1. Chemical Reactions

2. Describing Reactions Chemical Equations Balancing Chemical Equations
Using Equations to Represent Reactions
Conservation of Mass
Balancing Chemical Equations

3. Chemical Equations Reactants- substances that undergo a change
Products- New substances that form as a result of the change.

4. Using Equations to Represent Reactions
Reactants Products
Equation Analogy

5. Chemical Equations
A chemical equation is a representation of a chemical reaction in which the reactants and products are expressed as formulas.
Carbon + Oxygen Carbon Dioxide
C + O₂ CO
(is the above a “balanced” equation?)

6. Reading an Equation
The reaction of carbon and oxygen yields carbon dioxide.

7. Conservation of Mass
During a chemical reaction the mass of the reactants always equals the mass of the products.
Alka-Seltzer reaction
C6H8O7(aq) +
3NaHCO3(aq) →
3H2O(l)
3CO2(g)
Na3C6H5O7(aq)
citric acid
sodium bicarbonate
water
carbon dioxide
sodium citrate

8. Conservation of Mass
The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction.

9. Conservation of Mass

10. Conservation of Mass

11. Conservation of Mass

12. Balancing Equations
In order to show that mass is conserved during a reaction, a chemical equation must be balanced.
Change coefficients, NEVER change subscripts!
Number of atoms on the left must equal the number of atoms on the right.

13. Balancing Equations

17. Energy Changes in Reactions
Chemical Bonds and Energy
Breaking bonds
Forming bonds
Exothermic and Endothermic
Exothermic reactions
Endothermic reactions
Conservation of Energy

18. Chemical Bonds and Energy
Chemical energy is the energy stored in the chemical bonds of a substance.
Energy changes are determined by changes that occur in chemical bonding.
Chemical reactions involve the breaking of chemical bonds in the reactants and the formation of chemical bonds in the products.

19. Breaking Bonds
Breaking bonds requires energy.

20. Forming Bonds
The formation of chemical bonds releases energy.

22. Exothermic and Endothermic Reactions
During a chemical reaction, energy is either released or absorbed.

23. Exothermic Reaction
Exothermic Reaction is a chemical reaction that releases energy to its surroundings.
The energy released as the products formed is greater than the energy required to break the bonds in the reactants.

24. Exothermic Reactions

25. Endothermic Reaction
Endothermic reaction is a chemical reaction that absorbs energy form its surroundings.
More energy is required to break the bonds in the reactants than is released by the products formed.

26. Endothermic Reaction

27. Conservation of Energy
Exothermic reactions
Chemical energy of reactants is converted into heat plus chemical energy of products.
Endothermic reactions
Heat plus chemical energy of the reactants is converted into chemical energy of the products.