chem

1. RATES OF REACTION

2. 8.1 Factors affecting the rate of reaction
◦ It is nothing but the how fast or slow a reaction takes place. For e.g.: Precipitation reactions occur fast
while ripening of fruits/vegetables is slow.
◦ Few factors affect the rate of a reaction.
◦ 1. Concentration, which includes:
◦ A. Surface area of solid reactants B. Concentration of reactant solutions
◦ C. Pressure of reacting gases. D. Temperature in which the reaction is carried out
◦ 2. Use of a catalyst

3. 8.1 Factors affecting the rate of reaction
◦ 1. Effect of reactant concentration
◦ A. Surface area of reacting solids
◦ Greater the surface area, greater is the rate of reaction. A finely powdered reactant has large surface area compared to
chunks of reactant.
◦ With two solids, they are mixed together. With a solid and liquid, the reaction will take place at a faster rate if the
surface area of the solid is more.
◦ For. e.g.: Calcium carbonate in its two forms limestone or marble is made to react with dilute HCl
◦ CaCO3 (s) + HCl (aq)  CaCl2 (aq) + H2O (l) + CO2 (g)
◦ Two samples A (large pieces) and B (small pieces). Mass of the samples is measured of both samples (start with zero)
at regular time intervals. Note that the amount of A and B is same.
◦ Loss of mass is plotted with sample A and B against each other and a graph is plotted.

4. 8.1 Factors affecting the rate of reaction
◦ Following points are observed
1. The reaction is fastest at the start. However, curve B is steeper than curve A.
2. CO2 gas is released in both reactions in equal amount. Both the curves flatten out at the end of the reaction but curve B
reaches the horizontal part (plateau) first.
3. A numerical value for the rate of reaction can be obtained by drawing a tangent to the curve.
◦ This shows the rate of reaction increases with increase in surface area.
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5. 8.1 Factors affecting the rate of reaction
◦ B. Concentration of reacting solutions
◦ Same reaction of CaCO3 is taken which we name as C (concentrated=1 mol/dm3) and D (less conc =0.5 mol/dm3).
◦ Following points are seen
◦ C is steeper than D because of concentration
◦ Curve of C starts twice as steep as D. Thus, doubling the concentration doubles the rate of reaction.
◦ However, both the volumes of CO2 produced in both experiments are same.

6. 8.1 Factors affecting the rate of reaction
◦ C. Pressure of reacting gases
◦ Increase in pressure pushes the gas molecules closer and hence, they react more readily and this increases the rate of
reaction. For. E.g. combustion of fuel in a car engine
◦ D. Effect of temperature
◦ As temperature increases the rate of reaction increases.
◦ When we plot a graph, we see that:
◦ 1. At high temperatures, reactions take place faster.
◦ 2. The curve is not a straight line.

7. 8.1 Factors affecting the rate of reaction
◦ 2. Presence of a catalyst
◦ Catalyst: A substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical
change.
◦ For eg: Hydrogen peroxide decomposes to form water and oxygen: 2H2O2 (l)  2H2O (l) + O2 (g)
◦ This reaction occurs at room temperature but when Manganese (IV) oxide MnO2 is added, the reaction proceeds faster.
◦ Greater the amount of MnO2 added, faster the reaction. Also, powdered form increases rate of reaction.
◦ Eg: Enzymes help in breaking down food without undergoing chemical change.

8. 8.2 Collision theory of reaction rate
◦ Collision theory: It states that a chemical reaction takes place when particles of the reactants collide with
sufficient energy to initiate the reaction.
◦ Its collision will depend on:
1. Surface area (greater the surface area, greater the collision)
2. Concentration (greater the concentration, greater the collision)
3. Temperature (higher the temperature, greater the collision)
For gases only: Pressure (higher the pressure, greater the collision)

9. 8.2 Collison theory of reaction rate
◦ Action of a catalyst
◦ A catalyst reduces the amount of energy needed to break bonds (activation energy).
◦ Different reactions require different catalysts. Eg: Many catalysts work by absorbing reactants to a solid surface, which
weaken the bonds in reactants.