Chapter 3 Formulas, Equations and Moles

[Pages:20]Chapter 3 Formulas, Equations and Moles

Balancing Chemical Equations

What does mother nature lets you change and what doesn't she? H2 + O2 H2O

2H2 + O2 2 H2O Coefficients in black (bold) are changeable (numbers appearing before a substance) Subscripts in bold red are not changeable: Changing the subscript changes the chemical that is formed or used and you can't do that! Example: The combustion of butane: CH3CH2CH2CH3 + O2 CO2 + H2O

Remember: conservation of mass

Equivalent symbols: (means "goes to") can be replaced or is equivalent to a mathematical " = " sign

CH3CH2CH2CH3 + O2 = CO2 + H2O Application of the conservation of mass for carbon and hydrogen results in:

CH3CH2CH2CH3 + O2 = 4CO2 + 5H2O Balancing the oxygens:

CH3CH2CH2CH3 + 13/2O2 = 4CO2 + 5H2O or 2CH3CH2CH2CH3 + 13O2 = 8CO2 + 10H2O Notice that we made an assumption here, that we had as much oxygen as we needed. In reality, this may not be the case. Therefore you also need to know the limiting reagent.

Limiting reagent: the reagent present in the least amount based on the stoichiometry of the reaction.

The stoichiometry of the reaction is the ratio of reagents that are needed for the balanced reaction:

2CH3CH2CH2CH3 + 13O2 = 8CO2 + 10H2O

Sometimes by changing the coefficients in front of the reactants, you can change the chemistry, but not control it except superficially. Coefficients are changed by changing the relative amounts of reagents.

In the case of the reaction we just looked at, if O2 is the limiting reaction, the product composition may change:

In a more limited amount of air (O2)

CH3CH2CH2CH3 + O2 = CO + H2O

CH3CH2CH2CH3 + O2 = 4CO + 5H2O

CH3CH2CH2CH3 + 9/2O2 = 4CO + 5H2O

2CH3CH2CH2CH3 + 9O2 = 8CO + 10H2O

Less than the required amount of oxygen will lead to incomplete combustion and a mixture of CO and CO2, carbon soot, etc.

C + H2O = CO + H2

water gas

CO + H2 + O2 = CO2 + H2O + heat

water gas.mov

K + H2O KOH + H2 K + H2O KOH + 1/2H2 2K + 2H2O 2KOH + H2

Na + H2O NaOH + H2 2Na + 2H2O 2NaOH + H2

Li + H2O LiOH + H2 2Li + 2H2O 2LiOH + H2

movie MF Ch 6 07

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