Today-- Welcome Back!• Turn in “Chapter 6 Extra Credit”• Unexpected Changes Lab• Introduce Chapter 9 (LAST CHAPTER)• Outlining – NOPE!
Chapter 9 – Chemical Reactions and Equations
Learning Objectives:■ Be able to balance chemical equations by
applying the law of conservation of mass.
■ Be able to recognize synthesis, decomposition, single replacement, double replacement, combustion and neutralization reactions.
Law of Conservation of Matter• Conservation of Matter: in all chemical and
physical changes, matter is neither created or destroyed ■ The total mass in a chemical reaction remains constant
• Antoine Lavoisier:■ Made accurate and precise measurements
during chemical reactions
Reactants ―› Products• Reactants: the substances that enter
into the reaction
• Products: the substances that areproduced by the reaction
Why Do Reactions Occur• Think back to what we know about atoms
and specifically their electrons?
• Through chemical reactions, atoms have the opportunity to obtain complete sets of valence electrons and thus become more stable.
Five General Types of Chemical Reactions
• Direct Combination (Synthesis)• Decomposition• Single-Replacement• Double-Replacement• Combustion
By knowing the type of reaction that is occurring, you can predict the products that will be formed.
Chemical Equations• A method of describing chemical reactions
■ Word Equations• Calcium + Oxygen ―› Calcium Oxide
■ Formula Equations• 2Ca + O2 ―› 2 CaO
the arrow → separates the reactants from the products
Completing the Chemical Equation
• Complete the chemical equation by describing the physical state of each substance:
■ Solid (s)■ Liquid (l)■ Gas (g)■ Aqueous (aq) means dissolved in water
Symbols used in equations
• Double arrow indicates a reversible reaction
• shows that heat is supplied to the reaction
• is used to indicate a catalyst is supplied, in this case, platinum.
∆
Pt
What is a catalyst?
• A substance that speeds up a reaction, without being changed or used up by the reaction.
• Enzymes are biological catalysts.
• How can you physically speed up a reaction?
I. Direct Combination Reactions (also called synthesis reactions).
General form: A + B → AB (two reactants make a single product)
A, B = elements or compoundsAB = compound consisting of A
and B
■ This is the only type of chemical reaction in which there is a single product formed. This single product is always more complex than the reactants.
Examples of Synthesis Reactions
■calcium + oxygen yields calcium oxide 2Ca + O2 → 2CaO
■ Notice: All equations show two (or more) reactants, but only one product.
■ http://www.ric.edu/ptiskus/reactions/Index.htm
II. Decomposition Reactions
General form: AB → A + B (one reactant makes two or more
products)
AB = compoundA, B = elements or simpler
compounds
▪ This is the only type of chemical reaction in which there is a single reactant. This single reactant is always more complex than the products.
Decomposition Reactions: Examples
■ water yields hydrogen and oxygen2H2O → 2H2 + O2
■ marble (calcium carbonate) yields calcium oxide and carbon dioxideCaCO3 → CaO + CO2
■ Notice: all equations show a single reactant decomposing into two (or more) products.
■ http://www.ric.edu/ptiskus/reactions/Index.htm
Balancing Chemical Equations
• The Law of Conservation of Matter states that:■ Matter is neither created nor destroyed!
■ For mass to remain constant both before and after a reaction, the number of atoms must remain constant
Step 1: Balancing Equations
• Write the word equation that describes the reaction.
iron + oxygen ―› iron oxide
Step 2: Balancing Equations
2. Replace the words in the equation withsymbols and formulas.
Fe + O2 ―› Fe2O3
Do we have the same numbers of each atoms on both sides of arrow?
Does this follow the law of conservation of matter?
Step 3: Balancing Equations
3. Count the # of atoms of each elementon both sides of the equation.
Fe + O2 ―› Fe2O3
Step 4: Balancing Equations
4. Starting with elements that only occurin one substance on each side of theequation, make sure that each side of theequation has an equal # of that element.
Proceed with all elements.
Remember that changing the # of one element may alter elements that have already been balanced.
Fe + O2 ―› Fe2O3
Let’s try:
CH4 + O2 ―› CO2 + H2O
• Never change a subscript to balance an equation.■ If you change the formula you are describing a
different reaction.
■H2O is a different compound than H2O2
• Never put a coefficient in the middle of a formula■ 2 NaCl is okay, Na2Cl is not.
Never
Balancing Equations: Examples
■ H2 + O2 → H2O
■ Co + O2 → Co2O3
■ Pb(NO3)2 + K2S → PbS + KNO3
■ C2H6 + O2 → H2O + CO2
Balance the following
iron(II) chloride + sodium phosphate → sodium chloride + iron (II) phosphate
FeCl2 + Na3PO4 → NaCl + Fe3(PO4)2
Today
■ Look at Single-Replacement Reactions.
■ Begin “Single-Replacement Lab” set-up.
Single-Replacement Reactions
■ Copper metal and silver nitrate:
Cu(s) + AgNO3(aq) → Ag(s) + CuNO3(aq)
■ What do you observe about the reaction?
■ What do you notice about the chemical equation?
■ Cu must be more reactive than Ag in order for the reaction to take place.
Single-Replacement Reactions
General Form: A + BX → AX + BOne element and one compound
recombine (switch partners)AX, BX = ionic compounds
A, B = MetalsX = ion that switches partners
*Metal ‘A’ must be more reactive than ‘B’ for this to occur
Single-Replacement Lab
Today you will do the following:1. Formulate a question for the lab2. Formulate a hypothesis3. Design procedures4. Create a data table.
IV. Double-Replacement Reactions
General form: AX + BY → AY + BX(Positive ions in two compounds are exchanged)
A,B = positive ionsX,Y = negative ions
■ This is the only type of chemical reaction with two compounds as reactants and two compounds as products.
Double Replacement Examples
■ calcium carbonate and hydrochloric acid yield calcium chloride and carbonic acid
CaCO3 + 2HCl → CaCl2 + H2CO3
■Notice: in this reaction, two ionic compounds exchange ions to form two new ionic compounds
www.ric.edu/ptiskus/reactions/Index.htm
IV. Double-Replacement Reactions
General form: AX + BY → AY + BX(Positive ions in two compounds are exchanged)
A,B = positive ionsX,Y = negative ions
■ This is the only type of chemical reaction with two compounds as reactants and two compounds as products.
Double Replacement Examples
■ calcium carbonate and hydrochloric acid yield calcium chloride and carbonic acid
CaCO3 + 2HCl → CaCl2 + H2CO3
■Notice: in this reaction, two ionic compounds exchange ions to form two new ionic compounds
www.ric.edu/ptiskus/reactions/Index.htm
Rules of Double-Replacement Reactions
■ Reactants must be dissolved in water (releasing the ions).
■ Will occur if one of the products :• is a molecule (covalent), • a precipitate (solid comes out of
solution), or• an insoluble gas.
V. Combustion Reactions
General Form: CxHy + O2 → H2O + CO2
(hydrocarbon and oxygen react to form carbon dioxide and water)
■ This is the only type of chemical reaction where something reacts with oxygen and forms carbon dioxide and water
Combustion Examples
▪ Methane reacts with oxygen:CH4 (methane) + O2 → H2O + CO2
▪ Gasohol reacts with oxygen:C2H5OH (ethanol) + O2 → H2O + CO2
▪ Notice: in both cases, water and carbon dioxide are the products.
www.ric.edu/ptiskus/reactions/Index.htm
1. Write the word equation2. Write the balanced formula equation■ Solid iron (III) sulfide reacts with
gaseous hydrogen chloride to form iron (III) chloride and hydrogen sulfide gas.
1. Write the word equation2. Write the balanced formula equation
■ Nitric acid reacts with solid sodium carbonate to form liquid water and carbon dioxide gas and sodium nitrate.