chemical reactions

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Chemical Reactions

• Reactions involve chemical changes in matter that result in new substances.

• Reactions involve rearrangement and exchange of atoms to produce new molecules.

• Reactants Products

CH4 + 2O2 → CO2 + 2H2O

CH4

O2

CO2

H2O


Evidence of Chemical Reactions

• A chemical change occurs when new substances are made.

• Visual clues (permanent):– Color change, precipitate formation, gas bubbles,

flames, heat release, cooling, light

• Other clues:– New odor, permanent new state


Evidence of Chemical Reactions (cont.)


Chemical Equations

• Shorthand way of describing a reaction

• Provides information about the reaction:– Formulas of reactants and products

– States of reactants and products

– Relative numbers of reactant and product molecules that are required

– CH4(g)+ 2O2(g) → CO2(g) + 2H2O(l)– Can be used to determine weights of reactants used and

of products that can be made


Symbols Used in Equations

• Symbols used after chemical formula to indicate state:– (g) = gas; (l) = liquid; (s) = solid– (aq) = aqueous, dissolved in water

– e. g. NH3(aq) indicates ammonia dissolved in water


Conservation of Mass

• Matter cannot be created or destroyed.

• In a chemical reaction, all the atoms present at the beginning are still present at the end.

• Therefore, the total mass cannot change.


Combustion of Methane

• Methane gas burns to produce carbon dioxide gas and liquid water

CH4(g) + O2(g) CO2(g) + H2O(l)

H

HC

H

HOO+

O

O

C + OH H

1 C + 4 H + 2 O 1 C + 2 O + 2 H + O1 C + 2 H + 3 O


Combustion of Methane Balanced

• To show a reaction obeys the Law of Conservation of Mass, it must be balanced.

CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l)

H

HC

H

H

OO

+

O

O

C +

OH H

OO

+O

H H

+

1 C + 4 H + 4 O 1 C + 4 H + 4 O


Writing Equations

• Use proper formulas for each reactant and product.• Proper equation should be balanced.

– Obey Law of Conservation of Mass.

– All elements on reactants side also on product side.

– Equal numbers of atoms of each element on reactant side as on product side.

• Balanced equations show the relationship between the relative numbers of molecules of reactants and products. – Can be used to determine mass relationships


Balancing Chemical Equations

• If the equation is in words, write the formulas for the reactants and products (include physical states).

• Balance the equation by counting the atoms on both sides of the equation and changing the coefficients as needed. Never change the subscripts!

• This is done by trial and error. Start with the most complicated compound first.

• The best balanced equation is the one with the smallest integer coefficients (not fractions).


Balance the following equations:

• Solid potassium reacts with gaseous nitrogen to form solid potassium nitride.

• C3H8(g) + O2(g) → CO2(g) + H2O(l)

• CuSO4(aq) + KI(s) → CuI(s) + I2(s) + K2SO4(aq)

chemical reactions

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