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Flashcards in Thermochemistry Deck (27)
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What is energy?

The capacity to do work


Where does energy come from?

Through photosynthesis, energy from the sun can be converted into chemical energy


What is a closed system?

Energy can come in and out of system


What is an open system?

Energy and mass can come in and out of system


What is an isolated system?

No energy or matter can leave or enter the system


What is calorimetry?

A method used to determine the change in enthalpy of a chemical system


How is energy transferred in a calorimeter?

The energy lost by the chemical system will be gained by the calorimeter

The energy gained by the chemical system will be lost by the calorimeter


What are the assumptions about calorimeters?

The system is totally isolated (no exchange of energy or matter)

Specific heat capacity of water and dilute solutions is: 4.19

The density of water and dilute solutions is 1.00 g/mL


What is kinetic energy?

Moving electrons within atoms

The vibration of atoms connected by chemical bonds


What is potential energy?

Covalent bonds

Ionic bonds

Intermolecular forces


What is enthalpy?

The total kinetic and potential energy in the chemical system


What does enthalpy change represent?

The difference between the enthalpy of the products and reactants


What is an endothermic reaction?

Absorb energy from the surroundings

Positive enthalpy value

Requires more energy to break bonds than to form bonds

Potential energy increased, Kinetic energy decreased


What is a exothermic reaction?

Releases energy to the surroundings

Negative enthalpy value

Requires more energy to form bonds than it does to break bonds

Potential energy is decreased, kinetic energy is increased

All combustion reactions are exothermic


What is Hess’s Law?

States that the addition of chemical equations yields a net chemical equation whose enthalpy change is the sum of the individual enthalpy changes


What are the steps for using Hess’s Law?

1. Write the net reaction equation (the “want” or “goal”)

2. Manipulate the given equations so they will yield the net equation

3. Multiply, divide, and/or reverse the sign of the enthalpy

4. Cancel and add the remaining reactant and products to yield the net reaction equation

5. Add enthalpies to determine the enthalpy of the net reaction


What is the reference energy state?

SATP (Standard Ambient Temperature and Pressure) is an internationally accepted set of conditions that define a standard state

At SATP the potential energy of elements is zero, this is the reference energy state


What is thermal stability?

The tendency of a compound to resist decomposition when heated

A more negative enthalpy is very exothermic meaning the product has a very low potential energy, which means it’s more stable


What is activation energy?

The energy barrier that must be overcome for a chemical reaction to occur


What is an activated complex?

The chemical entity containing the collided reactants in collision reaction theory


What is a catalyst?

A substance that increases the rate of chemical reaction without being consumed itself in the overall process

Reduces the quantity of energy required to start a reaction

Results in a greater yield in same time period

Does not alter the net enthalpy change of reaction


What is bond energy?

Bond breaking requires energy

Bond making releases energy


What are the four ways of predicting change in enthalpy?


Hess’s Law

Molar Enthalpies of Formation

Bond Energies


How does calorimetry predict the change in enthalpy?

The charge in enthalpy (H) is equal to the change in thermal energy (Q)


How does Hess’s Law predict the change in enthalpy?

The change in enthalpy equals the sum of component enthalpy changes


How do molar enthalpies of formation predict the change in enthalpy?

The change in enthalpy equals the enthalpies of formation for the products minus the enthalpies of formation of reactants


How do bond energies predict the change in enthalpy?

The charge in enthalpy equals the energy released from bond making minus the energy required for bond breaking