CHEMICAL REACTIONS Reactants: Zn + I2 Product: Zn I2
Chemical EquationsTheir Job: Depict the kind of reactants and products and their relative amounts in a reaction.4 Al (s) + 3 O2 (g) ---> 2 Al2O3 (s)The numbers in the front are calledstoichiometric ____________The letters (s), (g), and (l) are the physical states of compounds.
Symbols We Use+ Added to→ Yields or Produces(s) Substance is a solid(l) Substance is a liquid(g) Substance is a gas(aq) Substance is aqueous↔ Reaction can go both ways (Reversible)Δ Heat if above arrows↑ Subs for (g) to show a gas is produced↓ Subs for (s) to show a precipitate is formed
IntroductionChemical reactions occur when bonds between the outermost parts of atoms are formed or brokenChemical reactions involve changes in matter, the making of new materials with new properties, and energy changes.Symbols represent elements, formulas describe compounds, chemical equations describe a chemical reaction
Parts of a Reaction EquationChemical equations show the conversion of reactants (the molecules shown on the left of the arrow) into products (the molecules shown on the right of the arrow). A + sign separates molecules on the same side The arrow is read as “yields” Example C + O2 CO2 This reads “carbon plus oxygen react to yield carbon dioxide”
The charcoal used in a grill is basically carbon. The carbon reacts with oxygen to yield carbon dioxide. The chemical equation for this reaction, C + O2 CO2, contains the same information as the English sentence but has quantitative meaning as well.
Chemical EquationsBecause of the principle of the conservation of matter,an equation must be balanced.It must have the same number of atoms of the same kind on both sides. Lavoisier, 1788
Balancing EquationsWhen balancing a chemical reaction you may add coefficients in front of the compounds to balance the reaction, but you may not change the subscripts. Changing the subscripts changes the compound. Subscripts are determined by the valence electrons (charges for ionic or sharing for covalent)
Chemical Equations4 Al(s) + 3 O2(g) ---> 2 Al2O3(s)This equation means4 Al atoms + 3 O2 molecules ---produces---> 2 molecules of Al2O3 AND/OR4 moles of Al + 3 moles of O2 ---produces--->
Steps in Balancing An EquationThere are four basic steps to balancing a chemical equation. 1. Write the correct formula for the reactants and the products. DO NOT TRY TO BALANCE IT YET! You must write the correct formulas first. And most importantly, once you write them correctly DO NOT CHANGE THE FORMULAS! 2. Find the number of atoms for each element on the left side. Compare those against the number of the atoms of the same element on the right side. 3. Determine where to place coefficients in front of formulas so that the left side has the same number of atoms as the right side for EACH element in order to balance the equation. 4. Check your answer to see if: The numbers of atoms on both sides of the equation are now balanced.
Balancing Equations 2 2 ___ H2(g) + ___ O2(g) ---> ___ H2O(l)What Happened to the OtherOxygen Atom?????This equation is not balanced!Two hydrogen atoms from a hydrogenmolecule (H2) combines with one of theoxygen atoms from an oxygen molecule(O2) to form H2O. Then, the remainingoxygen atom combines with two morehydrogen atoms (from another H2 molecule)to make a second H2O molecule.
Types of chemicalequations COMBINATION DECOMPOSITION DISPLACEMENT DOUBLE-DISPLACEMENT
COMBINATION REACTION• COMBINATION 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
EXAMPLE OF A COMBINATIONREACTION• Here is another example of a synthesis reaction
DECOMPOSITIONREACTION• 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
EXAMPLE OF A DECOMPOSITIONREACTION• Another view of a decomposition reaction:
DISPLACEMENT REACTION• 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 element + compound 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 intoH+ and OH- (not H+ and O-2 !!)
EXAMPLE OF A DISPLACEMENTREACTION• Another view:
DOUBLE-DISPLACEMENT REACTION• Double Replacement Reactions occur when a metal replaces a metal in a compound and a nonmetal replaces a nonmetal in a compound• Compound + compound compound+ compound• AB + CD AD + CB