chapter 11 what is a chemical reaction?. a chemical reaction describes a change in composition. in a...
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Chapter 11 What is a chemical reaction?
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• A chemical reaction describes a change in composition.
• In a chemical reaction, the original substances are the reactants, and the substances that are created are called the products.
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• Observations that suggests that a chemical reaction is taking place are the production of :
• Energy (heat, light, or sound)
• Gas
• Precipitate
• Change in color
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• Not all reactions are chemical.
• A physical change can occur rather than a chemical one.
• For example, water turning into ice.
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Reactions involve rearrangements of atoms.
• Reactants are converted into products.
• The law of conservation of mass states that mass cannot be created nor destroyed.
• This means that the number of atoms in the product must be equal to the original reactants.
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• Atoms do not become other kinds of atoms, nor do they appear or disappear.
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Concept check
• List four observations that signify a chemical change.
• What happens to atoms during a chemical reaction?
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Chemical reactions release or absorb energy
• Endothermic reaction- reaction in which energy is absorbed.
- Bond formation always requires energy.
• Exothermic reaction- reaction in which energy is released. – Bond breaking always releases energy.
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• Spontaneous reaction- when a reaction is said to occur naturally or unaided.
For example a forest fire.
Spontaneous reactions can be endothermic or exothermic.
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• Particles must collide for a chemical reaction to occur.
• Example, when a safety match is lit, the reaction begins when the two substances are brought together by striking the match head across the striking surface.
• If this collision happens with enough energy, the bonds in the reactants are broken, allowing new bonds to form between the atoms.
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Types of Reactions
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Types of Chemical Reactions
1) Synthesis (composition reaction)
2) Decomposition
3) Single replacement reactions
4) Double replacement reactions
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Synthesis Reactions
• Two or more substances combine to form a new compound
A + X AX
Where A and X can be elements or compounds. AX is a compound.
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Ex. A thin strip of magnesium metal is placed in an open flame, it burns with bright white light. When the metal strip is completely burned, only a fine white powder of magnesium oxide is left.
2 Mg (s) + O2 (g) 2 MgO (s)
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• Some metals, such as iron, combine with oxygen to produce two different oxides
2 Fe (s) + O2 (g) 2 FeO (s)
4 Fe (s) + 3 O2 (g) 2 Fe2O3 (s)
The first reaction is Fe +2The second reaction is Fe +3
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Decomposition Reactions
• Decomposition is a single compound undergoes a reaction that produces two or more simpler substances
AX A + X
AX is a compound. A and X can be elements or compounds
Most take place only when heat/electricity is added
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Decomposition of binary compounds
• Simplest type of decomposition
• Binary compound is decomposed into its elements
2 H2O (l) 2 H2 (g) + O2 (g)
Decomposition of a substance by an electric current is called electrolysis
electricity
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Decomposition of Metal Carbonates
• EX. When a metal carbonate is heated, it breaks down to produce a metal oxide and carbon dioxide gas
CaCO3 (s) CaO (s) + CO2
Δ means heated
Δ
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Single Replacement Reactions
• Single replacement reactions – one element replaces a similar element in a compound
A + BX AX + BOr
Y + BX BY + X
A, B, X, and Y are elements. AX, BX, and BY are compounds
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Replacement of a Metal in a Compound by Another metal
• A more active metal will replace a least active metal.
2 Al (s) + 3 Pb(NO3) (aq)
Looking at the activity series
Al is more reactive than Pb so Al WILL replace Pb to form:
3 Pb (s) + 2 Al(NO3)3 (aq)
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2 Al (s)+ 3 Pb(NO3) (aq) 3 Pb (s) + 2 Al(NO3)3 (aq)
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Double Replacement Reactions
• The ions of two compounds exchange places in an aqueous solution to form two new compounds.
• One of the compounds formed is usually a precipitate, an insoluble gas that bubbles out of solution or a molecular compounds, usually water.
• The other compound is often soluble and remains dissolved in solution
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Double Replacement Reaction
AX + BY AY + BX
A, X, B, and Y in the reactants represent ions. AY and BX represent ionic or molecular compounds
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Formation of a Precipitate
• The formation of a precipitate occurs when the cations of one reactant combine with the anions of another reactant to form an insoluble or slightly soluble compound.
2 KI (aq) + Pb(NO3)2 (aq) PbI2 (s) + 2 KNO3 (aq)
Verify the products with the solubility sheet
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Formation of a gas
• In some double replacement reactions, one of the products is an insoluble gas that bubbles out of the mixture
FeS (s) + 2 HCl H2S (g) + FeCl2 (aq)
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Formation of Water
• In some double replacement reactions, a very stable molecular compound, such as water, is one of the products
HCl (aq) + NaOH (aq) NaCl (aq) + H2O(l)
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Balancing Equations
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Balancing requires patience
• To satisfy the law of conservation of mass, you should insert coefficients into the chemical equation.
• This ensures that there are equal numbers of atoms for each element on each side of the equation when an equation is balanced.
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Testing an equation for balance Reactants Products
Balance?
Unbalanced
Formula equation CH3CH2OH +O2 CO2 + H2O
Carbon atoms 2 1 No
Hydrogen atoms 6 2 No
Oxygen atoms 3 3 Yes
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• To balance this equation, there are two carbon atoms in the reactant column and only one in the product column.
• To balance the number of carbon atoms you need to double the number of carbon dioxide molecules in the products.
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CH3CH2OH + ? O2 2CO2 + ? H2O
The 2 is called a coefficient and indicates that there are two carbon dioxide molecules.
Coefficients are normal whole numbers, when a coefficient is equal to 1, the 1 is not written for simplicity.
Coefficients are written in front of the formula and multiply the entire formula.
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Testing an equation for balance Reactants Products
Balance?
Unbalanced
Formula equation CH3CH2OH +O2 2CO2 + H2O
Carbon atoms 2 2 Yes
Hydrogen atoms 6 2 No
Oxygen atoms 3 5 No
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• The carbon atoms are now balanced by the hydrogen atoms are not. In addition the oxygen atoms have been thrown out of balance.
• Let’s start with hydrogen, there are six in the reactant column, so we must add three to the product column.
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Testing an equation for balance Reactants Products
Balance?
Unbalanced
Formula equation CH3CH2OH +O2 2CO2 + 3H2O
Carbon atoms 2 2 Yes
Hydrogen atoms 6 6 Yes
Oxygen atoms 3 7 No
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• The oxygen atoms are still unbalance.
• Multiplying the number of oxygen molecules by three results in an equal number of oxygen atoms.
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Testing an equation for balance Reactants Products
Balance?
Unbalanced
Formula equation CH3CH2OH + 3O2 2CO2 + 3H2O
Carbon atoms 2 2 Yes
Hydrogen atoms 6 6 Yes
Oxygen atoms 7 7 Yes
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• Now you have written a balanced chemical equation.
• Replacing the question marks in the formula equation with the right coefficients now give a correct chemical equation.
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Tips for balancing equations
• 1. Save the elements that appear in several
reactants or products. (usually oxygen and
hydrogen )• 2. If the same polyatomic ion appears on
both sides, treat them as single units. • 3. After considering the first two steps,
balance the equation from left to right. • 4. For ionic equations, be sure that charges are
balanced.
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Testing an equation for balance Reactants Products
Balance?
Unbalanced
Formula equation NH3 + O2 NO + H2O
Nitrogen atoms 1 1 Yes
Oxygen atoms 2 2 Yes
Hydrogen atoms 3 2 No
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Testing an equation for balance Reactants Products
Balance?
Unbalanced
Formula equation NH3 + O2 NO + H2O
Nitrogen atoms
Oxygen atoms
Hydrogen atoms
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Testing an equation for balance Reactants Products
Balance?
Unbalanced
Formula equation 4NH3 + 5O2 4NO + 6H2O
Nitrogen atoms 4 4 Yes
Oxygen atoms 10 10 Yes
Hydrogen atoms 12 12 Yes
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9-2 cont..How are chemical equations for
reactions written?
• A chemical equation-describes the type and number of atoms that are rearranged during a reaction.
• A word equation describes a reaction. ethanol + oxygen carbon dioxide + water
The arrow means “react to form” and the plus sign means “and”
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• A word equation can then be replaced with formulas to make a formula equation.
• The word ethanol can be replaced with its formula CH3CH2OH (l)
• The other three words in the equation are also replaced with the corresponding formula and symbol.
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Word equation:
ethanol + oxygen carbon dioxide + water
Formula equation:
?CH3CH2OH (l) + ? O2 (g) ?CO2 (g) +? H2O(l)
Question marks are used to signify that the number of molecules for each is not yet known.
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Numbers in Formulas
Coefficients Subscripts Superscripts
show show show
Amounts of reactants Number of Amount
and products in an each type of and type
Equation atom in a formula of charge on an ion.
Ex: 3NaOH Ex: H2O Ex: K+
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Sample problem p. 315
Sometimes it is useful to write the equation in the form of a sentence.
The first step in the commercial production of nitric acid is the combustion of ammonia in the presence of a catalyst. The exothermic reaction produces nitrogen monoxide and water vapor. Write and balance the equation for this reaction.
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• Step one: Write the word equation
ammonia + oxygen nitrogen monoxide + water
• Step two : Write the unbalanced formula equation.
? NH3 + ?O2 ?NO + ?H2O
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• Step 3: Count the number of atoms for each element and Balance