1. Unit 7 Chemical Reactions
Chapter 7 pp

2. What is a chemical reaction?
It is the process by which atoms of one or more substances are rearranged to form NEW different substances.
Consider the following demonstration…
What happens when an antacid tablet is added to water?
Let’s take a look…

3. How can you tell if a chemical reaction has occurred?
Temperature change – Adding a strong acid to water causes a dramatic increase in temp.
Color change – A rusty nail changes from silver to orange/brown
Odor – The smell of rotten eggs when you burn sulfur in the lab
Gas bubbles – When baking bread, the bread rises because of gas production
Precipitate or formation of a solid – A solid appears after adding two liquids together
Precipitate example: 2NaOH + CuCl2  2NaCl + Cu(OH)2

4. Chemical Equations In every chemical equation there are three parts.
Reactants
“Yield” symbol or “”
Products
Reactant 1 + Reactant 2  Product 1 + Product 2
Sometimes there are symbols that go above the “Yield” symbol. Often times these symbols represent catalysts or energy added or released from a reaction.

5. Symbols in chemical equations
Physical States
(s) - solid
(l) - liquid
(g) - gas
(aq) – aqueous or in solution
Symbols above the “”
Heat – heat energy
Δ – energy (often heat)
 - electricity
“Sy” – Elemental symbol acting as a catalyst

6. An important reminder…
The following elements exist as diatomic molecules in their natural state. H2 F2
Cl2
Br2 I2 O2 N2

7. Representing Chemical Reactions
Word Equations
Writing equations in the English language to explain the process of a chemical reaction.
Example:
iron(s) + chlorine(g)  iron(III)chloride(s)
“Solid Iron and gaseous chlorine react to produce solid iron(III)chloride.”

8. Writing Word Equations
Let’s look at another example.
Sodium(s) + Water(l)  Sodium Hydroxide(aq) + Hydrogen(g)
Now you try to write it correctly in English.
Answer:
Solid sodium and liquid water reacted to form aqueous sodium hydroxide and hydrogen gas.

9. Writing Skeleton Equations
Word equations are useful for describing chemical reactions, however, they are cumbersome and lack important information.
Let’s look at an example of the earlier iron reaction…
iron(s) + chlorine(g)  iron(III)chloride(s)
Skeleton equations use symbols in place of the names of atoms and compounds
Fe(s) + Cl2(g)  FeCl3(s)

10. Writing Skeleton Equations
Let’s look at an example:
Carbon(s) + Sulfur(s)  Carbon disulfide(l)
Now you try to write the skeleton equation
Answer:
C(s) + S(s)  CS2 (l)

11. Practice Write the following as Word Equations
hydrogen(g) + bromine(g)  hydrogen bromide(g)
carbon monoxide(g) + oxygen(g)  carbon dioxide(g)
Write the following as Skeleton Equations
Solid barium and oxygen gas react to produce solid barium oxide.
Solid iron and aqueous hydrogen sulfate (sulfuric acid) react to produce aqueous iron(III)sulfate and gaseous hydrogen.

12. Answers
Hydrogen gas reacted with gaseous bromine producing gaseous hydrogen bromide.
Gaseous carbon monoxide reacted with oxygen gas to produce carbon dioxide gas.
Ba(s) + O2(g)  BaO(s)
Fe(s) + H2SO4(aq)  Fe2(SO4)3(aq) + H2(g)

13. Chemical Reactions
Both of these types of equations are useful, but still something is missing…
When a reaction occurs it is important to remember the “Law of Conservation of Mass”.
Mass (atoms) cannot be created nor destroyed. Therefore, the previous types of equations won’t do for chemical analysis.

14. Balanced Chemical Equations
Let’s look at the iron equation one more time.
Fe(s) + Cl2(g)  FeCl3(s)
Notice anything strange?
Where did the extra chlorine atom come from in the iron(III)chloride?
To accurately represent this equation it is important to show that the number of atoms in the reactants is equal to the atoms of product.

15. Balancing Chemical Equations
Whenever the number of atoms of reactant(s) are equal to the number of atoms of product(s) we say that the equation is a Balanced Chemical Equation.
How do we get the reactants and products to balance out?
Coefficients – (different than subscripts!) Whole numbers placed in front of an atom or compound to indicate more than one of each.

16. Balancing Chemical Equations
Let’s look at the previously discussed iron equation when it is balanced…
2Fe(s) + 3Cl2(g)  2FeCl3(s)
Notice that there are 2 atoms of iron and 6 atoms of chlorine. (or 3 molecules of chlorine gas)
The same can be said for the atoms of iron and chlorine in the iron(III)chloride compound.

17. Steps for balancing equations.
Step 1: Write the skeleton equation for the reaction.
Step 2: Count the atoms of the elements in the reactants.
Step 3: Count the atoms of the elements in the products.
Step 4: Change the coefficients to make the number of atoms of each element equal on both sides of the equation.
Step 5: Write the coefficients in their lowest possible ratio.
Step 6: Check your work.

18. Steps for balancing equations example.
Hydrogen gas reacted with chlorine gas to produce hydrogen chloride.
Step 1: H2(g) + Cl2(g)  HCl(g)
Step 2: H Cl (2 atoms of H) (2 atoms of Cl)
Step 3: HCl ( 1 atom H + 1 atom Cl)
Step 4: H2(g) Cl2(g)  2HCl(g) (2 atoms H) (2 atoms Cl) (2 atoms H atoms Cl)

19. Steps for balancing equations example.
Step 5: The ratio 1 hydrogen molecule to chlorine molecule to 2 hydrogen chloride molecules (1:1:2) is the lowest possible ratio because the coefficients cannot be reduced and still remain whole numbers.
Step 6: Make sure the chemical formulas are written correctly. Then, check that the number of atoms of each element is equal on both sides of the equation.
That’s it!

20. OK, maybe that’s not all…
Sometimes equations are more complex than others, especially when polyatomic ions are involved. Here are a couple of extra tips.
Begin balancing with the most complex formula.
Balance polyatomic ions as a single unit.

21. Practice Time Write the balanced chemical equations for the following:
Aqueous sodium hydroxide and aqueous calcium bromide react to produce solid calcium hydroxide and aqueous sodium bromide.
Liquid carbon disulfide reacts with oxygen gas, producing carbon dioxide gas and sulfur dioxide gas.

22. Answers FeCl3(aq) + 3NaOH(aq)  Fe(OH)3(s) + 3NaCl(aq)
CS2(l) + 3O2(g)  CO2(g) + 2SO2(g)

23. Classifying Chemical Reactions

24. Why differentiate?
When you go to the library, you see many different types of books that are organized in a way to help you find them…
Fiction, Nonfiction, Mysteries, Biographies, History, Fantasy, etc.
Like books, there are also many different kinds of chemical reactions and scientists need a way to organize them.

25. Types of Chemical Reactions
Synthesis
Combustion
Decomposition
Single-replacement
Double-replacement

26. CaO(s) + H2O(l)  Ca(OH)2(s)
Synthesis Reactions
A chemical reaction in which two or more substances react to produce a single product.
A + B  AB
Example elements
2Fe(s) + 3Cl2(g)  2FeCl3(s)
Example compounds
CaO(s) + H2O(l)  Ca(OH)2(s)
Example element + 1 compound
2SO2(g) + O2(g)  2SO3(g)

27. Combustion Reactions
Oxygen combines with a substance and releases energy in the form of heat and light.
Example 1
2H2(g) + O2(g)  2H2O(g)
Example 2
C(s) + O2(g)  CO2(g)
Notice that both of these reactions are also Synthesis reactions as well...

28. CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
Combustion cont’d.
Not all combustion reactions are also synthesis types.
Sometimes oxygen will react with compounds called “hydrocarbons” that contain carbon and hydrogen.
In these reactions, such as the one below, carbon dioxide and water are produced.
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g)
This is the same type of reaction that powers a car. In that instance octane (C8H18), another hydrocarbon, is a reactant, but the products are the same.

29. Decomposition Reactions
A reaction in which a single compound breaks down into two or more elements or new compounds.
AB  A + B
Example
NH4NO3(s)  N2O(g) + 2H2O(g)
These types of reactions often require an energy source, such as heat, light, or electricity, to occur.

30. Decomposition cont’d.
One of the best examples of decomposition is in automobile airbags.
Airbags are inflated when an electrical signal starts a reaction with sodium azide (NaN3) pellets within the bag.
These pellets produce nitrogen gas which quickly inflates the bag.
2NaN3(s)  2Na(s) + 3N2(g)

31. Single Replacement Reactions
Atoms of one element replace atoms of another element in a compound.
When studying this type of reaction we must look closely at the element that we believe will replace another element
Some metals are more reactive than others.
If a metal is more reactive than the element it is replacing, it will take its place.
IF a metal is less reactive than the element it is replacing, ultimately, it will not replace it.

32. Single Replacement Cont’d.
How do we know if one element will replace another?
Activity Series of Metals – A table indicating the general level of reactivity of different elements

33. Single Replacement Example 3 Mg + LiNO3  No Reaction Example 1
Mg + Zn(NO3)2  Mg(NO3)2 + Zn
Example 2
Mg + 2AgNO3 Mg(NO3)2 + 2Ag
Example 3
Mg + LiNO3  No Reaction

34. Double Replacement
An exchange of positive ions between two compounds in a chemical reaction
Generally, these types of reactions occur in solution
Often times, in these types of reactions, one of the products will come out of solution as either, a gas, a pure liquid, or a solid.

35. Double Replacement Cont’d.
Example 1 – One product is soluble, the other is a precipitate.
Na2S(aq) + Cd(NO3)2(aq)  CdS(s) + 2 NaNO3(aq)
Example 2 – One product is a gas that bubbles out of the mixture
2NaCN(aq) + H2SO4(aq)  2HCN(g) + Na2SO4(aq)
Example 3 – One product is a molecular compound, such as water, which separates from the compounds in solution.
Ca(OH)2(aq) + 2HCl(aq)  CaCl2(aq) + 2H2O(l)