Rates of Chemical Reactions

During a chemical reaction, reactants collide with one another to form products, and the formation of these products do not occur at the same rates. Hence 

Rate of a chemical reaction can be defined as the number of moles of reactants that are converted, or products that are formed per unit time.

Mathematically 

Rate =      mass in grammes 

                      time taken

For instance, if 6g of zinc metal is placed in dilute tetraoxosulphate (VI) acid and it takes 3 mins to completely react, then the rate of chemical reaction is given by. 

Rate =      6g     = 2g/mins

                3min

That is, 2g of the zinc was converted to ZnCl₂ per minute 

Factors used for Measuring Rates of Reaction

The rate of a chemical reaction can be measured or can be determine by any one of the following.

i.   decrease in mass of reactants.

ii.  increase in volume of a gaseous product 

iii. change in pH

iv. change in colour intensity 

v. Change in pressure

 

These are all measurable factors.

 Factors Affecting the Rate of Chemical Reactions.

The following factors will a reaction to be fast or slow.

i. Nature of reactants

ii. Temperature

iii.. Concentration 

iv. Pressure 

v.  Presence of light

vi. Surface area 

vii. Presence of a catalyst

Before we discussed how each of these factors will affect the rate of a chemical reaction. It is important that we understand the concept of collision theory.

Collision theory assumes that for a chemical reaction to occur, there must be Collision between reactants particles, and these Collisions must be effective.

A collision is said to be effective when it leads to formation of products

What this concept is actually implying is that all collisions do not lead to the formation of a product, only the ones that are effective. So, the various factors that affect chemical reactions are factors that actually increase the number of effective collisions.

I. Effect of Nature of reactants: - this is one of the factors that affects the rate of a reaction, since different elements/ compounds behave differently.

The reactivity of metals varies as you go down the activity series and so when metals react with acids for example the reactions are not the same.

Example by virtue of their nature (their high reactivity) sodium and potassium will react explosively with dilut acids while metals like zinc and iron will react moderately with dilute acids.

II. Effect of Temperature: - An increase in temperature will cause reactants particles to gain more kinetic energy leading to an increase in effective collision and hence, an increase in the rate of reaction. An increase in temperature will also lead to an increase in the energy of the system.

III. Effect of Concentration: - An increase in the concentration of the reactants will lead to an increase in the number of reactants per unit area (the reactants becomes closer) thus, leading to overcrowding and increase in the effective collision of the reactants and hence an increase in the rate of the reaction.

IV. Effect of pressure: - An increase in pressure will lead to a decrease in volume that is, a decrease in the intermolecular space between the reactants particles leading to an increase in the effective collision and hence an increase in the rate of reaction. And vice versa 

V. Effect of presence of light: - Some reactions are photochemical, that is, affected by light. When such reactions are exposed to light the reactants, particles become more activated and collide more increasing the number of effective collisions and hence an increase in the rate of reaction.

VI. Effect of a catalyst: - A catalyst increases the rate of a reaction by creating a different pathway with a lower activation energy.

ACTIVATION ENERGY: see definition in chemistry.

  For a collision to be effective, the reactants must possess the minimum amount of energy needed to overcome the energy barrier (activation energy for the reaction) for that reaction. The higher the activation energy, the slower the rate of reaction; positive catalyst helps to lower the activation energy.

 Energy Profile Diagram 

Energy profile diagram is a diagram / graph that shows the pathway of a chemical reaction. whether the reaction is exothermic or endothermic.

                                          Figure1: Energy profile diagram of an exothermic reaction. 

The reactants have higher energy than the products, showing that heat is released during the reaction. The peak represents the activated complex, while Ea is the activation energy and ΔH is the heat of reaction.

                                         Figure2: Energy profile diagram of an endothermic reaction.

 The products have higher energy than the reactants, indicating that heat is absorbed during the reaction. The highest point is the activated complex, Ea is the activation energy, and ΔH represents the heat of reaction.

 Rate Curve

A rate Curve is the graph which shows the rate of a reaction. It is a graph of reaction against time.( that is, change in concentration against time, decrease in mass against time, e.t.c) 

The slope or the gradient of the curve is steep at the beginning because the reaction is fastest ( since  , it becomes less steep as the reaction progresses and slows down, then it finally becomes horizontal. 

The point at which the graph becomes horizontal indicates the end point of the reaction, when one of the reactants is completely used up

figure 1:  A typical rate curve showing how the mass of a reactant decreases with time during a chemical reaction. The steep part of the curve shows a fast reaction at the beginning, while the flatter part shows the reaction slowing down as the reactants are used up.

Figure 2: A graph showing how the volume of a gaseous product increases with time during a chemical reaction. The curve rises steeply at first, indicating a fast rate of gas production, and then levels off as the reaction nears completion.

ORDER Of REACTIONS

1. A first-order reaction is a chemical reaction in which the rate depends on the concentration of only one reactant raised to the power one.

In simple terms:
The speed of the reaction is directly proportional to the concentration of one reactant.

           Rate ɑ [A]
   or

           {Rate} = k[A]

Where:

• (A) = reactant

• (k) = rate constant

What this means

If the concentration of the reactant is doubled, the rate of reaction also doubles.
If the concentration is halved, the rate is halved.

Example

The decomposition of hydrogen peroxide:

H₂O₂ → H₂O + O₂

The rate depends only on the concentration of (H₂O₂), so it is a first-order reaction

A first-order reaction has a constant half-life — the time taken for half of the reactant to be used up does not change, no matter the starting concentration.

Example of a First-order reaction)

The half-life of a first-order reaction is 10 minutes.
If the initial concentration of the reactant is 80 mol dm⁻³, find the concentration after 20 minutes.

Solution

For a first-order reaction, the half-life is constant.

In 10 minutes → concentration becomes half

                                                         80 →40

In another 10 minutes (total = 20 minutes) → it halves again

40 → 20

Answer

The concentration after 20 minutes is 20 mol dm⁻³.

A second-order reaction is a chemical reaction in which the rate depends on the square of the concentration of one reactant, or on the product of two reactants.

It can be written as:

                                                Rate = k[A]²
                             or 
                                                        Rate = k[A][B]

what this means is that

• If the concentration of a reactant is doubled, the rate increases four times.

• If the concentration is tripled, the rate increases nine times.

Simple example

The reaction between nitrogen dioxide molecules:

                           2NO₂ →2NO + O₂

This is second order because the rate depends on ([NO₂]²).

Another example of a 2nd order reaction is 

The reaction between hydrogen and iodine:

                                H₂ + I₂ →2HI

This reaction is second order because experiments show that its rate depends on the product of the concentrations of both reactants:

Rate = k[H₂][I₂]

It means:

• If the concentration of H₂ is doubled, the rate doubles.

• If the concentration of I₂ is also doubled, the rate doubles again. So the overall rate becomes four times faster.

The reaction is second order because the rate depends on the concentrations of two reacting substances.

waec/neco keypoint

A second-order reaction does not have a constant half-life — its half-life changes as the concentration changes.

OBJECTIVE QUESTIONS 

1. Which of the following best describes the rate of a chemical reaction?

A. The amount of heat produced
B. The speed at which reactants are used up
C. The colour change in a reaction
D. The total energy of the reaction

2. Which of the following factors does NOT affect the rate of a chemical reaction?

A. Temperature
B. Pressure
C. Concentration
D. Density

3. An increase in temperature increases the rate of reaction because

A. molecules become heavier
B. molecules move faster and collide more
C. the volume of the gas increases
D. the pressure decreases

4. Which of the following will increase the rate of reaction between zinc and hydrochloric acid?

A. Using dilute acid
B. Using powdered zinc
C. Lowering the temperature
D. Using large zinc pieces

5. A catalyst increases the rate of a chemical reaction by

A. increasing the temperature
B. creating a different pathway with a lower activation energy
C. increasing the pressure
D. increasing the concentration

6. Which of the following is an example of a catalyst?

A. Copper (II) sulphate
B. Manganese (IV) oxide
C. Sodium chloride
D. Water

7. In a reaction between a solid and a liquid, the rate of reaction is increased by

A. decreasing the surface area of the solid
B. increasing the size of the solid
C. increasing the surface area of the solid
D. reducing the concentration of the liquid

8.  Which of the following will slow down a chemical reaction?

A. Increasing temperature
B. Increasing concentration
C. Adding a catalyst
D. Lowering temperature

9. The rate of reaction between magnesium and dilute hydrochloric acid can be measured by

A. change in colour
B. volume of gas produced
C. mass of magnesium used
D. number of bubbles formed

10. Which graph best represents a fast reaction?

A. One that rises steeply
B. One that rises slowly
C. One that is flat
D. One that slopes downward

11. Which of the following graphs best represents an exothermic reaction?
A. Products higher than reactants
B. Products lower than reactants
C. Reactants and products at same level
D. No activation energy

12. The peak of an energy profile diagram represents the
A. reactants
B. products
C. activated complex
D. catalyst

13. The minimum energy required for a reaction to occur is called
A. enthalpy
B. heat of reaction
C. activation energy
D. reaction rate

14. In an endothermic reaction, the heat of reaction (ΔH) is
A. zero
B. negative
C. positive
D. constant

15. If a reaction produces a gas and the mass of the reaction mixture decreases with time, this is because
A. heat is absorbed
B. gas escapes
C. the solid melts
D. the reaction stops

16. A steep slope on a rate of reaction graph indicates
A. a slow reaction
B. no reaction
C. a fast reaction
D. equilibrium

17. Which of the following will increase the rate of a chemical reaction?
A. Decreasing temperature
B. Removing a catalyst
C. Increasing concentration
D. Decreasing surface area

18. In an exothermic reaction, the products are
A. more energetic than the reactants
B. less energetic than the reactants
C. equal in energy to the reactants
D. unstable

19. The heat energy absorbed or released during a reaction is represented by
A. Ea
B. ΔH
C. K
D. pH

20. On a volume of gas versus time graph, the reaction is complete when
A. the curve is steep
B. the curve is straight
C. the curve becomes horizontal
D. the volume decreases 

21. The rate of a reaction is given by the expression
{Rate} = k[A]^2
If the concentration of (A) is increased from 0.2 mol dm⁻³ to 0.4 mol dm⁻³, how does the rate of reaction change?

A. It doubles
B. It triples
C. It increases four times
D. It remains the same

22. A finely divided form of a metal burn more readily in air than the rod form because the rod has  (a) higher molar mass (b).  smaller surface area (c) protective oxide coating (d) different chemical properties 

THEORY QUESTIONS.

1.(a)(i) Define rate of a chemical reaction?

     ii). mention three factors that can affect the rate of a chemical reaction

     iii). state the collision theory

b).  A sample of carbon is burnt at a rate of 0.50gper second for 30 minutes to generate heat.

    (i). write a balanced equation for the reaction

   (ii). determine the    I. volume of carbon (IV) oxide produced at s.t.p.  II. moles of oxygen used up in the process at s.t.p. [ C= 12.0, O= 16.0, Molar volume Vm = 22.4dm³

c. State how each of the following affects the rate of chemical reactions (i) surface area (ii) catalysts [waec]

2.a(I) What is meant by the rate of chemical reaction? 

(ii). Explain in terms of collision theory, the effect of temperature increase on reaction rate.

(b) When hydrogen peroxide is exposed to air, it decomposes (I) write an equation for the reaction (ii). Outline an experiment to illustrate the effect of a named catalyst on the rate of decomposition. 

(iii).         

3a(i) Sketch an energy profile diagram to show the effect of a catalyst on the reaction rate, given that it is exothermic.

b. The graph below is the ratio curve for the following reaction carried out in an open vessel 

MgCO3(aq) + 2HCl → MgCl2(aq) +H2O(l) + CO2(g)

(i)  For how long the reaction occurred 

(ii) why was there a loss in mass?

(iii) State whether the reaction rate was fastest at the beginning, the middle  or towards the end of the reaction.  Give reason for your answer.

(iv) List three reaction conditions that can affect the slope of the curve.(waec)

4(a)In an experiment, excess 0.050mol/dm3 HCl was added to 10g of granulated zincin a beaker. Other conditions remaining constant state how the reaction rate would be affected in each case if the experiment was repeated using 

(i) 1.0mol/dm₃ HCl

(ii) 8.0g of granulated zinc

(iii) 10g of zinc dust 

(iv) a higher volume of 0.5mol/dm₃ HCl 

(v) a reaction vessel dipped in crushed ice 

(vi) equal volumes of water and 0.50mol/dm3 HCl.

b. 

5. (a) What is meant by the rate of a chemical reaction?

     (b) State three factors that affect the rate of a chemical reaction.

6. Explain how each of the following affects the rate of a chemical reaction:

(a) Temperature
(b) Concentration
(c) Surface area

7. (a) What is a catalyst?

(b) Give two examples of catalysts.
(c) State two characteristics of a catalyst.

8. A piece of calcium carbonate reacts with dilute hydrochloric acid.

(a) Write the balanced chemical equation for the reaction.
(b) State two ways in which the rate of the reaction can be increased.
(c) Explain one of the ways stated in (b).

9. (a) Explain why powdered zinc reacts faster with dilute hydrochloric acid than a lump of zinc.

 (b) What is meant by activation energy?

(c) How does a catalyst affect the activation energy of a reaction?

10. During a chemical reaction, the volume of gas produced was measured with time.

 (a) What information can be obtained from such a graph?
(b) How can you tell from the graph that the reaction is fast?

11. State two industrial processes where catalysts are used and name  the catalyst used in each case.

 (b) Explain why an increase in pressure increases the rate of reaction between gases.

(12).  (i) Draw the energy profile diagram for the reaction H2(g) + I2(g) → 2HI(g) △H = -13kJmol-1 if the concentration of HI(g) increases from 0.001 to 0.002 mol/dm3 in 80 secs what is the rate of the reaction