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CHAPTER 6 CHEMICAL REACTIONS AND EQUATIONS Chemical reactions are responsible for just about everything that occurs around us all the time. How do you know a chemical reaction has taken place when a cake is baked? What are some other examples of chemical reactions in real life? MACROSCOPIC CHANES INDICATE A MICROSCOPIC CHANGE OR CHEMICAL REACTION HAS TAKEN PLACE. INDICATIONS OF A CHEMICAL REACTION 1. Gas is released (bubbles) 2. Light is released or absorbed 3. Odor is formed 4. Color change 5. Heat is released or absorbed 6. Precipitate is formed (solid from mixed liquids) • During a chemical reaction energy is either released or absorbed. • Endothermic reaction: energy is absorbed • Exothermic reaction: energy is released (explosions) - seen as heat and or light CHEMICAL REACTION (same as chemical change) change of one substance into another substance • original substance can not be recovered • proper amounts of starting materials are needed, and all must be used to make finished product • all amounts going in = amounts coming out ex: building a bicycle BICYCLE MATERIALS Frame, bolts, screws, nuts, wheels, handlebars, seat, gears, etc….. Would the bicycle work properly if there were nuts and bolts left over when it was built? In chemical reactions the LAW OF CONSERVATION OF MASS holds true C + O2 CO2 Same numbers of atoms must be on each side of reaction MASS IS CONSERVED activity CHEMICAL EQUATION • method of using chemical symbols to represent a chemical reaction on paper • describes what goes on in a chemical reaction • easier than writing out words Ex: 2 mols. of hydrogen 2 H2 + one mol. of oxygen + O2 becomes 2 mols. of water 2 H2O PARTS OF A CHEMICAL EQUATION reactants “goes to” product 2 H (g) + O (g) ----------------- 2 H2O “becomes” coefficient physical state subscript Reactant- chemicals that will react , left side of the arrow Product- chemicals that are produced, right side of arrow Physical state- solid, liquid, gas Arrow- shows direction of reaction and what reactants become Coefficient- large numbers in front of chemical formulas represent number of atoms of each element in molecule Subscript- small number at lower right of chemical symbols represent number of atoms of that element in molecule COUNTING ATOMS Rules 1. Multiply subscript x coefficient 2. If parenthesis is present multiply subscript of each x subscript each element outside inside parenthesis parenthesis 3. Add total atoms x coefficient COUNTING ATOMS 2 H2O 3 H2SO4 5 C6H12O6 H 2x2=4 O 1x2= 2 total 6 H 2x3=6 S 1x3=3 O 4 x 3 = 12 total 21 C 6 x 5 = 30 H 12 x 5 = 60 O 6 x 5 = 30 total 120 Mg(OH)2 Mg 1 x 1 = O 1x2x1= H 1x2x1= total 1 2 2 5 3 Na(PO4)3 Na 1 x 3 = 3 P 1x3x3= 9 O 4 x 3 x 3 = 36 total 48 BALANCING EQUATIONS When a chemical reaction occurs between reactants there must always be equal number of atoms of reactants and products. WHAT LAW DOES THIS DEMONSTRATE? REACTANT S NUMBER OF ATOMS PRODUCTS = NUMBER OF ATOMS BALANCING EQUATIONS Rules 1. Write all the reactants on the left side of arrow, products on right side of arrow 2. NEVER change subscripts (this will change the reactant substance), ONLY change coefficients Ex: H2O water H2O2 hydrogen peroxide 3. Find numbers of atoms of each element Ex: 2 H2O O=2 H=4 BALANCING EQUATIONS Rules, cont. 4. Make sure to add up all sources of same element on each side of arrow ex: 2 H2O + C2H4O6 do same with products C 2 = C2 H 4+4= H8 O 2+6= O8 5. Balance equation so that numbers of elements are equal on each side of the arrow (conservation of mass) 6. Simplify (reduce) equation to lowest terms (find common denominator) ex: 4 NH3 + 2 O2 4 NO3 + 6 H2O (all divisible by 2, so simplify) 2 NH3 + O2 2 NO3 + 3 H2O STEPS FOR BALANCING EQUATIONS H2 + O2 H2O 1. List out the atoms on each side of equation H H O O 2. Count number of each type of atom in reactants H O 2 2 3. Count number of each type of atom in products H O 2 1 4. Note which atoms are not balanced H2 + O2 H2O H 2 2 O 1 H O 2 5. Select one atom to balance - easiest to start with atom which is by itself on one side - order: metal, nonmetal, save H & O last H2O + O2 2 H2O 6. update atom counts on each side of equation H 2 H 4 O 2 O 1 7. continue to update atom counts until both sides have equal numbers of atoms of each element 2H2O + O2 2 H2O BALANCED EQUATION!!! 8. Reduce coefficients if possible 4 NH3 + 12 O2 4 NO3 + 6 H2O ALL COEFFICIENTS ARE DIVISIBLE BY 2 SO SIMPLIFY 2 NH3 + 6 O2 2 NO3 + 3 H2O Al + Fe2O3 Al 1 Fe 2 O 3 Al2O3 + Fe Al 2 unbalanced Fe 1 unbalanced Fe 1 2 Al + Fe2O3 Al2O3 + Fe Al Fe O 2 2 3 Al Fe O 2 1 unbalanced 3 2 Al + Fe2O3 Al2O3 + 2 Fe ALL ATOMS EQUAL ON BOTH SIDES EQUATION IS BALANCED ! Mg Cl2 + K2S MgS + KCl Mg 1 Mg 1 Cl 2 Cl 1 unbalanced K 2 Cl 1 unbalanced S 1 S 1 Mg Cl2 + K2S MgS + 2 KCl Mg 1 Mg 1 Cl 2 Cl 2 K 2 K 2 S 1 S 1 ATOMS ARE EQUAL ON BOTH SIDES EQUATION IS BALANCED ! P4O10 + H2O P 4 H 2 O 11 P4O10 + H2O P 4 H 2 O 11 P4O10 + P H O 6 H2O 4 12 16 H2PO4 P 1 H 3 O 4 4 H2PO4 P H O 4 12 16 4 H2PO4 P H O 4 12 16 PbO2 PbO2 PbO2 + Pb Cl H O + Pb Cl H O HCl 1 1 1 2 PbCl2 + Pb Cl H O Cl2 + 1 4 2 1 H2O 4 HCl PbCl2 + Cl2 + H2O 1 4 4 2 + 4 HCl Pb 1 Cl 4 H 4 O 2 PbCl2 Pb Cl H O 1 4 2 1 + Pb Cl H O Cl2 + 1 4 4 2 2 H2O Example of Parenthesis (NH4)2 Cr2 O7 N 2 Cr 2 H 8 O 7 (NH4)2 Cr2 O7 N 2 Cr 2 H 8 O 7 + + Cr2O3 N Cr H O + N2 + H2O 2 2 2 4 Cr2O3 + N2 N 2 Cr 2 H 8 O 7 EQUATION IS BALANCED ! + 4 H2O Practice Problems TYPES OF REACTIONS Reactions are classified into types because it makes it easier to predict what will happen during and at the results of that reaction. Types of Reactions Synthesis two or more substances or combine to form a new single product - most exothermic a + b ab 2H2 (g) + O2 (g) 2H2O (g) element + element = new cpd 4 Fe (s) + 3 O (g) 2 FeO (rust) cpd + cpd = new cpd CO2 (g) + H2 (g) H2CO3 element + cpd = new cpd O2 (g) + CO (g) CO2 (g) Types of Reactions Decomposition: compound breaks down into two or more simpler substances ab a + b 2H2O (l) 2H2 (g) + O2 NH4NO3 (s) N2O (g) + 2 H2O - most endothermic Types of Reactions Single displacement: - one element takes the place of another in a compound element can replace first or second part of a compound more reactive element will replace less reactive element - a + bc ac + b or d + bc bd + c Zn + Cu(NO3)2 Zn(NO3)2 + Cu ele Fe cpd + ele Cl2 CuSO4 cpd + 2NaBr new cpd FeSO4 new cpd 2NaCl new ele + Cu ele + Br2 Real Life Example of Single Displacement Reaction Salicylic acid (C7H6O3) Aspirin (C9H8O4) Types of Reactions Double displacement positive portions of 2 ionic compounds are interchanged ab + cd ad + cb AgNo2 + NaCl AgCl + NaNO3 cpd cpd cpd cpd Pb(NO3)2 + 2 KCL PbI2 + 2 KNO2 Types of Reactions Combustion: - Compound usually containing carbon rapidly combines with oxygen to form one or more oxides (compound containing oxygen) - called burning - heat is produced ab CH4 + O2 oxide of a + oxide of b + 2O2 CO2 + 2H2O 2C2H2 + 5O2 4CO2 + 2H2O + energy Most reactions fall into these five categories, and some may fall into more than one . NH4NO3 (s) N2O (g) + 2 H2O Combustion- two new products contain oxygen (oxides) Decomposition- one reactant is broken down into two simpler substances NATURE OF REACTIONS Reversible reaction • reaction that changes direction based on energy flow Moves right: more product is made Moves left: more reactants are formed • shown with double arrow each way or one arrow pointing both directions • • reactants do not become used up because as new product is formed other new products break down and supply new reactants reactions will try to reach a state of equilibrium equilibrium – state where no net change in amount of reactants or products (a stable state) Equilibrium Example equilibrium does not mean there are equal numbers of products and reactants, amount of material started with = amount of material in end Equilibrium Example Rechargeable Batteries Dynamic Equilibrium: state where 2 exactly opposite chemical reactions are occurring at the same place, same time, and same rate (speed) where reactions continuously occur •Two opposing forces are being exerted but they are in a state of balance •Amounts of all chemical entities are constant but do not have to be the same ex: tug of war where both teams are pulling eqally against each other Dynamic equilibrium example: • person rowing boat upstream at exactly same speed as current - if current speeds up, he has to speed up - if current slows, he has to slow down to stay in same place • NET RESULT: boat has NO net movement even though person is still rowing and stream is still flowing Equilibrium does not mean there are equal numbers of reactants and products It means: amount of material = amount of material started with in end CaCO3 CaO + CO2 Double arrow shows that reaction can go in either direction Process of Dynamic Equilibrium 1. at first reactions goes to right more quickly until more of the reactants are used up 2. as more product is formed , the reverse reaction starts to occur more frequently 3. eventually reaction moves at same speed in either direction 4. EQUILIBRIUM has been achieved • DYNAMIC because molecules are always moving back and forth • reactions try to stay at equilibrium because this is a stable state What happens if you add stress to a reaction? • If one side pulls harder there is more stress • Other side has to react and pull harder in order to keep stability (equilibrium) ** same is true of chemical reactions ** Le Chateliers Principle If a change of condition (stress) is applied to a chemical system, the reaction will return to equilibrium by shifting to counteract the stress 3 stresses 1. change concentration of reactants or products 2. change temperature 3. change pressure in system containing gases FACTORS THAT AFFECT DIRECTION OF REACTION 1. addition or removal of reactants or products • addition of reactants/ removal of products: pushes reaction to right • addition of products/ removal of reactants: pushes reaction to left FACTORS THAT AFFECT DIRECTION OF REACTION 2. addition or removal of energy - addition: endothermic reactions absorb energy so more product made (reaction moves to right) - removal: exothermic reactions give off energy to make more reactant (reaction moves to left) In order for any reaction to occur energy is needed. Activation energy: amount of energy needed to make atoms collide ex: H and O both highly reactive elements can coexist in the same container for years if just sitting quietly add spark (energy) and they will cause an explosion Hindenberg- spark ignited H with O and exploded Space shuttle- liquid O is fine to go into space, but exploded when heat energy was added due to loss of heat shield shingles Reaction rate: how fast a reaction occurs Rate of reaction = amt substance changed/ amount of time or amt of substance made in an amt of time Importance of Reaction Rate • Industry • Kitchen • Body processes • Nature FACTORS THAT AFFECT REACTION RATE 1. Temperature 2. Concentration of reactants 3. Catalysts 4. Inhibitors FACTORS THAT AFFECT REACTION RATE 1. Temperature temperature temperature rate rate 2. Concentration concentration concentration rate rate ** Higher amounts of substance and temperature will increase numbers of collisions ** limiting reactant: reactant which is depleated first (not the one with the lesser amount) • concentration of reactants is important because if we run out of one of the reactants it can limit and stop the whole reaction ex: How many smores can be made with the following? 6 graham crackers 3 marshmallows 2 pieces of chocolate Which ingredient is the limiting reactant? How many bicycles can be made if the following is needed? 1 bicycle = 1 frame + 2 wheels We have: 100 frames 120 Wheels Which component is the limiting reactant? How many bikes can be made? FACTORS THAT AFFECT REACTION RATE 3. Catalysts: - substance that speeds up a reaction without being itself changed or used up - lowers activation energy needed to start reaction - most powerful catalysts found in nature ex: enzymes 4. Inhibitors: Substance that slows down a reaction, doesn’t completely stop reaction Ex: stabalizers in products, retardents Study for the test !