CHEMICAL REACTION I
Week: Date: Time:
Period: Duration: 1 HR 20 MIN. Average age of learners: 16YEARS
Subject: CHEMISTRY Class: SS TWO
Topic: CHEMICAL REACTIONS -I
Sub topic: BASIC CONCEPT AND FACTORS
Reference materials:
(1) ESSENTIAL CHEMISTRY, TONALD PUBLISHERS, I. O ODESINA
(2) NEW SCHOOL CHEMISTRY, AFRICAN FIRST PUBLISHERS, OSEI YAW ABABIO
(3) INTERNET
Instructional materials: beaker, beam balance
Entry behavior: The students have been taught physical and chemical change.
Behavioural objective: At the end of the lesson the students should be able to:
1. Explain the basic concept in chemical reaction
2. State the factors affecting the rate of chemical reaction.
3. Explain any two factors affecting the rate of chemical reaction.
CONTENT
CHEMICAL REACTION
– I
BASIC CONCEPT
Reactants: this is the starting substances in a chemical reaction. It is also called reagents.
Product: these are the substance(s) formed as a of a chemical reaction.
Reaction time: is how long it takes a specified set of reactant (on a molecular level) to react to formed product.
Rate of reaction: is the number of moles of reactant converted or product formed per unit time
Rate of reaction =
Reaction rate: the change in the concentration of reactant or product with time.
A chemical kinetics is the area of chemistry concerned with the speed or rates at which a chemical reaction occurs. Here, kinetic refers to the rate of a reaction.
A chemical reaction involves the transformation of reactant to products. The rate of chemical reaction, is the number of moles of reactant converted or product formed per unit time i.e
A chemical kinetics is the area of chemistry concerned with the speed or rates at which a chemical reaction occurs. Here, kinetic refers to the rate of a reaction.
A chemical reaction involves the transformation of reactant to products. The rate of chemical reaction, is the number of moles of reactant converted or product formed per unit time i.e
Rate of reaction =
Example
If 2g of zinc granules was reacted with excess dilute HCl to evolve hydrogen gas which came to completion after 5 minutes. Calculate the rate of the chemical reaction in g hr-1.
Solution
Mass of zinc = 2g ; time = 5 mins = 5/60 = 0.0833hr
Rate of reaction =
FACTORS AFFECTING THE RATE OF CHEMICAL REACTION
It’s useful to be able to predict whether an action will affect the rate at which a chemical reaction proceeds. There are several factors that can influence the rate of chemical reaction. In general, a factor that increases the number of collisions between particles will increase the reaction rate and a factor that decreases the number of collisions between particles will decrease the chemical reaction rate. These factors are:
1. nature of the reaction 2. Concentration 3. Surface area of reactant
4. Temperature of reaction mixture 5. Present of light 6. Present of catalyst
These factors will influence:
a. The energy content of the particle b. the frequency of collision of the particle
c. The activation energy of the reaction.
1. NATURE OF THE REACTANTS: The rate of a chemical reaction is influenced by the nature of the reactants taking part in the reaction. For example, hydrogen combines vigorously with chlorine under certain conditions, but only slightly react with nitrogen under the same condition.
Also, when iron is placed in dilute hydrochloric acid, there is a slow evolution of hydrogen gas, with a piece of zinc, the reaction is rapid and with a piece of gold, there is no evidence of reaction.
2. CONCENTRATION OF REACTANTS: A higher concentration of reactants leads to more effective collisions per unit time, which leads to an increasing reaction rate. Similarly, a higher concentration of products tends to be associated with a lower reaction rate. Use the partial pressure of reactants in a gaseous state as a measure of their concentration.
Pressure affects the concentration of gaseous reactants. For example, a mixture of hydrogen and chlorine gases will react twice as fast, if the partial pressure of hydrogen or chlorine is doubled.
The concentration of solid and liquid reactants are unaffected by changes in pressure. For instance, The reaction between HCl and Mg.
Mg + 2HCl MgCl2 + H2
Hydrogen gas is given off. The rate of reaction is determined by measuring the time taken for effervescence. The concentration of the acid is varied, while the mass of the metal is kept constant.
3. SURFACE AREA OF THE REACTANTS: In solids, chemical reactions often take place on the surface of the solids. With a greater surface area of solid, like in finely divided (powdered) particles, collisions are far more frequent. For example, the rate of evolution of CO2 is faster when marble (CaCO3) powder is reacted with dilute HCl than when marble lumps are used. A greater surface area is exposed with marble powder than with the lump and as a result, the reaction with powder proceeds to completion more quickly. Zinc dust burns more quickly in oxygen or chlorine than zinc granules.
4. EFFECT OF TEMPERATURE: When the temperature of a reaction is increased, heat is supplied to the particles involved in the reaction and since heat is a form of energy we should expect that energy to go at least partly to increased the speed at which the particle travel i.e the particles acquire more kinetic energy if the particle travel at a greater speed when the temperature is increased, then they will collide with one another more frequently and at a higher speed, and the reaction would proceed at faster rate because more effective collisions will occur.
Temperature is an important factor in affecting the rate of chemical reaction which would not take place except the temperature is increased e.g CaCO3 will not decompose to yield CO2 until a certain temperature of decomposition is reached.
5. PRESENCE OF LIGHT: Light is a source of energy and can influence the rate of some chemical reaction, considerably by energizing the particles or molecules involved. For example, the reaction between H2 and Cl2 is negligible in the absence of light, moderately in day light and explosive in bright sunlight. This reaction that involves light is known as photochemical reaction. Other examples of these reactions are:
I. the combination of methane and chlorine II. the decomposition of hydrogen peroxide
III. conversion of silver halide to grey metallic silver used in photography IV. photosynthesis in plant.
6. EFFECT OF CATALYST: A catalyst is a substance which although present in small proportion, alter the rate of chemical reaction, but itself does not undergo change at the end of the reactions.
A catalyst has no effect on the equilibrium position but it only enables the equilibrium to be attained more rapidly. A catalyst which increases the rate of chemical reaction is known as POSITIVE CATALYST and this usually acts by lowering the activation energy or the energy barrier of a chemical reaction.
Some catalysts which slow down the rate of chemical reaction are known as NEGATIVE CATALYST OR INHIBITORS. Catalysts are very important in the chemical engineer to obtain their product, in the shortest time using the least amount of energy. Catalyst works by changing the activation energy of the reaction.
Characteristics of catalyst
I. It alter the rate of chemical reaction.
II. It is specific in action.
III. It cannot start a reaction, it is only effective in a reaction which is already in progress.
IV. It has no effect on the equilibrium position of a reversible reaction.
V. A catalyst remains unchanged in chemical nature and mass at the end of a reaction, although the physical feature such as colour and texture may be change.
ENZYMES: These are organic catalyst that are responsible for the catalysts of processes taking place in living organisms. Enzymes like ordinary inorganic catalyst do not alter the product formed, but alter the rate of their production. The enzymes are used to breakdown large molecules in the process of digestion.
Some of the common catalyst and their uses.
I. Vanadium (v) oxide (V2O5) for the manufacture of H2SO4 by the contact process.
II. Iron (Fe) for the production of ammonia by the harber process.
III. Oxygen in the production of polythene.
IV. Ethyl lead (Pb(C2H5)4) is a negative catalyst used to prevent knocking.
V. Nickel in the hydrogenation of alkene in magarine production.
VI. Manganese (iv) Oxide [MnO4] for the thermal decomposition of potassium trioxochlorate (v).
PRESENTATION
i.The teacher explains the basic concept of chemical reaction.
ii. The teacher lists the factors affecting the rate of chemical reaction.
iv. The students mention the factors affecting the rate of chemical reaction.
iii. The teacher explains all the factors affecting the rate of chemical reaction.
EVALUATION
The teacher evaluates the lessons by asking the following questions:-
1. Explain the basic concept in chemical reaction
2. State the factors affecting the rate of chemical reaction.
3. Explain any two factors affecting the rate of chemical reaction.
ASSIGNMENT
1.In an experiment, excess 0.50moldm-3 HCl was
added to 10g of granulated zinc in
beaker, other conditions remaining constant, state how the reaction rate would
be affected in each case. If the experiment was repeated using:
1.0moldm-3HCl (b)8.0g of granulated zinc. (c.)10g of zinc dust.
(d)A higher volume of 0.50moldm-3 HCl. (e)A reaction vessel dipped in crushed ice.
(f)Equal volume of water and 0.50moldm-3 HCl.
2a. Explain why coal burns more easily when it is broken into pieces than when it is in lump form.
b. Iron fillings corrodes faster than iron nails of the same mass.
3. Three breakers labeled A, B and C each contained zinc metal of the same mass but in different forms. A contained a length of zinc rod, B contained zinc dust while C contained zinc foil. 100cm3 of 5.0moldm-3 hydrochloric acid was added to each beaker to react with all the zinc.
(a) State the order in which the reaction came to completion in beakers A, B and C starting with the fastest.
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