1. Chemical Reactions

2. Chemical Reactions
A chemical reaction is the process by which a chemical change happens
All chemical reactions are accompanied by changes in energy

3. Chemical Reactions
All chemical reactions involve the conversion of starting materials, called reactants, into new substances, called products. The products have different properties than the reactants.

4. Chemical Reactions
For example, when solid magnesium is added to a solution of hydrochloric acid, bubbles of hydrogen gas are formed as well as aqueous magnesium chloride.

5. Chemical Equations
A chemical reaction is often described by writing a chemical equation.

6. Chemical Equations Every chemical equation must have: Reactant Product
Arrow directing reactant to product

7. Remember: Diatomic element (HOFBrINCl)
Chemical Equations
Chemical equations can be written in different ways.
Word Equation: describes the chemical equation using the names of the reactants and products.
Ex) Solid magnesium reacts with hydrochloric acid to produce magnesium chloride and hydrogen gas.
Skeleton Equation: describes the chemical reaction using the chemical formulas of the reactants and products.
Remember: Diatomic element (HOFBrINCl)
word equation:
magnesium + hydrochloric acid  magnesium chloride + hydrogen
Skeleton equation:
Mg(s) + HCl(aq)  MgCl2(aq) + H2(g)

8. Word vs. Skeleton Equations
Writing chemical equations is similar to math equations:
Two combines with three to make five.
Word Equation:
two + three  five
Skeleton Equation:
2 + 3  5
Words with + signs between reactants and an arrow to the products
Symbols with + signs between reactants and an arrow to the products

9. Chemical Equations
The chemical formulas in a chemical equation will often include the state of matter of each substance
(s) = solid
(l) = liquid
(g) = gas
(aq) = aqueous (substance is
dissolved in water)

10. The Law of Conservation of Mass
In a chemical reaction, the mass of the products always equals the mass of the reactants.
In other words, the mass is conserved.

11. The Law of Conservation of Mass
In a reaction, no atoms are destroyed and no new atoms are produced.
Only the chemical bonds between atoms are broken and rearranged

12. The Law of Conservation of Mass
For example:
hydrogen + oxygen  water
2H2(g) + O2(g)  2H2O(l)
4.04 g g  g
There are equal numbers of hydrogen atoms (4) and oxygen atoms (2) on both the reactant side and product side. The mass on each side of the reaction is also equal.

13. Balancing Chemical Equations
Chemical reactions that show the formulas of the reactants and products but are not balanced are called skeleton equations.

14. Balancing Chemical Equations
To balance a chemical equation, begin by counting the number of atoms of each element in the skeleton equation

15. Balancing Chemical Equations
Balance by placing coefficients in front of the chemical formulas.
Use only whole numbers as coefficients.
Check that all of the coefficients in the equation are the lowest common factor.
NEVER CHANGE A SUBSCRIPT in a formula to help make atoms balance!!!

16. Balancing Chemical Equations
Tips for Balancing:
Balance complicated looking formulas first.
Leave hydrogen for second last and oxygen for very last.

17. Balancing Chemical Equations
You may be able to treat polyatomic ions as a unit.
Example: If NO3- appears in the reactants and products of a skeleton equation, count the number of NO3- groups rather than the number of N and O atoms separately.

18. Balancing Chemical Equations
Example 1:
Balance the following chemical equation:
AlBr3(s) + Cl2(g)  AlCl3(s) + Br2(g)
1. Count the number of atoms in the reactants and products:

19. Balancing Chemical Equations
Example 1:
2AlBr3(s) + Cl2(g)  AlCl3(s) + 3Br2(g)
Balance the number of bromine atoms by adding a coefficient of 2 in front of AlBr3 and a coefficient of 3 in front of Br2. Adjust your atom count:
The number of aluminum atoms is no longer equal.

20. Balancing Chemical Equations
Example 1:
2AlBr3(s) + Cl2(g)  2AlCl3(s) + 3Br2(g)
Balance the number of aluminum atoms by adding a coefficient of 2 in front of AlCl3. Adjust your atom count:
The number of chlorine atoms is no longer equal.

21. Balancing Chemical Equations
Example 1:
2AlBr3(s) + 3Cl2(g)  2AlCl3(s) + 3Br2(g)
Balance the number of chlorine atoms by adding a coefficient of 3 in front of Cl2. Adjust your atom count:
The equation is now balanced!

22. Balancing Chemical Equations
Example 2:
Balance the following chemical equation:
BeCl2(aq) + AgNO3(aq)  Be(NO3)2(aq) + AgCl(s)

23. Balancing Chemical Equations
Example 3:
Balance the following chemical equation:
Mg3N2(s) + H2O(l)  MgO(aq) + NH3(g)