2. Indicators of Chemical Rxns. Abbreviations of State (g) gas (l) liquid (s) solid (aq) aqueous a substance dissolved in water

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1 Unit 6 Chemical Reactions 1. Parts of a Chemical Reaction 2. Indicators of a Chemical Reaction 3. Enthalpy 4. Balancing Chemical Equations 5. Word Equations 6. Classifying Chemical Reactions 7. Predicting Products 1. Parts of Chemical Reactions Can you think of any chemical reactions that occur naturally? Photosynthesis Respiration Corrosion Acid Rain Fermentation Combustion How to read a chemical equation 2 H 2 (g) + O 2 (g) 2 H 2 O(g) reactants products Reactants are on the left and products are on the right Subscripts tell how many atoms of each type of element are in the substance Coefficients show how many particles of each type of substance are needed for the reaction to take place reaction taking place + is used to separate reactant (and product) substances from each other Subscripts: H 2 O = H 2 O 1 A water molecule is made up of two hydrogens and one oxygen Coefficients: 3H 2 O There are three water molecules Abbreviations of State (g) gas (l) liquid (s) solid (aq) aqueous a substance dissolved in water 2. Indicators of Chemical Rxns evolution of a gas Mg (s) + 2HCl (aq) MgCl 2(aq) + H 2(g) change in colour FeCl 3(aq) + 3KSCN (aq) Fe(SCN) 3(aq) + 3KCl (aq) formation of a solid Pb(NO 3 ) 2(aq) + 2KI (aq) PbI 2(s) + 2KNO 3(aq) odour FeS (s) + 2HCl (aq) H 2 S (g) + FeCl 2(aq) temperature change in the surroundings NH 4 SCN + Ba(OH) 2 + energy NH 3 + H 2 O + Ba(SCN) 2 14 KMnO4 + 4 C3H5(OH)3 > 7 K2CO3 + 7 Mn2O3 + 5 CO H2O + HEAT 1

2 3. Enthalpy Energy Changes in Reactions When a chemical reaction occurs, energy must be absorbed in order for reactant bonds to break Once reactant bonds are broken, product bonds form and energy is released 2 outcomes: The energy absorbed is MORE than the energy released = net GAIN of energy in reaction The energy absorbed is LESS than the energy released = net RELEASE of energy in reaction Energy released when product bonds form Energy needed to break reactant bonds net gain of energy energy is absorbed ENDOTHERMIC Rxn Energy needed to break reactant bonds Energy released when product bonds form net loss of energy energy is released EXOTHERMIC Rxn Enthalpy (heat of reaction) H The change in Enthalpy (ΔH) is the energy difference between the reactants and the products, found by Product Energy Reactant Energy For an Endothermic reaction: product energy reactant energy = positive #, so ΔH is positive For an Exothermic reaction: product energy reactant energy = negative #, so ΔH is a negative Endothermic Reaction Endothermic reaction a reaction in which energy is absorbed The products are higher in energy than the reactants 2NH 4SCN (s) + Ba(OH) 2(s) Ba(SCN) 2(aq) + 2H 2O + 2NH 3(l) Is energy absorbed or released? Therefore, where should it be written in the equation? REACTANT energy + 2NH 4SCN (s) + Ba(OH) 2(s) Ba(SCN) 2(aq) + 2H 2O + 2NH 3(l) Exothermic Reaction Exothermic reaction a reaction in which heat is released The products are lower in energy than the reactants 2K (s) + 2H 2 O (l) 2KOH (aq) + H 2(g) Is energy absorbed or released? Where should the energy term be written in the equation? 2K (s) + 2H 2 O (l) 2KOH (aq) + H 2(g) + energy 2

3 Summary There are two ways to show an endothermic reaction in a chemical equation: 2N 2 + O kJ 2N 2 O Energy term on reactant side, OR 2N 2 + O 2 2N 2 O ΔH = +58kJ Positive 'delta' H *delta is a greek letter meaning 'change in' There are two ways to show an exothermic reaction in a chemical equation (suppose 120kJ of energy released): 1. CH 4 + 2O 2 CO 2 + 2H 2 O +120 kj Energy term written on product side 2. CH 4 + 2O 2 CO 2 + 2H 2 O H = 120kJ 'Delta' H is a negative term Hebden p # Balancing Chemical Equations Parts of an Equation: Reactant Product 2Ag + H 2 S Ag 2 S + H 2 Reaction symbol Reactant The chemical(s) you start with before the reaction. Product The new chemical(s) formed by the reaction. 3

4 Subscript In a chemical equation, the number that shows how many atoms of an element are in a specific molecule. H 2 O There is one atom of oxygen in this water molecule There are two atoms of hydrogen in this water molecule Coefficient In a chemical equation, the number that shows how many molecules there are of this chemical. 3 H 2 O There are three molecules of water A balanced chemical equation has the same number of atoms on both sides of the equation. 4Al + 3O 2 2Al 2 O 3 4 Aluminum 6 Oxygen 4 Aluminum 6 Oxygen Balancing Rules Matter can never be created nor destroyed. A subscript can never be added, removed or changed. You may change only the coefficients in the equation. You must end up with the same number and type of atoms on the reactant and product sides of the equation. Generally, balance main elements first, then try hydrogens, then try oxygens 2 ** Balancing Demo H 2 + O 2 2 H 2 O Reactants Products H O You try! use a tally if needed! C + O 2 CO 2 4Fe + 3O 2 2Fe 2 O 3 2N 2 O 2N 2 + O 2 2Fe + O 2 2FeO 4

5 Balance this C 4 H 10 + O 2 C 4 H 10 + O 2 C 4 H 10 + O 2 CO 2 + H 2 O 4CO 2 + H 2 O 4CO 2 + 5H 2 O Polyatomic Ions If you have the same polyatomic ion on both sides of the equation, balance the whole polyatomic rather than each element in the polyatomic.see below Na 2 SO 4 + Al(NO 3 ) 3 NaNO 3 + Al 2 (SO 4 ) 3 3Na 2 SO 4 + Al(NO 3 ) 3 NaNO 3 + Al 2 (SO 4 ) 3 3Na 2 SO 4 + Al(NO 3 ) 3 6NaNO 3 + Al 2 (SO 4 ) 3 C 4 H O 2 4CO 2 + 5H 2 O But we can t have a decimal coefficient so multiply by 2 3Na 2 SO 4 + 2Al(NO 3 ) 3 6NaNO 3 + Al 2 (SO 4 ) 3 2C 4 H O 2 8CO H 2 O BALANCING CHEMICAL EQUATIONS Answers 1. Na 2 SO 4 (aq) + BaCl 2 (aq) _BaSO 4 (s) + _2_NaCl(aq) 2. _2_Al(s) + _3_H 2 SO 4 (aq) Al 2 (SO 4 ) 3 (aq) + _3_H 2 (g) Balancing Chemical Equations I 3. _4_NO 2 (g) + _2_H 2 O(l) + O 2 (g) _4_HNO 3 (aq) 4. _2_Al(s) + _6_HCl(aq) _2_AlCl 3 (aq) + _3_H 2 (g) 5. C 3 H 8 (g) + _5_O 2 (g) _3_CO 2 (g) + _4_H 2 O(g) 5. Word Equations we must be able to build balanced formula equations from word equations hints: keep in mind that seven elements naturally occur as diatomic molecules: H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2 acids can be found on the back of periodic table ammonia is the common name for NH 3 yields is another word for produces Example hydrogen and nitrogen yields ammonia H 2 + N 2 NH 3 Now balance: 3H 2 + N 2 2NH 3 5

6 Another aluminum sulphate + phosphoric acid yields aluminum phosphate + sulfuric acid Al 2 (SO 4 ) 3 + H 3 PO 4 AlPO 4 + H 2 SO 4 Balancing Chemical Equations II Balance Al 2 (SO 4 ) 3 + 2H 3 PO 4 2AlPO 4 + 3H 2 SO 4 6. Classifying Chemical Reactions Classifying Types of Chemical Reactions Six types of chemical reactions. Classified according to patterns of reactants & products Study pattern to predict products Synthesis combining 2 (or more) substances into 1 can start with elements or compounds follow rules for writing formulas for product E.g. 2Na + Cl 2 2NaCl Single Replacement element reacts with compound single element replaces one element in compound metal replaces metal, non metal replaces non metal e.g. Fe + CuSO 4 FeSO 4 + Cu Cl 2 + 2KBr 2KCl + Br 2 2. Decomposition 1 substance breaking down into 2 (or more) products are elements or simpler compounds E.g. 2H 2 O 2H 2 + O 2 6

7 Double Replacement 2 compounds switch partners 2 elements replace each other new compounds will be ionic (+ ion/ ion) follow formula writing rules e.g. CaCl 2 + Na 2 CO 3 CaCO 3 + 2NaCl Acid Base Neutralization (special kind of Double Replacement) H + from acid and OH from base neutralize to form H 2 O Acid + Base Salt + Water e.g. HBr + LiOH LiBr + HOH (H 2 O) Combustion burning of hydrocarbons always requires oxygen gas products are always CO 2 and H 2 O e.g. CH 4 + 2O 2 CO 2 + 2H 2 O Classifying Reactions 7. Predicting Products of Chemical Reactions What can we do so far from this unit? We can: make formulas from names balance equations give reaction types for different reactions Now we ll use these tools to build reactions and predict the products! Predict the products: Ca(OH) 2 + Al 2 (SO 4 ) 3 Step 1: Determine the reaction type using only the reactants. i.e. Is this synthesis, decomposition, single replacement, double replacement, neutralization, combustion? Step 2: Build the products using ion charges Ca(OH) 2 + Al 2 (SO 4 ) 3 Ca SO 4 + Al ( OH) 3 Step 3: Balance the equation 3Ca(OH) 2 + Al 2 (SO 4 ) 3 3CaSO 4 + 2Al(OH) 3 The reactants are both compounds, but there is no acid present, therefore the type is.. DOUBLE REPLACEMENT! 7

8 Try these (use the 3 steps) 2H 2 + O 2 2H 2 O 2NaCl 2Na + Cl 2 2C 2 H 2 + 5O 2 4CO 2 + 2H 2 O Predicting Single Replacement If you have reactants composed of a single element reacting with a compound, you most likely have a single replacement reaction There are two outcomes to a single replacement reaction: The reaction proceeds as you would predict The reaction will not occur H 2 SO 4 + 2NaOH 2H 2 O + Na 2 SO 4 Some elements are more reactive than others If a more reactive element is replacing a less reactive element in a compound, the reaction WILL proceed If a less reactive element is attempting to replace a more reactive element in a compound, the reaction WILL NOT occur We have a reactivity list called an ACTIVITY SERIES Activity Series Metals Lithium most active Potassium Calcium Sodium Magnesium Aluminum Zinc Chromium Iron Nickel Tin Lead Hydrogen Copper Silver Mercury Platinum Gold least active Nonmetals Fluorine Chlorine Bromine Iodine An element higher on the list will replace an element lower on the list in a single replacement reaction. NOTE: separate lists for metals & non metals Try these 2Al + 3NiCl 2 2AlCl 3 + 3Ni Pb + LiBr no rxn F 2 + CaCl 2 Cl 2 + CaF 2 Br 2 + AICI 3 no rxn 1. Classifying & Predicting 2. Activity Series 8