Chemical Reactions

Ms. HautBiology Engineering 2

Chemical Reactions

• Reactants—starting material

• Products—ending material

• Chemical Equation

Reactants → Products

Chemical Reactions• Occur when bonds

between the outermost parts of atoms are formed or broken

• Involve changes in matter, the making of new materials with new properties, and energy changes

Chemical Equations

• Show the kind of reactants and products and their relative amounts in the reaction

4 Al + 3 O2 → 2 Al2O3

Chemical Equations• Because of the Because of the

principle of the principle of the conservation of conservation of mattermatter, ,

• an an equation must equation must be balanced.be balanced.

• It must have the same It must have the same number of atoms of number of atoms of the same kind on both the same kind on both sidessides

Balancing Chemical Equations

• You may add coefficients in front of the compounds to balance the reaction, but you may notnot change the subscripts.

3 CO2

• Changing subscripts changes the compound.

Subscripts vs. CoefficientsSubscripts vs. Coefficients• Subscripts tell you Subscripts tell you

how many atoms of a how many atoms of a particular element are particular element are in a compound. in a compound.

• The coefficient tells The coefficient tells you about the you about the quantity, or number, quantity, or number, of molecules of the of molecules of the compound.compound.

4 Al + 3 O2 → 2 Al2O3

This equation means4 Al atoms + 3 O2 molecules →produces→ 2 molecules of Al2O3

Four basic steps to balancing a chemical equation.• Write the correct formula for the reactants and the products in

an unbalanced equation.• Inventory the number of each kind of atom on both sides of the

unbalanced equation.• Determine where to place coefficients in front of formulas to

balance the equation.• Take another inventory to determine if:

– The numbers of atoms on both sides of the equation are now balanced.

– The coefficients are in the lowest possible whole number ratios.

Write and balance the chemical equation for:

methane reacting with oxygen gas to produce carbon dioxide and water

1) What are the formulas of the reactants and products?

REACTANT PRODUCT

Methane: CH4 Carbon dioxide: CO2

Oxygen gas: O2 Water: H2O

2) Write the general formula of the reaction

CH4 + O2 CO2 + H2O

Let’s Practice…

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

Count the number of same elements on both sides

ELEMENT REACTANT PRODUCT

C 1 1

H 4 2

O 2 3

C is balanced by H and O are not…H is off by 2 and O is off by 1

CH4 + O2 CO2 + H2O

C is balanced but H and O are not…H is off by 2 and O is off by 1

For H: What number do you multiply the 2 H on the product byto get 4H? 2

CH4 + O2 CO2 + 2 H2O

So now there are 4 H’s on both sidesNow there are 2 O on the reactant side and 4 O on the product side

For O: what number do you multiply the 2O on the reactantside to get 4O? 2

CH4 + 2 O2 CO2 + 2 H2O

CH4 + 2 O2 CO2 + 2 H2O

ELEMENT REACTANT PRODUCT

C 1 1

H 4 4

O 4 4

Reaction is balanced

Now Balance this Reaction

• N2 + H2 → NH3

N = 2H = 2

N = 1H = 3

2

2

6

3

6

Now Balance this Reaction

• Na + Cl2 → NaCl

Na = 1Cl = 2

Na = 1Cl = 1

2

2

2

2

2

Now Balance this Reaction

• Al + O2 → Al2O3

Al = 1O = 2

Al = 2O = 3

4

2

6

2

2

3

64

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