Energy and Chemical Reactions

1. Energy and Chemical Reactions Chapter 17 and 18

2. CHEMICAL Changes Changes in Matter PHYSICAL Changes Changes that affect physical properties of substance Requires energy Example: changing size or shape, changing state (melting/freezing) Changes that transform one substance into a new substance Requires energy Example: baking/cooking, Iron and O2 to form rust

3. Eating Eating involves BOTHphysical and chemical changes Physical: breaking food into smaller pieces or warming it up Chemical: breaking down molecules in food to create energy or mixing ingredients to make a new substance

4. Chemical Reactions Any time there is a chemical change a chemical reaction has taken place Chemical reaction is the process of breaking chemical bonds in one or more substances and the reforming of new bonds to create new substances

5. Evidence for a Chemical reaction 1. Bubbling indicates formation of a gas 2. Cloudiness indicates the formation of a precipitate (solid that is forming) 3. Color change indicates a new substance is forming 4. Temperature change indicates energy being released (heat) or absorbed (cold)

6. Evidence

7. Chemical Reaction Parts of a chemical reaction (like a recipe) Reactants – the substances that you begin with in a chemical reaction Products – the new substance formed as a result of a chemical reaction

9. Law of the Conservation of Mass Established by Antoine Laurent Lavoisier Law of the Conservation of Mass states that the total mass of the reactants EQUALS the total mass of the products in a chemical reaction It must be performed in a CLOSED system

10. Law of Conservation of Mass Closed System

11. Law of Conservation of Mass Open System

12. Formula Mass Chemical Formula can help you calculate the mass of 1 unit of the compound Formula Mass is the sum of ALL the atomic mass values of the atoms in a chemical formula Determined by adding up all the atomic masses Measured in amu

13. Formula Mass What is the formula mass of H2O? Atomic Mass of H = 1.008 amu (x2) Atomic Mass of O = 15.999 amu FORMULA MASS = 1.008 x2 + 15.999 = 18.015 amu

14. Avogadro’s Number When performing chemical reactions it is helpful to measure compounds in grams amu’s and grams are related using Avogadro’s number (6.02 x 1023); it is the number of molecules in the formula Also called the MOLE

15. Mole 6.02 x 1023 equals the number of molecules in the formula mass of a compound when the mass is expressed in grams Example: 18.015 amu = 1 molecule of H2O 18.015 g = 6.02 x 1023 molecules of H2O

16. Mole 1 mole = 6.02 x 1023 molecules of any substance Molar mass equals the mass in grams of 1 mole (6.02 x 1023) of a compound

17. Problem What is the molar mass of one mole of CaCO3? Atomic Mass of Ca = _____________ amu Atomic Mass of C = ______________ amu Atomic Mass of O = ______________ x3 amu Formula Mass = _________________ amu Molar Mass = __________________ grams

18. Answer

19. Chemical Equations A chemical equation is when a chemical reaction is written using chemical formulas and symbols The arrow is between the reactants and products The arrow means “to produce” or “yield” The Law of the Conservation of Mass is applied to chemical equations to produce

20. Chemical Equations

21. Counting Atoms Subscript # tells you the number of atoms present in the molecule No Subscript # tell you 1 atom is present Counting atoms is necessary to balance equations

22. Counting Atoms How many C atoms? How many H atoms? How many O atoms?

23. Balancing Equations To balance chemical equations you can ADD coefficients in front of the chemical formula A coefficient is applied to ALL the atoms in that chemical formula If no coefficient is present only 1 molecule is present in that chemical formula

24. Balancing Equations Steps to Balancing Equations: 1. Write the word form of equation 2. Write the chemical equation using chemical formulas and symbols 3. Count the number of each type of atom on both sides 4. Add coefficients to balance the equation

25. Balancing Equations

26. Balancing Equations

27. Problem __Cu2S + ___O2 → ___Cu2O + ___SO2

28. Answer

29. Energy ALL chemical reactions involve energy Some GIVE OFF energy (in form of light or heat) Some USE energy

30. Energy Energy is involved in chemical reactions in 2 ways: 1. At the start of a chemical reaction it is used to break bonds between the reactants 2. Energy is released when new bonds form as the atoms recombine to make new products

31. Energy Chemical reactions are classified based on: 1. How much energy is used to break bonds? 2. How much energy is released when products are made?

32. Classifying Reactions Chemical Reactions can be classified as: 1. Exothermic 2. Endothermic

33. Exothermic Reactions Exothermic Reactions release MORE energy than is used in the chemical reaction Exothermic reactions releaseenergy in the form of light OR heat as a product Indicated by an increase in temperature of products

34. Exothermic Reactions They occur because the products have lower energy (due to energy released) and are more stable than the reactants Exothermic reactions tend to keep going longer because they supply energy to neighboring atoms Examples: respiration (the breakdown of food into energy) and combustion (burning)

36. Endothermic Reactions Endothermic reactions uses MORE energy during the chemical reaction to break bonds than it releases Endothermic reactions absorb energy as a reactant Indicated by a decrease in temperature

37. Endothermic Reactions Endothermic reactions need more energy to keep going Example: photosynthesis (need more sunlight to keep going), refinement of ores to make useful metals, cold packs