CHEMICAL REACTION

Chemical Reactions

        Now that we know the how and why of chemical bonding, we can look at some chemical reactions.  Chemical reactions happen all around us: when we light a match, start a car, eat dinner or walk the dog.  A chemical reaction is the pathway by which two substances bond together.  In fact we have already discussed several chemical reactions.  One we have mentioned is the reaction of hydrogen with oxygen to form water.  To write the chemical reaction you would place the reactants (the substances reacting) on the left with an arrow pointing to the the products (the substances being formed).  Given this information, one might guess that the reaction to form water is written: H + O  H₂O However there are 2 problems with this chemical reaction.  First, because atoms like to have full valence shells, single H or O atoms are rare (and unhappy) creatures.  As we saw in the previous lesson, both hydrogen and oxygen react with themselves to form the molecules H₂ and O₂, respectively.  These hydrogen and oxygen molecules are much more common.  Given this correction, one might guess that the reaction looks like this:
H₂ + O₂ H₂O But we still have one problem.  As written, this equation tells us that 1 hydrogen molecule (with 2 H atoms) reacts with 1 oxygen molecule (with 2 O atoms) to form 1 water molecule (with 2 H atoms and 1 O atom).  In other words, we seem to have lost 1 O atom along the way!  To write a chemical reaction correctly, the number of atoms on the left side of a chemical equation has to be precisely balanced with the atoms on the right side of the equation.  How does this happen in our example?  In actuality, the O atom that we 'lost' reacts with a 2nd molecule of hydrogen to form a second molecule of water.  The reaction is therefore written:
   2H₂ + O₂ 2H₂O In the chemical reaction above, the number in front of the molecule (called a coefficient) indicates how many molecules participate in the reaction.  A simulation of the reaction can be viewed by clicking below (the atoms are represented as spheres in the animation: red = hydrogen, blue = oxygen):

(~61k animation opens in a new window)

        In order to write a correct chemical reaction, we must balance all of the atoms on the left side of the reaction with the atoms on the right side.  Let's look at another example.  Natural gas is primarily methane.  Methane (CH₄) is a molecule in which 4 hydrogen atoms are bonded to one carbon atom.  If you have a gas stove, lighting the stove causes the methane to react with oxygen in the atmosphere to release heat and the atoms recombine to form carbon dioxide and water vapor.  The unbalanced chemical reaction would be:
CH₄ + O₂ CO₂ + H₂O Look at the reaction atom by atom.  On the left side we find 1 carbon atom, and 1 on the right.  There are 4 hydrogen atoms on the left, but only 2 on the right.  Therefore, you know 2 water molecules must be formed.  Adding this coeffiecient we get:
CH₄ + O₂ CO₂ + 2H₂O  Now we have to balance the oxygen atoms.  On the left you find 2 atoms, on the right 4 (2 in the CO₂ molecule and 1 in each of 2 H₂O molecules).  Therefore we need to start with 4 oxygen atoms, or 2 molecules.  The balanced equation would then be:
CH₄ + 2O₂ CO₂ + 2H₂O Try balancing the following equations on your own (note: while you don't need to write the coefficient 1 in a chemical reaction, these examples will not work unless you input a 1 where needed):
1) Na + Cl₂NaCl

2) N₂ + H₂NH₃

3) Fe + O₂Fe₂O₃

4) Cu + AgNO₃Ag + Cu(NO₃)₂ (a Cu atom bonded to 2 NO₃ groups)

5) H₂SO₄ + NaCN HCN+ Na₂SO₄

The Mole and Molecular Weights         Up until this point we have been talking about atoms and molecules.  The problem with this approach is that atoms and molecules are very small things.  In a single drop of water for example, there are trillions and trillions of water molecules.  A reaction between a single molecule of hydrogen and a single molecule of oxygen, as we discussed above, would be undetectable.  Instead of talking about single molecules in science, we talk about groups of molecules.  You can think of it like buying eggs.  You don't go to the store and buy an egg - you buy a dozen.  Contained within that dozen are the individual eggs.  Its the same thing when we talk about molecules.  We don't talk about single units, we talk about groups.
        But even a dozen molecules is a tiny amount.  What we need is a big number - a huge number!  That number is the mole.  The mole is the scientific community's baker's dozen.  One mole equals 6.02 x 10²³ (also known as Avogadro's number).  A 6 followed by 23 zeros.  Now that's a pretty big number.  But that's all it is, a number.  You can't just have a mole, you have to have a mole of something.  A mole of atoms.  A mole of water molecules.  A mole of pennies (which would make you richer than you can imagine).  Why the mole?  As it turns out, the mole has some interesting properties.  One mole of hydrogen atoms (6.02 x 10²³ H atoms) weighs 1 g.  From the periodic table we know that an He atoms weighs 4 times as much as an H atom, so go figure, 1 mole of He atoms weighs 4 g.  In fact, one mole of any element is equal to the atomic mass of that element (in grams).
        Let's think about that for a second.  If we know the molar mass of an element, and we know how many elements make up a specific molecule, then you can calculate the molar mass of a compound by adding up the atomic weights.  Huh?  Take water for example.  How much does a mole of water weigh?  Well, one mole of water contains one mole of oxygen atoms and two moles of hydrogen atoms.  A mole of hydrogen weighs 1 g and a mole of oxygen weighs 16 g (look at the atomic mass in the periodic table).  So to calculate the weight of one mole of water:
(2 moles H * 1 g per mole) + (1 mole O * 16 g per mole) = 18 g One mole of water weighs 18 grams!
        The mole is also useful in chemical reactions.  Though you can't measure out an atom of hydrogen, you can measure out a mole.  Since the mole is just a constant number, the coefficients in a balanced chemical reaction give you the molar proportions of reactants and products.  In other words:
   2H₂ + O₂ 2H₂O tells us that: 2 H₂ molecules react with 1 O₂ molecule to form 2 H₂O molecules. It also tells us that:
2 moles of H₂ molecules react with 1 mole of O₂ molecules to form 2  moles of H₂O molecules.

          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. 

Aluminum and Bromine

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.

Sodium and Chlorine

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.

Zinc and Oxygen

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.

Sodium and Potassium in Water

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    +       B

Iron (III) Oxide and Aluminum Reaction 2

Thermite 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.

Copper (II) Oxide and Carbon

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.

Silver Nitrate and Copper

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.

Tin (II) Chloride and Zinc

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   +      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.

Sodium chloride and Silver nitrate

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      +      B

Water into Hydrogen and Oxygen

Electrolysis 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.

Nitrogen Triiodide

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 H₂O

Hydrogen and Oxygen Reaction II

The 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

Various Substances with Oxygen

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.

Phosphorous and Oxygen

The combustion of yellow phosphorus occurs in an oxygen atmosphere.  The main product of this reaction is phosphorus pentoxide.