1. Chapter 6: Chemical Reactions and Equations
Section 6.1: Chemical Equations

2. Objectives: Relate chemical changes and macroscopic properties, demonstrate how chemical equations describe chemical reactions, Illustrate how to balance chemical reactions by changing coefficients

3. Identify the changes that take place when a cake is made:

4. A Chemical Reaction is…
When a substance undergoes a chemical change
After a chemical reacts, it no longer has the same chemical identity

5. How Do You Recognize Chemical Change?
A precipitate (solid) forms
Heat is produced or absorbed
Odor changes
Gas is given off
Color change and bubbles given off indicates a good chance a chemical change has occurred, but they are not proof positive.

6. Which of the following is a chemical change?
growth of a tree
melting butter
the use of food in the body
CO2 escaping from a can of soda
separation of the components of crude oil through distillation
a lake freezing

7. Chemical Reaction (RXN): a process by which one or more substances are changed into one or more different substances; represented by an equation.
Chemical Equation (EQN): shows what change takes place, and shows the relative amounts of various elements/compounds that take part in these changes.

8. Reactions can involve single/ or many reactants or products
REACTANT: Substance that undergoes a reaction (change)
PRODUCT: New substance formed
Example:
Iron and oxygen (reactants) → iron (III) oxide
(Fe3+) (O2) (Fe2O3)
Reactions can involve single/ or many reactants or products

9. Word Equations
Reactants are placed to the left of an arrow and products are placed to the right
Plus signs are used to separate reactants and also to separate products
Example:
hydrochloric acid + sodium hydroxide → water + sodium chloride
(reactants) (products)

10. Chemical Equations
Word equations can be converted into chemical equations
Chemical formulas replace the names of compounds or elements

11. Chemical Equations
Symbols in parentheses are put after formulas to indicate the state of the substance (s) = solid (l) = liquid (g) = gas (aq) = aqueous Example: HCl(l) + NaOH(s) → H2O(l) + NaCl(aq)

12. Energy and chemical equations
Energy is often released or absorbed during chemical reactions
If energy is absorbed → endothermic reaction
If energy is released → exothermic reaction

13. Energy and chemical equations
For reactions that absorb (need) energy, the word energy is often written along with the reactants in a chemical equation
Example: 2H2O(l) + energy → 2H2(g) + O2(g)

14. Energy and chemical equations
For reactions where energy is released, the word energy is often written along with the products in a chemical equation Example: CH4(g) + 2O2(g) → CO2(g) + 2H20(g) + energy

15. Energy and chemical equations
Note: The word energy is not always written. If it is important to know if energy is absorbed or released or in some cases where the reaction would not occur without the addition of energy

16. Conservation of mass
The same amount of matter is contained in the products and reactants (matter is neither created nor destroyed) Mass of reactants = Mass or products In chemical reactions, atoms do NOT change, they rearrange!

17. Conservation of atoms
The number and kinds of atoms present in the reactants of the chemical reaction are the same as those present in the products Counting atoms: The subscript after the symbol for an element represents how many atoms of that element are present

18. Balancing Chemical Equations
For a chemical equation to accurately represent a reaction, the same number of each kind of atom must be on the left side of the arrow as are on the right side → It is said to be balanced

19. Balancing Chemical Equations
To represent more than one unit taking part or being formed in a reaction, use a COEFFICIENT: a number is placed in front to indicate the number of units involved To balance equations, coefficients can be added or changed on the reactants and products Subscripts CAN’T be changed – changing subscripts changes the identity of that subs

20. Steps to write a balanced equation
Analyze
Set Up
Solve
Check

21. Balancing Chemical Equations
1. Magnesium metal and water combine to form solid magnesium hydroxide and hydrogen gas
Analyze: (word equation)
magnesium + water → magnesium hydroxide + hydrogen
Set up: (chemical equation)
Mg(s) + H2O(l) → Mg(OH)2(s) + H2(g)

22. Balancing Chemical Equations
Solve: (count atoms and then balance)
Left: Mg = 1, H = 2, O = 1
Right: Mg = 1, H= 4, O = 2
Mg(s) + 2H2O(l) → Mg(OH)2(s) + H2(g)

23. Check: (count atoms) Left: Mg = 1, H = 4, O = 2 Right: Mg = 1, H= 4, O = 2 The equation is properly balanced.

24. Create and Balance…
Hydrogen and oxygen combine to form gaseous water and release energy.
Word equation:
Hydrogen + oxygen -> water + energy
Chemical equation:
H2(g) + O2(g) -> H2O (g) + energy
Balance:
2H2(g) + O2(g) -> 2H2O (g) + energy

25. Practice Problems P. 199 (#1,3)
Write word equations and chemical equations
1) Magnesium and water combine to form solid magnesium hydroxide and hydrogen gas.
3) When energy is added to solid manganese(II) sulfate heptahydrate crystals, they break down to form liquid water and solid manganese(II) sulfate monohydrate

26. Practice Problems
P. 199 (#2,4) Write word equations and chemical equations 2) An aq. Solution of dihydrogen dioxide and solid lead (II) sulfide combine to form solid lead (II) sulfate and liquid water. 4) Solid potassium reacts with liquid water to produce aq. Potassium hydroxide and hydrogen gas