Chemical Reactions

Using Everyday Equations Every minute of the day chemical reactions are taking place both in and around you. What makes something a chemical reaction? A chemical reaction involves one or more substances, the reactants, changing into one or more substances, the products. Reactants Products “ ” means “yields” or “reacts to produce.”

Chemical reactions can be described many ways Chemical reactions can be described many ways. One is by using a word equation. A word equation is an equation where reactants and products are represented by words. Example: methane + oxygen carbon dioxide + water Word equations, while useful, are cumbersome. To better describe a reaction, writing the formulas is used. A chemical equation uses formulas of reactants (on the left) separated by an arrow from products (on the right).

Example: Fe + O2 Fe2O3 (Skeleton equation) - does not indicate the relative amounts of reactants and products Most often symbols are used to describe the state (i.e., gas, solid, etc.) of each substance

Types of Reactions The four types of chemical reactions we will talk about are: Synthesis reactions Decomposition reactions Single displacement reactions Double displacement reactions You need to be able to identify the type of reaction and predict the product(s)

Steps to Writing Reactions Some steps for doing reactions Identify the type of reaction Predict the product(s) using the type of reaction as a model Balance it Don’t forget about the diatomic elements! (BrINClHOF) For example, Oxygen is O2 as an element. In a compound, it can’t be a diatomic element because it’s not an element anymore, it’s a compound!

1. Synthesis reactions Synthesis reactions occur when two substances (generally elements) combine and form a compound. (Sometimes these are called combination or addition reactions.) reactant + reactant  1 product Basically: A + B  AB Example: 2H2 + O2  2H2O Example: C + O2  CO2

2. Decomposition Reactions Decomposition reactions occur when a compound breaks up into the elements or in a few to simpler compounds 1 Reactant  Product + Product In general: AB  A + B Example: 2 H2O  2H2 + O2 Example: 2 HgO  2Hg + O2

3. Single Displacement Reactions Single Replacement Reactions occur when one element replaces another in a compound. A metal can replace a metal (+) OR a nonmetal can replace a nonmetal (-). element + compound product + product A + BC  AC + B (if A is a metal) OR A + BC  BA + C (if A is a nonmetal) (remember the cation always goes first!) When H2O splits into ions, it splits into H+ and OH- (not H+ and O-2 !!)

We have looked at several reactions: Fe + CuSO4  Cu + Fe2(SO4)3 Li + H2O  LiOH + H2 Such experiments reveal trends. The activity series ranks the relative reactivity of metals. It allows us to predict if certain chemicals will undergo single displacement reactions when mixed: metals near the top are most reactive and will displacing metals near the bottom. Q: Which of these will react? Fe + CuSO4  Ni + NaCl  Li + ZnCO3  Al + CuCl2  K Na Li Ca Mg Al Zn Fe Ni Sn Pb H Cu Hg Ag Au No, Ni is below Na Yes, Li is above Zn Yes, Al is above Cu Yes, Fe is above Cu Cu + Fe2(SO4)3 NR (no reaction) Zn + Li2CO3 Cu + AlCl3

A: No for cold, yes if it is hot/steam Mg + H2O  H2 + Mg(OH)2 acid H is the only nonmetal listed. H2 may be displaced from acids or can be given off when a metal reacts with H2O (producing H2 + metal hydroxide). The reaction with H2O depends on metal reactivity & water temp. Q: will Mg react with H2O? K Na Li Ca Mg Al Zn Fe Ni Sn Pb H Cu Hg Ag Au cold H2O hot H2O steam A: No for cold, yes if it is hot/steam Mg + H2O  H2 + Mg(OH)2 acid Q: Zn + HCl  H2 + ZnCl2 Complete these reactions: Al + H2O(steam)  Cu + H2O  Ca + H2SO4  Na + H2O  H2 + Al(OH)3 NR H2 + CaSO4 H2 + NaOH

Other Activity Series Information All metals will have a specific place in the activity series. For simplicity, only the most common metals are shown. The metals near the top of the activity series are more reactive because their valence electrons are more easily removed. On tests and exams the activity series may appear as K, Na, … Ag, Au; you must remember that K is reactive, Au is not. If the valence of a metal is not indicated in the question, use its most common valence (in bold on your periodic table) to determine the correct chemical formula.

4. Double Displacement Reactions Double Replacement Reactions occur when a metal replaces a metal in a compound and a nonmetal replaces a nonmetal in a compound Compound + compound  product + product AB + CD  AD + CB

Double Displacement Reactions Think about it like “foil”ing in algebra, first and last ions go together + inside ions go together Example: AgNO3(aq) + NaCl(s)  AgCl(s) + NaNO3(aq) Another example: K2SO4(aq) + Ba(NO3)2(aq)  KNO3(aq) + BaSO4(s) 2