Download Key - MCHS Science

8.1 Describing Chemical Reactions
Chemical reaction- process by which one or more substances are changed into one or
more different substances
Reactants: original substance
Products: resulting substance
Chemical Equation: represents, with symbols and formulas, the identities and relative
amounts of the reactants and products in a chemical reaction.
Indications of a Chemical Reaction
1. Evolution of heat and light
2. Production of a gas
3. Formation of a precipitate (a solid that is produced as a result of a chemical reaction
in solution and that separates from the solution)
4. Color change
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Chemical equation gives no indication of whether a reaction will naturally occur
or not, the speed of the reaction, or if the reaction would even take place.
Formula equation- represents the reactants and products of a chemical reaction
by their symbols or formulas
Diatomic Molecules- Hydrogen, nitrogen, oxygen , fluorine, chlorine, bromine,
iodine (H2,N2, O2, F2, Cl2, B2, I2)
Balancing Chemical Equations
1. Write a formula equation by substituting correct formulas for the names of the
reactants and the products
Ex: H2O(l) → H2(g) + O2(g)
2. Balance the formula equation according to the law of conservation of mass
 Numbers of each type of atom are the same for both the products and the
reactants, the equation is balanced
 Trial and error
o Balance the different types of atoms one at a time
o First balance the atoms of elements that are combined and that
appear only once on each side of the equation
o Balance polyatomic ions that appear on both sides of the equation
as a single unit
o Balance H atoms and O atoms after atoms of all other elements
have been balanced
Ex: 2H2O(l) → 2H2(g) + O2(g)
3. Count the atoms to be sure that the equation is balanced
Ex: 2H2O(l) → 2H2(g) + O2(g)
(4H + 2 O) = (4 H) + (2O)
Note: Coefficients- small whole number that appears in front of a formula in a
chemical equation
If the coefficients do not represent the smallest possible whole-number ratio of
reactants and products, divide by their greatest common factor
**YOU NEVER BALANCE EQUATIONS BY CHANGING SUBSCRIPTS**
1.
Sample Problems:
Balance the following reactions:
a. Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
b. 2HNO3(aq) + Mg(OH)2(s) → Mg(NO3)2(aq) + 2H2O (l)
c. Al4C3(s) + 12H2O(l) → 3CH4(g) + 4Al(OH)3 (s)
2. Write a balanced chemical equation for the following: Solid sodium combines
with chlorine gas to produce solid sodium chloride.
2Na(s) + Cl2(g) → 2NaCl(s)
3.
Write the chemical equation for the following: ammonium chloride +
calcium hydroxide → calcium chloride + ammonia + water
2NH4Cl + Ca(OH)2 →CaCl2 + 2NH3 + 2H2O
8.1 Describing Chemical Reactions
___________________ _______________- process by which one or more substances
are changed into one or more different substances
_____________________: original substance
_____________________: resulting substance
______________________ ________________: represents, with symbols and formulas,
the identities and relative amounts of the reactants and products in a chemical reaction.
Indications of a Chemical Reaction
1. Evolution of ___________ and _______________
2. Production of a ____________
3. Formation of a __________________________ (a solid that is produced as a result of
a chemical reaction in solution and that separates from the solution)
4. __________________ change
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Chemical equation gives no indication of whether a reaction will naturally occur
or not, the speed of the reaction, or if the reaction would even take place.
_____________________ _______________- represents the reactants and
products of a chemical reaction by their symbols or formulas
___________________ ________________- Hydrogen, nitrogen, oxygen ,
fluorine, chlorine, bromine, iodine (H2,N2, O2, F2, Cl2, B2, I2)
Balancing Chemical Equations
1. Write a formula equation by substituting correct formulas for the names of the
reactants and the products
Ex:
2. Balance the formula equation according to the law of conservation of _________
 Numbers of each type of atom are the same for both the products and the
reactants, the equation is balanced
 ___________and _______________
o Balance the different types of atoms one at a time
o First balance the atoms of elements that are combined and that
appear only once on each side of the equation
o Balance _____________________ __________ that appear on
both sides of the equation as a single unit
o Balance ______ atoms and ____ atoms after atoms of all other
elements have been balanced
Ex:
3. Count the atoms to be sure that the equation is balanced
Ex:
Note: _________________- small whole number that appears in front of a
formula in a chemical equation
If the coefficients do not represent the smallest possible whole-number ratio of
reactants and products, divide by their _______________ _______________
___________________
**YOU NEVER BALANCE EQUATIONS BY CHANGING
_______________**
Sample Problems:
1. Balance the following reactions:
a. _____Zn(s) + _____HCl(aq) → ______ZnCl2(aq) + ______H2(g)
b. _____HNO3(aq) + _____Mg(OH)2(s) → ____Mg(NO3)2(aq) +
___H2O (l)
c. _____Al4C3(s) + _____H2O(l) → ______CH4(g) + _____Al(OH)3 (s)
2. Write a balanced chemical equation for the following: Solid sodium
combines with chlorine gas to produce solid sodium chloride.
3.
Write the chemical equation for the following: ammonium chloride +
calcium hydroxide → calcium chloride + ammonia + water