1. Ch. 3: Classifying Reactions Chemicals in Balance

2. Ch. 3: Chemicals in Balance
3.1: Chemical Equations

3. 3.1: Chemical Equations Chemical reaction: a chemical change
Reactants: substances that start a reaction (on the left side)
Products: substances formed in a reaction (on the right side)
Law of Conservation of Matter- All atoms present in reactants must also be present in products. Atoms cannot be created nor destroyed.
Chemical bonds connections between atoms in the reactants are broken and new bonds are formed as atoms rearrange to form products.
REACTANTS  PRODUCTS
Above the arrow could be heat, catalyst, etc.;
( , , etc.)
same number of atoms on reactants and products side

4. Chemical equation: Shorthand expression for a chemical reaction showing mathematical relationships between reactants & products
Format:
Reactants (written on left) are separated from products (on right) with an arrow (→), which shows direction of reaction. Plus signs (+) placed between substances when needed.
Ex: H2 + O2 → H2O
2. Coefficient: whole number placed in front of substances to show how many units (atoms, molecules, ions, moles) are present. When no number is shown, it is understood to be “1”.
2 H2 + O2 → 2 H2O
3. States of all chemicals are listed
Solid = (s) Liquid = (l) Gas = (g)
Aqueous solution (dissolved in water) = (aq)
+ = reacts with (reactants or products side)  = yields, produces, forms

5. Chemical reactions can be represented as:
word equations:
Identifies the reactants and products of a chemical reaction by name.
water  hydrogen oxygen
ii) skeleton chemical equations:
Lists the chemical formula of each reactant on the left side , separated by a +, and followed by an arrow
H2O (l)  H2 (g) O2 (g)
iii) balanced chemical equations:
This shows that there is the same number of each kind of atom on both sides of the equation.
2H2O (l)  2H2 (g) O2 (g)

6. 4.1: Balancing Chemical Equations
Balanced chemical equation: contains same number of each kind of atom on each side of the arrow.
2 H2 (g) + O2 (g) → 2 H2O (l)
2 x 2 H = 4 H 2 x 2 H = 4 H
1 x 2 O = 2 O 2 x 1 O = 2 O
Steps:
Write the unbalanced equation.
Inventory the atoms in the reactants and in the products.
Balance the equation.
Strategies:
Balance elements left-to-right on the Periodic Table (metals first, then nonmetals)
Leave H and O until last.

7. __Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g)
1. Write the unbalanced equation.
__Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g)

8. __Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g)
2. Inventory reactants and products.
__Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g) Zn Cl H 1 Zn Cl H 1 2

9. __Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g)
3. Balance the equation (by changing coefficients.)
__Zn (s) + __HCl (aq) → __ZnCl2 (aq) + __ H2 (g) 1 2 1 1 Zn Cl H 1 Zn Cl H 1 2
BALANCED ! 2
BALANCED !

10. Try to balance this reaction:
__CH4 (g) + __O2 (g)  __CO2 (g) + __H2O (l) 2 2 1 1 C H O 1 4 2 C H O 1 2 3 4 4 4

11. K Cl O 1 3 2 K Cl O 1 2 2 2 2 6 6 __KClO3 (s)  __KCl (s) + __O2 (g) 2
Try to balance this reaction:
__KClO3 (s)  __KCl (s) + __O2 (g) 2 2 3 K Cl O 1 3 2 K Cl O 1 2 2 2 2 6 6

12. Try to balance this reaction (using atoms):2 3 6
__Al(OH)3(s) + __H2SO4(aq)  __Al2(SO4)3(aq) + __H2O(l) 1 Al S H O 1 5 7 2 Al S H O 2 3 13 3 8 12 12 10 18 18

13. Try to balance this reaction (grouping ions):2 3 6
__Al(OH)3(s) + __H2SO4(aq)  __Al2(SO4)3(aq) + __HOH(l) 1 Al OH H
SO4 1 3 2 2 Al OH H
SO4 2 1 3 6 6 6 6 3

14. What does a Chemical Equation mean? In terms of molecules and atoms:
C3H8 (g) + 5 O2 (g) → 3 CO2 (g) + 4 H2O (l)
1 molecule C3H8
3 C atoms &
8 H atoms
5 molecules O2
10 O atoms
3 molecules CO2
3 C atoms
&
6 O atoms
4 molecules H2O
8 H atoms
&
4 O atoms

15. Balance these equations
__ Mg + __ O2 → __ MgO
__ KClO3 → __ KCl + __ O2
__ Cu + __ AgNO3→ __ Cu(NO3)2 + __ Ag
__KOH + __ H3PO4 → __ K3PO4 + __ H2O
2 Mg + 1 O2 → 2 MgO
2 KClO3 → 2 KCl + 3 O2
1 Cu + 2 AgNO3→ 1 Cu(NO3)2 + 2 Ag
3 KOH + 1 H3PO4 → 1 K3PO4 + 3 H2O
Even better…
2 Mg (s) + 1 O2 (g) → 2 MgO (s)