1. Chemical Reactions: An Introduction Chapter 61

2. Chemical Reactions Reactants  Products
Reactions involve chemical changes in matter resulting in new substances
Reactions involve rearrangement and exchange of atoms to produce new molecules
Reactants  Products 2

3. 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 3

5. 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
Can be used to determine weights of reactants used and of products that can be made 4

6. 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
Therefore the total mass of the reactants will be the same as the total mass of the products 5

7. CH4(g) + O2(g)  CO2(g) + H2O(l)
Combustion of Methane
methane gas burns to produce carbon dioxide gas and liquid water
whenever something burns it combines with O2(g)
CH4(g) + O2(g)  CO2(g) + H2O(l) H C O +
1 C + 4 H O
1 C + 2 O H + O
1 C + 2 H + 3 O 6

8. Combustion of Methane Balanced
to show the reaction obeys the Law of Conservation of Mass it must be balanced
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(l) H C O +
1 C H O
1 C H O 7

9. Describing Chemical Change
Atoms themselves are never created or destroyed, but simply rearranged from the reactants to the products.
Word equations – describe in words the reactants and products of a chemical reaction.
Chemical equation – uses chemical formulas for the reactants and the products to represent the reaction that is taking place.

10. 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 equation shows the relationship between the relative numbers of moles of reactants and products
can be used to determine mass relationships 8

11. Symbols Used in Equations
symbols used after chemical formula to indicate state
(g) = gas; (l) = liquid; (s) = solid
(aq) = aqueous, dissolved in water 9

12. Recognizing Reactants and Products
when magnesium metal burns in air it produces a white, powdery compound magnesium oxide
burning in air means reacting with O2
Metals are solids, except for Hg which is liquid
write the equation in words
identify the state of each chemical
magnesium(s) + oxygen(g) magnesium oxide(s)
write the equation in formulas
identify diatomic elements
identify polyatomic ions
determine formulas
Mg(s) + O2(g)  MgO(s) 10

13. Writing Word Equations Sample
Lithium hydroxide and barium sulfate react to form lithium sulfate and barium hydroxide
Aqueous solutions of calcium chloride and silver nitrate react to produce a precipitate of silver chloride and an aqueous solution of calcium nitrate

14. The Seven Diatomic Substances (plus two friends)
Hydrogen, H2, Nitrogen, N2,,Oxygen, O2, Fluorine, F2, Chlorine, Cl2, Bromine, Br2, Iodine I2
In addition to the above, phosphorous is P4 and sulfur is S8.

15. Writing Word Equations Sample
Aqueous sodium bromide reacts with fluorine gas to produce aqueous sodium fluoride and liquid bromine
Nitrogen gas combines with oxygen gas to produce dinitrogen pentoxide gas.

16. Balancing Chemical Equations
To balance chemical equations use coefficients – whole numbers in front of each formula.
You cannot change the formulas for the compounds to balance the equations.
Always check your work at the end to make sure your equation is balanced.

17. Rules for Balancing Equations
Write the formulas for the reactants on the left and the formulas for the products on the right, separating them by an arrow.
Count the number of atoms of each element in the reactants and products.
polyatomic ions may be counted as one
“element” if it does not change in the reaction
Al + FeSO4 Al2(SO4)3 + Fe
1←SO4→3
b. if an element appears in more than one compound on the same side, count each separately and add
CO + O2  CO2
← O → 2

18. Rules for Balancing Equations
3. Balance the elements one at a time using coefficients (must be whole numbers). When no coefficient is present, it is assumed to be Balance the elements that appear only once on each side of the equation first. You may not change subscripts to balance an equation. 5. Balance Oxygen and Hydrogen last. 6. Make sure all the coefficients are in the lowest possible ratio that balances.

19. Balancing Equations Examples
__ Cl2 + __ NaF  __ NaCl + __ F2
__ C3H8 + __ O2  __ CO2 + __ H2O
__ Al(NO3)3+__ Fe3(PO4)2__ Fe(NO3) 2+ __ AlPO4
__ NH3 + __ O2  __ NO2 + __ H2O