Topic 3 (Part B): Chemical Equations

Moles A Pair is 2 A dozen is 12 A gross is 144 A ream is 500 A mole is a number, that measures the amount of a substance A mole is So it’s just a really big number, often expressed in scientific notation as 6.02 x mole = n = 6.02 x This is Avogadro’s number

It is too hard to weigh an individual atom, so we weigh many of them (6.02 x ) in order to work in grams! When you measure the mass of 6.02 x nucleons from an atom of carbon-12, the mass is 12 grams! What is the mass of a mole of carbon-12 atoms? Why 6.02 x Answer: 12 grams!! 1 Mole = 6.02 x = Avogadro’s number

Molar Mass Molar mass is the mass (in grams) of one mole of any substance. The molar mass of every element is given on your periodic table. Molar mass should be expressed in grams per mole (g/mol). The molar mass of iron is 55.85

Molar mass of molecules and compounds To find the molar mass of a molecule or compound you must add up the molar mass of each atom in the formula. Find the molar mass of iron (III) sulphate Fe 2 (SO 4 ) 3 (s) Molar mass = 2(M of Fe) + 3(M of S) + 12(M of O) = 2(55.85) + 3(32.07) + 12(16.00) = g/mol Don’t forget the units!!!

Using molar mass to find moles How many HCl (g) molecules (in moles (n)) make up a 35.0g sample ? To find moles, we use the formula… number of moles of a substance (mol) Molar mass of The substance (g/mol) Mass of the substance (g) This formula is on page 10 of your data booklet!

Using molar mass to find moles How many HCl (g) molecules (in moles (n)) make up a 35.0g sample ? 1. Write down what you know and what you want to find out. Steps to find moles: 2. Substitute and solve. n = mol

Using molar mass to find grams To find moles, we use the formula… number of moles (mol) Molar mass (g/mol) Mass (g) This formula is on page 10 of your data booklet! 3. What is the mass of a 0.960mol sample of HCl (g) ?

Using molar mass to find grams 1. Write down what you know and what you want to find out. Steps to find moles: 2. Substitute and solve. 3. What is the mass of a 0.960mol sample of HCl (g) ?

Evidence of a chemical reaction Recall: Evidence that a chemical reaction is occurring includes … Sound HeatBubbles Light Smell Color Change Smoke

Endothermic vs. Endothermic Reactions Exothermic: Energy is released to the surroundings Endothermic: Energy is absorbed from the surroundings. There are 2 different types of reactions:

Conservation Rules When we say that certain things are conserved, we mean that they stay the same. During a chemical reaction three things are conserved: –Mass –Energy –Number of atoms We will rely on conservation rules when we learn to balance equations later on in the course.

Types of Chemical Reactions 1.Formation: Element (E) + Element  Compound (C) Ex. 2Mg + O 2  2MgO 2.Decomposition: Compound  Element + Element Ex. 2MgO  2Mg + O 2 3.Single Replacement: E + C  New E + New C Ex. Cl 2 + NaI  I 2 + NaCl These are listed on page 9 of your data booklet

Types of Chemical Reactions 4. Double Replacement: C + C  New C + New C Ex. HCl + KOH  HOH + KCl 5. Acid Base Neutralization (ex of double replacement) Acid + Base  Salt + Water Ex. HCl + NaOH  NaCl + HOH 6. Hydrocarbon Combustion: HC + O 2(g)  CO 2(g) + H 2 O (g) Ex. CH 4 + 2O 2  CO 2 + 2H 2 O These are listed on page 9 of your data booklet (except acid-bas neutralization)

1. Formation Reactions Formation of water (H 2 O) Formation of table salt (NaCl) Element + Element  Compound

Formation Reactions Element + Element  Compound C (s) + 2H 2(g)  CH 4 (g) N 2(g) + H 2(g)  NH 3(g) Al (s) + Cl 2(g)  AlCl 3(s) Na (s) + Cl 2(g)  NaCl (s) Memorize molecular examples Ionic compounds are predicted using charges

2. Simple Decomposition Compound  Element + Element (the exact reverse of Formation!!) Decompostion of water (H 2 O) Decompostion of ammonia (NH 3 ) Decompostion of aluminum chloride (AlCl 3 ) Decompostion of table salt (NaCl)

2. Simple Decomposition Compound  Element + Element (the exact reverse of Formation!!) As with formation, you don’t have to worry about solubility since there is no solvent present.

3. Single Replacement Element + Compound  New Compound +New Element Zn (s) + AgNO 3(aq)  Zn(NO 3 ) 2(aq) + Ag (s) If the element is a metal, it will take the place of the metal in the compound. Zn 2+ NO 3 - Use charges to figure out the formula for the new compound. Elements by themselves have no charge

Cl 2(g) + AlI 3(aq)  If the element is a nonmetal, it will take the place of the nonmetal in the compound. Al 3+ Cl - Use charges to figure out the formula for the new compound. How many chlorines are needed to balance the charge of aluminum? Three! 3Cl 2(g) + 2AlI 3(aq)  2AlCl 3(aq) + 3I 2(s) How do you balance this equation? Balancing Example

Balancing Equations When balancing, balance groups of atoms (like polyatomic ions) together. Zn (s) + AgNO 3(aq)  Zn(NO 3 ) 2(aq) + Ag (s) 22 State: Since single replacement reactions always take place in aqueous solution (in water) you must use your solubility table to determine if the compound in the product is “aq” or “s”. How do you balance this equation? Solubility Table

Double Replacement Compound + Compound  New Compound + New Compound Ca(NO 3 ) 2(aq) + Na 2 CO 3(aq)  NaNO 3(aq) + CaCO 3(s) The ions in the compounds to the left (reactants) switch places to produce the new compounds on the right (products). Inside combines with inside Outside combines with outside Remember that it is easier to balance in large chunks than with individual atoms. How is this equation balanced? 2

Ca(NO 3 ) 2(aq) + Na 2 CO 3(aq)  2NaNO 3(aq) + CaCO 3(s) Use your solubility table to look up the states of the products. Notice the solid state on calcium carbonate (called a precipitate). Double Replacement

Na 2 S (aq) + Cd(NO 3 ) 2(aq)  CdS (s) + NaNO 3(aq) CdS (s) 2 Double Replacement

5. Acid Base Reactions Acid + Base  Salt + water HCl (aq) + NaOH (aq)  NaCl (aq) + HOH (l) Salty water AcidBase Acid base reactions are also called neutralization reactions Notice the similarity to double replacement (Compound + Compound). To predict the products you can treat an acid base reaction just like double replacement.

5. Acid Base Neutralization hydrochloric acid + sodium hydroxide sulfuric acid + sodium hydroxide

HCl (aq) + Ba(OH) 2(aq)  Write water as HOH to make balancing easier for single replacement, double replacement and acid base reactions. BaCl 2(aq) + HOH (l) 2 2 Can you balance the equation ? 5. Acid Base Neutralization

6. Combustion Reactions A reaction were something reacts with oxygen to form oxides is a combustion reaction. (something + O 2(g) means combustion) 2Mg (s) + O 2(g)  2 MgO (s) CH 4(g) + 2 O 2(g)  CO 2(g) + 2H 2 O (g)

6. Combustion 4Al (s) + 3 O 2(g)  2Al 2 O 3(S) 4Fe (s) + 3O 2(g)  2Fe 2 O 3(s) Combustion reactions typically give off lots of heat and light. We use the word oxidation to describe combustion processes which are slow (like rusting)

Balancing Combustion Equations Example: Write a balanced chemical equation for the complete combustion of heptane. Step 1: Write the equation as you normally would. Step 2: Now balance the carbons 7 carbons here 1 carbon here Place a 7 in front of carbon dioxide. 7

Balancing Combustion Equations Example: Write a balancing chemical equation for the complete combustion of heptane. Step 3: Now balance the hydrogens. 16 hydrogens here 2 hydrogens here Place an 8 in front of water (because 8 x 2 = 16) 78

Balancing Combustion Equations Example: Write a balancing chemical equation for the complete combustion of heptane. Step 4: Now balance the oxygens 7x2 = 14 oxygens here8x1 = 8 oxygens here There are = 22 oxygens on the right hand side. We have to have 22 on the left hand side, we have 2 already What multiplied by 2 will give you 22 oxygens??

Predicting Combustion Products Combustion reactions produce oxides A complete combustion will always produce the most common oxide of the element. ElementMost common Oxide CCO 2(g) SSO 2(g) NNO 2(g) HH 2 O (g) metalMetal Oxide

ElementMost common Oxide CCO 2(g) SSO 2(g) NNO 2(g) HH 2 O (g) metalMetal Oxide 4NH 3(g) + 7O 2(g)  4NO 2(g) + 6H 2 O (g) 2H 2 S (g) + 3O 2(g)  2H 2 O (g) + 2SO 2(g) C (s) + O 2(g)  CO 2(g) S 8(s) + 8O 2(g)  8SO 2(g) Predicting Combustion Products

What is the most common oxide of a metal? Metals with only one charge can only form one kind of oxide: 2Mg (s) + O 2(g)  2 MgO (s) How do you balance this equation? Predicting Combustion Products

When more than one charge is possible (multivalent metals) use the most common charge; this will be the charge that is listed first on your periodic table. 4Fe (s) + 3O 2(g)  2Fe 2 O 3(s) Iron (III) is used because it is the most common charge Cu (s) + O 2(g)  What is the product of this reaction? CuO (s) What is the balanced equation? 22 Predicting Combustion Products

Balance & Identify reaction type

Identify reaction type Formation Single Replacement Double Replacement Specifically: Acid/Base Neutralization

Mole Ratios The ratio of the coefficients in a balanced chemical equation Coefficients: The number IN FRONT of the compound

Mole Ratios Example: The coefficient on oxygen is 1.

Mole Ratios Determine the amount of silver required to make mol of silver sulfide using the chemical equation below. Step 1: Determine Mole Ratio:

Mole Ratios Determine the amount of silver required to make mol of silver sulfide using the chemical equation below. Step 2: Calculate number of moles of silver required:

Mole Ratio Example 2

Mole Ratio Example Solution