1. Chemical Equations & Reactions
Chemical Reaction - Study Questions
1. What is a chemical reaction?
2. What is evidence a reaction has occurred?
3. What is a chemical equation?
4. Define reactants.
5. Define products.
6. Define catalyst.
7. What information is found in an equation?
8. What symbols are used to represent the states of matter?
9. What is the meaning of “aq”?
10. How do you indicate a catalyst is being used in a reaction?
11. What is activation energy?
12. What the three things does conservation of matter require of chemical equations?
13. What is the procedure for balancing a chemical equation?
14. Balance the equation for the reaction of magnesium chloride and silver nitrate to form magnesium nitrate and silver chloride.
15. What is a synthesis reaction?
16. How is a combustion reaction related to a synthesis reaction?
17. What type of reaction is an explosion?
18. Compare decomposition and dissociation.
19. What determines whether one metal will replace another in a single displacement reaction?
20. What is the general form of a double displacement reaction?
21. What type of reaction is each of these?
2Na(s) + Cl2(g)  2NaCl(s)
PCl5(s)  PCl3(s) + Cl2(g)
2Al(s) + Fe2O3(s)  2Fe(s) + Al2O3(s)
C6H12O6(s) + 6O2(g)  6CO2(g) + 6H2O(l)
BaCl2(aq) + H2SO4(aq)  2HCl(aq) + BaSO4(s)
22. What is the oxidation number of an element?
23. What is the oxidation number of the fluorine ion?
24. What is the sum of the oxidation numbers in a compound?
31. Can a redox reaction form a molecule? Explain your answer.

2. Describing a Chemical Reaction
Indications of a Chemical Reaction
Evolution of heat, light, and/or sound
Production of a gas
Formation of a precipitate
Color change
Objective:
To state four observations that are evidence for a chemical reaction.
Photo of precipitate:

3. Signs of Chemical Reactions
There are five main signs that indicate a chemical reaction has taken place:
release
input
change in color
change in odor
production of new
gases or vapor
input or release
of energy
difficult to reverse

4. Chemical Equations Depict the kind of reactants and products
aluminum oxide
Depict the kind of reactants and products
and their relative amounts in a reaction.
reactants
product
4 Al(s) O2(g) Al2O3(s)
A chemical equation is an expression that gives the identities and quantities of the substances in a chemical reaction
Chemical formulas and other symbols are used to indicate the starting material(s) or reactant(s), which are written on the left side of the equation, and the final compound(s) or product(s), which are written on the right side. An arrow, read as yields or reacts to form, points from the reactants to the products.
Abbreviations are added in parentheses as subscripts to indicate the physical state of each species:—(s) for solid, (l) for liquid, (g) for gas, and (aq) for an aqueous solution.
A balanced chemical equation is when both the numbers of each type of atom and the total charge are the same on both sides. A chemical reaction represents a change in the distribution of atoms but not in the number of atoms.
The letters (s), (g), and (l) are the
physical states of compounds.
The numbers in the front are called
stoichiometric coefficients.

5. 4 Al atoms + 3 O2 molecules yield 2 molecules of Al2O3
Chemical Equations
aluminum oxide
sandpaper
4 Al(s) O2(g) Al2O3(s)
4 g Al g O2 yield 2 g Al2O3
This equation means:
4 Al atoms O2 molecules yield 2 molecules of Al2O3

6. Chemical Equations Because the same atoms are present
in a reaction at the beginning (reactants)
and at the end (products), the amount
of matter in a system does not change.
The Law of Conservation of Matter
Kotz web

7. Chemical Equations
Because of the principle of the conservation of matter,
An equation must be balanced.
It must have the same number of atoms
of the same kind on both sides.
Lavoisier, 1788

8. Chemical Equations
Reactants – the substances that exist before a chemical change (or reaction) takes place.
Products – the new substance(s) that are formed during the chemical changes.
CHEMICAL EQUATION indicates the reactants and products of a reaction.
Objectives:
To identify seven elements that occur naturally as diatomic molecules, H2, N2, O2, F2, Cl2, Br2, I2. [HOBrFINCL twins or BrINClHOF brothers]
To write a chemical equation from the description of a chemical reaction.
REACTANTS  PRODUCTS

9. Word Equations
A WORD EQUATION describes chemical change using the names of the reactants and products.
Write the word equation for the reaction of methane gas with oxygen gas to form carbon dioxide and water.
methane oxygen
carbon dioxide water
Reactants
Products
CH4 + 2 O2
CO2 + 2
H2O

10. Unbalanced and Balanced EquationsH Cl Cl H Cl Cl H H
H2 + Cl2  HCl
(unbalanced)
H2 + Cl2  2 HCl
(balanced)
reactants products H Cl
reactants products H Cl 2 1 2 2 2 1 2 2

11. Visualizing a Chemical Reaction2
Na Cl NaCl 2
___ mole Na 10 10
___ mole Cl2 5 5
___ mole NaCl 10 10 ?

12. Visualizing a Chemical Reaction2
Na Cl NaCl 2

13. Meaning of Chemical Formula
Symbol Meaning Composition
H2O One molecule
of water:
Two H atoms and one O atom
2 H2O Two molecules
of water:
Four H atoms and two O atoms
H2O2 One molecule
of hydrogen
peroxide:
Two H atoms and two O atoms

14. Balancing Chemical Equations
Balanced Equation – one in which the number of atoms of each element as a reactant is equal to the number of atoms of that element as a product
What is the relationship between conservation of mass and
the fact that a balanced equation will always have the same
number of atoms of each element on both sides of an equation?
Objective:
To write balanced chemical equations by inspection.
Balanced chemical equation
– Provides qualitative information about the identities and physical states of the reactants and products
– Provides quantitative information because it tells the relative amounts of reactants and products consumed or produced in the reaction
– The number of atoms, molecules, or formula units of a reactant or product in a balanced chemical equation is the coefficient of that species
– Mole ratio of two substances in a chemical reaction is the ratio of their coefficients in the balanced chemical equation
Determine whether the following equation is balanced.
2 Na + H2O  2 NaOH + H2
2 Na H2O  2 NaOH + H2

15. CH4 + 2 O2  CO2 + 2 H2O Reactants Products 1 C atom 1 C atom
4 H atoms H atoms
4 O atoms O atoms

16. Reactants  Products
catalyst – speeds up reaction 2 2
Unbalanced

17. Showing Phases in Chemical Equations
H2O(s) H2O(l) H2O(g)
Solid Phase – the substance is relatively rigid and has a definite volume and shape. NaCl(s)
Liquid Phase – the substance has a definite volume, but is able to change shape by flowing. H2O(l)
Gaseous Phase – the substance has no definite volume or shape, and it shows little response to gravity. Cl2(g)

18. Additional Symbols Used in Chemical Equations
“Yields”; indicates result of reaction
Used to indicate a reversible reaction
A reactant or product in the solid state;
also used to indicate a precipitate
Alternative to (s), but used only to indicate a precipitate
A reactant or product in the liquid state
A reactant or product in an aqueous solution
(dissolved in water)
A reactant or product in the gaseous state
(s)
(l)
(aq)
(g)

19. Additional Symbols Used in Chemical Equations
Alternative to (g), but used only to indicate a gaseous product
Reactants are heated
Pressure at which reaction is carried out, in this case 2 atm
Pressure at which reaction is carried out exceeds normal
atmospheric pressure
Temperature at which reaction is carried out, in this case 0 oC
Formula of catalyst, in this case manganese (IV) oxide,
used to alter the rate of the reaction D
2 atm
pressure
0 oC
MnO2