1. Chemical Reactions
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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 stampedes
Chemical – produces a new substance
stampedes made + steps

6. 3. Describe reactants and products
Chemical Change
Reactants Products
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 H2O
Ex: C + H + O C12H22O11
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?
Write the equation
Count the atoms
Use coefficients to balance atoms
Look back and check

29. Example #1
HgO Hg + O (unbalanced)
2HgO Hg + O2 (balanced)

30. Example #2 Mg + HCl H2 + MgCl2 (unbalanced)
Mg + 2HCl H2 + MgCl2 (balanced)