Friday, March 11, 2011

Chem Think: Chemical Reactions

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 in 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 same atoms at the end of 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 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 two atoms of H and two atoms of O as reactants. This would make one molecule of H2O, but we’d have one 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 was 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:
2 H2 + 1 O2 = 2 H2O
Which is the same as:
# of Atoms in Reactants Element # of Atoms in Reactants
2 H 2
4 O 4

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) In the unbalanced equation, there are: Reactants = Products
Cu atoms-1 Cu atoms-1
O atoms-2 O atoms-1

15) To balance his 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 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
This is the same thing as saying: Reactants = Products
# Cu atoms-2 = # Cu atoms-2
# O atoms-2 = # O atoms-2

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
# of Atoms in Reactants Element # of Atoms in Reactants
1 C 1
4 H 2
2 O 3

19) What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.)
1 N2 + 3 H2 = 2 NH3
# of Atoms in Reactants Element # of Atoms in Reactants
2 N 1
2 H 3

20) What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.)
2 KCLO3 = 2 KCI + 3 O2
# of Atoms in Reactants Element # of Atoms in Reactants
1 K 1
1 CI 2
3 O 3

21) What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.)
2 Al + 2 O2 = 3 Al2O3
# of Atoms in Reactants Element # of Atoms in Reactants
1 AI 1
2 O 2

Summary

1) Chemical reactions always involve breaking bonds between atoms and rearranging them in a particular order to create the product.

2) The Law of Conservation of Mass says that the same atoms must be present in the product as they were in the reactants. This is a reworded explanation of the French Chemist Antoine Lavoisier in 1789, which simply states that mass is neither created nor destroyed in any ordinary chemical reaction. Also, “the mass of substances produced by a chemical reaction is always equal to the mass of the reacting substances.”

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 atoms in both reactants and products.

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