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Science form4 - chapter 5

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Science form4 - chapter 5

  1. 1. 5.1 PHYSICAL AND CHEMICAL CHANGES. Two types of changes involved in matter: *Physical changes *Chemical changes a) Physical Changes 1. Physical change are defined as the changes that only affect the physical properties of substances. 2. The characteristics of physical change are:  Involves only physical changes such as shape or size of a substance.  Involves changes in the state of matter; solid, liquid and gas.  Chemicals composition and chemical properties of substances remain unchanged so that no new substances are formed.  Less energy is needed.  The changes are reversible. 3. Physical changes in our daily life are: (a) *Melting of ice: (b) Freezing: (c) *Evaporation of water: (d) Natural water cycle.
  2. 2. (e) Condensation: (f) Dissolving salt and sugar in water. (g) Formation of dew on the grass. 4. Physical changes being studied in a laboratory are: (a) Sublimation: b) Chemical Changes 1. Defined as the changes that affect the chemical composition and chemical properties of the substances. 2. The characteristics of chemical change are:  New substances known as products.  Chemical composition and chemical properties of the new substances are different from the original substance which is known as reactant.  Is usually fixed and irreversible.  Needs large amount of energy. 3. Chemical changes in our daily life are:  Respiration.  Burning of paper or fossil fuels produces carbon dioxide and ash.  Digestion of food.  Washing dishes with detergent or washing hair using shampoo.  Change in colour of peeled apple when exposed to air.  Decomposition of animal carcasses.  Making bread from wheat flour.  Photosynthesis.
  3. 3. 4. Examples of chemical changes being studied in a laboratory are: (a) Respiration Oxygen + glucose --------> carbon dioxide + water vapour + energy (b) Neutralisation: Acid + alkali + -------> salt = water (c) Electrolysis of water. COMPARING AND CONTRASTING PHYSICAL AND CHEMICAL CHANGES 5.2 HEAT CHANGE IN CHEMICAL REACTIONS a) Chemical Reaction Involves Heat Change 1. Chemical reactions involves changes in energy in the form of heat energy. 2. Chemical reactions:-  energy is aborded to break bonds in the reactants.  energy is released when new bonds sre formed in the products. 3. Chemical reaction that absorbs energy is dissolving ammonium chloride in water. When ammonium chloride (Salt) is put into a beaker of water which is held with our hands, we can feel that the temperature of water falls. This is because the reaction in the beaker absorbs energy from the water and our hands. 4. Chemical reactions that releases energy is dissolving sodiumhydroxide in water. When sodium hydroxide (SOAP) is put into a pail of water and stirred using our hands, we can feel that the soap solution becomes hot. This is because the reaction releases heat into the surroundings. b) Reaction Involving Heat Loss And Heat Gain. Chemical reactions can be classified into two groups:  Exothermic reactions.  Endothermic reactions
  4. 4. Exothermic reactions 1. Exothermic reactions is defined as a reaction which releases energy in the form of heat into the surrounding (heat loss), and causes the surrounding temperature to increase. 2. The characteristics of exothermic reactions are:  Heat is released into the surroundings.  Energy is released when bonds are formed because the energy needed to break the bond in the reactants is less than the energy released when new bonds are formed in the products.  Surrounding temperature increases due to the release of energy but the temperature of mixture decreases.  Energy content of the reactant(s)  Higher energy content of the product(s) Endothermic reactions 1. Absorb energy or gain heat (surrounding temperature to decrease). 2. The characteristics of exothermic reaction are:  Heat is absorbed.  Energy is aborded.  Surrounding temperature decreases and temperature of the mixture increases.  Energy content of the reactant(s) is lower than energy content of the product(s) 3. Endothermic reactions are:  Boiling water.  Heating copper sulphate crystals.  Melting wax.  Dissolving ammonium chloride in water.  Photosynthesis.  Decomposition of mercury oxide by heat and decomposition of copper carbonate by heat. c)Heat Changes In Industrial Chemical Reactions  Haber Process  Contact Process Haber Process 1. The production of ammonia through the Haber process involves the reaction between nitrogen and hydrogen gases mixed in the proportional 1:3 2. The Haber process is an exothermic reaction. It is a reversible reaction (forward and backward reaction). 3. Catalyst (iron) at a temperature of 450-500°C and a pressure of 200-250 atmospheres.
  5. 5. The Haber Process Contact Process 1. The Contact process is an industrial process to produce sulphuric acid 2. Catalyst (vanadium(V) oxide) at a temperature of 450-500°C and a pressure 1 atm. 3. Sulphurid acid is used:-  In the production of chemical fertilisers  In the production of detergent  As the electrolyte in accumulators (batteries)  As a dehydrating agent The Contact Process 5.3 THE REACTIVITY OF METALS.  Reactions of metal with water  Reactions of metal with acid  Reactions of metal with oxygen
  6. 6. Reactions of metal with water. 1. The most reactive metals can react with cold water to produce alkaline solutions and hydrogen. 2. There are only three metals that can react with water: potassium, sodium and calcium Reactions of metal with acid 1. Most metals (i.e. metals that are placed above hydrogen in the electrochemical series) can react with acids 2. During the reaction, the metal displaces hydrogen from the acid to form a salt and hydrogen gas 3. Metal + acid = salt + hydrogen Reactions of metal with oxygen. 1. Metal + oxygen = metal oxide 2. The reactivity of metals with oxygen decreases down the reactivity series of metals. 3. The metal oxide produced a different colour compared to the original metal.
  7. 7. COMPARING AND CONTRASTING THE REACTIVITY OF METALS WITH WATER, ACIDS AND OXYGEN. The Reactivity Series Of Metals. The reactivity series order of reactivity with oxygen
  8. 8. The Position Of Carbon In The Reactivity Series Of Metals. 1. The reactivity series of metals shows the inclination of a metal to react with oxygen. 2. The operational definition of this experiment is carbon reduces a metal oxide to its metal if carbon is more reactive than the metal. Metal oxide X + carbon = carbon dioxide + metal X 3. On the other hand, carbon does not react with a metal oxide if it is less reactive than the metal. 5.4 APPLICATION OF THE REACTIVITY SERIES OF METALS a) Method of Extraction of Metals  Depend on their positions in the reactivity series.  Metals placed below carbon in the reactivity series can be extracted from their ores by heating their ores with carbon  Electrolysis is only used to extract metals from their ores if the metals are more reactive than carbon
  9. 9. b) The importance of the reactivity series are:  Determine the method of extracting the metal.  Predict the reactivity of a particular metal.  Predict whether a particular metal can remove oxygen from another metal oxide. 5.5 ELECTROLYSIS 1. Electrolysis is the decomposition of electrolytes by electric current 2. During electrolysis, electrical energy changes to chemical energy Term Explanation Electrolyte a substances (in liquid or molten state) that allows electricity to flow through it. Electrode carbon rod or platinum sheet dipped in the electrolyte Anode positive electrode connected to the positive source of the battery Cathode negative electrode connected to the negative source of the battery Anions negative ions that are attracted to the anode Cations positive ions that are attracted to the cathode 3. The process that occurs during electrolysis  Cations are attracted to the cathode. At the cathode, cations receive electrons and are discharged to form neutral atoms.  Anions are attracted to the anode. At the anode, anions release electrons and are discharged to form neutral atoms. Electrolysis In Industry  Extraction of metals  Purification of metals  Electroplating of metals
  10. 10. Extraction of metals 1. Metals placed above carbon in the reactivity series of metals can be extracted from their oxides by electrolysis 2. Examples:  Extraction of aluminium from aluminium oxide (bauxite)  Extraction of sodium from sodium chloride Purification of metals 1. The following conditions must be satisfied:  Anode: the strip of impure metal  Cathode: the strip of pure metal  Electrolyte: a salt solution containing the ions of the metal that is to be purified. 2. Example: purification of copper metal  Anode: the impure copper  Cathode: the pure copper  Electrolyte: copper sulphate solution Electroplating of metals 1. Electroplating is defined as the process of covering the surface of a metal with another metal with electrolysis. 2. The reasons for electroplating are:  To prevent and protect metals from corrosion and rusting  To make a metal surface look more attractive 3. In electroplating, the following conditions must be satisfied  Anode: the plating metal  Cathode: the object that is to be electroplated  Electrolysis: a salt solution containing the plating metal 4. Example: electroplating an iron spoon with silver  Anode: silver metal  Cathode: iron spoon  Electrolysis: silver nitrate solution 5.6 THE PRODUCTION OF ELECTRICAL ENERGY FROM CHEMICALS REACTIONS Simple cells 1. A simple cells consists of two metal electrodes which are different in reactivity and immersed in an electrolyte 2. The polarity of the cell depends on the positions of the two metals in the reactivity series
  11. 11. 3. Various types of cells Type of cell Users Advantages Disadvantages Dry cell used in radios, torches, toys, electric shavers, cameras, alarm clock  Light and can be found in variety of sizes  Electrolytes cannot spill  The current produced is firm because polarization is overcome  Cannot last long  The electric current that is produced is small  Cannot be recharged when it is fully discharged Lead-acid accumulator used to supply energy to cars, lorries, and hospital emergency equipment  It can be recharged when fully discharged  The current and voltage produced are large and constant  Long lasting  It is heavy and difficult to carry  It can be easily spoilt and can lose its charge if it is not used for a long time  Its electrolyte cell spill easily Alkaline battery used to supply a large amount of electrical energy, useful for clocks and games equipment  Long lasting  The current and voltage produced are large and constant non rechargeable silver oxide- mercury battery used in watches, calculators and hearing aids  It is small in size and easy to carry  The current and voltage produced are large and constant  Long lasting  It is expensive  non rechargeable nickel cadmium battery used in electronic devices  It can be recharged  Long lasting it is expensive
  12. 12. 5.7 CHEMICAL REACTIONS THAT OCCUR IN THE PRESENCE OF LIGHT. a) Photosynthesis 1. In photosynthesis, sunlight is absorbed by the chlorophyll in the leaves and a biochemical reaction is carried out Carbondioxide +water --------------------------- > glucose + oxygen 2. Importance: to maintain the percentage of composition of oxygen and carbon dioxide in the atmosphere and to provide food to plants. b) Effect of Light on a Photosensitive Chemicals 1. Photographic films are made of plastic strips covered with an emulsion consisting of gelatin and silver bromide crystals 2. To snap a picture, the filmmust exposed to light 3. The part of the filmthat is exposed to light will undergo chemical reaction 4. The exposed filmis then processed by using sodium thiosulphate solution to remove the unexposed layer 5. The product is a negative image on the photographic film. 6. Storing:  Film must be wrapped in black plastic wrapper or put in a black box to prevent it from exposed to light  Some medicine must be stored in dark bottles to prevent them from being spoilt 7. Examples of photosensitive chemicals:  Silver salt  Hydrogen peroxide  Chlorine water  Sodium hypochlorite 5.8 USE OF CHEMICAL REACTIONS AS SOURCES OF ENERGY 1. Chemical reactions enable us to extract active metals from their ores using electrolysis 2. Electroplating can produce attractive product that can resist corrosion and rusting 3. The changes of chemical energy to electrical energy enables us to build various types of chemical cells for supply of power

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