1. General Chemistry – Unit 7 Chemical Equations
Chapter 7

2. What is a chemical rxn?
A process where one or more substances changes to one or more other substances.
Exchange of electrons!!!

3. Parts of a chemical reaction
Reactants the original substances
Products the resulting substances

4. ASK YOURSELF:
Which are the products and which are the reactants in these equations?
Na+ + Cl- NaCl
2H2O  2H2 + O2

5. Quick Demo
Magnesium

6. A word equation
Magnesium metal reacts with oxygen gas to form white magnesium oxide solid.
We must learn how to translate these!

7. Balancing equations Tips
__Fe2O3 + __H2SO4  __Fe2(SO4)3 + __H2O
If possible treat polyatomic ions as 1 unit
Balance hydrogen 2nd last
Balance oxygen last
If the last element does not balance double everything

8. Ways to Represent Reactions
Chemical equation: symbols that describe a chemical reaction and show what atoms (and how many, relatively speaking) are involved.
Word equation: the names of each product/reactant are written out. Doesn’t show how many of each

9. Write out the word equation
Magnesium metal reacts with oxygen gas to form magnesium oxide solid.
Mg + O  MgO

10. Remember BrINClOFH Br2 – a liquid I2 - a solid N2 - a gas Cl2 - a gas
O2 - a gas
F2 - a gas
H2 - a gas

11. Check for BrINClOFH
Any time you see 1 of the 7 diatomic elements alone, write them with a 2 subscript
Mg + O2  MgO

12. Balance the charges of ionic comp
In our reaction we have already done this, but make sure to use proper formulas (for molecular comp too!):
MgO = Mg2+ O2-
Mg + O2  MgO

13. Balance number of atoms/ions on each side
In ordinary chemical changes, atoms just rearrange
This is the LAW OF CONSERVATION OF MATTER
We do this with coefficients
2Mg O2  2MgO

14. Example
If you take Ca(NO3)2 and change it to 2Ca(NO3)2, how many of each element do you have?

15. Put in physical states Symbols used in chemical equations: (s) solid
(l) liquid
(g) gas
(aq) aqueous solution (dissolved in H2O)

16. Put in physical states
I will give you physical states in word problems:
Magnesium metal reacts with oxygen gas to form magnesium oxide solid
2Mg(s) O2(g)  2MgO(s)

17. Word Equation example
Hydrogen gas and Oxygen gas combine to form liquid water.
2H2(g) + O2(g)  2H2O(l)

18. Example:
Sodium solid reacts with chlorine gas to form sodium chloride solid.
2Na(s) + Cl2(g)  2NaCl(s)

19. Try this one:
Dicarbon dihydride reacts with oxygen gas to produce carbon dioxide gas and water

20. Evidence of rxns All rxns involve chemical change
Look for evidence of those changes:
Light, heat, gas, color change, precipitate

21. Other Symbols  precipitate forms  gas forms  “produces” or “yields”
 products can reform in to reactants – end result is a mix of the two.
Δ – means that heat was added

22. Chemistry – Chemical Equations
Part II

23. Experiment 17 Please read over steps 1-8 carefully
Get the proper safety gear, and the proper equipment
Do steps 1-8, then set the solid aside to dry (step 9)
Return to your seat – while the solid dries, you can work on A7
At the end of class – record the mass of your beaker and solid.

24. Balanced equations show RATIOS
The coefficients of a balanced equation reveal the RATIO of each reactant to each product:
2H2 + O2  2H2O means there is a 2:1:2 ratio of H2 to O2 to H2O.
This is the same as a 4:2:4 ratio, etc. etc.

25. The activity series
Writing an equation doesn’t mean it will actually happen
How do we know it will happen?
Single replacement reactions are predicted with the activity series
This activity series has been created through experiment.

26. The activity series
Things on the list will displace anything below but not above other things on the list
There are two lists – one for elements that become positive ions, and one for elements that become negative ions.

27. Element
Reactivity
Li Rb K Ba Ca Na
React with cold H2O and acids, replacing hydrogen
F2 Cl2 Br2 I2
Listed from most reactive to least reactive
Mg Al Mn Zn Cr Fe
React with acids or steam but usually not liquid water, to replace hydrogen
Ni Sn Pb
All react with acids but not water, to replace hydrogen
H2 Cu Hg
All react with oxygen to form oxides
Ag Pt Au
Mostly unreactive

28. ? When a rxn releases energy, where does it come from?
You have to put energy in to break bonds
If bonding is stronger in the products they will be more stable
More stable = lower energy
Lower energy means energy must be RELEASED
This is an exothermic reaction
Is this an example of an ENDOthermic or EXOthermic reaction?

29. If bonding is weaker in the products, they will be less stable, have more energy, and will therefore absorb it.
Is this an endo or exothermic rxn?