Balancing equations: 1 st …review what we already know.

Balancing equations:

1st …review what we already know.

A formula is a representation of a molecule of an element or a molecule of a compound.

42

2

SOH

O

112212 OHCThe subscript

represents the number of atoms of a particular element in the molecule.

12 atoms of carbon

22 atoms of hydrogen

11 atoms of oxygen

42SOHIf no subscript is

shown. An understood one is indicated.1 molecule of sulfuric acid

2 atoms of hydrogen

1 atom of sulfur

4 atoms of oxygen

A coefficient is a number in front of a formula and represents the number of molecules.

If no coefficient is shown, one molecule is indicated.

112212 OH2C

2 molecules of sucrose

24 atoms of carbon

44 atoms of hydrogen

22 atoms of oxygen

wateroxygen Hydrogen

22 222

OHOH

An equation is a sentence showing what is happening in a chemical

reaction.

O2H2 222 OHThe reactants are the substances

(elements or compounds) that will react. They are on the left or starting side.

OH 222 2O 2H

The products on the right side of the equation are what are formed or are produced during the reaction.

• Chemical reactions do not create or destroy atoms, but only rearrange them

•Law of Conservation of Mass–During a chemical reaction matter is neither created nor destroyed–There must be as many atoms of each type on the left side of the equation as on the right side

• Proper coefficients are placed in front of formulas to balance atoms• Subscripts cannot be changed

–S8 + 8O2 8SO2

http://www.nisd.net/secww/science/science-taks/quiz23/mass%20quiz.htm 5 questions about conservation of mass. Allow pop-ups to view answers.

Balancing Chemical Reactions

Reactants Products

Element # Atoms # Atoms

N

O

N2 + O2 ---> N2O

2 2

1 1

1x2 1x2

1

1x21

1x1

2 1These don’t match. So….

Balancing Chemical Reactions

Reactants Products

Element # Atoms # Atoms

N

O

N2 + O2 ---> N2O1 1

1x2 1x2

1

1x21

1x1

2

2x1

2 1

2x2

4

2x2

2

4

2

Number of reactants equals number of products.

N2 + O2 ---> N2O 2, 1 ---> 2

NN

OO O

NN

NN O

NN

2 diatomic nitrogens and 1 diatomic oxygen become nitrous oxide or “laughing gas”.

Balancing Chemical Reactions

Reactants Products

Element # Atoms # Atoms

Al 1 1

H 1 2

Cl 1 3

6 21 1 1 1

To fix the Cl

3

To fix the H

2

To fix the Al

2 2

6 6

6 6

Which coefficients are needed to balance this equation?

2

To fix both the NO3 and the K

1 1 1 1 1 1 11 11

To finish fixing the K and try out the CrO4

Reactants Products

Element # Atoms # Atoms

Pb 1 Pb 1 Pb

N 2 N 2 N

O 10 O 10 O

K 2 K 2 K

Cr 1 Cr 1 Cr

1 1 1 2

Which coefficients are needed to balance this equation?

3

To fix our Cl1 1 1 1 1 1 1

3 1

To fix our Ag

1

Which coefficients are needed to balance this equation?

11113

To fix the Cl

Now we have 3 H’s and 2 H’s

Which coefficients are needed to balance this equation?

11116

To fix the Cl

2

To fix the H

3

To fix the Al

2

Which coefficients are needed to balance this equation?

3To fix the SO4

111 1

1 1 1 1 1

3

To fix the Ca

2To fix the Al and the OH

1

Finishes up the balanced equation.

Check: Reactants3Ca, 6O, 6H, 2Al, 3S and 12O

Products: 3Ca, 3S, 12O, 2Al, 6O, 6H

Which one is already balanced correctly?

The Al don’t match.

The H don’t match.

1 11x3 for S and 4x3 for O

Which one is already balanced correctly?

Al don’t match.

Cl don’t match.

Cl don’t match.

http://www.creative-chemistry.org.uk/gcse/revision/equations/02.htm

This hyperlink has you balance equations.

Instantly find out if you are correct or not.

As + NaOH →

Na3AsO3 + H2

2

12

2

2

3 6

36

22

24 1

2 331

44

41

1

36?32

2

22 1

When the above equation is balanced, the

coefficient for magnesium chloride is —

A 0

B 1

C 2

D 4

2

’04 Su #25

What are the coefficients that will balance this chemical equation?A 2, 1, 1B 3, 4, 2C 2, 2, 1D 4, 3, 2

’03 10 #45

1 lead 1 lead2 oxygens ≠ 3 oxygens

Put a 2 here to make an even number of oxygens.

1 1 122 1

1 lead 2 lead2 oxygens ≠ 4 oxygens

2 lead 2 lead4 oxygens = 4 oxygens

What is the coefficient for H2O when the equation is balanced?A 1B 2C 3D 4 ’03 11 #19

1 1 1 1 1

To make the hydrogens ‘even’, We put the coefficient 2 here.And have a sum of 4 hydrogens, 2 potassiums and 2 oxygens

2

Then we’ll need to putcoffecients on this side of the arrow to finish balancing it out.

2 2

To produce 4 molecules of sugar, a plant needs —

F 6 molecules of hydrogenG 12 molecules of ATPH 18 molecules of waterJ 24 molecules of carbon dioxide

’03 10#10

Multiply the whole reaction by 4.

14( )4 x 6 carbon dioxide

Which chemical equation supports the law of conservation of mass?

F 2H2O(l) H2(g) + O2(g)G Zn(s) + HCl(aq) ZnCl2(aq) + H2(g)H Al4C3(s) + H2O(l) CH4(g) + Al(OH)3(s)J CH4(g) + 2O2(g) CO2(g) + 2H2O(g)

’04 11 #32

4 hydrogens ≠ 2 hydrogens 2 oxygens 2 oxygens

1 zinc 1 zinc

1 hydrogens ≠ 2 hydrogens 1 chlorine 2 chlorines

4 aluminums 1 aluminum3 carbons 1 carbon

2 hydrogens ≠ 3 hydrogens 1 oxygen 3 oxygens

1 carbons 1 carbon

4 hydrogens = 4 hydrogens 4 oxygens 4 oxygens

Why is the sum of the products’ energy in this reaction less than the sum of the reactants’ energy?A Energy is given off as heat.B The products absorb available energy.C Energy is trapped in the reactants.D The reactants’ energy is less than the melting point of glucose. ’04 11 #37

+ = +≠Reactants = Products

The energy not stored in the bonds of the carbon dioxide and water must have left in the form of heat.

According to the law of conservation of mass, how much zinc was present in the zinc carbonate?A 40 gB 88 gC 104 gD 256 g

’03 11 #39

64 + 192 = 152 + ? 64 + 192 – 152 = ?

If all the reactants in a chemical reaction arecompletely used, which of the followingstatements accurately describes therelationship between the reactants and theproducts?

F The products must have a differentphysical state than the reactants.G The total mass of the reactants mustequal the total mass of the products.H The reactants must contain more complexmolecules than the products do.J The density of the reactants must equalthe density of the products.

Reactants become Products 2H2 + O2 2H2O20 grams + 10grams = 30 grams

’04 Su #40

The chemical equation shows CaCO3 being heated. Which of these statements best describes the mass of the products if 100 g of CaCO3 is heated?A The difference in the products’ masses is equal to the mass of the CaCO3.B The sum of the products’ masses is less than the mass of the CaCO3.C The mass of each product is equal to the mass of the CaCO3.D The sum of the products’ masses equals the mass of the CaCO3.

’04 11 #29

Reactants → Products

100 g

When 127 g of copper reacts with 32 g of oxygen gas to form copper (II) oxide, no copper or oxygen is left over. How much copper (II) oxide is produced?F 32 gG 95 gH 127 gJ 159 g

’04 11 #40

127 + 32 = ?

The illustrations show a conservation-of-massexperiment. The solution in the beaker lost mass because —F materials have less mass at high temperaturesG the mass of the reactants and products was less than 100 gH sodium sulfate (Na2SO4) is lighter than airJ some of the water molecules turned into gas

’0410#30

Before

Before(100 g)

After

After( less

than 100 g)

Some of the water evaporated because of the heat.

REACTANTS →Products The sum of the reactants = the sum of the products.

According to this information, what is the chemical formula for aluminum sulfate?A AlSO4

B Al2(SO4)3

C Al3(SO4)2

D Al6SO4 ’04 11 #23

+1 +2 +3 +4 -3 -2 -1 0

+1 +2 +3 +4 -3 -2 -1 0

Na P

Li C

+1 +2 +3 +4 -3 -2 -1 0

Na+1 P-3

Li C

+1 +2 +3 +4 -3 -2 -1 0

Na+1 P-3

3 1Li C

+1 +2 +3 +4 -3 -2 -1 0

Na+1 P-3

3 1Li C

+1 +2 +3 +4 -3 -2 -1 0

Na+1 P-3

3 1Li C

+1 +2 +3 +4 -3 -2 -1 0

Na3P

Li C

Balanced!

+1 +2 +3 +4 -3 -2 -1 0

Na3P

Li+1C± 4

+1 +2 +3 +4 -3 -2 -1 0

Na3P

Li+1C± 4

4 1

+1 +2 +3 +4 -3 -2 -1 0

Na3P

Li+1C± 4

4 1

+1 +2 +3 +4 -3 -2 -1 0

Na3P

Li4CBalanced!

According to this information, what is the chemical formula for aluminum sulfate?A AlSO4

B Al2(SO4)3

C Al3(SO4)2

D Al6SO4 ’04 11 #23

The chemical formula for calcium chloride is —F Ca2ClG CaClH CaCl2J Ca2Cl3

’04 11 #38