1. UNIT 1 Matter Day 32 October 13, 2017
Counting Atoms
8th Grade Science

2. Pick up the two sheets from the lab table, and be seated.
Copy the E.Q. onto page 23.
E.Q. - What is the Law of Conservation of Matter (Mass)?
October 13, 2017

3. Plan for today:
Trout in the Classroom
Lesson on chemical formulas and chemical equations
Practice counting atoms in compounds

4. Homework:
Worksheet “How to Count Atoms” and “Counting Atoms”.

5. E.Q Page _____
What is the Law of Conservation of Matter (Mass)?

6. Review Lab

7. Trout in the Classroom
../Trout in the Classroom /Trout in the Classroom 2017.pptx

8. Copy these 3 examples. Chemical Formulas
Def. Chemical formula is a combination of chemical symbols and numbers to represent a substance.It shows how many atoms of each kind of element are present in a molecule.
Subscript: a number written to the lower right of a chemical symbol. It tells how many atoms of that element are present.
Copy these 3 examples.

9. chemical Equations
Def. Use chemical symbols and formulas to describe a chemical reaction.
Ex: Copy this
Chemical equation.
C and O2 are the reactants. CO2 is the product.
yields

11. Law of Conservation of Mass
Def. This law states that matter(mass) CANNOT be created or destroyed in ordinary physical or chemical changes.
The total mass of the reactants equals the total mass of the products.
This can be proven in a closed system… one in which nothing enters or leaves the system.

13. Let’s look at the Law of Conservation of Mass and Chemical Equations presented Brainpop’s way.
Brainpop - Law of Conservation of Matter

14. Now let’s learn how to balance chemical equations.
A Quick Review
elements
Chemical symbols represent __________.
compounds
Chemical formulas represent __________.
chemical reactions.
Chemical equations represent __________.
Now let’s learn how to balance chemical equations.

15. Let’s practice counting atoms in preparation for learning how to balance chemical equations.

16. Rules for counting atoms:
Subscripts (the little numbers) tell you how many atoms of that element are present. H2O = 2 atoms of hydrogen and one atom of oxygen
If a coefficient (big number) is written in front of a chemical formula or symbol, you have to multiply that number by all the elements in the compound.
Ex. 2NaCl (think about the distributive property in math) 2(NaCl) – this would mean 2 atoms of sodium (Na) and 2 atoms of chlorine (Cl) OR molecules of sodium chloride
If a compound contains subscripts and coefficients, you have to multiply the coefficients by the subscripts.
Ex. 2Na2SO4 or 2(Na2SO4) = 4 sodium (Na) atoms, 2 Sulfur atoms, and 8 oxygen atoms

17. *Coefficient: a number that is placed in front of a chemical symbol or formula in order to balance a chemical equation. Subscripts (the little numbers) CANNOT be changed to balance an equation. Why?
H2O 2H20 -2 molecules of water
CO2 4CO2 – 4 molecules of carbon dioxide
C6H12O6

18. How to Balance Chemical Equations: 3 Simple Steps
Step 1: Count the number of atoms for each element on both sides of the equation. Look for elements that are not balanced.
Ex. Mg O2 MgO
Mg- Mg-
O- O-
_____ is unbalanced.

19. Step 2: Add a coefficient in front of the formula that contains the unbalanced element(s). The O was unbalanced. Place a coefficient 2 in front of the MgO. ***Note: This may throw off the balance of other elements, so you have to keep working with it.
Ex. Mg O2 __ MgO
This will balance the O, but unbalance the Mg. So…add another coefficient 2 in front of the Mg in the reactant.
Ex. ___Mg O2 2 MgO
*This should balance the Mg but be sure to do Step 3.

20. Step 3: Check the number of atoms on both sides of the equation to be certain they are balanced.
2Mg O 2MgO
Mg- Mg-
O- O-
Is it balanced? If so, you have supported the Law of Conservation of Matter (Mass).

21. How does balancing equations illustrate and support the Law of Conservation of Mass/Matter?
*Because they require that you have the same mass after the reaction as you do before the reaction. This means…
*The # and type of atoms are the same in the reactants and products (just in different combinations)
*Although the state of matter of the products may be different from the reactants, the overall mass doesn’t change.

22. In a chemical reaction, no atoms are gained or lost
In a chemical reaction, no atoms are gained or lost. They just get rearranged.
Because this Law of Conservation of Matter (Mass) has been proven, chemical equations (which represent a chemical reaction) MUST be balanced (same number of atoms on both sides).
mass of the reactants=mass of the products.

23. Let’s look at Chemical Equations presented Brainpop’s way.
Brainpop-Chemical Equations

24. Chemical Formulas
Def. Chemical formula is a combination of ____________ ___________and ____________ to represent a ___________.It shows how many __________ of each kind of ___________ are present in a ____________.
Subscript: a number written to the _________ __________ of a chemical ___________ . It tells how many ___________ of that element are
present.

25. Chemical Equations
Def. - Use chemical ____________ and ____________ to describe a ____________ ____________.

26. Law of Conservation of Mass
Def. This law states that ______________ CANNOT be _____________ or _____________ in ordinary ______________or ________________ ________________.
The total __________________________________________
______________________________________________________.
This can be proven in a __________ ___________…one in which nothing enters or leaves the system.

27. Now let’s learn how to balance chemical equations.
A Quick Review
Chemical symbols represent __________.
Chemical formulas represent __________.
Chemical equations represent __________.
Now let’s learn how to balance chemical equations.

28. *Coefficient: a number that is placed ____________ a chemical _________ or _________ in order to __________ a chemical ___________. Subscripts __________ be _________ to balance an equation. Why?
H2O 2H20 -2 molecules of water
CO2 4CO2 – 4 molecules of carbon dioxide
C6H12O6

29. More O2 in the reactants.
Count to be sure the number of atoms of each element is the same in the reactants and products.

30. Let’s practice balancing some chemical equations
Let’s practice balancing some chemical equations. “Balancing Act” practice sheet.
Use pencil. You may have to change coefficients a few times.

31. Wrap-up:
Any questions?