1. Chemical Reactions Physical Science 8 th Grade

2. 1. What is a chemical change? a change in matter that produces one or more NEW SUBSTANCES with different properties

3. 2. What is the difference between physical changes and chemical changes. Physical – alters form or appearance stampedes Chemical – produces a new substance stampedes made + steps

6. 3. Describe reactants and products Reactants Products Chemical Change Substances that undergo change New substances formed from the reaction

7. 4. When does a chemical change occur? When chemical bonds break and new bonds form.

8. 5. How can you tell if a chemical reaction occurs? Look for clues! Change in properties Energy change

9. 6. Compare the properties of a compound with the properties of the elements that make up that compound. The properties of the new compound IS DIFFERENT from the properties of the original elements. Ex: H + O H 2 O Ex: C + H + OC 12 H 22 O 11 Ex: Na + Cl NaCl

13. 7. How does energy change during a chemical reaction? Chemical reactions usually absorb heat or liberate (release) heat.

14. 8. What is an exothermic reaction? A chemical reaction in which energy is released “liberate” or release heat Feels warm!

15. 9. What is an endothermic reaction? A chemical reaction in which energy is taken in. “absorb” heat Feels cool!

16. 10. What is a chemical equation? Chemical symbols and formulas as a short way to describe a chemical reaction. _____ + _____ reactants yield products

20. 11. What is the law of the conservation of mass? During a chemical reaction, matter is neither created nor destroyed.

22. Lavoisier Determined (among other things) that a rusting object doesn’t lose weight, as everyone had long assumed, but gains weight – an extraordinary discovery. Somehow, as it rusted the object was attracting elemental particles from the air. It was the first realization that matter can be transformed but not eliminated.

23. Also, if we were to burn this book now, its matter would be changed into ash and smoke, but the net amount of stuff in the universe would be the same! - Bill Bryson

25. 12. What does this mean? The number of atoms stays the same no matter how they are arranged. So their total mass stays the same Atoms in = atoms out

26. 13. Why must equations be balanced? To satisfy the law of conservation of mass A chemical equation must show the same number of each type of atom on both sides of the equation.

27. 14. What are the steps to balancing equations? 1.Write the equation 2.Count the atoms 3.Use coefficients to balance atoms 4.Look back and check

29. Example #1 HgO Hg + O 2 (unbalanced) 2HgO 2Hg + O 2 (balanced)

30. Example #2 Mg + HCl H 2 + MgCl 2 (unbalanced) Mg + 2HCl H 2 + MgCl 2 (balanced)