Slide 1 : chracterstics of chemical reactions class 8th science & types of chemical reactions
Slide 2 : Observations are an important factor in the determination of whether or not a chemical reaction has occurred. Some types of evidence that may indicate a chemical reaction include the following observations:
Formation of a gas: This may be indicated by fizzing or bubbling.
The reaction of carbonates or bicarbonates with acids to produce carbon dioxide gas is an example of this type of evidence.
Formation of a solid:
The formation of a solid (precipitate) is evidenced by the reaction of silver ions with chloride ions to form the insoluble salt, silver chloride.
Disappearance of a solid:
When "acid rain" destroys marble statues or limestone structures a chemical reaction is occurring. The carbonates that form marble or limestone are dissolved away by the acids resulting from environmental pollutants.
Change in color:
When copper carbonate (a bluish-green compound) is heated, carbon dioxide is given off and copper oxide (a black compound) is formed.
Heat is given off or absorbed:
Slide 3 : There are many different types of chemical reactions. Chemists have classified the many different reactions into general categories. The chemical reactions we will explore are a representation of the types of reactions found in each group. Synthesis Reaction (Combination Reaction)
In a synthesis reaction, two or more substances combine to form a new compound. This type of
reaction is represented by the following equation. types of chemical reactions
Slide 4 : synthesis reaction A + B AB
A and B represent the reacting elements or compounds while AB represents a compound as the product.
Slide 5 : The following examples are representative of synthesis reactions.
Aluminum and BromineFormation of Aluminum Bromide: When Al is placed on the surface of liquid Br2 an exothermic reaction occurs. The Al is oxidized to Al3+ by the Br2, which is reduced to Br - ions. The ionic product, AlBr3, can be observed on the watch glass after the reaction.
Sodium and ChlorineFormation of Sodium Chloride: Molten sodium burns when it is put into a container of chlorine gas. In the reaction a sodium ion loses an electron to form a sodium cation and a chlorine atom simultaneously gains an electron to form a chloride anion. The product of the reaction is the ionic compound sodium chloride, which is the white solid observed. examples of synthesis reactions
Slide 6 : Combustion - When organic compounds like propane are burned, they react with the oxygen in the air to form carbon dioxide and water. The reason why these combustion reactions will stop when all available oxygen is used up is because oxygen is one of the reactants. The basic form of the combustion reaction is shown below;
hydrocarbon + oxygen ----> carbon dioxide and water
Some examples of combustion reactions are;
CH4(g) + 2O2(g) ----> 2H2O(g) + CO2(g)
2C2H6(g) + 7O2(g) ----> 6H20(g) + 4CO2(g)
C3H8(g) + 5O2(g) ----> 4H examples of combustions
Slide 7 : ) Decomposition: A decomposition reaction is the opposite of a synthesis reaction - a complex molecule breaks down to make simpler ones. These reactions come in the general form:
AB ---> A + B
One example of a decomposition reaction is the electrolysis of water to make oxygen and hydrogen gas:
2 H2O ---> 2 H2 + O2 decomposition reactions
Slide 8 : Single-Replacement Reaction
In a single-replacement reaction (displacement reaction) one element replaces a similar element
in the compound. Single-replacement reactions can be represented by the following equations.
AB + C AC + B
Iron (III) Oxide and Aluminum Reaction 2Thermite Reaction: In the thermite reaction, Al reduces Fe2O3 to Fe in an extremely exothermic reaction in which Al is oxidized to Al2O3. The reaction produces enough heat to melt the iron. Because of the extreme heat produced in the thermite reaction, it is used industrially to weld iron. replacement reactions
Slide 9 : Double-Replacement Reaction
In a double-replacement reaction, the ions of two compounds exchange places in an aqueous solution
to form two new compounds. A double-replacement reaction can be represented by the following equation.
AB + CD AC + BD
Calcium carbonate and Sulfurous Acid This marble statue has been eroded by acid rain. Marble is a material having CaCO3 as its primary component. Acids react with and dissolve the marble. The acid comes from sulfur dioxide in the atmosphere combining with water to form sulfurous acid.
Lead (II) Nitrate and Potassium Iodide An aqueous solution of Potassium Iodide is added to an aqueous solution of Lead (II) Nitrate forming lead (II) iodide. The formation of a precipitate occurs when the cations of one reactant combines with the anions of the other reactant to form an insoluble or slightly insoluble compound. double displacement reactions
Slide 10 : Acid - Base
A reaction between an acid and a base to form a salt and water as the only products.
Hydrochloric acid and sodium hydroxide ---> sodium chloride and waterHCl + NaOH ---> NaCl + H2O
Sulphuric acid and potassium hydroxide ---> potassium sulphate and waterH2SO4 + 2KOH ---> K2SO4 + 2H2O
Nitric acid and calcium hydroxide ---> calcium nitrate and water2HNO3 + Ca(OH)2 ---> Ca(NO3)2 + 2H2O neutrilization reactions
Slide 11 : Endothermic ReactionsChemical reactions in which energy is absorbed are endothermic. Energy is required for the reaction to occur. The energy absorbed is often heat energy or electrical energy. Adding electrical energy to metal oxides can separate them into the pure metal and oxygen. Adding electrical energy to sodium chloride can cause the table salt to break into its original sodium and chlorine parts.
Exothermic ReactionsChemical reactions in which energy is released are exothermic. The energy that is released was originally stored in the chemical bonds of the reactants. Often the heat given off causes the product(s) to feel hot. Any reaction that involves combustion (burning) is an exothermic chemical reaction. The next two pages include labs for both endothermic and exothermic reactions!