ChemThink: Chemical Reactions

1. C H E M I C A L R E A C T I O NS T U T O R I A L Q U EST I ON S

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 reaction




4. All reactions have one thing in common: there is a chemical change.




5. Chemical reactions always involve a rearrangement.




6. In all reactions we still have all of the same 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 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 one atom of O as reactants. This would make one molecule of H20, but we'd have no oxygen atoms left. However, this reaction only makes H20.




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:




2 H2 +2 O2 = 1 H20




Which is the same as:







# of atoms in Reactant	Element	# of atoms in Products
2	H	2
2	O	1




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




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




13. What is the balanced equation for this reaction? before the reaction (in the reactants)




2 Cu+ 1 O2 = 1 CuO




14. In the unbalanced equation there are:




Reactants>




Cu atoms 1




O atoms 2







Products>




Cu atoms 1




O atoms 1




15. To balance the equation, we have to add CuO molecules to the products, because this reaction doesn't make lone 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 two Cu atoms to the reactants.




17. The balanced equation for this reaction is




2 Cu + 2 O = 2 CuO




This is the same thing as saying:




Reactants




# Cu atoms 2 = # Cu atoms 2




# O atoms 2 = # O atoms 2







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 H20 + 1 CO2







# of atoms in Reactants	Elements	# of atoms in the Products
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	Elements	# of atoms in Products
2	N	1
2	H	3







1. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.)




2 KDIO3 -> 2 KCI + 3 O2




1. 













1. What is the balanced equation for this reaction? (Use the table to keep track of the atoms on each side.)




4 Al + 3 O2 -> 2 Al2O3







# of atoms in Reactants	Elements	# of atoms in Products
1	Al	2
2	O	3










SU M M A R Y




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.