Download Rules for Balancing Equations

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Rules for Balancing Equations
The symbols for atoms represent their mass.
The symbol (s) indicates a soluble material.
The mass of reactants is more than the mass of products in a chemical
equation.
Subscripts on formulas in chemical equations can be changed to balance
the equation.
The coefficient "1" is generally not written but understood in chemical
equations.
The coefficients in chemical equations are generally written as whole
numbers.
The molar mass of an element equals the atomic weight expressed in
grams.
The molecular mass for H2 is less than the molecular mass for CH4.
A mole of hydrogen contains the same number of molecules as a mole of
anything else.
The mole is a unit devised to count particles by measuring mass.
The molar mass to the nearest whole number for hydrogen, H2, is 2.
grams.
The mass of a mole of sodium is more than the molar mass of lithium, Li.
The equation is balanced CHCH + H2 ---> CH3CH3
The law of conservation of mass says the total mass of matter in the
universe is constant.
There are 8 "S" atoms in a formula unit of alum, KAl(SO4)2• 12 H2O
The oxidation number for pure elements is zero.
If you dried 100 g of alum the weight would stay constant.
There are 24 "H" atoms in a formula unit of alum, KAl(SO4)2• 12 H2O
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19. Hydrogen, H2, is a product in the equation CHCH + H2 ---> CH3CH3
The mole ratio of hydrogen to acetylene is 1 to 1 in the equation CHCH +2
20. H2 ---> CH3CH3
21. The oxidation number for fluorine in compounds is always "-1".
22. Oxidation is the loss of electrons.
23. Reduction is the gain of electrons.
24. This is a neutralization reaction HCl + NaOH ---> NaCl + H2O
Reactions where a solid is formed that "drops out" of solution are precipitation
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reactions.
1. T The symbols for atoms represent their mass.
2. F The symbol (s) indicates a soluble material.
3. T The mass of reactants is more than the mass of products in a chemical
equation.
4. F Subscripts on formulas in chemical equations can be changed to balance
the equation.
5. T The coefficient "1" is generally not written but understood in chemical
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8. F
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equations.
The coefficients in chemical equations are generally written as whole
numbers.
The molar mass of an element equals the atomic weight expressed in
grams.
The molecular mass for H2 is more than the molecular mass for CH4.
A mole of hydrogen contains the same number of molecules as a mole of
anything else.
The mole is a unit devised to count particles by measuring mass.
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T
11. F The molar mass to the nearest whole number for hydrogen, H2, is 1.
grams.
12. The mass of a mole of sodium is more than the molar mass of lithium, Li.
T
13. F The equation is balanced CHCH + H2 ---> CH3CH3
14. The law of conservation of mass says the total mass of matter in the
T
universe is constant.
15. F There are 8 "S" atoms in a formula unit of alum, KAl(SO4)2• 12 H2O
16. The oxidation number for pure elements is zero.
T
17. F If you dried 100 g of alum the weight would stay constant.
18.
There are 24 "H" atoms in a formula unit of alum, KAl(SO4)2• 12 H2O
T
19. F Hydrogen, H2, is a product in the equation CHCH + H2 ---> CH3CH3
The mole ratio of hydrogen to acetylene is 1 to 1 in the equation CHCH
20. F +2 H2 ---> CH3CH3
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The oxidation number for fluorine in compounds is always "-1".
T
22.
Oxidation is the loss of electrons.
T
23. F Reduction is the loss of electrons.
24.
This is a neutralization reaction HCl + NaOH ---> NaCl + H2O
T
25. Reactions where a solid is formed that "drops out" of solution are precipitation
reactions.
T
Balancing equations.
The letter symbols that represent atoms and molecules in equations are treated like
objects. Balancing chemical equations literally means counting the number of times
atom symbols appear in the reactants and products to make sure the counts are the
same on both sides. Conservation of mass is linked to "conservation " of element
symbols.
The law of conservation of mass is met when the count of element symbols on
reactants side is equal to the count of element symbols on the product side. This rests
on the additional assumption that the symbols also represent the masses of the
elements.
A balanced equation has equal counts (number) of atoms of each element in both
reactants and products.
Equations are balanced by adjusting the multipliers (coefficients) in front of formula
symbols so the counts of atoms are the same in reactants and products. The
subscripts are not changed.
Subscripts in chemical formulas are NEVER changed in the balancing process.
Changing the subscripts changes the identity of compounds and the sense of the
equation.
Example:
The reaction between solid sulfur and oxygen is summarized in the unbalanced
equation below.
reactant
combine
reactant
condition product
sulfur solid +
oxygen gas
heat
sulfur dioxide gas
+
S6 (s)
O2 (g)
SO2 (g)
Changes of coefficients are done so multiplying coefficients and subscripts gives the
same number for both sides of the equation for an element. Picking coefficients is
done by "inspection" followed by an organized trial and error process.
Step 1 : Identify the substance that has the most influence on the equation and insert
a coefficient for that formula. Here insert a "1" "1"in front of the S6(s) . This means
there are 1 x 6 sulfur atoms in reactants.
sulfur solid +
1 S6 (s)
+
1 x 6 sulfur
oxygen gas
heat
sulfur dioxide gas
O2 (g)
??? SO2 (g)
?? x 2 oxygen
??? x 1 sulfur
?? x 2 oxygen
Step 2: The number of sulfurs must be equal to six in the products. The subscript on
"S" is a one in sulfur dioxide. You have to decide how many SO2(g) molecules are
needed to give a count of 6 sulfur atoms. ??? x 1 = 6
sulfur atom in reactants =
sulfur atom in
products
coefficient x subscript =
1 x 6 "S" atoms
1 x 6 "S" atoms
6 "S" atoms
1 S6 (s)
+
1 x 6 sulfur
=
=
=
coefficient x
subscript
??? x 1 "S" atoms
??? x 1 "S" atoms
??? x 1 "S" atoms
?? O2 (g)
?? x 2 oxygen
6 SO2 (g)
6 x 1 sulfur
?? x 2 oxygen
Step 3: Check to see if subscripts on an atom are the same in formulas in reactants
and products. If the subscripts are the same the coefficient for the formulas must be
the same. The "O" atoms have a subscript of "2" in both reactants and products. The
coefficient must be "6 " for both O2 and SO2.
1 S6 (s)
+
3 O2 (g)
6 SO2 (g)
1 x 6 sulfur
6 x 2 oxygen
6 x 1 sulfur
6 x 2 oxygen
When the unbalanced equation is illustrated with ball and stick models we can see
the numbers of atoms of various elements.
S6 (s)
O2 (g)
SO2 (g)
two oxygen atoms -
---------- two oxygen
atoms
six sulfur atoms ------ -------------------------- ---------- one sulfur
----atom
The ball and stick formulas showing the correct counts and a balanced equation are
shown below.
1 S6 (s)
6 O2 (g)
6 SO2 (g)
two "O" atoms in a molecule
6 molecules x 2 "O" atoms
6 "S" atoms per molecule
equal
two "O" atoms
in a molecule
---------- 6 molecules x 2
"O" atoms
equal
one "S" atom in
a molecule
1 x 6 "S" atoms------------------------------------------------------------------- 6 x 1 "S" atoms
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