The rate of reaction is defined as the change in concentration of reactants or products per unit time. The rate of reaction means the speed at which a chemical reaction proceeds.
Suppose a reaction in which reactant A is converted into product B.
Rate of reaction= change in conc. of reactants or product/time
Rate of reaction=-=
d=change in concentration of reactants
d=change in a concentration of products
The negative sign indicates that the concentration of the reactant is decreasing with time. It does not mean that rate is negative.
The rate of reaction is measured in mol dm-3sec-1.
Some of the reactions occur at a very slow rate. For example, rusting of iron is a very slow process. On the other hand, the reaction between acid and base is very fast.
Reaction rates are explained by collision theory.
Postulates of CT:
- In order for a chemical reaction to take place, the reactants must collide.
- The collision transfers the kinetic energy needed to break the bonds so that new bonds are formed.
- All collisions do not lead to a chemical reaction. Collisions that caused the chemical reaction are called effective collisions.
There are two conditions for effective collision.
- There must be a head-on collision between reactants molecules.
- Reactants molecules must have enough energy to cross a barrier and form products.
4. The rate of reaction is directly proportional to the number of collisions per unit volume per second.
5. Variation in rate of reaction with temperature or pressure is due to the fact that these factors change the frequency of collisions between reactants molecules.
6. A catalyst increases the number of effective collisions just by lowering the activation energy barrier.
Factors that affect the reaction rate
There is a number of factors that affect the reaction rate. Some of them are given below.
Chemical reactions are very sensitive to temperature and must be studied under constant temperature. It is commonly observed that increasing the temperature of the system increases the rate of reaction greatly.
Temperature is a measure of the average kinetic energy of the reactants. As the temperature increases, the motion of reactant molecules increases which increases their kinetic energy. When the reactant molecules move faster, the probability of effective collision between them increases. So the chances of formation of products increase, which in turn results in the rate of reaction increases.
Rates of some reactions even doubled or triple when the temperature is increased by 10oC.
For a chemical reaction to takes place, the reactants molecule must have sufficient energy to cross a barrier. The minimum energy required to convert the reactant into a product is called activation energy.
Activation energy is the difference between the energy of the transition state and the energy of reactants. An increase in temperature just increases the baseline of reactants so they need less activation energy to cross the barrier.
The first graph is at low temperature and the other is at high temperature. The graph represents that reaction is exothermic because reactants have more energy than products.
The rate of reactions is studied as a function of temperature because the rate constant depends only on temperature. It does not depend on concentration because it remains constant.
It means it is a rate constant that incorporates information about the effect of temperature on the rate of reaction.
There are different types of behavior are observed when the rate of reactions is studied as a function of temperature. Three of the most common cases are shown in the figure.
Most of the reactions follow the first case. In this, the rate of reaction increases exponentially with the increase in temperature.
The second shows the behavior of some materials that become explosive at a certain temperature. When the temperature is below the explosive limit, the rate is unaffected by temperature. when the temperature reaches the explosive limit then a very slight change in temperature causes an enormous increase in the rate of reaction.
In the 3rd case, the rate of reaction first increases and reaches a maximum value at a certain temperature. After that, the further increase in temperature causes a decrease in the rate of reaction.
Enzyme catalyzed reactions show such type of behavior. This is because at higher temperatures enzymes become denatured and the rate of reaction decreases.
Arrhenius equation is another proof of the statement that the rate of reaction varies as a function of temperature.
k= rate constant
A= Arrhenius factor
Ea= activation energy
R= general gas constant
This equation states that when the temperature increases the term Ea/RT decreases. But the overall exponential term increase is due to negative signs. As the rate constant is directly proportional to the equation so it also increases. So the rate of reaction increases.
Catalyst is a substance that increases the rate of reaction by lowering its activation energy. The mass and chemical composition of the catalyst remains unchanged at the end of the reaction. So it can be reused. It is required in a very small amount to change the rate of reaction. In the reversible reaction, it changes the rates of forward and reverse steps.
Most catalysts cannot start a reaction. But now it has been observed that a catalyst can initiate reactions.
Catalyst does not change the equilibrium position. It lowers the activation energy and gives a new path to the reaction.
From the graph, it is clear that the presence of catalyst lowers the energy of activation. So reactants can easily cross the energy barrier and form a product. So the rate of reaction increases.
When we change the temperature of the chemical reaction the activity of the catalyst increase. So the rate of reaction increases.
Sometimes, another substance called promotor (which increases the activity of catalyst) is added in reaction to increase its rate.
All catalysts do not increase the rate of reaction, some also decrease the rate of reaction. The catalyst which increases the rate of reaction is called a positive catalyst. While the catalyst which decreases the rate of reaction is called a negative catalyst.
- Concentration of reactants
When the concentration of the reactants increases, the rate of reaction also increases. When the reactant molecules having energy equal to or greater than activation energy combine, the chemical reaction occurs. With the increase in concentration, the number of molecules with minimum required energy will increase therefore the rate of reaction will increase.
By doubling the concentration of reactants the rate of reaction also double.
Increasing the concentration of reactants increases the frequency of collision between them. More collision means more opportunities for reaction.
- Surface area of solid reactants
In the case of heterogeneous catalysis, the surface area of the solid reactant affects the rate of reaction. On the surface of the solid reactant, adsorption of liquid reactant takes place and chemical reactions occur. Solid and liquid contact each other only at the solid-liquid interface, which is on the surface of solid.
The solid molecules present in the bulk of the solid cannot react. Therefore, increasing the surface area of solid will more expose more solid molecules to the liquid which cause more collision. So the rate of reaction increases.
The surface area of solid reactants can be increased by decreasing their particle size. For example in daily life, powdered sugar dissolves fast in water than crystal sugar.
This figure shows that by increasing the surface area of solid, it allows more chances of effective collision so reaction rate increase.
- Nature of reactants
When the nature of the two reactants is different then the rate of reaction is also affected. In the case of heterogeneous catalysis, one reactant is solid and the other is liquid. The collision between the reactants takes place only at the interface.
When the particle size of solid reactant is small then reaction occurs at a fast rate as compared to the large size of reactants. Because small size has more surface area exposed to liquid. So a number of collision increase and rate of reaction also increases.
Some reactions are naturally fast as the particles of solid move slowly in solution as compared to gaseous particles.
The rate of reactions in solutions and unimolecular reactions in the gas phase is dependent on pressure. The rate of gaseous reaction increase with the increase in pressure. The molecules of gas are very spread out. They are at a very large distance from each other. For a chemical reaction to take place the collision must take place between the molecules.
When the pressure increases the molecules comes close to each other and number of collision between them increases which increase the rate of reaction. Its effect is the same just as the effect of increasing concentration. The pressure can be increased by reducing the volume.
Pressure has no effect on the rate of reaction if no gaseous reactants molecules are present.
- Intensity of light
Photochemical reactions occur under the influence of radiation. Conventional sources of radiation, and modern flash and laser photolysis techniques are both extensively used.
Uv or visible radiation can initiate or catalyze the particular reaction. As the intensity of light increase more reactant molecule are able to gain energy and react which increase the rate of reaction.
Light is in the form of energy. When the intensity of light increase more molecule absorbs energy and chemical reaction occur at a fast rate.
- Orientation of reacting species
For a chemical reaction to take place there should be an effective collision between reactants molecules occur. For this, there must be the proper orientation of reactants(head-on collision).
2HCl + Mg → MgCl2 +H2
Mg H—Cl H-Cl Cl MgH
Wrong orientation correct orientation
- What are the 5 factors that affect the rate of reaction?
The main five factors that affect the rate of reactions are
- Concentration of reactants
- Surface area of reactants
2. How does temperature affect reaction rate?
An increase in temperature causes an increase in the reaction rate. When the temperature increases the kinetic energy of reactants molecules increases so collision between them increases. Now more reactants molecules need less energy for effective collision. So reaction rate increases.
An increase in temperature increases the rate constant which increases the rate of reaction.
3. How does surface area affect reaction rate?
The surface area affects the reaction rate. Greater the surface area greater the reaction rate. When the surface area of solid reactants increases, the chances of collision between reactants increase because now more particles of solid are exposed to the liquid reactants. So reaction rate increases.
4. What are the factors affecting E1 reactions?
E1 is an elimination unimolecular reaction. It takes place in two steps. In the first step ionization of alkyl halide take place to form a carbocation. In the second step, a proton is removed from carbocation to form a double bond.
Tertiary alkyl halide undergoes E1 reaction. The main factors that affect E1 reactions are
- Stability of carbocation
- The concentration of alkyl halide
- Nature of solvent
- Nature of leaving group
5. What is the relationship between concentration and rate of reaction?
The rate of reaction and concentration of reactants are proportional to each other. An increase in the rate of reaction increases the rate of reaction. When the concentration of reactants increases, the frequency of collision between them increases. More reactants are able to convert into products. So the rate of reaction increases.
6. What are the factors on which rate constant depends?
The main factors on which rate constant depends are
- Presence of catalyst
- Activation energy
An increase in temperature causes increases in rate constant according to the Arrhenius equation.
k= rate constant
A= Arrhenius factor
Ea= activation energy
R= general gas constant
Activation energy depends on the catalyst. The presence of catalyst lowers the activation increase and increases the rate constant.
7. How does the activation energy of a reaction affect the overall rate of the chemical reaction?
Activation energy is the minimum amount of energy required for reactants to form products. It is the difference between the energy of the transition state and the energy of reactants. Activation energy determines the speed of reaction.
Higher activation energy means reactants require high energy to cross the barrier to form products. So the rate of reaction is slow and vice versa.
8. What are the units of the rate of reaction in chemistry?
The rate of reaction is the speed at which the reaction proceeds. It is determined by a change in the concentration of reactants or products per unit time.
The unit of concentration is moldm-3 and time is second. So the unit of rate of reaction is moldm-3sec-1 or simply Ms-1.
9. What are factors that affect cellular respiration?
The factors that affect cellular respiration are
- The concentration of oxygen and CO2 in the atmosphere
- Water supply
- Toxic and stimulating substance
10. Why is radioactivity a first-order reaction?
In the first order reaction the rate is directly proportional to first power of concentration of reactants. In radioactivaity, the rate at which radioactive nuclei decay is directly proportional to the number of radioactive atoms present at that time.
Since radioactivity follows the first order rate law so it is first order reaction.
11. What is a Ziegler-Natta catalyst?
It is a catalyst which is formed from mixture of organic compound. It is used in polymerization of olefins to form high molecular weights and high structures.
12. What is the effect of temperature in an endothermic reaction?
13. Does a catalyst increase the number of products formed in a reaction?
A catalyst does not affect the number of products formed in a reaction. The role of catalyst in a reaction is just to lower the activation energy and give alternate path to the reaction. So increase the rate of reaction and product takes less time to form.
13. How does concentration affect the chemical equilibrium?
14. What happens when a chemical reaction reaches equilibrium?
At equilibrium the rate of forward reaction becomes equal to the rate of reverse reaction and the concentration of reactants and products do not change.
15. What do up and down arrows in a chemical equation indicate?
The up arrow indicates that product is a gas evolved during the reaction. And down arrow near the substance shows that substance has precipitated out.
16. What is a key feature of a chemical change?
The characteristics of chemical change are
- Change of colour
- Formation of new substance
- Evolution of gas
- Change of smell
- Change of temperature
- Formation of precipitate
Principles Chemical Kinetics_By James E House.pdf