Physical vs. Chemical Changes
Ice melting is what type of change?
Physical change.
In the formula H2O, what does the subscript “2” mean?
Two Hydrogen Atoms
Energy cannot be created or destroyed. What law is this?
Law of Conservation of Energy (1st Law of Thermodynamics).
Balance: H2+O2→H2O.
2H2+O2→2H2O
Balance: Al+Cl2→AlCl3.
2Al+3Cl2→2AlCl3
Burning wood is what type of change?
Chemical change.
In 3CO2, how many oxygen atoms are present?
6 oxygen atoms
*** Daily Double ***
A reaction that releases heat is called…
Exothermic
Balance: N2+H2→NH3.
N2+3H2→2NH3
Balance: KClO3→KCl+O2.
2KClO3→2KCl+3O2
Why is dissolving sugar in water considered a physical change?
No new substance forms; sugar can be recovered.
*** Daily Double ***
What part of a chemical formula can be changed when balancing: coefficients or subscripts?
Coefficients
A reaction that absorbs heat is called…
Endothermic
*** Daily Double ***
Balance: Mg+O2→MgO.
2Mg+O2→2MgO
Balance: Na+H2O→NaOH+H2.
2Na+2H2O→2NaOH+H2
***Daily Double***
You eat a bagel and your stomach digests the carbohydrates and sugars in the bagel.
Chemical Change
In the reaction 2H2+O2→2H2O, how many total hydrogen atoms are on the product side?
4 Hydrogen Atoms
At equilibrium, what is true about the reactant and products sides?
They are equal
Balance: Fe+O2→Fe2O3.
4Fe+3O2→2Fe2O3
*** Daily Double ***
Balance: Pb(NO3)2→PbO+NO2+O2.
2Pb(NO3)2→2PbO+4NO2+O2
Rusting is a chemical change because…
A new substance (iron oxide) forms.
Why must chemical equations be balanced?
To obey the Law of Conservation of Matter.
What must be present and in what state for a reaction to be considered a combustion reaction?
O2(g) must be present and in gaseous form.
Balance: C3H8+O2→CO2+H2O.
C3H8+5O2→3CO2+4H2O
Balance: C2H6+O2→CO2+H2O.
2C2H6+7O2→4CO2+6H2O