 1496

Experiment 3:-

Word Image 1497

Word Image 1498

Word Image 1499

Word Image 1500

Word Image 1501

Word Image 1502

2.  The following data were obtained for;

Word Image 1503

Word Image 1504
Word Image 1505
Word Image 1506
Word Image 1507
Word Image 1508
Word Image 1509
Word Image 1510
Word Image 1511
Word Image 1512
Word Image 1513
Word Image 1514
Word Image 1515
Word Image 1516
Word Image 1517
Word Image 1518
Word Image 1519
Word Image 1520
Word Image 1521
Word Image 1522
Word Image 1523

Determine the rate law for the above reaction

Solution:

Word Image 1524

The value of a is found when concentration of y is constant.

Word Image 1525

Word Image 1526

Divide;

Word Image 1527

Word Image 1528

The value of b;

Word Image 1529

Word Image 1530

Divide;

Word Image 1531

Word Image 1532

Word Image 1533

Word Image 1534

Word Image 1535

Word Image 1536

Word Image 1537

Word Image 1538

Word Image 1539

Word Image 1540

:.

Word Image 1541

3.      The data below was obtained from the reaction;

At 298K and 1atm

Word Image 1542

Word Image 1543
Word Image 1544
Word Image 1545
Word Image 1546
Word Image 1547
Word Image 1548
Word Image 1549
Word Image 1550
Word Image 1551
Word Image 1552
Word Image 1553
Word Image 1554
Word Image 1555
Word Image 1556
Word Image 1557
Word Image 1558
Word Image 1559
Word Image 1560
Word Image 1561
Word Image 1562

Determine:-

i)   Rate law

ii)  Velocity constant

iii)  Rate of reaction in experiment 4

Solution;

i)

Word Image 1563

The value of b:-

Word Image 1564

Word Image 1565

Divide equation 1 by 2

Word Image 1566

Word Image 1567

The value of a:-

Word Image 1568

Word Image 1569

Word Image 1570

Word Image 1571

ii)   Velocity constant, K

Word Image 1572

Word Image 1573

Word Image 1574

iii)  Rate of the reaction in experiment 4

Word Image 1575

Word Image 1576

Word Image 1577

Note :- If the rate of reaction is not given directly, it can be found from concentration and time. The graph of concentration against time is always a curve for first and second order reaction

Chemical Kinetic

Choose points, draw the tangent and find the slopes

slope = Rate of reaction 

Rate = K[A]m  where m= order of reaction

……………….1

Word Image 1580

………………2

Word Image 1581

Using these two equations order of reaction can be obtained. For more than two slopes, introduce log on both sides of rate equation

Read Also  TOPIC 7 : BONDING, FORMULA AND NOMENCLATURE QUESTIONS WITH ANSWERS ~ CHEMISTRY FORM 2

i.e.

Word Image 1582

Therefore a graph of log R against log [A] gives a straight line where m = slope (Order of reaction)

Alternatively:-

Word Image 1583

Assume m=1, show

Word Image 1584

m=2, show

Word Image 1585

m=3,

Word Image 1586

Example:

1.      For the reaction   the following data was obtained at 450C

Word Image 1587

Word Image 1588
Word Image 1589
Word Image 1590
Word Image 1591
Word Image 1592
Word Image 1593
Word Image 1594
Word Image 1595
Word Image 1596
Word Image 1597
Word Image 1598
Word Image 1599
Word Image 1600
Word Image 1601
Word Image 1602
Word Image 1603

a)   Plot a graph of [A] against t:

b)   Take tangents to obtain a slope at [A] = 2, 1.167 and 1moldm-3

c)    The rate law for the above reaction can be

Word Image 1604

Use of the result in (b) to find which of the rate law actually holds.

2.      The following data was obtained from the reaction;

Word Image 1605

Word Image 1606
Word Image 1607
Word Image 1608
Word Image 1609
Word Image 1610
Word Image 1611
Word Image 1612
Word Image 1613
Word Image 1614
Word Image 1615
Word Image 1616
Word Image 1617

i.  Determine the value of n and K in the equation

Word Image 1618

ii.  Obtain rate constant and the rate of reaction when concentration of

Word Image 1619

Word Image 1620

iii.   Use the above example to explain the fact that the order of reaction must be determined experimentally and not from the co-efficient of balanced equation

Answers
       0.5128           9.4966 * 10-4 = 0.00094966
-0.9676          9.99 *10-4 = 0.00099   
       4.5099          1.193 * 10-3 = 0.001193=11.93*10-4

ARRHENIUS EQUATION

Chemical Kinetic

From the collision theory, the reaction rate depends the energy and number of collision between molecules and whether the collisions have correct geometry and temperature.

These requirements are summarized by Arrhenius into what is known as the Arrhenius Equation.

K=Ae-EaRT

Word Image 1621

Where; K =Rate /velocity constant.

R= Universal gas constant

A= Frequency factor (per time)

e-EaRT = Fraction of molecules having minimum energy required for the reaction to take place.

Word Image 1622

Frequency factor (A) –Express number of collisions occurring and a fraction of them that has the right geometry.

Application of Arrhenius Equation

i)   Determination of Activation Energy
Introduce In on both sides of

Word Image 1623

ii)   The collision must be energetic enough to break the bond between molecules.

Word Image 1624

I.e The collision between 2 molecules will not lead to reaction if the kinetic energy cannot provide the necessary activation energy.

As the temperature rises, the number of molecules with energies greater than minimum energy required (ACTIVATION ENERGY) increases.

This factor causes the reaction to be sensitive temperature.

You cannot copy content of this page