Copy this slide! The Law of Conservation of Mass Mass is never gained or lost in a chemical reaction. When a reaction happens, the atoms in the chemicals are rearranged into different compounds. No new atoms are created and no atoms can disappear. Copy this slide!

Molymod kits On the next few slides there is going to be a couple of words. You will have a short amount of time to hold up the correct thing. Think of this as a race! When you have made each ‘thing’ don’t take them apart!

Hold up a single hydrogen atom.

Can you make a hydrogen molecule (H2)?

Hold up a single oxygen atom.

Can you make a water molecule (H2O)?

Can you make an oxygen molecule (O2)? Hint: use the long bendy grey sticks (bonds)

Out of the things you made can you hold up an element?

Out of the things you made can you hold up a compound?

Lay out your molecules like this on the desk in front of you. Hydrogen Oxygen Water When hydrogen and oxygen react together, what new compound do you think they make?

Notice how we use an arrow and not a = sign!! We can write this chemical reaction as an equation. Notice how we use an arrow and not a = sign!! Oxygen + Hydrogen  Water H2 + O2  H2O In the symbol equation the little numbers after the symbol tell us how many of those atoms are in each molecule

2H2 + O2  2H2O Balancing Equations Take your hydrogen molecule and your oxygen molecule molymod. Using only these two molecules can you rearrange them to make as many water molecules as you can? + You should only have been able to make one water molecule… and you have an oxygen atom left over. In reactions we don’t like to have atoms left over… so we change the numbers of whole molecules to balance it out. + 2H2 + O2  2H2O

You don’t have to draw or use models to figure out how to balance an equation. 2 2 Mg + O2  MgO 1. Divide the symbol equation where the arrow is. - 1 - 2 2 2. List the type of atoms on each side: make sure they are in the same order on each side. Mg O Mg O - 1 2 3. Count the number of each type of atom on each side. 4. If the numbers are the same then it is balanced. If not place BIG numbers in front of the molecules. Then recount. Keep putting the big numbers in front of whole molecules and recounting until the numbers on each side are equal.

H2 + Cl2  HCl Na + Cl2  NaCl Mg + O2  MgO C2 + H2  C2H6 Your turn. Balance these equations. Then write out the word equation for each. H2 + Cl2  HCl Na + Cl2  NaCl Mg + O2  MgO C2 + H2  C2H6 ZnCl2 + Na  NaCl + Zn

H2 + Cl2  HCl Na + Cl2  NaCl Mg + O2  MgO C2 + H2  C2H6 Your turn. Balance these equations. Then write out the word equation for each. H2 + Cl2  HCl Na + Cl2  NaCl Mg + O2  MgO C2 + H2  C2H6 ZnCl2 + Na  NaCl + Zn Answers: H2 + Cl2  2 HCl 2 Na + Cl2  2 NaCl 2 Mg + O2  2 MgO C2 + 3H2  C2H6 ZnCl2 + 2 Na  2 NaCl + Zn

If you melted 25g of ice what mass of water would you get? Your turn (part 2). Balance these equations. Then write out the word equation for each. Al + O2    Al2O3 Al + CuO  Al2O3 + Cu H2 + N2  NH3 Mg + P4  Mg3P2 Other questions: If you reacted 2g of hydrogen with 16g of oxygen, what mass of water would you get? If you melted 25g of ice what mass of water would you get?