Chapter 7 Chemical Reactions

Chemical Reactions Chemical reaction (chem. rxn) Chemical change. Substances change into other substances. Key words: Produces, Yields, Forms Review Question: How is a chemical reaction different from a phase change (melting, freezing, etc)? Answer: In a phase change, no new substances are formed.

Reactants and Products Enter into rxn (left side of arrow) “Ingredients” Products Produced by rxn (right side of arrow) Reactants  Products In a chem rxn: existing bonds are broken atoms are rearranged new bonds are formed

A+B→C+D REACTANTS PRODUCTS Chemical Equations Chemical equation Combination of symbols and numbers Represents a chem rxn In a chem equation: “+” means “and” “” means “yields” or “produces” A+B→C+D REACTANTS PRODUCTS

Examples Br2(aq) + 2NaI(aq) → 2NaBr(aq) + I2(aq) Bromine and sodium iodide produce sodium bromide and iodine. 2H2(g) + O2(g) → 2H2O(l) Hydrogen and oxygen yield water.

Phase Symbols (s) = solid (l) = liquid (g) = gas (aq) = aqueous (dissolved in H2O) Phase symbols are not always necessary, but are sometimes included to provide extra information about the rxn.

Balancing Chemical Equations Law of Conservation of Mass In a chemical reaction, matter is not created or destroyed. Atoms can be rearranged. The amount of each atom that begins a rxn must equal the amount of each atom that ends a rxn 4H 2O 4H 2O

Balancing Chemical Equations If the reaction starts with 2 atoms of H & 2 atoms of O, then it must end with 2 atoms of H & 2 atoms of O Coefficients Numbers placed in front of symbols & formulas in chem. equation Coefficient - # of units of each substance Pb(NO3)2(aq)+2KI(aq)→PbI2(s)+2KNO3(aq)

Structure of an Equation: yields

Describing Coefficients individual atom = “atom” 2Mg = 2 atoms of magnesium covalent substance = “molecule” 3CO2 = 3 molecules of carbon dioxide ionic substance = “unit” 4MgO = 4 units of magnesium oxide

Counting Atoms in an Equation If no subscript present it is assumed to be 1 atom If elements in brackets or parenthesis, treat same as in math. Coefficients multiply the entire molecule atoms You must add all reactant molecules together & compare w/ all molecules in the products Ca3(PO4)2 Ca=3 P=2 O=8 2Ca3(PO4)2 Ca=6 P=4 O=16 CaCl2 Ca=1 Cl=2 It’s best to list the # of atoms under the molecules as we are doing in these examples

Review Counting Atoms What atoms/ions make up 1 unit of Ba3(PO4)2? 3 barium ions 2 phosphorus atoms 8 oxygen atoms What atoms make up 1 molecule of C6H12O6? 6 carbon atoms 12 hydrogen atoms 6 oxygen atoms

Balancing Chemical Equations Steps: 1) List all elements on the left side. 2) List all elements on the right side. 3) Count the number of atoms of each element on both sides. 4) Compare the numbers. If the numbers are equal, the equation is already balanced. If the numbers are not the same, balance using coefficients. 5) A coefficient multiplies an entire formula and all the atoms in it. Must re-count atoms every time you add or change a coefficient.

Balancing Equations, Ex.1 H2(g) + O2(g)  H2O(l) Hydrogen (gas) + oxygen (gas)  water (liquid) The phase symbols don’t affect the balancing. Atoms on left: H = 2 O = 2 Atoms on right: H O = 2 = 1 This equation is not balanced!

Balancing Equations 2 oxygen atoms on left; 1 on right. Multiply H2O by 2: H2(g) + O2(g)  2H2O(l) Recount: Left side H = 2 O = 2 Right side H = 4 O = 2 Remember: Coefficients multiply every atom in a formula.

Balancing Equations The equation is now balanced! Right side H = 4 Now there are 4 hydrogen atoms on the right, but only two on the left. Multiply H2 by 2. 2H2(g) + O2(g)  2H2O(l) Recount: Left side H = 4 O = 2 Right side H = 4 O = 2 The equation is now balanced!

2H2(g) + O2(g)  2H2O(l)

Check for Understanding How many oxygen atoms are represented in: H2O Answer: 1 oxygen atom NaNO3 Answer: 3 oxygen atoms Al2(SO4)3 Answer: 12 oxygen atoms 3KNO3 Answer: 9 oxygen atoms 4CO2 + 5H2O Answer: 13 oxygen atoms

Check for Understanding Balance the following chemical equations: Na + Cl2  NaCl KClO3  KCl + O2 C3H8 + O2  CO2 + H2O BaCl2 + K3PO4  Ba3(PO4)2 + KCl Al + CuCl2  AlCl3 + Cu

Na + Cl2  NaCl

KClO3  KCl + O2

C3H8 + O2  CO2 + H2O

BaCl2 + K3PO4  Ba3(PO4)2 + KCl

Al + CuCl2  AlCl3 + Cu

Check for Understanding Balance the following chemical equations: Na + Cl2  NaCl Answer: 2Na + Cl2  2NaCl KClO3  KCl + O2 Answer: 2KClO3  2KCl + 3O2 C3H8 + O2  CO2 + H2O Answer: C3H8 + 5O2  3CO2 + 4H2O BaCl2 + K3PO4  Ba3(PO4)2 + KCl Answer: 3BaCl2 + 2K3PO4  Ba3(PO4)2 + 6KCl Al + CuCl2  AlCl3 + Cu Answer: 2Al + 3CuCl2  2AlCl3 + 3Cu

Rates of Change *Massive, bulky molecules react slower To increase the rate (speed) of a reaction (in most cases): Increase temperature Increase surface area Concentrated solutions Increase pressure *Massive, bulky molecules react slower

https://www.youtube.com/watch?v=2Juem0lc ifE

Balancing equations activity!