1. Unit 10: Energy in Chemical Reactions
Cypress Creek High School
Chemistry 1K
Chapter 10

2. States of Matter
Some chemical equations indicate the state of matter of the substances
Solid (s) - baking soda
Liquid (l) - water
Gas (g) - carbon dioxide
Aqueous (aq) - vinegar and sodium acetate
Aqueous means the substance is dissolved in water. For example, NaCl (aq) means table salt dissolved in water.   

3. Chemical Equations
The simplest way to represent this reaction is by using words
Chemical equations replace chemical names with chemical formulas
Reactants (LEFT)
substances that undergo a reaction
Yield sign (MIDDLE)
represents the change/reaction
Products (RIGHT)
new substances formed in a reaction

4. Chemical Equations Practice
What is the chemical equation for the following reactions?
What are the reactants and products?
1) Water and sulfur trioxide combine to form sulfuric acid
2) Potassium chlorite decomposes to form potassium chloride and oxygen
H2O + SO3 → H2SO4
reactants product
KClO2 → KCl + O2
reactant products

5. Review of Chemical Equations
Label the following chemical equation:
reactants
products
SnO2(s) + 2H2(g)  Sn(s) + 2H2O(g)
subscript
coefficient
state of matter
yield sign

6. Law of Conservation of Mass
The LCM says matter is neither created nor destroyed
The mass of the reactants is always the same as the mass of the products.
The number of atoms of each element is always the same in the reactants and the products.

7. Law of Conservation of Mass Examples
Hydrogen gas and oxygen gas combine to create water.
2H2 + O2  2H2O
The large numbers, known as coefficients, indicate how many moles of each compound there are. This helps to satisfy the LCM!
They apply to every element in the compound (2H2O means 4 H’s & 2 O’s)
They are the coefficients should be in the lowest whole number ratio
Particles are rearranged, not created or destroyed!

8. Law of Conservation of Mass Examples
This should be a review - just count moles of atoms!
How many carbon atoms are conserved? Hydrogen atoms? Oxygen atoms?

9. LCM Examples
Calcium chloride and sodium sulfate combine to form calcium sulfate and sodium chloride. CaCl2 + Na2SO4  CaSO4 + 2NaCl
Before and After:
Does the mass change?
Do the number of atoms change?
# atoms in the reactants
# atoms in the products Ca 1 Cl 2 Na S O 4
Total 10

10. Balancing Chemical Equations
Because the Law of Conservation of Mass (LCM) states that matter is not created or destroyed, the number of atoms of each element must be identical in the reactants and products. This means the chemical equation is balanced.
Example 1:
# atoms in the reactants
# atoms in the products H 2 C 1 O 3
Total 6
This equation is balanced because it satisfies the LCM
Writing coefficients is not necessary because each compound has a coefficient of 1

11. Balancing Chemical Equations
Example 2:
# atoms in the reactants
# atoms in the products Na 1 2 O 3 4 H C
Total 6 9
This equation is not balanced because it does not satisfy the LCM
Writing coefficients is necessary
So we have to balance the equation…

12. Balancing Chemical Equations
Example 2 continued:
The process of balancing chemical equations requires trial and error.
You may not alter the subscripts because it changes the substance!
You may only place coefficients in front of the compounds. These act as multipliers to balance the number of atoms.
# atoms in the reactants
# atoms in the products Na 2 O 4 H C 1
Total 9
By placing a 2 in front of NaOH, the equation now satisfies the LCM and is balanced
***Tip: balance hydrogen and especially oxygen last - they often fall into place!

13. Balancing Chemical Equations Practice
Write a balanced chemical equation for the following reactions. Place coefficients in the blanks. Chemical equations never actually write the coefficient “1”, but for this activity write a number in every blank.
A) ___ K(s) + ___ H2O(l)  ___ H2(g) + ___ KOH(aq)
B) ___ CaCl2(aq) + ___ Na2CO3(aq)  ___ CaCO3(s) + ___ NaCl(aq)
C) ___ N2(g) + ___ H2(g)  ___ NH3(g)