10.1 Apply
Name__________________________
Endothermic vs. Exothermic
Period______Date________________
Classify the following reactions as endothermic or exothermic. 5.0 g of five different solids were added
to 20.0 mL of water and the temperature was monitored until the entire solid dissolved.
Beginning Water
Temperature (◦C)
Final Water
Temperature (◦C)
Change in Water
Temperature (◦C)
NH4NO3
25.4
22.1
-3.3
CaCl2
25.4
28.9
+3.5
LiCl
25.4
23.9
-1.5
NaCl
25.4
25.1
-0.3
NaOH
25.4
29.5
+4.1
Solid
Questions:
1. List the solids that underwent exothermic reactions:_____________________________________
2. List the solids that underwent endothermic reactions: ____________________________________
3. Would the temperature changes be the same if you used a mole of each solid? ________
4. Would the water still increase or decrease in temperature? _________
5. The equation for dissolving NaOH is NaOH(s) → Na+(aq) + OH-(aq). Rewrite the equation and
include the word “energy” on the correct side.
______________________________________________________________________________
6. The equation for dissolving LiCl is LiCl(s) → Li+(aq) + Cl-(aq). Rewrite the equation and include
the word “energy” on the correct side.
______________________________________________________________________________
7. Explain what happens to the energy that “disappears” from the water in an endothermic reaction:
______________________________________________________________________________
______________________________________________________________________________
8. Explain what happens to the energy in an exothermic reaction:
______________________________________________________________________________
10.2 Apply – Which is better, Natural Gas or Propane?
Many homes heat and cook with either methane (natural gas, CH4) or propane (C3H8). Here are the
equations for their combustion including the standard enthalpy changes.
CH4
+ 2 O2
C3H8 + 5 O2
CO2 +
2 H2 O
∆H◦ = -890 kJ
→ 3 CO2 +
4 H2 O
∆H◦ = -2043 kJ
→
Questions:
1. Calculate the amount of heat released when 5.50 g of methane is burned.
2. Calculate the amount of heat released when 5.50 g of propane is burned.
3. Which product gives off more heat per gram? _______________
4. What is the amount of heat given off when 1 mole of methane is burned? ________________
5. What is the amount of heat given off when 1 mole of propane is burned? ________________
6. Which fuel gives off more heat per mole? _______________
7. Heating fuel is typically rated in terms of British Thermal Units (BTUs) instead of kilojoules.
Given: 1 kJ = 0.948 BTU, calculate the heat released during combustion of one mole of methane
and one mole of propane in BTUs.
8. Heating fuel is typically sold by mass. Based on this, which gas is the more economical heating
fuel? _________________ Explain!
_____________________________________________________________________________
_____________________________________________________________________________