chapter 6: chemical reactions. identify a chemical reaction. write balanced chemical equations
TRANSCRIPT
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Section 1: Reactions and Equations
Chapter 6: Chemical Reactions
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Learning Goals
Identify a chemical reaction.Write balanced chemical equations.
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Evidence of Chemical Reactions
If we could see the atoms and molecules that compose matter, we could easily identify a chemical reaction: Atoms combine with other atoms to form
compounds.
New molecules form.
The original molecules decompose.
Atoms in one molecule change places with atoms in another.
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Evidence of Chemical Reactions
Visible evidence of a chemical reaction include: A color change The formation of a solid (precipitate) in a previously clear solution
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Evidence of Chemical Reactions
The formation of a gas when you add a substance to a solution
A change in energy (heat, light)
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Evidence of Chemical Reactions
Chemical reactions may occur without any obvious signs, yet chemical analysis may show that a reaction has indeed occurred.
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Practice
Which changes involve a chemical reaction? Ice melting The formation of hydrogen and oxygen gas from passing electricity through water
Iron rusting Bubbles forming when opening a pop can
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Practice
Determine if a chemical reaction has occurred in the following:
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Practice
Determine if a chemical reaction has occurred in the following:
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Practice
Determine if a chemical reaction has occurred in the following:
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Chemical Equations
Chemical reactions are represented with chemical equations.
We often specify the state of each reactant or product in parentheses next to the formula.
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Chemical Equations
The reaction occurring in a natural gas flame is methane reacting with oxygen to form carbon dioxide and water.
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Chemical Equations
How many oxygen atoms are on each side of the equation?
How many hydrogen atoms are on each side of the equations?
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Chemical Equations
Atoms cannot simply appear or disappear in chemical equations. We must account for the atoms on both sides of the equation.
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Balancing Chemical Equations
To balance an equation, we insert coefficients (not subscripts) in front of the chemical formulas as needed to make the number of each type of atom in the reactants equal to the number of each type of atom in the products.
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Balancing Chemical Equations
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Balancing Chemical Equations
Solid aluminum reacts with aqueous sulfuric acid to form aqueous
aluminum sulfate and hydrogen gas.
1. If needed, write a skeletal equation by writing correct chemical formulas for each of the reactants and products.
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Balancing Chemical Equations
2. If an element occurs in only one compound on both sides of the equation, balance it first. If there is more than one such element, balance metals before nonmetals. Al(s) + H2SO4(aq) Al2(SO4)3
+ H2(g)
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Balancing Chemical Equations
3. If an element occurs as a free element on either side of the chemical equation, balance it last. Always balance free elements by adjusting the coefficient on the free element.
Al(s) + 3H2SO4(aq) Al2(SO4)3 + H2(g)
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Balancing Chemical Equations
4. Check to make certain the equation is balanced by summing the total number of each type of atom on both sides of the equation. 2Al(s) + 3H2SO4(aq) Al2(SO4)3
+ 3H2(g) Al Al
H H
S S
O O
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Balancing Chemical Equations
Remember, change only the coefficients to balance a chemical equation; never change the subscripts.
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Practice
Write a balanced equation for each of the following: Solid silicon dioxide reacts with solid
carbon to produce solid silicon monocarbide and carbon monoxide gas.
Liquid octane (C8H18) and oxygen gas to form gaseous carbon dioxide and gaseous water.