1. Chemical Reactions

2. Chemical Change How do you know if a chemical change has taken place?
Form a solid (called a precipitate)
Form a gas (bubbles or a smell)
Change in temp
Unexpected color change
All chemical changes are results of chemical reactions.

3. Chemical Reactions
Chemical Reactions: occur when bonds are broken and a new product is formed. (Chemical Change)
Involve the rearrangement of atoms, not the creation of atoms

4. Chemical reactions
Chemical formulas are used to write chemical equations.
The chemical compounds used to start the reaction are Reactants.
The chemical compounds at the end of the reaction are Products.

5. Reaction Symbols There are several types of Reaction symbols used.
+ plus (added to)
 forms (produces or yields)
g – gas
s – solid
l – liquid
aq—aqueous solution
(dissolved in water)

6. Law of Conservation of Mass
Matter cannot be created or destroyed. It is always conserved in a reaction.
Atoms cannot simply appear or disappear.
The amount used at the start of the reaction is equal to the amount at the end of the reaction.
# of atoms of REACTANTS = # of atoms of PRODUCTS

7. Take a look at this…
Does it look balanced?

8. We must the get the O2 to balance..
Is this balanced now?

9. And now….Is it balanced?
YES!

10. Why H2 and O2?
There is something special about SEVEN of the elements in their neutral, pure forms.
These are called the diatomic atoms because they will always exist as a two-atom molecule when pure.
They are N2, O2, F2, Cl2, Br2, I2, and H2
Pull out your periodic tables…

11. Practice Counting Atoms
Mg= 1 atom of magnesium
MgCl2
Cl= 2 atoms of chlorine
H= 2
H2SO4
S= 1
O= 4

12. More counting! N= 1 NH4Cl H= 4 Cl= 1 Mg= 1 Mg(NO3)2 N= 2 O= 6
(NH4)3PO4
N= 3
P= 1
H= 12
O= 4

13. Balanced Equations
In order to conserve atoms, we must balance the number and type of atoms.
CAN’T CHANGE SUBSCRIPTS!
Can only change the numbers of atoms on each side of the equation by adding COEFFICIENTS!
Coefficients: the numbers that come in front of a chemical formula.

14. Let’s Try to Figure out Coefficients
How many do you have of each?
3 H2O 2 H2O3
4 Fe2O3 6 NaCl
NO2

15. Steps to Balancing
First, count the number and type of each atom on both sides
If there are different numbers of atoms for an element on both sides, you must write coefficients in front of compounds to change the number of atoms

16. Second, figure out what coefficient is needed in order to make the numbers of atoms equal.
REMEMBER: IF YOU CHANGE A COEFFICIENT, ALL ELEMENTS IN THAT COMPOUND ARE AFFECTED.

17. Magnesium + Oxygen (g) Magnesium Oxide
Mg + O2 MgO

18. Now try on your own…
C + Cl2 CCl4
Al2O3 Al + O2

19. Helpful Hints to Balancing
If an element appears in more than one compound on the same side of the equation, leave that element until LAST to balance

20. Other Helpful Hints
If there is an even number of atoms of an element on one side of the equation and an odd number on the other, place a coefficient of 2 in front of the ODD compound

21. Energy in Reactions
Energy is either released or absorbed in a chemical reaction.
Exothermic reactions produce energy (RELEASE!) so the temperature rises!
Endothermic reactions require a lot of energy (ABSORBS!) so the temperature drops!

22. Types of Reactions Synthesis or Composition Decomposition
Single Displacement
Double Displacement
Acid Base
Combustion

23. Composition (Synthesis)
Two or more reactants combine to form a new product
A + B  AB
Example: Rust formation:
Fe(s) + O2(g)  Fe2O3

24. Decomposition
Compounds are broken down into two or more smaller compounds
AB  A + B
SiCl4  Si + Cl2

25. Single Displacement One element replaces another in a compound
AX + B  BX + A
Ca + HCl  CaCl2 + H2

26. Double Displacement Ions switch places; two elements switch places
AB + CD  AD + CB
AgNO3 + NaCl 
AgCl + NaNO3

27. Acid Base Reactions
The reactants will always be an acid and a base producing 2 specific products; Water and a Salt.
Acids always start with an H-
Bases always end in an –OH
Water is H2O
HCl + KOH  H2O + KCl

28. Combustion
Hydrocarbons react with oxygen to release carbon dioxide and water and energy
Carbon cmpd + O2 CO H2O + energy!
C2H6 + O2  CO2 + H2O

29. Let’s practice- Type _______ HgO → Hg + O2
_______ CH4 + O2 → CO2 + H2O
_______ NaCl + AgNO3 → NaNO3 + AgCl
_______ Mg + HCl → MgCl2 + H2
_______ Zn + H2SO4 → ZnSO4 + H2
_______ C6H12O6 + O2→ CO2 + H2O
_______ NaOH + HCl → NaCl + H2O
_______ Al2(SO4)3 + Ca(OH)2 → Al(OH)3 + CaSO4
_______ H2 + O2 → H2O
_______ Cl2 + NaBr → NaCl + Br2
_______ KClO3 → KCl + O2
_______ C2H2 + O2→ CO2 + H2O
_______ Na2O + CO2 → Na2CO3

30. Categorizing Chemical Reactions
All chemical reactions involve something moving, so we can categorize the types of reactions we just learned to help us predict the products in a reaction!
Moving electrons = synthesis, decomposition, single displacement, combustion.
Moving ions = double displacement
Moving protons = acid/base reaction (we’ll get into this in a later unit)

31. Moving Electrons
In the following reactions, one or more electrons move from one atom to another
This allows the atoms to act differently than in their original compounds, and we form new substances.
Ex:
Mg + Br2  MgBr2
NaCl  Na + Cl2
Zn + CuSO4  Cu + ZnSO4

32. In the first example, think about the Lewis Structures that we’ve drawn.
The Magnesium atom loses electrons and the Bromine atom gains electrons.
There is a transfer of electrons to create the ionic bond.

33. In the second example, we break apart the compound by transferring the electrons back to the individual elements.

34. Zn + CuSO4  Cu + ZnSO4
Remember CuSO4 is an ionic compound, so it really is made of Cu+2 and SO4-2 ions.
Zn is starting off neutral in the reactants.
During this reaction, Zn transfers its electrons to Cu+2 ion.
Zinc loses electrons and becomes a Zn+2 ion and Cu gains electrons to become neutral.
Now that Zn+2 has a charge, it will form the ionic compound with SO4-2 because the opposite charges ions will attract.

35. In this case, did the sulfate ion see an change of electrons?
No, so these electrons were not involved in the reaction and were not shown.
This is an example of something called a “Spectator Ion” and we’ll talk about it later.

36. Reduction and Oxidation
In these reactions, one atom or element is reduced and the other is oxidized. So you’ll hear these called Redox.
A substance that is reduced has gained electrons and the charge is reduced. (more negative)
A substance that is oxidized has lost electrons and the charge is increased. (more positive)

37. 2 Fun Ways to Remember Oxidation Is Losing elections Reduction
OIL RIG
LEO the lion says GER
Oxidation Is
Losing elections
Reduction
Gaining electrons
Losing
Electrons is
Oxidation
Gaining
Reduction

38. And now, the tricky part…
A substance that has been oxidized is called a reducing AGENT because it caused another element to take their electrons.
A substance that has been reduced is called an oxidizing AGENT because it caused another element to give up its own electrons

39. Br starts with 0 charge and ends with -1
The charge has gone down, because Br has gained electrons.
Mg starts with 0 charge and ends with +2
The charge has gone up, because Mg has lost electrons.
This means Br has been ______ and is the _______ agent.
reduced oxidizing
This means Mg has been ______ and is the _______ agent.
oxidized reducing

40. Na starts with a +1 charge, ends with 0 charge
Charge has gone down because it gained electrons
Cl starts with a -1 charge, ends with 0 charge
Charge has gone up because it lost electrons
This means Na has been _______ and is the ______ agent.
reduced oxidizing
This means Cl has been ______ and is the _______ agent.
oxidized reducing

41. Zn started with 0 charge, ended with +2
Charge has gone up because it lost electrons
Cu started with +2 charge, ended with 0
Charge has gone down because it gained electrons
SO4 started and ended with -2, no electrons changed here
Zn has been oxidized and is the reducing agent
Cu has been reduced and is the oxidizing agent
SO4 wasn’t really involved in the reaction. (Spectator)