ChemThink: Chemical Reactions

CHEMICAL REACTIONS TUTORIAL QUESTIONS:

1. Starting materials in a chemical reaction are called reactants

2. The ending materials in a chemical reaction are called products

3. The arrow indicates a chemical change has taken place.

4. All reactions have one thing in common: there is a rearrangement of chemical bonds.

5. Chemical reactions always involve breaking old bonds, forming new bonds, or both.

6. In all reactions we still have all of the atoms at the end that we had at the start.

7. In every reaction there can never be any missing atoms or new atoms

8. Chemical reactions only rearrange bonds in the atoms that are already there.

9. Let’s represent a reaction on paper. For example, hydrogen gas (H2) reacts with oxygen gas (O2) to form water (H2O):

H2 + O2→H2O If we use only the atoms shown, we’d have 2 atoms of H and 2 atoms of O as reactants. This would make 1 molecule of H2O, but we’d have 1 atom of O leftover. However, this reaction only makes H2O.

Remember: reactions are not limited to 1 molecule each of reactants. We can use as many as we need to balance the chemical equation

 A balanced chemical reaction shows:

a) What atoms are present before (in the reactants) and after (in the products)

b) How many of each reactant and product is present before and after. 

10. So to make H2O from oxygen gas and hydrogen gas, the balanced equation would be:

2H2 + 2O2 → 2 H2O Which is the same as:

4 H atoms in the reactants

4 H atoms in the products

2 O atoms in the reactants

2 O atoms in the products

11. This idea is called the: Law of Conservation of Mass

12. There must be the same mass and the same number of atoms before the reaction (in the reactants) and after the reaction (in the products).

13. What is the balanced equation for this reaction? 2 Cu + 1 O2 → 2 CuO

14. on the unbalanced equation there are: Reactants: Cu atoms 1/O atoms 2 → Products: Cu atoms 1/O atoms 2

15. To balance this equation, we have to add 2 molecules to the products, because this reaction doesn’t make lone oxygen atoms.

16. When we added a molecule of CuO, now the number of oxygen atoms is balanced but the number of copper atoms don’t match. Now we have to add more copper atoms to the reactants.

17. The balanced equation for this reaction is  2Cu + O2= 2CuO

This is the same thing as saying:

2 Cu atoms in the reactants

2 Cu atoms in the products

2 O atoms in the reactants

2 O atoms in the products

20. 2 KCIO3  +  2 KCI →  3 O2 21. 4Al  + 3O2  →  2Al2O3

18. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.) 1 CH4  +   2 O2 →  2 H2O + 1 CO2

19. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.) 1N2+ 3H2 →  2NH3

20. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.) 2 KCIO3  →  3O2+  2 KCI

21. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.) 4Al + 3O2→2Al2O3

SUMMARY

1. Chemical reactions always involve breaking bonds.
2. The Law of Conservation of Mass says that the same atoms must be present before and after the reaction.
3. To balance a chemical equation, you change the coefficients in front of each substance until there are the same number of each type of atom in both reactants and products.