Exothermic Reactions
Endothermic Reactions
Enthalpy
Stoichiometry (Math) Problems
Models
100
Will an exothermic reaction feel warm or cold to the touch?
Warm
100
Will an endothermic reaction feel warm or cold to the touch?
Cold
100
What signs can enthalpy be?
+/-
Positive or Negative
100
How many moles of oxygen gas (O₂) are produced when 4.0 moles of potassium chlorate (KClO₃) decompose?
2KClO3→2KCl+3O2
We are going from moles to moles, so only one step, the mole ratio.
4.0 Mol KClO3 x 3 Mol O2 / 2 Mol KClO3
Answer: 6 Moles O2
100
How can you show me that bonds are breaking? Also which bonds in a reaction are breaking? (model it & don't forget labels)
Bonds from reactants
Bonds break when energy is absorbed, so arrows drawn TOWARDS the bonds to represent energy absorbing
200
What is an example of an exothermic reaction?
campfire
200
What is an example of an endothermic reaction?
Ice, Cold Pack
200
What does a positive change in enthalpy mean?
Energy is absorbed from the surroundings to the system
The products have higher energy than the reactants
More energy was used to break bonds than energy used to form bonds (Endothermic)
200
How many moles of aluminum oxide (Al₂O₃) are produced when 10.0 grams of aluminum react with excess oxygen?
4Al+3O2→2Al2O3
We are going from grams to moles, so this is a two step problem, molar mass of the given, and then the mole ratio.
10.0g Al x 1 Mol Al / 26.98g Al x 2 Mol Al2O3 / 4 Mol Al
Answer: 0.19 Moles Al2O3
200
How can you show me bonds are forming? Also which bonds in a reaction are forming? (model it & include appropriate labels)
Bonds from products
Bonds form when energy is released, so arrows drawn AWAY from the bonds to represent energy releasing
300
How does energy transfer? (What is the flow of heat?)
From the system to the surroundings
Energy is released to the surroundings.
300
How does energy transfer? (What is the flow of heat?)
From the surroundings to the system.
Energy is absorbed by the system from the surroundings
300
What does a negative change in enthalpy mean?
Energy is released from the system to the surroundings
The reactants have higher energy than the products
More energy is used to form bonds than energy used to break bonds. (Exothermic)
300
How many grams of sodium chloride (NaCl) are produced when 1.5 moles of chlorine gas (Cl₂) react with excess sodium?
2Na+Cl2→2NaCl
We are going from moles to grams, so this is a two step problem. Mole ratio, then molar mass of the unknown.
1.5 Moles Cl2 x 2 Mol NaCl / 1 Mol Cl2 x 58.44g NaCl / 1 Mol NaCl
Answer = 175.32g NaCl
400
Is the amount of energy USED higher for breaking bonds or forming bonds?
The amount of energy USED for forming bonds is higher
Key word, USED
400
Is the amount of energy USED higher for breaking bonds or forming bonds?
The amount of energy used for breaking bonds is higher than forming bonds.
Key word, USED
400
How do you solve for change in enthalpy?
Bond energies of products - Bond energies of reactants
400
How many grams of calcium chloride (CaCl₂) are produced when 20.0 grams of calcium react with excess chlorine gas?
Ca+Cl2→CaCl2
We are going from grams to grams, so this is a 3 step problem. Molar mass of the given (always first if you need to use two molar masses), mole ratio, then molar mass of the unknown.
20g Ca x 1 Mol Ca / 22.99g Ca x 1 Mol CaCl2 / 1 Mol Ca
x 93.89g CaCl2 / 1 Mol CaCl2Answer: 81.68g CaCl2
500
What does the energy diagram for an exothermic reaction look like? (Label all parts)
500
What does the energy diagram for an endothermic reaction look like? (Label all parts)
500
What does this symbol mean?
Delta enthalpy
Change in enthalpy
Change in energy between products & reactants
500
You discover there is 115g of carbon dioxide (C3H8) released when propane explodes. How many kJ of energy is released?
NH4Cl + 2H2O --> NH4+ + Cl- + 2H2O
𝚫H = 14.78 kJ / 1 mol H2O
C3H8 + 5O2 --> 3CO2 + 4H2O
𝚫H = -2220 kJ / 1 mol H2O
MgSO4 + 3H2O --> Mg2+ + SO42- + 3H2O
𝚫H =-91.20 kJ / 1 mol H2O
First decide which reaction you are going to look at based on the question. The question provides C3H8 as a molecule, so the third reaction will be looked at. This is a 3 step problem going from grams to kJ. First molar mass, mole ratio, then 𝚫H.
115g C3H8 x 1 mol C3H8 / 44.11g C3H8 x 4 mol H2O / 1 mol C3H8 x -2220kJ / 1 mol H2O
Answer: -23,151.21 kJ released