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.
4. All reactions have one thing in common: there is a rearrangement of bonds.
5. Chemical reactions always involve breaking bonds, forming 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 atoms or new atoms.
8. Chemical reactions only rearrange the 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):
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:
10. So to make H2O from oxygen gas and hydrogen gas, the balanced equation would be:
Which is the same as:
11. This idea is called the Law of Conservation of Mass.
12. There must be the same atoms 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 + 2 O2 -> 2 CuO
14. In the unbalanced equation there are:
15. To balance this equation we have to add CuO molecules to the products, because this reaction doesn't loan O atoms.
16. When we added a molecule of CuO, now the number of O atoms is balanced but the number of Cu atoms don’t match. Now we have to add more Cu atoms to the reactants.
17. The balanced equation for this reaction is:
2 Cu + 1 O2 = 2 CuO
That is the same thing as saying:
18. What is the balanced equation for this reaction? 1 CH4 + 2 O2 -> 2 H2O + 1 CO2
19. What is the balanced equation for this reaction? 1 N2 + 3 H2 ->2 NH3
20. What is the balanced equation for this reaction? 2 KDIO3 -> 2 KCI + 3 O2
21. What is the balanced equation for this reaction? 4 Al + 3 O2 -> 2 Al2O3
Summary
1. Chemical reactions always involve breaking bonds, making bonds, or both.
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.
2. The ending materials in a chemical reaction are called products.
3. The arrow indicates a chemical change.
4. All reactions have one thing in common: there is a rearrangement of bonds.
5. Chemical reactions always involve breaking bonds, forming 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 atoms or new atoms.
8. Chemical reactions only rearrange the 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:
4 H2 + 2 O2 -> H2O
Which is the same as:
| # of atoms in reactants | Element | # of atoms in products |
| 4 | H | 4 |
| 2 | O | 2 |
11. This idea is called the Law of Conservation of Mass.
12. There must be the same atoms 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 + 2 O2 -> 2 CuO
14. In the unbalanced equation there are:
| Reactants | Products |
| 1 Cu atom | 1 Cu atom |
| 2 O atoms | 2 O atoms |
15. To balance this equation we have to add CuO molecules to the products, because this reaction doesn't loan O atoms.
16. When we added a molecule of CuO, now the number of O atoms is balanced but the number of Cu atoms don’t match. Now we have to add more Cu atoms to the reactants.
17. The balanced equation for this reaction is:
2 Cu + 1 O2 = 2 CuO
That is the same thing as saying:
| Reactants | Products |
| 2 Cu atoms | 2 Cu atoms |
| 2 O atoms | 2 O atoms |
18. What is the balanced equation for this reaction? 1 CH4 + 2 O2 -> 2 H2O + 1 CO2
19. What is the balanced equation for this reaction? 1 N2 + 3 H2 ->2 NH3
20. What is the balanced equation for this reaction? 2 KDIO3 -> 2 KCI + 3 O2
21. What is the balanced equation for this reaction? 4 Al + 3 O2 -> 2 Al2O3
Summary
1. Chemical reactions always involve breaking bonds, making bonds, or both.
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.
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