1. 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.

2. Task 1 – Investigate Conservation of Mass

3. Calcium carbonate (CaCO3) hydrochloric acid (HCl)
and
hydrochloric acid (HCl)
What will happen to the mass when the reaction happens?
Can you explain why you have predicted this?
What do you think the chemical reaction is?

4. Calcium carbonate (CaCO3) hydrochloric acid (HCl)
and
hydrochloric acid (HCl)
This time we will try to measure the mass of the carbon dioxide gas given off. Will it have mass?

5. Potassium Iodide + Lead Nitrate
Potassium Iodide + Lead Nitrate Lead iodide + Potassium nitrate solution
What are the products?
What are the reactants?
What do you notice about the quantities of chemical before and after?

6. In a neutralisation reaction, for example,
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
There are the same number of atoms on each side of the equation

7. The atoms rearrange + + + + H Na O H Cl Hydrochloric acid
Sodium hydroxide H O Cl Na + +
Sodium chloride
water

8. Burning Magnesium
Why is there an apparent increase in mass if atoms have not been created?
Oxygen from the surrounding air joins with the magnesium to form a compound.
All atoms have mass.

9. Again they have rearrangedMg Mg O O + + O Mg Mg O +
magnesium
Oxygen (from air)
Magnesium oxide

10. Acid + Carbonate What is the general equation:
Acid + carbonate salt + carbon + water
dioxide
Use the equation to help explain what you would expect to happen to the mass in the above reaction. Discuss for 1 minute.

11. In a closed container:
The mass should be the same before and after the reaction: Ca C O H Cl + H Cl

12. In a closed container: + + + + Cl Ca C O H O Calcium chloride Carbon
dioxide +
water

13. But what about in an open container?Cl Ca C O H O + +
Calcium chloride +
Carbon
dioxide +
water
The carbon dioxide can escape therefore the mass in the reaction vessel decreases

14. But what about in an open container?Cl Ca H O +
escapes C O
Calcium chloride +
water
Carbon dioxide

15. Hydrogen gas has escaped 0.2g
Checkpoint
From the air
9.6g
9.4g
Hydrogen gas has escaped
0.2g

16. Moly mod kits
Making models of atoms and molecules

17. Hold up a single hydrogen atom.

18. Can you make a hydrogen molecule (H2)?

19. Hold up a single oxygen atom.

20. Can you make a water molecule (H2O)?

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

22. 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?

23. H2 + O2  H2O We can write this chemical reaction as an equation.
Oxygen + Hydrogen  Water
H O2  H2O
In the symbol equation the small numbers after the symbol tell us how many of those atoms are in each molecule

24. 4 4 2 2

25. Take your hydrogen molecule and your oxygen molecule.
Balancing Equations
Take your hydrogen molecule and your oxygen molecule.
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. +
2H O  2H2O

26. 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.

27. In a neutralisation reaction, for example,
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
There are the same number of atoms on each side of the equation

28. Task
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

29. Task
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

30. Al + O2  Al2O3 Al + CuO  Al2O3 + Cu H2 + N2  NH3 Mg + P4  Mg3P2
Task 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

31. Task 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
Answers:
4Al + 3O2    2Al2O3
4Al + 6CuO  2Al2O3 + 6Cu
3H2 + N2  2NH3
6Mg + P4  2Mg3P2

32. What will happen when the reaction happens?
Checkpoint
What will happen when the reaction happens?
Can you explain why you have predicted this?
What do you think the chemical reaction is?
prism
Heat proof mat