Chemical Equations and Reactions

Chemical Reactions A chemical rxn involves a chemical change in the identity of one or more chemical species. A chemical rxn involves a chemical change in the identity of one or more chemical species. EX. Rusting of iron (Fe) is a chemical rxn btwn water (H 2 O) and iron. Reactions involve the breaking of bonds and the formation of new bonds. Reactions involve the breaking of bonds and the formation of new bonds. Chemical Equations - shorthand notation for a chemical reaction

Evidence of Chemical Reactions Release of a gas. Formation of a solid (precipitate) in solution. Heat is produced or absorbed (temp changes) Color changes. Light is absorbed or emitted. Changes in electrical or magnetic properties.

The Chemical Equation Reactants: starting materials that undergo change in the reaction. Reactants: starting materials that undergo change in the reaction. Products: substances produced by the reaction. Products: substances produced by the reaction. Law of Conservation of Mass: matter cannot be gained or lost in a chemical reaction. Law of Conservation of Mass: matter cannot be gained or lost in a chemical reaction. Total mass of products = total mass of reactants. Total mass of products = total mass of reactants. a A(g) + b B(g) c C(g) + d D(g) ReactantsProducts

Info in the Chemical Equation Products written on the right Reactants written on the left of arrow Reactants and products specified using chemical symbols Physical states shown in parentheses  - means heat is needed.

Remember the Conservation of Mass The equation must be balanced. –All atoms in reactants must also be in products!! Info in the Chemical Equation Coefficients: tells how many molecules of each substance are involved in the reaction.

Balancing Equations Steps: 1)Must have the same type of elements on each side of the equation. 2)Check for the same # atoms of each element on each side of the equation. 3)Use coefficients in front of each substance to balance the equation.

Additional Tips for Balancing NEVER change subscripts, place coefficients in front of the chemical formula Treat polyatomic ions as a single unit if it is not changed during the rxn If an element appears in more than one cmpd on the same side of the equation, balance it last If there is an even # of atoms of an element on one side of the equation & an odd # on the other side, place a 2 in front of the cmpd with the odd # of atoms

Mg + NaCl  MgCl 2 + Na Mg 1 1 Na Cl x 2 2

Balancing Chemical Equations Consider the following reaction: H 2 (g) + O 2 (g)  H 2 O(l) Is the equation balanced? NO It must be balanced by changing coefficients RIGHT: 2 H 2 (g) + O 2 (g)  2 H 2 O(l) WRONG: H 2 (g) + O 2 (g)  H 2 O 2 (l)

The Right Way!! 2 HCl (aq) + Ca (s)  CaCl 2 (aq) + H 2 (g) + Ca Cl Ca + H H Cl H H Change the number of HCl RIGHT WAY!! HCl (aq) + Ca (s)  CaCl 2 (aq) + H 2 (g) NOT BALANCED Cl H + Ca Cl Ca + H H

The Wrong Way!! H 2 Cl 2 (aq) + Ca (s)  CaCl 2 (aq) + H 2 (g) WRONG WAY!! + Ca Cl Ca + H H Cl H H HCl (aq) + Ca (s)  CaCl 2 (aq) + H 2 (g) NOT BALANCED Cl H + Ca Cl Ca + H H This compound does not exist!!

Balance the following equations: 1. C 2 H 2 + O 2  CO 2 + H 2 O 2. AgNO 3 + FeCl 3  Fe(NO 3 ) 3 + AgCl 3. C 2 H 6 + O 2  CO 2 + H 2 O 4. N 2 + H 2  NH 3

3 types of Rxns involving Energy: Endothermic - thermal energy absorbed during rxn Exothermic - thermal energy given off during rxn Chemiluminescence - light given off during rxn

4 Types Chemical Reactions: Synthesis element + element  compound ex/ 2H 2 + O 2  2H 2 O

Synthesis Examples Mg (s) + O 2(g)  MgO (s) Mg (s) + O 2(g)  MgO (s) 2Mg (s) + O 2(g)  2MgO (s) 2Mg (s) + O 2(g)  2MgO (s) Ba (s) + S 8 (s)  BaS (s) Ba (s) + S 8 (s)  BaS (s) 8Ba (s) + S 8 (s)  8 BaS (s) 8Ba (s) + S 8 (s)  8 BaS (s) Na (s) + Cl 2(g)  NaCl Na (s) + Cl 2(g)  NaCl 2 Na (s) + Cl 2 (g)  2NaCl 2 Na (s) + Cl 2 (g)  2NaCl

Decomposition compound  element + element ex/ 2H 2 O 2  2 H 2 O + O 2

Decomposition Examples H 2 0 (l)  H 2 (g) + O 2 (g) H 2 0 (l)  H 2 (g) + O 2 (g) 2H 2 0 (l)  2H 2 (g) + O 2 (g) 2H 2 0 (l)  2H 2 (g) + O 2 (g) HgO (s)  Hg (g) + O 2 (g) HgO (s)  Hg (g) + O 2 (g) 2HgO (s)  2Hg (g) + O 2 (g) 2HgO (s)  2Hg (g) + O 2 (g) CaCO 3 (s)  CaO (s) + CO 2 (g) CaCO 3 (s)  CaO (s) + CO 2 (g)

Single Replacement elem + comp  elem + comp replace either the cation or the anion Mg + 2HCl  H 2 + MgCl 2 Ba + FeBr 2 BaBr 2 + Fe

Single Replacement Examples 2Al (s) + 3Pb(NO 3 ) 2 (aq)  3Pb (s) + 2Al(NO 3 ) 3 (aq) Al replaces Pb Al replaces Pb Cl 2(g) + 2 KBr (aq)  2KCl (aq) + Br 2(g) Cl 2(g) + 2 KBr (aq)  2KCl (aq) + Br 2(g) Cl replaces Br Cl replaces Br

Double Replacement comp + comp  comp + comp ex/ FeCl 3 + NaOH  NaCl + Fe(OH) 3

Double Replacement Examples 2K I (aq) + Pb(NO 3 ) 2(aq)  2 KNO 3(aq) + Pb I 2 (s) K cation replaces Pb cation K cation replaces Pb cation I anion replaces nitrate anion I anion replaces nitrate anion FeS (s) + 2HCl (aq)  H 2 S (g) + FeCl 2(aq) Fe cation replaces H cation Fe cation replaces H cation S anion replaces Cl anion S anion replaces Cl anion

CHO : Combustion Reactions A substance combines with oxygen releasing a large amount of energy in the form of heat and light. A substance combines with oxygen releasing a large amount of energy in the form of heat and light. Hydrocarbon + O 2  CO 2 + H 2 O Hydrocarbon + O 2  CO 2 + H 2 O Balance carbon first, then hydrogen and finally oxygen. Balance carbon first, then hydrogen and finally oxygen. C 2 H 6 + O 2  CO 2 + H 2 O C 2 H 6 + O 2  CO 2 + H 2 O 2C 2 H 6 +7 O 2  4CO 2 + 6H 2 O 2C 2 H 6 +7 O 2  4CO 2 + 6H 2 O

Combustion Example: CH4 (g) + O2 (g)  CO2 (g) + H2O (g) CH4 (g) + 2O2 (g)  CO2 (g) + 2H2O (g) C3H8 (g) + O2 (g)  CO2 (g) + H2O (g) C3H8 (g) + 5 O2 (g)  3CO2 (g) + 4H2O (g)

Word & chemical equation Solid sodium oxide is added to water (at room temp.) and forms sodium hydroxide which is dissolved in water. Solid sodium oxide is added to water (at room temp.) and forms sodium hydroxide which is dissolved in water. Sodium oxide(s) + water(l)  sodium hydroxide (aq) Na 2 O(s) + H 2 O(l)  NaOH(aq) 2Na2O(s) + 2H2O(l)  4NaOH(aq)

Word Equations Write a word equation to describe the reaction of methane as it burns in the air to produce carbon dioxide and water vapor. Write a word equation to describe the reaction of methane as it burns in the air to produce carbon dioxide and water vapor. Methane + oxygen  carbon dioxide + water

Word & chemical equations Hydrogen peroxide in an aqueous solution decomposes to produce oxygen and water. Hydrogen peroxide in an aqueous solution decomposes to produce oxygen and water. Hydrogen peroxide  oxygen + water H 2 O 2 (aq)  O 2 (g) + H 2 O(l) 2H 2 O 2 (aq)  O 2 (g) + 2H 2 O(l)

Factors Affecting Reaction Rate Catalysts Catalysts Temperature Temperature Concentration Concentration Surface Area Surface Area

Catalysts – speed up chemical rxn but are not used up in the rxn Inhibitors – combine w/ one of the reactants so it can’t react

Temperature If we increase the temperature of a reaction by 10 0 C the rate will double this means the reaction will be complete in half the time. If we increase the temperature of a reaction by 10 0 C the rate will double this means the reaction will be complete in half the time.

Concentration If we increase the concentration of a reactant the number of particles increase that in turn increases the chance of a collision and initiates a chemical reaction. If we increase the concentration of a reactant the number of particles increase that in turn increases the chance of a collision and initiates a chemical reaction.

Surface Area The larger the particle size the smaller the relative area the slower the reaction. The larger the particle size the smaller the relative area the slower the reaction. The smaller the particle size the greater the relative surface area and the faster the reaction. The smaller the particle size the greater the relative surface area and the faster the reaction.

Write a word & chemical equation Solid copper metal reacts with aqueous silver nitrate to produce solid silver metal and aqueous copper(II) nitrate. Solid copper metal reacts with aqueous silver nitrate to produce solid silver metal and aqueous copper(II) nitrate. Copper (s) + silver nitrate (aq)  silver (s) + copper(II)nitrate (aq) Copper (s) + silver nitrate (aq)  silver (s) + copper(II)nitrate (aq) Cu (s) + AgNO 3 (aq)  Ag (s) + Cu(NO 3 ) 2 (aq) Cu (s) + AgNO 3 (aq)  Ag (s) + Cu(NO 3 ) 2 (aq) Cu (s) + 2AgNO 3(aq)  2Ag (s) + Cu(NO 3 ) 2 (aq) Cu (s) + 2AgNO 3(aq)  2Ag (s) + Cu(NO 3 ) 2 (aq)

Write a word & balanced chemical equation Solid zinc metal reacts with aqueous copper(II) sulfate to produce solid copper metal and aqueous zinc sulfate. Zinc(s) + copper(II)sulfate(aq)copper (s) + zinc sulfate (aq) Zn(s) + CuSO4 (aq) Cu(s) + ZnSO4(aq)

Write a Balanced Chemical Equation: Zinc(s) + hydrochloric acid(aq) zinc chloride (aq) and hydrogen gas. Zn(s) + HCl (aq)  ZnCl2(aq) + H2(g) Zn(s) + 2 HCl(aq)  ZnCl2(aq) + H2(g)

Aluminum carbide, Al 4 C 3(s) + water  methane gas, CH 4 + aluminum hydroxide (s) Al 4 C 3 (s) + H 2 O (l)  CH 4(g) + Al(OH) 3(s) Al 4 C 3 (s) + H 2 O (l)  CH 4(g) + Al(OH) 3(s) Al 4 C 3(s) + 12 H 2 O (l)  3CH 4(g) + 4Al(OH) 3(s) Al 4 C 3(s) + 12 H 2 O (l)  3CH 4(g) + 4Al(OH) 3(s)

Write a Balanced Chemical Equation: Aluminum sulfate + calcium hydroxide  aluminum hydroxide + calcium sulfate Aluminum sulfate + calcium hydroxide  aluminum hydroxide + calcium sulfate Al 2 (SO 4 ) 3 + Ca(OH) 2  Al(OH) 3 + CaSO 4 Al 2 (SO 4 ) 3 +3Ca(OH) 2  2Al(OH) 3 + 3CaSO 4