Thermochemistry Flashcards
When is the Q=mcT equation used?
Q-mcT us used to calculate heat transferred in a process where there is no phase change
Where is a vast majority of all the energy within Earth’s biosphere derived from?
Solar energy
How does the sun produce energy?
Hydrogen atoms aret converted into helium atoms in a highly exothermic nuclear fusion reaction
How do plants and animals store chemical energy?
In the molecular bonds of a number of organic compounds such as starches, sugars, and carbohydrates.
What does thermal energy refer to?
The TOTAL kinetic energy of the molecules
What happens to energy when a bonds form and break?
Bonds breaking: Energy required
Bonds forming: Energy released
Chemical reactions will have a change in energy because of the energy changes that occur when __________________.
old bonds are broken and new bond form
What is temperature?
The AVERAGE kinetic energy of the molecules in the sample
What is heat?
the transfer of kinetic energy from one object to another
What is enthalpy?
The change in potential energy between the reactants and products at a constant pressure. It is equal to the heat absorbed or released by the system.
What is molar enthalpy?
Molar enthalpy is the enthalpy per mole of a specified substance in the reaction equation.
Will exothermic and endothermic reactions have positive or negative enthalpies?
Endothermic: positive
Exothermic: Negative
If the coefficients of the reaction change, the enthalpies of the reaction change _________.
by the same factor
Photosynthesis is an __________(1) process. Cellular respiration is an ______________(2) process. Hydrocarbon combustion is an _____________(3) process.
(1) Endothermic
(2) Exothermic
(3) Exothermic
What is an activated complex?
The activated complex is an unstable intermediate in a transition state between reactants and products. The peak of the energy diagram
When more bond are being formed than broken, an _________ reaction is occurring.
exothermic
When more bonds are being broken than formed, an _________ reaction is occurring.
endothermic
What is collision theory?
To react, molecules must collide with the correct orientation and enough energy to overcome the activation energy barrier.
In an ___________ reaction, the products contain more potential energy in their bonds than the reactants
endothermic
In an __________ reaction, the products contain less potential energy in their bonds than the reactants.
exothermic
What is a catalyst?
Catalysts cause the rates of reactions to increase. They do NOT affect the value of the enthalpy change.
Catalysts lower the activation energy by changing the reaction pathway. They lower both the reverse and forward activation energy. They also increase the probability of successful collisions.
What are the 4 ways to increase the rate of reaction?
- Increase temperature
- Increase pressure
- Increase concentration
- Add a catalyst
How does increasing the temperature cause an increase in the rate of a reaction?
The number of collisions increases and the energies of the collisions have more energy to overcome the activation energy
Does a change in temperature affect endothermic or exothermic reactions more?
Since endothermic reactions require a greater amount of activation energy, an increase in temperature affects endothermic reactions more than exothermic reactions
How does increasing the pressure of gases increase the rate of reaction?
Reducing the volume results in an increase in the rate of reaction by increasing the number of reactions(doesn’t change orientation or activation energy)
ONLY AFFECTS GASES IN A REACTION!!
What impact does adding an inert gas have on the rate of reaction?
An inert gas is a non-reactive gas so it has no effect on rate of reaction.
How does increasing concentration increase the rate of reaction?
Adding more reactant into the bottle leads to more molecules causing more collisions.
What is the first law of thermodynamics?
Energy can not be created or destroyed, only transformed
If a reactions potential energy energy increases, the surrounding kinetic energy __________(1). If a reactions potential energy ___________(2), the surrounding kinetic increases.
(1) decreases
(2) decreases
What does a system refer to?
- The tiny bit of matter that is being studied
- Can be a specific reactant/product/reaction
What is an open system?
Matter and energy can transfer, like a can of pop
What is a closed system?
Only energy can move, matter can not, like a closed can of pop
What is an isolated system?
No matter and no energy is transferred
In chemistry, we assume that systems are isolated, although this is actually impossible to fully achieve
What type of system is a beaker of liquid?
- Over a short period of time, a beaker of liquid can act like a closed system(matter lost is insignificant)
- Over a long period of time, it be comes an open system
What do surroundings refer to?
Everything in the universe that is not the system
An endothermic reaction feels ______(1) as energy is __________________(2) and going into the ________________(3) of the reaction. The change in the chemical potential of the reaction is ______________(4).
(1) cold
(2) being taken from the surroundings
(3) chemical potential energy
(4) postive/increasing
An exothermic reaction feels ______(1) as thermal energy is __________________(2). The change in the chemical potential of the reaction is ______________(3).
(1) hot
(2) transferred to the surroundings
(3) negative/decreases
What is calorimetry?
The process of measuring heat change in a chemical reaction
What is an assumption with calorimetry?
The system is isolated
How do you calculate percent efficiency?
(experimental)/(theoretical) x 100%
How do you calculate molar enthalpy?
(delta)H = -Q(cal)/n(limiting)
The amount of ____________(1) the water gained or lost is ___________________(2) to the amount of energy the reaction ______________(3).
(1)energy(heat)
(2) equal and opposite
(3) gained/lost
(delta)kinetic energy of water = -(delta)H run
In the formula Q=mcT, mass is measured in _________(1).
grams
What is specific heat capacity?
How much energy it takes to heat 1g of substance to increase by 1 degree celsius.
If the calorimeter is not made of ____________(1), it’s heat should __________(2).
(1) polystyrene
(2) not be ignored
What are the four different ways to communicate enthalpy changes?
- As molar enthalpy
- As a (delta)H term at the end of a balanced reaction
- Write it as an energy term in the balanced reaction (reactant if endothermic, product if exothermic)
- As a potential energy diagram
What does the potential energy diagram look like for an endothermic reaction?
up to down
What does the potential energy diagram look like for an exothermic reaction?
up to down
What are the labels on the x and y axis for a potential energy diagram?
x axis: reaction coordinate(no units)
y axis: potential energy(kJ)
What is modified Hess’ Law?
Refer to notebook
While increasing the pressure of gases in a system leads to more collisions with the correct orientation, it doesn’t actually change the ____________ of collisions.
proportion
In comparison to exothermic reactions, endothermic reactions require ___________(1) to create an activated complex.
(1) more kinetic energy
