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Energy
Ability to do work, which is measured in Joules
Potential energy
Relation to position, being vital for bonds that change in chemical rxns, and hence changing attractions which change in phase changes
Kinetic energy
Related to motion, changes when the temperature changes.
Heat
Flow of energy from hot to cold objects until they reach the same temperature, which is thermal equilirbrium
Temperature
AVERAGE kinetic energy of the particles
Thermal energy
TOTAL kinetic energy of the particles
Endothermic
Energy enters a system. Energy gained will make particles speed up or break/weaken attractions and bonds
Exothermic
Energy exits a system. Energy lost can make particles slow down or form/strengthen bonds
Specific heat capicity
The amount of energy needed to change the temperature.
Materials with high specific heats will…
Heat slowly and cool slowly
Equation for determining heat energy
q=mc Delta T
What can the energy gained or lost do in a physical change?
Change temperature which is the speed, or the phase which changes the position of the particles
Change in enthalpy(Delta H)
Heat absorved or released during the process at constant pressure, applies to phase and chemical changes.
Remember, as attractions break, energy is…
Absorbed, which accounts for a positive enthalpy(Endothermic)
Remember, as attractions form, energy is…
Released, which can account for a negative enthalpy(Exothermic)
Phase changes involving a solid always class the Delta H as…
Fusion(Melting, Freezing)
Phase changes involving a gas always class the Delta H as…
Vaporization(Condense, boil)
Equation to calculate energy during a phase change
q=n delta H, note that N MUST BE IN MOLES
Heat/cool curve main points
Involving both temp and phase changes, each step must be calculated seperatly. All energies must be in KJ. Flat lines indicate a phase change, and on a heating curve that does not mean speed up
Delta H Rnx
The difference between the input and output
If energy required to break bonds is greater than the energy released when forming bonds, Delta H Rnx is…
Positive and endothermic
If the energy required to break bonds is less than the energy released when forming bonds, Delta H Rnx is…
Negative and exothermic
Endothermic vs Exothermic graph
Calculate Delta H rnx with bond energy
Identify the bonds:
Analyze the balanced chemical equation to identify all the bonds present in the reactants and products.
Look up bond energies:
Use a table of average bond energies to find the energy value for each type of bond involved in the reaction.
Calculate the energy of bonds broken:
Multiply the number of each type of bond broken in the reactants by its corresponding bond energy and add them together.
Calculate the energy of bonds formed:
Multiply the number of each type of bond formed in the products by its corresponding bond energy and add them together.
Calculate delta H:
Subtract the energy of bonds formed from the energy of bonds broken: ΔH = (Energy of bonds broken) - (Energy of bonds formed).
Calorimetry
Determining the heat of a reaction by transferring the energy released to a sample of water.
Calorimetry formula
RXN———>H20, so -Q RNX=Q H20
Reactants___, Products ____
Enter(+), and exit(-)
Change in enthalpy for reactions. What does it say?
It is the Hproducts-Hreactants, so how we go from products to reactants is not needed to be known.
Hess’s law
For any reaction that can be written in steps, the sum of standard heats of them is the same as the standard heat of rnx of the OVERALL reaction. This is useful for determining the heat of a reaction that is difficult to measure directly
How do do Hess’s law
Given the steps, you look at the overrall reaction and match it by altering the steps. You can either flip or divide/multiply sides to cancel certain things out or get desired values.
Reversing a reaction
Flipping the reaction from both sides of the arrow will require you to also flip the sign of the Delta H
Multiplying a reaction
When you double or cut in half the number values of a reaction(front ones) you must do the same thing to the Delta H
Hess’s law: Opposing sides___ and same sides____
Cancel, add up
