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Lesson 8: Chemical Reactions





Online Glossary




Types of Chemical Reactions


Combination Reactions


Combination reactions are those in which individual elements react together to form compounds. The reaction of elemental hydrogen gas (H2) and oxygen gas (O2) to form water is an example:


2 H2 + O2 ® 2 H2O


There are many more examples. Sodium and chlorine gas react to form sodium chloride:


2 Na + Cl2 ® 2 NaCl


Carbon (as in charcoal or graphite) reacts with oxygen to form carbon dioxide:


C + O2 ® CO2


Magnesium and oxygen react to form magnesium oxide:

Mg + O2 ® MgO2


Combination reactions are also called synthesis reactions. Simple compounds can be used to form more complex compounds. In this case, carbon dioxide (CO2) reacts with water to form carbonic acid (H2CO3):


2 NaO + H2O ® 2 NaOH


Combination reactions have this general form:
A + B ® AB.



Decomposition Reactions


Decomposition reactions are essentially the reverse of combination processes. One or more compounds react to form two or more elements. One example of a decomposition reaction is water decomposing into hydrogen gas (H2) and oxygen gas (O2):


2 H2O ® 2 H2 + O2.


Sometimes decomposition reactions occur with one compound breaking down into two or more simpler compounds, as in the thermal decomposition of calcium carbonate (CaCO3) into calcium oxide (CaO) and carbon dioxide (CO2):


CaCO3 ® CaO + CO2


Decomposition reactions take the general form:
AB ® A + B.



Combustion Reactions


Combustion reactions are well known to us. A substance undergoing a combustion reaction reacts with oxygen to form products that are all combined with oxygen. The examples we are most familiar with are probably those that occur in our automobiles, in which hydrocarbon compounds burn to produce water and carbon dioxide. In this case octane, C8H8, reacts with oxygen gas (O2) to form carbon dioxide (CO2) and water:


2 C8H8 + 25 O2 ® 18 H2O + 16 CO2


Combustion reactions are very useful to us because another result is the release of thermal (heat) energy. We harness this energy to do work in our cars, homes, factories and power plants. We commonly think of combustion as “burning in air”.


Other types of compounds also burn in air. Because ethanol is a renewable source of energy, it is sometimes added to gasoline (forming gasohol). Ethanol burns in air to produce carbon dioxide and water (and remember to include oxygen gas, O2, as a reactant):


C2H5OH + 3O2 ® 2CO2 + 3H2O


Methane (CH4) is the fuel found in natural gas. Many peoples’ homes are heated by natural gas heat; they may also have ranges, clothes dryers, and other appliances fueled by natural gas.  The combustion of methane is as follows:


CH4 + 2 O2 ® CO2 + 2 H2O


You may have already noticed that combustion reactions all have carbon dioxide and water as products.




Single Displacement Reaction


In a single displacement reaction, one element takes the place of another element, as part of a chemical compound. The reactants will therefore include one element and one compound. The products will include a new product and an element. Zinc and copper(II) sulfate react to form zinc sulfate and copper:


Zn + CuSO4 ® ZnSO4 + Cu.


The general form for single replacement reactions is:
A + BX ® AX + B.


Chlorine gas and potassium iodide react to form potassium chloride and iodine:


Cl2 + 2 KI ® 2 KCl + I2


Iron reacts with copper(II) sulfate to form iron(II) sulfate and copper:


Fe + CuSO4 ® FeSO4 + Cu.




Double Displacement Reaction


Double displacement reactions are also known as metathesis reactions. The reactants are compounds that form products by recombining elements. Each product is a compound that is different from the reactants and that has a different set of chemical and physical properties. We commonly observe differences in solubility of reactants and products with this type of reaction. Often one product is insoluble in water while the other is soluble.


The general form for double displacement reactions is:
AX + BY ® AY + BX.


Here are some examples:


Calcium oxide (CaO) reacts with hydrochloric acid (HCl) to form calcium chloride (CaCl2) and water:


CaO + 2 HCl ® CaCl2 + H2O


Silver nitrate (AgNO3) reacts with sodium chloride (NaCl) to form silver chloride (AgCl) and sodium nitrate (NaNO3):


AgNO3 + NaCl ® AgCl + NaNO3


Symbols indicating the state of matter (s = solid; l = liquid; g = gas), or whether the compounds are water soluble (aq = in aqueous solution) are needed to show that products have different physical properties than the reactants.


In the equation:


FeCl2 (aq) + K2S (aq) ® FeS (s) + 2 KCl (aq)


both reactants are water soluble and are initially found in aqueous solution. The product iron(II) sulfide is NOT water soluble, and settles out of solution as a solid, indicated "(s)", as soon as the reaction occurs. The other product, potassium chloride (KCl) is water soluble and remains dissolved in solution.




How to Recognize Types of Reactions


Try to identify the type of reaction and the products before reading the answers.  Note that the reactants appear to the left of the arrow while products appear to the right.



1.) 2 H2O2 ® 2 H2O + O2


Decomposition: one reactant and two products; the reactant is a compound, not an element.


2.) 2 AgNO3 + CaF2 ® 2 AgF + Ca(NO3)2


Double displacement: two reactants and two products; one product has been given and shows elements from each reactant combined in a new way.


3.) CaCl2 + F2 ® CaF2 + Cl2


Single displacement: two reactants, one of which is an element, and two products.


4.) C3H8 + 5 O2 ® 3 CO2 + 4 H2O


Combustion: the hydrocarbon reactant reacts with something; the products are both combined with O2 and are carbon dioxide and water, which are always associated with combustion.


5.) 2 Na + Cl2 ® 2 NaCl


Combination: two reactants, one of which is an element; the one product is composed of only two elements.


6.) 2 HCl + Zn ® H2 + ZnCl2


Single displacement: two reactants, one of which is an element; one of the products is an element that was in a compound on the reactant side.




Balancing Chemical Equations


Examine the unbalanced chemical equation:


Al(NO3)3 + FeCl2 ® Fe(NO3)2 + AlCl3


To find the names of the reactants, begin on the left. Identify the elements present, then decide if any endings must be changed or if common ions are present. The first reactant is aluminum nitrate. The second reactant is iron chloride, but we must be more specific, since iron is in the center of the periodic table and therefore a transition metal. Transition metals can take on different charges.  We must specify the charge on the iron ion. The most common method is the stock system, which uses roman numerals inside parentheses. Remember that chloride has a charge of -1.  So, two chloride atoms have a charge of -2.  This means that the iron atom must have a charge of +2.  So, we call this compound iron(II) chloride. Following the same steps, our products are iron(II) nitrate, and aluminum chloride.


To being balancing this example, begin with the most complex formula first. You would probably choose Al(NO3)3. Check for the number of each element on each side of the equation. If a common polyatomic ion (such as NO3-) shows up in reactants and products, balance the common ion as a group. Since we are dealing with an odd number of nitrate ions on the reactant side, and an even number on the product side, we’ll multiply the reactant by two. At this step, we have:


2 Al(NO3)3 + FeCl2 ® Fe(NO3)2 + AlCl3.


This gives us a total of 6 nitrate groups on the reactant side. To have six on the product side, we must multiply 2 by 3, with a coefficient in front of the formula containing the nitrate group:


2 Al(NO3)3 + FeCl2 ® 3 Fe(NO3)2 + AlCl3.


We now have 6 nitrate groups on each side. Let’s move to the next reactant, FeCl3. There is one Fe on the reactant side and three on the product side. We must multiply the FeCl2 by 3:


2 Al(NO3)3 + 3 FeCl2 ® 3 Fe(NO3)2 + AlCl3.


Count on each side, and there are now 6 NO3 groups, and 3 Fe ions on each side. Go back to the beginning, and check on the Al. There are two on the left, and one on the right. Multiply the product by 2 and check our results:


2 Al(NO3)3 + 3 FeCl2 ® 3 Fe(NO3)2 + 2 AlCl3


We now have 2 Al, 6 NO3, 3 Fe, and 6 Cl on each side. The equation is balanced.



If you cannot balance an equation, first check to be sure you have the correct formulas for all reactants and products. An equation with incorrect formulas will not balance. Keep in mind the importance of balancing equations: The Law of Conservation of Matter tells us that matter can neither be created nor destroyed in ordinary chemical reactions. A reaction that shows more Al atoms (for example) on one side than the other, would show a change in mass. For mass to be conserved, the number of atoms of each element must be the same.


To review, the correct way to balance an equation is to begin with correctly identified reactants and products and their correct formulas. Then check for balancing by counting the number of atoms of each type on each side of the equation. If the equation is not balanced, choose the most complicated formula to begin balancing. Leave simpler formulas until later, and lone elements until last.


A chemical equation is an important tool for relating the macroscopic world (which we can easily see with the unaided eye) to the microscopic world of atoms. The coefficient in a balanced equation tell us the atom to atom ratios in an individual reaction, as if we could see for ourselves. These same number also tell us bulk (macroscopic) ratios. Physical states are often shown as part of chemical equations. To indicate a solid use (s); for liquid, (l); for gas (g); for an aqueous solution (aq).




Hints for Assignment

Here is a reminder of how the coefficient affects the number of atoms in the molecule.


When the coefficient is 1, as in this formula, Al(NO3)3, the number of atoms is 1 aluminum, 3 nitrogen, 9 oxygen. When the coefficient is 3, as in this formula, 3 Al(NO3)3, the number of atoms is multiplied by 3 and there are 3 aluminums, 9 nitrogens, 27 oxygens. The answer is not 3.


Any time you see the words "to produce," you insert the arrow in the equation. The reactants are before the arrow.




Practice Problems


Try the practice problems on your own, before examining the answers. The names of these reagents may have appeared in earlier chapters. By now you should be familiar with many elements, as well as common compounds and polyatomic ions. The correct formulas and names are given for each question before the balanced equation is shown, to encourage you to try to balance the equation on your own first.

  1. Hydrogen gas: H2 (g); nitrogen gas N2 (g); ammonia gas NH3 (g)

    Balanced equation: 3 H2 (g) + N2 (g) ® 2 NH3 (g)

  2. hydrogen bromide: HBr (g); oxygen gas: O2 (g); water: H2O; bromine gas: Br2(g)

    balanced equation: 4 HBr (g) + O2 (g) ® 2 H2O + 2 Br2 (g)

  3. magnesium: Mg; hydrochloric acid: HCl (aq); hydrogen gas: H2 (g); magnesium chloride: MgCl2

    Balanced equation: Mg (s) + 2 HCl ® H2 (g) + MgCl2

  4. hydrochloric acid: HCl (aq); calcium carbonate: CaCO3; calcium chloride: CaCl2; carbon dioxide: CO2; water: H2O

    Balanced equation: 2 HCl + CaCO3 ® CaCl2 + CO2 (g) + H2O

  5. carbon disulfide: CS2; oxygen gas: O2 (g); carbon dioxide: CO2; sulfur dioxide: SO2

    Balanced equation: CS2 + 3 O2 (g) ® CO2 (g) + 2 SO2

  6. Magnesium oxide: MgO; hydrochloric acid: HCl; manganese(II) chloride, MnCl2; water liquid: H2O (l); chlorine gas: Cl2 (g)

    Balanced equation: MnO2 (s) + 4 HCl ® MnCl2 + 2 HCl (l) + Cl2 (g)