1. Chemical Equations and Reactions
CHEMISTRY – Chapter 8
Chemical Equations and Reactions

2. Chapter 8 – Section 1 Objectives:
1. List three observations that suggest that a chemical reaction has taken place.
2. List three requirements for a correctly written chemical equation.
3. Write a word equation and a formula equation for a given chemical reaction.

3. Chemical Reactions
A process by which one or more substances are changed into one or more different substances
Reactants – the original substances
Products – the resulting substances
Mass is always conserved

4. Chemical Equation
Represents the identities and relative amounts of reactants and products in a chemical rx

5. Chemical Reactions A new substance must be produced
The following changes indicate a chemical reaction:
Heat and light – release of E
Production of gas – bubbles
Formation of a precipitate – a solid produced in a solution that settles
Color change

6. Characteristics of Chemical Equations
Chemical Equations must do the following:
Represent known facts
Contain the correct formulas for the reactants and products
Diatomic molecules, etc.
Law of conservation of mass must be satisfied
Atoms are not created or destroyed, just rearranged

7. Word Equations
Equations where products and reactants are represented by words
Qualitative
Does not give the whole story
No quantities are given
ex. methane + oxygen → carbon dioxide + water

8. Formula equation
Represents reactants and products of a chemical rx with symbols or formulas
Use coefficients to balance
Small whole number that appears in front of a formula in an equation
ex. CH4 + 2 O2 → CO2 + 2 H2O

9. Symbols in Chemical Equations
→ - yields
↔ - reversible reaction – chem rx in which the products reform the originals reactants
(s) – solid state
↓ - precipitate
(l) – liquid
(aq) – aqueous solution (dissolved in water)
(g) – gas
heat→ - reactants are heated
0°→ - temperature at which reaction is carried out
MnO2→ - catalyst – substance used to alter the rate of rx

10. Review and Assignment Assignment: HW 8-1 and HW 8-2
1. List three observations that suggest that a chemical reaction has taken place.
2. List three requirements for a correctly written chemical equation.
3. Write a word equation and a formula equation for a given chemical reaction.
Assignment: HW 8-1 and HW 8-2

11. Objectives: 1. Balance a chemical formulas by inspection.
Chapter 8 - Section 2
Objectives:
1. Balance a chemical formulas by inspection.

12. Chemical Equations Give us quantitative information
Coefficients indicate relative, not absolute amounts of reactants and products
H2 + Cl2 → 2 HCl
1 mole of H2 and 1 mole of Cl2
Yields 2 moles HCl
Relative masses of the reactants and products of a chemical rx can be determined from the coefficients
1 mole H2 = 2 g H2
1 mole Cl2 = 71 g Cl2
Yields 2 moles HCl = 73 g HCl
Reverse reaction for a chemical equation has the same relative amounts of substances as the forward reaction

13. Balancing Chemical Reactions
1. Identify the names of the reactants and the products, and write a word equation.
2. Write a formula equation by substituting correct formulas for the names of reactants and products.
Don’t forget diatomic molecules (H2, N2, O2, F2, Cl2, Br2, I2)
3. Balance the formula equation.
Balance the different types of atoms one at a time
1st balance atoms of elements that are combined and appear only once on each side
Balance polyatomic ions that appear on both sides
Balance H and O atoms
4. Count atoms to be sure that the equation is balanced.

14. Example 1. zinc + hydrochloric acid → zinc chloride + hydrogen
2. Zn(s) + HCl(aq) → ZnCl2(aq) + H2(g)
3. Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
4. 1 Zn + 2H and 2 Cl → 1 Zn and 2 Cl + 2 H

15. Demo. Zinc + hydrochloric acid → zinc chloride + hydrogen gas
Aluminum sulfate + calcium chloride → aluminum chloride + calcium sulfate
Balance the equations from the previous demonstrations
Zn + HCl → ZnCl2 + H2
Zn + 2 HCl → ZnCl2 + H2
Al2(SO4)3 + CaCl2 → AlCl3 + CaSO4
Al2(SO4)3 + 3 CaCl2 → 2AlCl3 + 3 CaSO4

16. Review and Assignment Assignment: HW 8-3 and HW 8-4
1. Balance a formula equation by inspection.
Assignment: HW 8-3 and HW 8-4

17. Chapter 8 - Section 2 Objectives:
Define and give general equations for synthesis, decomposition, single-replacement, and double-replacement reactions.
Classify a reaction as synthesis, decomposition, single-replacement, and double-replacement, or combustion.

18. Types of Chemical Reactions
1. Synthesis reaction
A + X → AX
2. Decomposition
AX → A + X
3. Single-Replacement
A + BX → AX + B
Y + BX → BY + X
4. Double-Replacement
AX + BY → AY + BX

19. Synthesis 2 or more substances combine to produce a new compound
Also known as composition reaction
Forms oxides with oxygen and metals

20. Decomposition
A single compound undergoes a reaction that produces two or more simpler substances
Most are endothermic when energy is supplied by electricity or heat
ex. electrolysis – decomposition of a substance by an electric current

21. Single-Replacement
One element replaces a similar element in a compound
Also called displacement
Many are in aqueous solutions
Less E required
The more active element replaces the less active one
Most active metals (group 1) react w/water and produce metal hydroxides

22. Double-Replacement
Ions of two compounds exchange places in an aqueous solution to form two new compounds
One compound formed is usually a precipitate, an insoluble gas, or a molecular compound (usually water)
Other compound remains dissolved

23. Combustion Reaction
A substance combines with oxygen, releasing a large amount of E in the form of light and heat
Always have oxygen as a reactant

24. Review and Assignment
1. Define and give general equations for synthesis, decomposition, single-replacement, and double-replacement reactions.
2. Classify a reaction as synthesis, decomposition, single-replacement, and double-replacement, or combustion.
Assignment: WS, HW 8-5, HW 8-7

25. Chapter 8 - Section 3 Objectives:
Explain the significance of an activity series.
Use the activity series to predict whether a given reaction will occur and what the products will be.

26. Chemical Activity The ability of an element to react
Metals lose e- easily
Non-metals gain e- easily

27. Activity Series
A list of elements organized according to ease with which the elements undergo certain chemical reactions
Metals – greater activity = greater ease to lose e-
Non-metals – greater activity = greater ease to gain e-
Order is determined by single-replacement reactions
The metal at the top can replace anything beneath it
Predicts if a reaction will actually take place
d/o what they are reacting with (water vs. acid)

28. Examples 2 Al (s) + 3 ZnCl2(ag) → 3 Zn(s) + 2AlCl3(aq)
Al will replace Zn
This reaction will occur
Co(s) + 2 NaCl(s) → no reaction
Cr(s) + H2O(l) → no reaction (b/c water, not steam)

29. Examples Zn(s) + H2O(l) 50°C→ Sn(s) + O2(g) → Cd(s) + Pb(NO3)2(aq) →
No reaction (b/c water is not steam)
Sn(s) + O2(g) →
yes, Sn reacts w/oxygen
Cd(s) + Pb(NO3)2(aq) →
yes, Pb is below Cd
Cu(s) + HCl(aq) →
no, Cu is below H2

30. Review and Assignment Assignment: HW 8-8
Explain the significance of an activity series.
Use the activity series to predict whether a given reaction will occur and what the products will be.
Assignment: HW 8-8