1. Chemical Reactions and Balancing Equations

2. Evidence of Chemical Reactions
Reactions and Balancing Equations
Evidence of Chemical Reactions
A chemical change occurs when new substances are made.
Visual clues:
Color change
precipitate formation
gas bubbles
flame
heat release
cooling
light
Other clues:
New odor
permanent new state 3

3. Evidence of Chemical Reactions (cont.)
Reactions and Balancing Equations
Evidence of Chemical Reactions (cont.)
Heat produced or absorbed
Change in colour
Change of state
Change of state
Name these compounds
Write these equations
Methane reacts with oxygen to produce heat
HCl + Co(NO3)2
Na2Cr2O7 + Pb(NO3)2 forming a solid
Calcium reacts with water to create hydrogen gas

4. Reactions and Balancing Equations
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.
The number of atoms doesn’t change, just the arrangement
Therefore the total mass cannot change. 5

5. Reactions and Balancing Equations
Chemical Reactions
Reactions involve chemical changes in matter that result in new substances.
Reactions involve rearrangement and exchange of atoms to produce new compounds.
Reactions may involve various states of matter: solid, liquid, gas 2

6. Reactions and Balancing Equations
Chemical Reactions
Zn(s) + 2HCl(aq) H2(g) + ZnCl2(aq)
On the reactants side we have solid zinc and (aqueous) hydrochloric acid
all acids are aqueous
aqueous solutions are water based solutions
On the products side we have gaseous hydrogen and aqueous zinc chloride
1 mole : 2 = 1 : 1

7. Chemical Reactions (con.)
The number of atoms on the reactant side of the equation must to be the same as the number of atoms on the product side of the equation
the equation must be balanced
This is a reiteration of the Conservation of Mass

8. Reactions and Balancing Equations
Chemical Reactions
For example:
methane + oxygen yields carbon dioxide + water
CH4(g) + O2(g) CO2(g) + H2O(g)
This equation is unbalanced
Compare the number of hydrogens and the number of oxygens on both side
Discuss – match up atoms on both sides

9. Reactions and Balancing Equations
Chemical Reactions
As seen on the previous slide:
We need the same number of hydrogens on the both sides
The same goes for carbon and oxygen
Discuss – match up atoms on both sides

10. Balancing Reactions
We will cover this on the board in class

11. Reactions and Balancing Equations
Chemical Equations
Shorthand way of describing a reaction.
Provides information about the reaction:
Formulas of reactants and products
States of reactants and products
Relative moles of reactant and product compounds (atoms) that are required
Can be used to determine masses of reactants used and of products made
Write an eqn on the board 4

12. The Four Types of Reactions
Type of Reaction
Characteristic
Element Arrangement
Combination
Reactants form one product
Decomposition
One reactant forms two or more products
Single Displacement
One element displaces another in a compound
Double Displacement
Compounds “trade” elements

13. Examples of Combination Reactions

14. Examples of Decomposition Reactions

15. Examples of Single Displacement Reactions

16. The Activity Series
Used to determine the outcome of a single displacement reaction
See the next slide

17. Metals K Ca Na Mg Al Zn Fe Ni Sn Pb H Cu Ag Hg
In a reaction an element in the activity series will displace an element below it from a compound.
Metals K Ca Na Mg Al Zn Fe Ni Sn Pb H Cu Ag Hg
increasing activity

18. Examples of Double Displacement Reactions

19. The following may result from a double displacement reaction:
formation of a precipitate
release of gas bubbles
release or absorption of heat
formation of water

20. Calculations from Chemical Equations
Known as Stoichiometry

21. The Importance of a Balanced Chemical Equation
Indicates the mole ratios of compounds in the reaction
both on the reactant side and the product side

22. Mole Ratios Consider the following reaction:
What are the mole ratios of:
Al to H2?
Al to HCl?
AlCl3 to H2?

23. Stoichiometry
The area of chemistry that deals with the quantitative relationships between reactants and products.

24. Mole Ratios in a Chemical Equation
The molar coefficients in a balanced chemical equation form the basis for the solution of stoichiometry problems
We use the ratios between them, known as the “molar ratios”, as the basis for solving the problem

25. Mole Calculations
Stoichiometry problems may involve moles only. See the next two examples below.

26. For the synthesis of ammonia given below:
How many moles of hydrogen gas are needed to react with 15.5 moles of nitrogen gas?
How many moles of ammonia are produced?

27. In the following double displacement reaction, how many moles of PbCl2 are formed if 5.00 moles of NaCl react?

28. Mass Calculations
In stoichiometry problems the quantities of the compounds involved could be expressed in terms of their masses.
These problems require converting between mass and moles.
See the two examples below.

29. Example 1, Mass Calculation
Calculate the number of grams of H2 required to form 12.0 moles of NH3.

30. Example 2, Mass Calculations
Consider the following reaction:
Say we have 16.8 g of Fe. How many grams of hydrogen gas are produced?