Thermochemistry Flashcards
Thermochemistry:
The study of heat changes associated with physical, chemical and nuclear processes.
Potential Energy:
Stored energy.
Kinetic Energy:
The energy of motion.
Exothermic Processes:
Energy is released to its surroundings. It is mathematically negative.
Endothermic Processes:
Energy is absorbed from its surroundings. It is mathematically positive.
If energy is a reactant it is ___________.
endothermic
If energy is a product it is __________.
exothermic
Photosynthesis is (endo/exo)thermic.
endo
Cellular respiration is (endo/exo)thermic
exo
What is total enthalpy affected by?
Molecular structure of the substance and the mass of the substance
What are the units for enthalpy?
kJ
What are the units for molar enthalpy?
kJ/mol
Describe the endothermic chemical reaction.
Bonds of the reactants are broken and energy is required.
Describe the exothermic chemical reaction.
Bonds of the products are reformed and energy is released.
Does an exothermic visual diagram start high or low?
High.
Does an endothermic visual diagram start high or low?
Low.
What is the bump in a reaction diagram?
Activation energy.
What is necessary for a reaction diagram?
Title, labelled axis’, enthalpy change, reactants and products, units.
What three things affect kinetic energy?
Temperature, amount, and type of substance.
How do you calculate Ek?
mc(change in)t
How do you calculate molar enthalpy?
Ep/n
What are the units for specific heat capacity?
J/gC
Specific heat capacity:
The amount of heat energy needed to raise the temperature of 1 g of a substance 1C/1K
Which has a higher heat capacity, metal or rubber?
Rubber
When Ek is negative, a substance is _______ ____.
cooling down
When Ek is positive, a substance is _______ ____.
heating up
How do you do calorimetry with a styrofoam calorimeter?
Calculate Ek, multiply by -1 to get Ep, Ep/n
What has a 1:1 ratio with 1g?
1 mL
How do you do calorimetry with a metal can calorimeter?
Ek = mct(water) + mct(can), * -1 = Ep, Ep/n
When in a combustion reaction is water a liquid?
In a bomb calorimeter or a living system ie. human urine
When in a combustion reaction is water gas?
In an open environment ie. boiling a pot of water
What are the Molar Enthalpy of Formation values derived from?
Bomb calorimeter data.
Commonly, molar enthalpy of formation reactions are (end/ex)othermic.
end
How do you find the molar enthalpy of a decomposition?
Flip the sign from the data booklet.
What is the molar enthalpy of formation for elements assumed to be?
Zero.
When Ep is released in formation, what is the result?
A more thermally stable compound.
Which is more thermally stable and why?
SO3(l) H = -441.0 kJ
SO3(g) H = -395.7 kJ
The liquid one because more Ep was released.
Catalysts:
Can be used to speed up reactions that are exothermic but react slowly. They reduce the activation energy required to move the reaction forward. (They provide an alternate reaction path.)
When a catalyst is added, does the Molar Enthalpy of Formation/Decomposition value change?
No.
Which compound is more thermally stable, glucose or sucrose?
Sucrose.
Basic rules to apply to Hess’s law:
- Balanced equation with molar enthalpy
- Decompose one mole of each reactant and change the enthalpy sign. Multiply enthalpy by coefficients
- Form the correct number of moles for the products and multiply by coefficients
- Ignore elements as they will typically sort themselves out
- Add the equations, and they should add up to the original equation.
How do you use Hess’s Law Shortcut?
- Write a balanced equation for which H is needed for one mol
- Add all the H values of the compounds on the products side
- Add all the H values on the reactant side
- Multiply H values by coefficients if needed
- Subtract the sum of the reactants from the sum of the products
