Chemical Kinetic / Rate of reaction - KCSE Chemistry Practicals / Chemistry Paper 3

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Defination of terms

  • Chemical kinetics
  • Rate of reaction
  • SI
  • Stop watch/clock
  • Concentration
  • Temperature
  • Collision frequency/frequency of collision
  • Endothermic
  • Exothermic

Chemical kinetics is the branch of chemistry which deals with the study of rates (or fastness) of chemical reactions, the factors affecting it and the mechanism by which the reactions proceed. How different conditions affect the rate of reactions.

Graph is plotted between the concentration of reactants (vertical axis) and time of reaction (horizontal axis)

Remember:

  • Reactants are those initial substances whose reaction is being tested
  • Products are the final substsances formed due to the reaction.

For example in the reaction to form water:

  • 2H2 + O2 -> 2H2O
  • 2H2 + O2 => are the reactants
  • 2H2O => are the products

The rate of a chemical reaction can be defined as the time taken for a known amount of reactants to form known amount of products.

Graphical representation

chemical kinetics

  • Where the slope of products is positive and that of the reactants is negative indicating that the concentration of reactants and products, decreases and increases respectively
  • The SI unit of time is seconds. Minutes and hours are also common .
  • The vertical axis represents any factor that can affect the rate of reaction
  • NOTE: This is an example and the practical question may ask you to plot a graph of time against concentration. In that case, you should plot your graph with time on the vertical axis and concentration of reactants on the horizontal axis So, always read the questions carefully before progressing with the experimengt and answering it.

Some reactions are too slow to be determined e.g weathering others are instantaneous

Time is determined using a stop watch/clock

Candidates using stop watch/clock should learn to:

  1. Press start button concurrently with starting off determination of a reaction using one hand each.
  2. Press stop button when the reaction is over.
  3. Record all times in seconds unless specified.
  4. Press reset button to begin another timing
  5. Ignore time beyond seconds for stop clock/watch beyond this accuracy
  6. Avoid accidental pressing of any button before recording

It can be very frustrating repeating a whole procedure.

The following factors theoretically and practically alter/influence/affect/determine the rate of a chemical reaction:

  1. Concentration of reactants
  2. Temperature

Other factors that affect the rate of reaction include:

  1. Nature of reactants
  2. Physical state of reactants
  3. Surface area of reactants
  4. Effect of solvent 
  5. The presence of a catalyst


Concentration

An increase in concentration increases the rate the rate of reaction by reducing the time taken to completion.

Theoretically, increase in concentration is a decrease in distance between reacting particles which increases their collision frequency.

Practically decreasing concentration is diluting/adding water.

To demonstrate the effect of concentration on reaction rate

You are provided with

  1. sodium thiosulphate containing 40gdm -3 solution labeled A
  2. 2M hydrochloric acid labeled solution B

You are required to determine the rate of reaction between solution A and B

Procedure

  • Measure 40cm3 of solution A into 100 cm3 glass beaker.
  • Place it on top of a pen-mark “X”.
  • Measure another 40cm3 of solution B.
  • Simultaneously put solution B into solution A and start off a stop watch/clock.
  • Determine the time taken for the pen-mark “X” to be invisible/obscurred from above.
  • Repeat the procedure by measuring 35cm3 of solution B and adding 5cm3 of water.
  • Complete the table 1 below by using other values os solution B and water.

Volume of solution A (cm3)

 
40  40  40 40 40 40

Volume of solution B (cm3)

40 35 30 25 20 15

Volume of water (cm3)

0 5 10 15 20 25

Time taken for x to be invisible(seconds)

14 17 21 25 50 120

1/t

0.0714

0.0588

0.0351

0.04

0.02

0.0083

Calculate 1/t in each case

Plot a graph of 1/t (y-axis) against volume of solution B.

concentration on reaction rate

Sample questions

  1. Explain the shape of the graph

    (Straight line graph from the origin)
    Decrease in concentration decreases the rate of reaction.
    The higher the concentration of solution B the less time taken for mark x to be obscurred/invisible due to increased collision frequency between the reacting particles.

  2. From the graph determine the time taken for the mark to be invisible at 37cm3  
    At 37cm3 then 1/t  =>  1/37  =  0.027

    From a well plotted graph:
    1/t = 0.027  => 16.2602 seconds

  3. From graph determine the volume of solution B  at 100 seconds

    100 seconds => 1/t  =  1/1000  = 0.01

    From a well plotted graph:
    At 1/t = 0.01  =>  the volume of B  = 17.0cm

  4. State  another factor that would alter the rate of the above reaction.
    Temperature
  5. State  another factor that would not alter the rate of the above reaction.
    Surface area
    Pressure
    Catalyst



Temperature

An increase in temperature increases the rate of reaction.

An increase of 10 oC/10K practically doubles the rate of a chemical reaction/reduces time of completion by 1/2.

An increase in temperature increase the kinetic energy of reacting particles increasing their collision frequency

Practically ,increase in temperature involves heating the reactants The results and presentation should be as in the effect of concentration.

Increased temperature reverses the table I time results  i.e less time as temperature increases.

Note: 

  • If the temperature is increased, the number of collisions between reactant molecules per second (frequency of collision). Increases, thereby increasing the rate of the reaction.
  • But depending on whether the reaction is endothermic or exothermic increase in temperature increases the rate of forward or backward reactions respectively.
  • Remember:
    • Endothermic reactions - reactions in which the reactants absorb heat energy from the surroundings to form products
    • Exothermic reactions - chemical reaction that involves the release of energy in the form of heat or light
  • In a system where more than one reaction is possible, the same reactants can produce different products under different temperature conditions

Other factors affecting the rate of reaction

Nature of reactants

The reaction rate also depends on the types of substances that are reacting. If we consider acid/base reactions, salt formation and ion exchange, they are mostly fast reactions. During the formation of a covalent bond between the molecules that results in the formation of larger molecules the reaction that takes place is usually slower. Furthermore, the nature and strength of bonds in reactant molecules significantly affect the rate of their transformation into products.

Physical state of reactants

The physical state of a reactant whether it is solid, liquid, or gas can greatly affect the rate of change. To discuss it further, if reactants are in the same phase let’s say they are in an aqueous solution, here the thermal motion will bring them together. If they are in different phases then the reaction will be limited to the interface between the reactants. The reaction mainly occurs only at their area of contact; in the case of a liquid and a gas, at the surface of the liquid.

Surface area of reactants

If we take two solids, in this case, the particles that are at the surface will take part in the reaction. Likewise, if we were to crush a solid into smaller parts more particles will be present at the surface. What it means is that the frequency of collisions between these and reactant particles will most likely increase. As a result, the reaction will occur more rapidly.

When two or more reactants are in the same phase of fluid, their particles collide more often than when either or both are in the solid phase or when they are in a heterogeneous mixture. In a heterogeneous medium, the collision between the particles occurs at an interface between phases. Compared to the homogeneous case, the number of collisions between reactants per unit time is significantly reduced, and so is the reaction rate. 

The presence of a catalyst

Catalysts alter the rate of the reaction by changing the reaction mechanism. There are two types of catalysts namely, promoters and poisons which increase and decrease the rate of reactions respectively.

Lab Safety/Precautions

  • Wear your eye protection.
  • Do not inhale any fumes.
  • Dispose of the solution immediately after the experiment following your teacher's instructions.
  • Wash your hands thoroughly when finished.

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