Lecture Notes:

Chemical Reactions and Equations

• Chemical Equations
  •  A chemical equation communicates the mass relationships between reactants and products in a reaction; you may recall that mass is never created or destroyed: a chemical equation must follow this rule
  • The coefficients in front of each formula tell you how many moles of each reactant are required for the reaction to go to completion
  • Therefore the mass relationship is indirect and implied; to find the mass relationships you must find the molar masses of each substance
  • For an equation to be considered ‘balanced’ it has to have the same amount of each element on the left and on the right
  • The substances on the left side undergo a reaction: they are the reactants
  • The substances on the right side of the equation are the products of the reaction
  • One special type of chemical equation is called a total ionic equation in which all the ions are shown separately since they are really separate when they are in solution
  • This type of chemical reaction can be reduced to a net ionic equation by removing all the components that do not actually react but rather stay in solution
  • Here is an example of a net ionic equation series:
    2KI + Pb(NO₃)₂ → PbI₂ + 2KNO₃ regular equation
    2K⁺ + 2I^- + Pb²⁺ 2NO₃^- → PbI₂ + 2K⁺ + 2NO₃^- total ionic equation
    2I^- + Pb²⁺ → PbI₂ net ionic equation
• Balancing Chemical Equations
  •    There are a few simple steps that will enable you to balance any chemical equation
    1. Count the atoms of each element in the equation in its unbalanced form. Make a table for the reactant side (left) and one for the product side (right) showing the quantity of each element or polyatomic ion.
    2. Balance the atom or polyatomic ion that appears only once on each side by inserting a coefficient in front of the whole formula. Note: you cannot change the subscripts in the formulas since this would change the substance in the reaction! Sometimes an element or polyatomic ion will appear more than once on one side. Save these instances for later.
    3. Balance the other elements and polyatomic ions one by one, leaving O and H for last.
    4. Update your table for each change you make to the equation.
    5. When you are done, each element or polyatomic ion will have the same number next to it in both the reactant table and the product table.
  • Some other things to keep in mind:
    • Treat polyatomic ions as units if they appear on both sides of an equation, as they typically will. Do not count the atoms inside the polyatomic ion since you might get them confused with the same atoms that are not part of the ion.
    • Sometimes a change you make in the middle of the balancing process will cause one of the other elements or polyatomic ions to go out of balance. Don’t let this bother you: you will just go back and update the coefficients as you go along.
    • Think in terms of the mathematical concept of least common multiples (LCMs). You can figure out how many atoms or ions are on each side of the equation; just find the LCM of the two numbers to balance that unit.
    • The concept of the LCM is important because the ratios between substances in the reaction should be expressed in lowest terms
    • Finally, make sure you have the correct formulas for all of your reactants: incorrect formulas lead to incorrect equations!
• Chemical Reactions
  • Some changes, such as melting, evaporating, or freezing are purely physical changes
  • Physical changes involve changes of state and are accompanied by either a loss or a gain in energy
  • Chemical reactions also involve changes in energy and very often changes in state
  • A chemical reaction can be recognized for what it is if the chemical properties of the products are different from those of the reactants
  • For instance, you know a chemical reaction has occurred when you strike a match; once you have lit it and blown it out, you cannot relight it: the chemical properties of the stuff in the match’s head have changed
  • This change in chemical properties comes about because of the rearrangement of atoms into new compounds
• Information in Chemical Equations
  • Chemical equations tell you the molar ratios among all the substances involved
  • In the equation 2H₂ + O₂ → 2H₂O you could say that two molecules of hydrogen react with one molecule of oxygen to form two molecules of water
  • You could also say that two moles of hydrogen react with one mole of oxygen to form two moles of water
  • Say you wanted to know how many moles of hydrogen you would need (there is plenty of oxygen in the air so we don’t need to worry about that) to get 10 moles of water
  • Look at the coefficients in the chemical equation: there is a 1:1 relationship between hydrogen and water
  • To get 10 moles of water you would need 10 moles of hydrogen; how many moles of oxygen did we use?
  • One thing to note is that reactions seldom go to completion; that is, it is seldom true that you have all product and no more reactants
  • The amount of each product shown in the chemical equation represents the theoretical yield of the reaction
  • The amount of each product actually collected represents the actual yield
• Chemical Reaction Types
  • Combustion: the combination of an element with oxygen to form an oxide; it is usually accompanied by the release of a large amount of heat; hydrocarbon combustion reactions usually result in the formation of CO₂ and H₂O; if ashes remain after such a reaction it is because the reaction did not go to completion and some of the carbon failed to combust
  • Synthesis: the combination of simpler materials to form more complex materials; combustion is actually a type of synthesis; another example is photosynthesis: 6CO₂ + H₂O → C₆H₁₂O₆ + 6O₂
  • Decomposition: the breaking down of one substance into several others; an example is the electrolysis of water which reverses the synthesis reaction already referred to
  • Displacement: a single displacement results in one element taking the place of another; for example take the reaction of zinc with HCl to form zinc chloride and hydrogen gas; it is possible to predict displacement reactions with an activity series; there is a table in every chemistry book that you can refer to; elements at the top of the list are more reactive than elements at the bottom and if two elements are ‘competing’ in a reaction, the one that is higher up will react first; also, elements at the top of the list are capable of displacing elements below them but the reverse is not true