What is a chemical reaction?
Achemical reaction is the change of a substance into a new one that has a different chemical identity.
How can I tell if a chemical reaction is occurring?
A chemical reaction is usually accompanied by easily observed physical effects, such as the emission of heat and light, the formation of a precipitate, the evolution of gas, or a color change. Absolute confirmation of a chemical change can only be validated by chemical analysis of the products!
Take a look at the following image and describe what you see going on? What are the key indicators of a chemical change? The key is observation!
Click on Sherlock Holmes as a warm to test your powers of observation!
Oh …..don’t forget to check out the barking dog!
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. There is a general description of the main reaction types and specific examples provided in the selection boxes.
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.
A + B AB A and B represent the reacting elements or compounds while AB represents a compound as the product.
The following examples are representative of synthesis reactions.
Formation 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. |
Formation 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. |
Formation of Zinc Oxide: Oxidation is a loss of electrons and reduction is a gain of electrons. The oxidation of metallic Zn by O2 to form ZnO(s) is illustrated at the molecular level. The transfer of electrons from Zn to O2 is shown. Atoms can be observed to change as they are oxidized or reduced, respectively to their ionic forms. |
Formation of Sodium Hydroxide and Potassium Hydroxide: When a small piece of Na is added to a solution containing an indicator, evidence of the reaction can be observed by the change in the color of the solution as NaOH is formed, by the melting of the Na and by the movement of the Na caused by formation of hydrogen gas. K is more reactive than Na as demonstrated by its reaction with water. This reaction produces enough heat to ignite the H2 produced. |
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 + BThermite 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. |
Reduction of CuO: When black carbon and black copper oxide are heated together the Cu2+ ions are reduced to metallic Cu and a gas is evolved. When the gas is collected in Ca(OH)2 a white precipitate of CaCO3 is formed. The reaction which occurs involves the reduction of Cu2+ ions by carbon which is oxidized to CO2. |
Formation of Silver Crystals: When a copper wire is placed in a solution of AgNO3, the Cu reduces Ag+ to metallic Ag. At the same time, Cu is oxidized to Cu2+. As the reaction progresses Ag crystals can be seen to form on the Cu wire and the solution becomes blue as a result of the formation of Cu2+ ions. |
Formation of Tin Crystals: Oxidation-reduction chemistry of Sn and Zn. When acidified Sn(II)Cl2 is added to a beaker containing a piece of Zn, some of the Sn2+ reacts with H+ in the solution to produce H2 gas. Immediate changes can also be observed on the surface of the Zn as it quickly becomes coated with Sn crystals. After the reaction has progressed for a time needles of Sn can be observed on the surface of the Zn. |
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 + BDThis 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. |
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. |
An aqueous solution of Sodium Chloride is added to an aqueous solution of Silver Nitrate forming silver chloride. |
Decomposition Reaction
In a decomposition reaction, single compound undergoes a reaction that produces two or more simpler
substances. A decomposition reaction can be represented by the following equation.
AB A + BElectrolysis of Water: When a direct current is passed through water it decomposes to form oxygen and hydrogen. The volume of hydrogen gas produced at the negative electrode is twice the volume of the oxygen gas formed at the positive electrode. This indicates that water contains twice as many hydrogen atoms as oxygen atoms, which is an illustration of the law of constant composition. |
Decomposition of Nitrogen Triiodide:Nitrogen triiodide is extremely unstable when it is dry. Touching it with a feather causes it to decompose explosively. The explosion occurs as chemical energy is released by the decomposition of nitrogen triiodide to N2 and I2. Violet iodine vapor can be observed after the explosion. |
Combustion Reaction
In a combustion reaction, a substance combines with oxygen, releasing a large amount of energy in the form
of light and heat. For organic compounds, such as hydrocarbons, the products of the combustion reaction are carbon dioxide and water.
CH4 + 2 O2 CO2 + 2 H2OThe combustion of hydrogen yields water vapor as a reaction product. Three balloons of hydrogen and one balloon mixed with hydrogen and oxygen form an explosive mixture |
Reactions with Oxygen. Magnesium, steel wool, white phosphorous, and sulfur are burned in oxygen. The resulting reactions are combination reactions in which two substances react to form one product. The products formed in these reactions are MgO, Fe2O3, P4O10 and SO2. All of these combustion reactions are very exothermic. |
The combustion of yellow phosphorus occurs in an oxygen atmosphere. The main product of this reaction is phosphorus pentoxide. |