Reactions and Stoichiometry

Chapters 11-12

Reactions

Reactants Products

Balancing Reactions

Reactions must maintain conservation of mass, charge, and energy

Reactants and Products must have the same number of atoms of each element2H2 + O2 2H2O

Balancing Reactions

Reactions must maintain conservation of mass, charge, and energy

Reactants must have the same total charge as ProductsCu+1 + Fe+3 Cu+2 + Fe+2

Balancing Reactions

To balance a reaction:Do NOT change chemistry

(compounds, subscripts)Only change coefficients (big numbers

in front of chemicals)Coefficients can only be whole

numbers2H2 + O2 2H2O

Balancing Reactions

4Na + O2 2Na2O

2Al + 3Br2 2AlBr3

4Ni + 3O2 2Ni2O3

2HNO3 + Ca(OH)2 Ca(NO3)2 + 2H2O

Reaction Types

Synthesis Decomposition Single Replacement Double Replacement Combustion

Synthesis

Chemical change in which two or more substances react to form a single new substance (1 product)

Also called Combination

2Mg + O2 2MgO

Decomposition

Chemical change in which a single compound breaks down into two or more simpler products1 reactant

NaHCO3 NaOH + CO2

Single Replacement

Chemical change in which one element replaces a second element in a compoundMetal replaces metal (hydrogen included)

Nonmetal replaces nonmetal

Zn + 2HCl ZnCl2 + H2

Double Replacement

Chemical change involving an exchange of positive ions between compounds

AgNO3 + NaCl NaNO3 + AgCl

Combustion

Chemical change in which an element or a compound reacts with oxygen, often producing energy in the form of heat and light

2Mg + O2 2MgO

Combustion

Complete combustion of a hydrocarbon produces carbon dioxide and water

2C2H6 + 7O2 4CO2 + 6H2O

Spontaneous Reactions

A single replacement reaction will only occur if:The single element in the reactants is

more active than the element it replaces in the compound

Table J

The more active element:Does not want to be

aloneWants to be

combined with someone else

Spontaneous or not?

ZnCO3 + Mg MgCO3 + Zn

PbSO4 + Mn MnSO4 + Pb

3KNO3 + Al Al(NO3)3 + 3K

2HCl + Zn ZnCl2 + H2

2NaBr + I2 2NaI + Br2

YES

NO

YES

YES

NO

Spontaneous Reactions

A double replacement reaction will only occur if:A precipitate (solid) is producedA liquid is produced

• H2O(l)

A gas is produced

Table F

Spontaneous or not?

NaCl + AgNO3 NaNO3 + AgCl

K2CO3 + MgSO4 K2SO4 + MgCO3

NH4OH + NaNO3 NaOH + NH4NO3

Yes

Yes

No

Stoichiometry

GoalThe role of coefficients in balanced

formula equations• Mole ratios in a balanced formula• Mole-mole stoichiometry problems

Stoichiometry

Calculations of quantities in chemical reactions

Food Example

How many pancakes can be made with 8 cups flour, excess eggs and milk?

pancakescupmilkeggscupsflour 10124

ratio 4 2 1 10

8 Xamount

X = 20 pancakes

Process for Solving

Balance reaction Copy coefficients from reaction into

ratio row Place numbers from question in row

above ratio row (Setting up proportion)

Solve for X using a proportion

Food Example

How many eggs are needed to make 25 pancakes?

pancakescupmilkeggscupsflour 10124

ratio 4 2 1 10

25Xamount

X = 5 eggs

Chemical Example

How many moles of NH3 can be made with 6 moles H2 and excess N2?

322 23 NHHN

ratio 1 3 2

6 Xamount

X = 4 mol NH3

=

Chemical Example

How many moles of N2 are needed to produce 7 moles of NH3?

322 23 NHHN

ratio 1 3 2

7Xamount

X = 3.5 mol N2

Chemical Example

How many moles of O2 are needed to produce 9 moles of H2O?

OHCOOHC 22283 435

ratio 1 5 3

9Xamount

X = 11.25 mol O2

4

Endo/Exothermic

Endothermic – Energy is absorbedEnergy term is on the left side

Exothermic – Energy is releasedEnergy term is on the right side

Treat just like a coefficient

Example

kJNHHN 8.9123 322

How much energy is produced when 6 moles H2 reacts with excess N2?

ratio 1 3 2

6 Xamount

91.8

X =183.6 kJ