Lesson Notes By Weeks and Term v3 - Senior Secondary 2
Chemical Reactions
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Chemical Reactions
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Subject: Chemistry
Class: Senior Secondary 2
Term: 1st Term
Week: 1
Theme: The Chemical World
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A chemical reaction is a process that involves the rearrangement of the atomic, ionic, or molecular structure of substances, leading to the formation of new substances. During a chemical reaction, chemical bonds in the original substances are broken, and new chemical bonds are formed.
Reactants: These are the starting substances in a chemical reaction. They are written on the left side of a chemical equation.
Example:* In the burning of charcoal (carbon) in air, carbon and oxygen are the reactants.
Products: These are the new substances formed as a result of a chemical reaction. They are written on the right side of a chemical equation.
Reaction Time: This is the total time taken for a chemical reaction to go to completion or for a significant change in reactant or product concentration to occur. It is often measured from the start of the reaction until no further change is observed.
Reaction Rate: This refers to the speed at which a chemical reaction proceeds. It is defined as the change in concentration of a reactant or a product per unit time. Rate = (Change in concentration of reactant or product) / (Change in time) Units are typically mol dm−3 s−1 (or M/s).
Relationship: Reaction rate and reaction time are inversely related. A faster reaction rate means the reaction takes a shorter reaction time to complete. A slower reaction rate means the reaction takes a longer reaction time to complete.
Analogy: If a danfo (commercial bus) drives fast (high rate), it reaches its destination in a shorter time. If it drives slowly (low rate), it takes a longer time. Collision theory explains how chemical reactions occur and why reaction rates differ. For a reaction to take place, reactant particles (atoms, ions, or molecules) must: Collide: The particles must physically come into contact with each other. Possess Sufficient Energy (Activation Energy): The colliding particles must have kinetic energy equal to or greater than a minimum energy called the activation energy (Ea). This energy is required to break existing bonds in the reactants. Collisions with less than the activation energy are ineffective and do not lead to a reaction.
Have Correct Orientation: The particles must collide in a way that allows the specific parts of the molecules involved in bond breaking and formation to come into contact. An incorrect orientation, even with sufficient energy, will result in an ineffective collision.
Impact on Reaction Rate and Time: A higher frequency of effective collisions (collisions with sufficient energy and correct orientation) leads to a faster reaction rate and therefore a shorter reaction time. Factors that increase the number of effective collisions will increase the reaction rate. Several factors can influence how fast a chemical reaction proceeds: Nature of Reactants: Physical State: Gases generally react faster than liquids, and liquids react faster than solids, because particles in gases and liquids are more mobile and can collide more frequently.
Surface Area (for Solids): Increasing the surface area of solid reactants increases the number of particles exposed and available for collisions. Finely powdered solids react much faster than larger lumps of the same substance.
Example:* Powdered chalk (calcium carbonate) reacts much faster with dilute acid than a solid lump of chalk. This is relevant in industries where raw materials are ground finely to speed up reactions, like in cement production or ore processing.
Chemical Reactivity: Some substances are intrinsically more reactive than others.
Example:* Sodium reacts explosively with water, while iron reacts very slowly (rusting).
Concentration/Pressure: Concentration (for solutions): Increasing the concentration of reactants means there are more reactant particles per unit volume. This leads to a higher frequency of collisions, thus increasing the reaction rate.
Example:* A more concentrated bleaching solution cleans faster than a dilute one.
Pressure (for gases): Increasing the pressure of gaseous reactants forces the gas particles closer together, effectively increasing their concentration. This leads to a higher frequency of collisions and a faster reaction rate.
Example:* Industrial processes often use high pressure to speed up reactions involving gases, such as the synthesis of ammonia.
Temperature: Increasing the temperature of a reaction system increases the kinetic energy of the reactant particles.
This has two main effects: Increased Collision Frequency: Particles move faster, leading to more frequent collisions. Increased Proportion of Effective Collisions: A larger fraction of colliding particles will possess energy equal to or greater than the activation energy (Ea), leading to more effective collisions.
Therefore, increasing temperature generally increases the reaction rate significantly. A common rule of thumb is that for many reactions, the rate doubles for every 10°C rise in temperature.
Example:* Food spoils faster at room temperature than in a refrigerator because reactions causing spoilage are slower at lower temperatures. Cooking is faster at higher temperatures.
Catalysts: A catalyst is a substance that increases the rate of a chemical reaction without being chemically changed or consumed in the overall reaction. Catalysts work by providing an alternative reaction pathway with a lower activation energy (Ea). This means that a larger proportion of reactant particles will have enough energy to react, leading to more effective collisions and a faster reaction rate.
Types: Homogeneous catalyst: In the same phase as reactants (e.g., enzymes in biological reactions).
Heterogeneous catalyst: In a different phase from reactants (e.g., solid metals in gaseous reactions).
Example:* Manganese(IV) oxide (MnO2) catalyzes the decomposition of hydrogen peroxide (H2O2). Enzymes in our bodies are biological catalysts that speed up digestion. Catalytic converters in vehicles use platinum, palladium, and rhodium to convert harmful exhaust gases into less toxic ones, crucial for reducing air pollution in Nigerian cities. Chemical reactions involve energy changes, typically in the form of heat.
Exothermic Reactions: Definition: Reactions that release heat energy into the surroundings.
Characteristics: The temperature of the surroundings increases. The enthalpy change (ΔH) for exothermic reactions is negative (ΔH 0) because the products have higher energy than the reactants, and energy is absorbed from the surroundings.
Examples:* Photosynthesis (plants absorb solar energy), dissolving ammonium chloride in water (causes cooling), thermal decomposition (e.g., limestone decomposition). N2(g) + O2(g) → 2NO(g) ; ΔH = +180 kJ/mol (formation of nitrogen monoxide) Instant cold packs often contain ammonium nitrate and water, which react endothermically when mixed.
Food Processing and Preservation (Local Context): Fermentation: The production of ogi, garri, or palm wine involves biochemical reactions catalyzed by enzymes from microorganisms. Understanding reaction rates helps in optimizing fermentation time and conditions for desired product quality and shelf life. For instance, controlling temperature affects the rate of yeast activity in palm wine fermentation.
Cooking: Increasing temperature speeds up cooking of staple foods like yam, rice, and beans. Pressure cookers achieve even higher temperatures and faster cooking times by increasing pressure. Cutting food into smaller pieces (increasing surface area) also speeds up cooking. Refrigeration slows down spoilage reactions, preserving food for longer. Industrial Manufacturing (Nigerian Industries): Fertilizer Production: The Haber process (N2(g) + 3H2(g) ⇌ 2NH3(g)) is a prime example of applying Le Chatelier's principle. To maximize ammonia yield (a key component of fertilizers essential for Nigerian agriculture), high pressures and carefully controlled temperatures (moderate, as a compromise between rate and equilibrium position) are used, along with an iron catalyst.
Cement Production: The decomposition of limestone (CaCO3 → CaO + CO2) is an endothermic reaction requiring high temperatures. Understanding the energy changes and reaction rates is critical for efficient kiln operation.
Environmental Management: Rusting of Metals: Iron rusting (4Fe(s) + 3O2(g) → 2Fe2O3(s)) is a slow chemical reaction accelerated by water and electrolytes. Understanding its factors allows for protective measures like painting, galvanizing, or cathodic protection, which are crucial for maintaining infrastructure like bridges, pipelines, and roofing sheets (zinc is used for galvanization in Nigeria).
Waste Treatment: Many waste treatment processes rely on chemical reactions (e.g., oxidation, precipitation) to convert harmful substances into less toxic ones. Understanding reaction rates helps in designing efficient treatment plants.