Unit 4 - FINAL REVIEW

First law of thermodynamics

total energy in the universe is constant(energy is conserved)

Energy is…

the capacity to do work or produce heat

E = q + w

stored PE is the result of position or arrangement(what happens to PE when there is less vs. more conflict)

• more “conflict” based on position → less stability → more PE

• less “conflict” based on position → more stability → less PE

Heat(q) is…

the flow of energy (PE+KE) as a result of temperature differences and is measured in joules (J)

An endothermic system ______ heat; what is the q of the system and q of surroundings?

GAINS; +q system = -q surroundings

An exothermic system ______ heat; what is the q of the system and q of surroundings?

LOSES; -q system = + q surroundings

Work is…

the energy to move against a force

Enthalpy(delta H) measures

the change of energy during a process at constant pressure

• depends on substance and change taking place(physical changes, chemical change, solution formation, etc.)

• typically measured in kj/mol

Second law of thermodynamics states

energy prefers to move in “favorable” direction (HIGH TO LOW)

Thermal equilibrium

when particles have the same kinetic energy

• having the same kinetic energy does not mean substances experienced the same delta T

adding energy to a system includes:

• warming = +deltaKE/+delta T

• phase change = +deltaPE/+deltaH

• Endothermic process

• feel cool to touch due to system absorbing energy from surroundings

removing energy from system includes:

• cooling = -deltaKE/-deltaT

• phase change = -deltaPE/-deltaH

• exothermic process

• feels hot to touch due to energy being released to surroundings

when is q=mcat used and what units does each variable

• when measuring energy transferred when heating or cooling a substance and there are NO PHASE CHANGES

• q = heat in J

• m = mass in g

• c = specific heat in J/g degrees C → reflects the energy needed to raise the temperature of 1g sample by 1 degree C

• delta T = change in temperature (T final - T initial = delta degrees C)

What is latent heat?

the energy needed to change phases; is unique to that substance due to differences in IMFs and particle organization

equation for latent heat(phase changes)

q = m * delta H

• m = mass in g

• delta H = enthalpy of phase change (J/g)

enthalpy of fusion =

melting or freezing → ± delta Hfus

enthalpy of vaporization =

vaporization or condensation → ± delta Hvap

What happens to PE, KE, and delta H when moving from solid, to liquid, to gas?

• +PE due to separation of particles and decrease in stability

• + KE due to increase of speed of particles due to separation

• + delta H fus when going from solid to liquid

• + delta H vap when going from liquid to gas

What happens to PE, KE, and delta H when moving from gas, to liquid, to solid?

• -PE as the particles are allowed to come back together based on IMFs which improves stability

• -KE because there is a decrease in the speed of the particles as they become closer together

• - delta Hvap when going from gas to liquid

• - delta Hfus when going from liquid to solid

Calorimetry:

measures energy transfer by observing the temperature change of water before and after a process occurs

• relies on 1st and 2nd laws of thermodynamics

How does energy flow when a sample placed in water is warmer than the water?

Energy flows from the sample to the water, warming it, as it will lose energy to reach equilibrium

How does energy flow when a sample placed in water is cooler than the water?

Energy flows from the water to the sample as the water will lose heat in order to attain thermal equilibrium

Heat of solution (delta H solution):

the energy change associated with making an aqueous solution with a specific solute

• separating solute and solvent particles requires energy (+PE)

• putting particles together in a favorable arrangement releases energy (-PE)

What does a +PE imply?

separation of particles, which requires energy(decreases stability)

What does -PE imply?

establishing favorable arrangements improve stability and releases energy

More stable bonds have…

more stored energy

When does an ideal position(bond length) occur?

when the attraction/repulsion forces become balanced

Details on Graph of bond lengths:

• minimum value represents the ideal distance between nuclei

• the “well” reflects all possible positions the bond can exist at (average distance bond length)

• atoms move more as temperature increases and strain the bond, causing the bond to break

delta H rxn in terms of bond breaking and making equation

sum of delta H breaking - sum of delta H making

• count the number of bonds that are broken and bonds that are formed(draw structures)

Enthalpy of formation (delta H f):

• defined to be the PE of bonded structure when 1 mole is formed from elements in their standard states

• Elements in their standard state are defined to have delta H f = 0 kj/mol

Delta H rxn in terms of products and reactants

sum of delta H products - sum of delta H reactants

Heat of neutralization ( delta H neut):

energy associated with acid/base reactions

Heat of Combustion (delta Hc):

energy associated with combustion reactions

What is Hess’s law used for?

determining delta H

• delta H is the sum of all the individual steps

• process requires manipulation of given stepwise reactions to reach the target reaction (multiply by a coefficient, reverse, etc.)