Lecture: Reactions and Equations

Reaction Equations

Key terms

Energy, exothermic reaction, endothermic reactionPhysical reactions, chemical reactions, phase transitionsReactants, productsReaction stoichiometry

Skills to develop
To distinguish chemical changes from physical changes.
To write chemical equations to describe a chemical reaction.
To balance chemical equations.
To calculate the quantities of reactants required or the quantities produced in a chemical reaction.

Chemical Reaction Equations

Changes in a material or system are called reactions, and they are divided into chemical and physical reactions.
Energy is the driving force of all changes, both physical and chemical reactions. Energy is always involved in these reactions. If a system is more stable by losing some energy, a reaction takes place, releasing energy. Such a reaction is said to be exothermic. Supplying energy to a system also causes a reaction. Energy absorbing reactions are called endothermic reactions. Sometimes, the amount of energy involved in a reaction may be so small that the change in energy is not readily noticeable.

An equation can be used to describe a physical reaction, which involves a change of states. For example, melting, sublimation, evaporation, and condensation can be represented as follow.

In these equations, (s) stands for solid, (l) for liquid (l), and (g) for gas,

H2O(s) ® H2O(l) . . . melting
H2O(s) ® H2O(g) . . . sublimation
C2H5OH(l) ® C2H5OH(g) . . . evaporation
NH3(g) ® NH3(l) . . . condensationIn these changes, no chemical bonds are broken or formed, and the molecular identities of the substances have not changed.

Is the phase transition between graphite and diamond is a chemical or physical reaction?
C(graphite) ® C(diamond).

The crystal structures of diamond and graphite are very different, and bonding between the carbon atoms are also different in the two solid states. Because chemical bonds are broken and new bonds are formed, the phase transition of diamond and graphite is a chemical reaction.
Chemicals or substances change converting to one or more other substances, and these changes are called chemical reactions. At the molecular level, atoms or groups of atoms rearrange resulting in breaking and forming some chemical bonds in a chemical reaction. The substances undergoing changes are called reactants, whereas substances newly formed are called products. Physical appearances of products are often different from reactants. Chemical reactions are often accompanied by the appearance of gas, fire, precipitate, color, light, sound, or odor.

These phenomena are related to energy and properties of the reactants and products. For example, the oxidation of propane releases heat and light, and a rapid reaction is an explosion,
C3H8 + 5 O2 ® 3 CO2 + 4 H2O A balanced equation also shows a macroscopic quantitative relationship. This balanced reaction equation shows that five moles of oxygen reacts with one mole of propane generating three moles of carbon dioxide and four moles of water, a total of 7 moles of products in the combustion reaction.

At the molecular level, this equation shows that for each propane molecule, 5 oxygen molecules are required. The three carbon atoms are converted to three molecules of carbon dioxide, whereas the 8 hydrogen atoms in propane are oxidized to 4 water molecules. The numbers of H, C, and O atoms are the same on both sides of the equation.

We study properties of substances so that we know how to make use of them. Tendencies of a substance to react, either by itself or with others, are important chemical properties. Via properties, we understand chemical reactions, which are best studied by experimentation and observation. After you have performed many experiments, you may generalize certain rules and facts. Knowing these rules and facts enable you to solve problems that you have not yet encountered.

The most important aspect of a chemical reaction is to know what are the reactants and what are the products. For this, the best description of a reaction is to write an equation for the reaction. A chemical reaction equation gives the reactants and products, and a balanced chemical reaction equation shows the mole relationships of reactants and products. Often, the amount of energy involved in the reaction is given. Dealing with the quantitative aspect of chemical reactions is called reaction stoichiometry.

For example, when clamshells, CaCO3, are heated, a gas CO2 will be released, leaving a white powder (solid CaO) behind. This reaction is represented by the reaction as depicted in the picture, and the equation of the reaction is written as:
CaCO3 ® CaO + CO2The equation indicates that one mole of CaCO3 gives one mole each of CaO and CO2. Amounts of substances represented by chemical formulas have been introduced on the two previous pages, and these concepts should help to figure out the stoichiometry of reactions when a reaction equation is given.


Example 1
When 10.0 g pure calcium carbonate is heated and converted to solid calcium oxide CaO, how much calcium oxide should be obtained? If the only 5.0 grams CaO is obtained, what is the actual yield? Hint:Under ideal condition, amounts of substance in the reaction equation is as indicated below:
CaCO3 ® CaO + CO2100.0 . . . . . 56 . . . 44 g/mol (formula weights) 1 mol CaCO3 1 mol CaO 56 g CaO
10.0 g CaCO3 ------------ ----------- --------- = 5.6 g CaO
100 g CaCO3 1 mol CaCO3 1 mol CaO
DiscussionAn inefficient conversion is given here, but the method shows the details of consideration. If the amount of CaO obtained is not 5.6 g, one can conclude that the sample may not be pure.

Example 2
When 10.0 g pure calcium carbonate is heated and converted to solid calcium oxide CaO, how much CO2 at standard condition is released? Hint:
CaCO3 ® CaO + CO2 1 mol CO2 22.4 L CO2
10.0 g CaCO3 ----------- ----------- = 2.24 L CO2
100 g CaCO3 1 mol CO2

Discussion

We have taken a short cut in this formulation compared to Example 1. Example 1 and 2 illustrate the evaluation of quantities in g and in L.

Writing Equations for Chemical Reactions
Chemical reaction equations truly represent changes of materials. For many reactions, we may only be able to write equations for the overall reactions. For example, common sense tells us that when sugar is fully oxidized, carbon dioxide and water are the final products. The oxidation reaction is the same as the combustion reaction. Thus we write
C12H22O11 + 12 O2 ® 12 CO2 + 11 H2O This illustrates the methods used for writing balanced reaction equations:

Determine the reactants and productsIn this case, the products are CO2 and H2O, determined by common sense. We know that.

Apply the fundamental principle of conservation of atomsNumbers of atoms of each kind must be the same before and after the reactions.
Balance one type of atoms at a timeBR> We may use H or C to begin. Since there are 12 C atoms on the left, the coefficient is 12 for CO2. Similarly, 22 H atoms produce 11 H2O molecules.
Balance the oxygen atoms on both sides. There are a total of 35 O atoms on the right hand, and the coefficient for O2 should be 11.

Example 3
The compound N2O5 is unstable at room temperature. It decomposes yielding a brown gas NO2 and oxygen. Write a balanced chemical reaction equation for its decomposition. Hint:The first step is to write an unbalanced equation indicating only the reactant and products:
N2O5 ® NO2 + O2 A N2O5 molecule decomposes into two NO2 molecule, and half of O2.
N2O5 ® 2 NO2 + 1/2O2 In order to give whole number stoichiometric coefficients to the equation, we multiply all the stoichiometric coefficients by 2.
2 N2O5 ® 4 NO2 + O2 DiscussionThis example illustrate the steps used in writing a balance equation for a chemical reaction. This balanced equation does not tell us how a N2O5 molecule decompose, it only illustrate the overall reaction.

Example 4
When solutions of CaCl2 and AgNO3 are mixed, a white precipitate is formed. The same precipitate is also observed when NaCl solution is mixed with AgCH3CO2 solution. Write a balanced equation for this the reaction between CaCl2 and AgNO3. Hint:The common ions between NaCl and CaCl2 are Cl- ions, and Ag+ ions are common between the two silver containing compounds. The question illustrates a scientific deduction used in the determination of products. The product is AgCl, and the balanced reaction is
CaCl2 + 2 AgNO3 ® 2 AgCl + Ca(NO3)2 DiscussionIn reality, solutions of salts contain ions. In this case, the solutions contain Ca2+, Cl-, Ag+, and NO3- ions. The Cl- and Ag+ ions form an insoluble solid, and a precipitate is formed,
Cl- + Ag+ ® AgCl(s) Ca2+ and NO3- are by-stander ions.

Chemical Reactions
One of the most important topics in chemistry is chemical reaction. In this page, we only concentrate on the stoichiometry conveyed by reaction equations.

Other topics related to chemical reactions are:
Excess and Limiting Reagents or reactants left over or used upFeatures of chemical reactions or classification of reactionsChemical kinetics or reaction ratesReaction mechanism or how actually reaction proceedThe first two topics are included in this group, but the later topics will be discussed in another course (CHEM123).
Balancing Redox Reactions Balancing oxidation and reduction reaction equations is a little more complicated than what we discussed here. You have to have the skills to assign oxidation states, explain oxidation and reduction in terms of oxidation-state change, and write half reaction euqations. Then you will be able to balance redox reactions. All these are given in the next module on Chemical Reactions.

http://www.science.uwaterloo.ca/~cchieh/cact/c120/reaction.html