Chem Exam 3 Material

Lecture 18-24

What is thermodynamics?

The study of energy and its interconversions, which we can use to predict if a reaction can occur and is spontaneous

What does spontaneity depend on? What breaks the tie for conflicting terms?

1. Energy/Enthalpy (ΔH) - The heat released or gained by a reaction at constant pressure. We have exothermic and endothermic

2. Entropy (ΔS) - The measure of disorder (ordered to disorder)

   • Temperature breaks the tie

Describe what Δ E < 0 and ΔE >0 means in terms of system, surroundings, exothermic, and endothermic?

1. ΔE<0, exothermic, energy goes from system to surroundings, and is favored

2. ΔE>0, endothermic, energy enters the system from the surroundings

What is ΔH?

The heat released or gained by a reaction at constant pressure.

1. Exothermic (released)

2. Endothermic (gained)

What is the equation for thermodynamics?

ΔH = ΔE + Δ(PV)

• Where ΔPV = P2V2-P1V1

What is the difference between system, surroundings, and universe?

System: What we’re studying

Surroundings: Everything else

Universe: System + surroundings

What is internal energy (E)? Why do we use ΔE

Sum of potential and kinetic energy; where potential energy is due to position and kinetic energy is to do motion?

• It is too hard to determine E absolutely, so we use ΔE by using an equation

How many joules are in 1L/atm? How many J are in a KJ

101.3, 1KJ = 1000J

What equation can be used to describe ΔE? Describe the variables

ΔE = q+w

• q = heat flow due to temperature difference

• w = Work (force/distance)/-sum of Fdx

How can we determine sign conventions for thermodynamic quantities?

All quantities (ΔE, ΔH, q, w, etc) is positive if added to the system by surroundings, and negative if removed from the system to the surroundings

What is PV work?

Work associated with the expansion or compression of gases

• W=force over distance

How can you determine PV work? What if p is constant?

P = force per unit area (F/A) and w = -sum of Pdv

• If pressure is constant, work = -PΔV

What is expansion?

positive ΔN, and positive ΔV, negative work (system does work on the surroundings)

What is compression?

Δn<O, ΔV is negative, so work is positive (surroundings work on the system)

What is the only thing you count when considering compression or expansion?

Gases

What is the relationship between ΔV and work?

Inversely proportional

Describe the differences between path and state functions?

Path: Quantity which depends on how a system gets from its initial state to its final state (lowercase letters such as work and heat)

State: Quantity which is path independent (only depends on initial and final state, such as Δ not how)

What is the first law of thermodynamics?

Energy of the universe is constant/Whatever happens to the system, the opposite happens to the surroundings

• ΔE universe = O

• ΔSystem = -Δsurroundings

What is enthaply?

H = E + PV or ΔH = qp (where lowercase p means constant pressure)

Describe how heat and bonds change from a system to surroundings (exothermic) vs surroundings to system (endothermic)

Exothermic: Going from high potential energy to low (weak to strong bonds)

Endothermic: Going from low potential energy to high (strong to weak bonds)

What is the relationship between bond strength and potential energy?

Inversely related

What is the difference between extensive and intensive properties?

Extensive: A property that depends on the quantity of substance present. Mass, volume, enthalpy change, etc

Intensive: A property that does not depend on the quantity of substance present. Density, melting point, etc

Describe the components of ΔE = q + w

ΔE = Change in internal energy of the system

q = Energy exchanged between the system and the surroundings as heat

w = energy exchanged between the system and the surroundings as work

How can you determine the sign conventions for q, w, and ΔE

q > 0 = system absorbs heat

q < 0 = system evolves heat

w > 0 = surroundings does work on system

w < 0 = system does work on surroundings

What are the three possible equations for work?

w = -sumFdx, -sum of PdV, and, at constant pressure, w = -PΔV

In endothermic reactions, ΔH is positive or negative? Exothermic?

Endothermic: ΔH>0 (positive)

Exothermic: Δ<0 (negative)

How can you determine ΔV

Final volume - initial volume

Is enthalpy a state or path function?

State

What are the two quantities that make up thermodynamics?

1. A number (magnitude of change)

2. A sign (direction)

What are some common exothermic reactions?

1. Combustion

2. Freezing

3. Deposition (gas to solid)

4. Condensation (gas to liquid)

5. Precipitation

6. Fuck Da Connecticut County Police Department

What are some common endothermic reactions?

1. Melting

2. Evaporation

3. Sublimation (solid to gas)

What is calorimetry?

Science of measuring heat capacity (the amount of heat required to raise the temperature of a substance 1 degree celcius

• Can relate that qsys = -qsur or qsurr=-qsys

What do you have to assume when doing a calorimetry problem?

Assume that the heat capacity of the solution = heat capacity of water (4.184J/g degrees celsius)

Assume that qcalorimeter = 0

Worry about signs at the end

Exothermic reaction: T increases. Endothermic exn: Temperature decreases

ΔH is extensive so must convert qsys to ΔHsys using molar mass and balanced reaction

When temperature increases, is it exo or endothermic?

Exothermic

What is the difference between a coffee cup calorimeter and a bomb calorimeter?

Coffee cup calorimeters are run under constant pressure conditions where the heat is released at constant pressure (qp) is determined, and can be used to calculate ΔH

Bomb calorimeters are run under constant volume conditions where the heat released at constant volume (qv) is determined, and can be used to calculate ΔE

What is heat capacity, specific heat capacity, and molar heat capacity? What are their respective equations

Heat capacity: Amount of heat required to raise the temperature of a certain amount of substance 1 degree celsius

• q = J/mol*C times ΔT

Specific heat capacity: heat capacity per gram of substance (J/g C)

• q = s times mass times ΔT

Molar heat capacity: Heat capacity per mole of substance (J/mol C or J/mol K)

• J/mol*C times mol times ΔT

What is Hess’s Law? Why does it work?

If a reaction can be carried out in a series of steps, delta H for the reaction equals the sum of the enthalpy changes for all individual steps

• Works because enthalpy changes are state functions; they are path independent

• Hard puzzle problems

What is standard heat of formation?

ΔH _f o is the enthalpy change that accompanies the formation of 1 mol of a compound from its elements in their standard states (where ΔH _fo rxn= sum of ΔH _f (prod) - ΔH _f. (react)

What are bond energies?

Energy it takes to break or form a bond

What are the rules when doing a Hess’s Law problem?

1. When you reverse the reaction, change the sign of ΔH

2. When you multiply the equations in a balanced reaction by some number, you must also multiple ΔH by that number

What is the reaction for in terms of standard enthalpies of formation?

The reaction is for the production of 1 mol of that compound from its elements in their standard states (standard state = how we find that element at 25 degrees celsius and 1 atm).

What value does any element in their standard state have for their enthalpy of formation? Why?

The value is zero because its forming one mol of that substance from the elements that make up that substance in their standard state. So its the same initial and final states

The equation to calculate ΔH for a reaction is: ΔHrxn = ΔH prod - ΔH react, why does this work?

Because of Hess’s law

What is the heat gained/released at constant pressure equal to? (𝑞𝑃=?) What is the heat gained/released at constant volume equal to (𝑞𝑉=?)? Explain why Δ𝐻 is obtained directly from a coffee-cup calorimeter, whereas Δ𝐸 is obtained directly from a bomb calorimeter.

qP = Δ𝐻 and qv=ΔE a coffee-cup calorimeter is at constant (atmospheric) pressure. The heat released or gained at constant pressure is Δ𝐻. A bomb calorimeter is at constant volume, and the heat released or gained at constant volume is ΔE

What does D represent?

Dissociation energy = bond energy

Is endothermic breaking or forming bonds? What about exothermic?

Endothermic: Breaking bonds, positive, less stable

Exothermic: Forming bonds, negative, more stable

What is the relationship between bond energies and ΔH? What does it assume?

It gives a good estimate of ΔH (especially for gas reactions)

It assumes that enthalpy changes are due to energy differences between product bonds and reactant bonds

What is the process of reactants to atoms to products and if they break bonds and add energy

What are the main differences between exo and endothermic reactions?

Exothermic:

|ΔHform| > |ΔHbreak|

Product bonds are stronger than reactant bonds giving a negative ΔHvalue

Endothermic:

|ΔHbreak| > |ΔHform|

Reactant bonds are stronger than product bonds giving a positive ΔH value

Why are bond energies only an estimate?

1. They ignore intermolecular forces (which is why they work well for gases

2. They are average values

Be able to draw out a heating curve

okay

What equations are used for plateau’s and slopes?

Plateaus:

Use ΔH (kj/mol)

q = ΔH X mole

ΔHfusion = solid to liquid = 6.02kj/mol

ΔHvaporization = liquid to gas = 40.7kj/mol

Slopes:

Use specific heat capacity

q= s X m X ΔT

Ssolid = 2.03 J/g C

Sliquid = 4.184 K/g C

Sgas = 2.03 J/g C

What does each part of the heating curve for water represent?

-20 to 0: Heat is added and the heat is used to raise its temperature

Ice and water: Temperature doesn’t change because heat (energy) is used in transforming solid into liquid, instead of being used in raising its temperature

0-100: Heat is used to raise the temperature

Water and steam: Temperature doesn’t change because heat (energy) is used in transforming liquid into gas instead of being used in raising its temperature

Steam: Heat is used to raise the temperature

What is the equation for bond energies?

ΔHrxn = | ΔHBonds broken (reactants/put in energy) | - ΔH form bonds (products/release energy)

True or false, when water boils, the system does PV work on the surroundings?

True

True or false, compounds with strong bonds have low potential energy?

True

True or false, In an endothermic reaction, the potential energy increases as reactants are converted to products?

True

True or false, when two atoms come together to form a bond, this is an exothermic process?

True

When you are asked to find the standard enthalpy of something and are given the standard heats of formation, what equations do you use?

Sum of the products - sum of the reactants (multiplied by the coefficients)

What is the true endpoint of a reaction? Why?

Not completion; equilibrium. Because both the forward and reverse reaction can and do occur

What is equilibrium?

When the rate of two opposing processes (forward and reverse reactions) are equal to each other (we have some of everything at the end, so there is no net change in the concentration of reactants and products)

When does the rate of concentration end?

At equilibrium

What does dynamic mean?

The reaction is still occurring on the molecular level

What is equilibrium position?

Refers to the relative amounts of reactants and products at equilibrium, each reaction has its own specific equilibrum position.

What is the equilibrium constant K?

A number that gives a measure of the extent that the forward and reverse reactions occur. K value is constant for a specific reaction at a specific temperature, with K= [products]^x/[reactants]^y, with [] signifying molarity

When K is greater than 1, less than 1, or equal to 1

How can you solve for Δn?

Moles gaseous products - moles gaseous reactants

What is the equation to solve for Kp? (partial pressure)

Kp = K(RT)^Δn

When does K=Kp?

When ΔN is 0

What phases do not affect equilibrium? Which ones appear in the K expression?

Solids and liquids do not affect equilibrium, so only gases and solutes appear in a k expression

What are the rules for manipulating reactions and K values?

When the reaction is reverse, it becomes 1/original K

When the coefficients are multiplied by some number, K becomes raised to that power

What is the reaction quotient (Q)?

It has the same expression as K but initial concentrations are used to calculate Q instead of equilibrium concentrations

What does Q being equal to, less than, or greater than K mean?

What does it mean when K « 1? What can you use if so?

Not a lot of products are present at equilibrium, so 5% rule

How can you check for the 5% rule?

Divide the x you calculated by the original value and multiply by 100.

What is LeChatelier’s Principle?

When a reaction at equilibrium is stressed (changed somehow) the reaction responds to relieve the stress and return the reaction back to equilibrium. The typical stresses include:

1. Changing one of the reactants or product concentrations

2. Changing the volume of the reaction container

3. Changing the temperature

What is the only effect on K?

Temperature, there is no effect if its constant

What is the (exothermic) response of the stressors of

1. adding a reactant

2. removing a product

3. increasing pressure

4. decreasing volume

5. increasing volume

6. increasing temperature

7. decreasing temperature

1. Shift right

2. Shift right

3. Nothing

4. Shift to side with fewer moles of gas

5. Shift left, side with more moles of gas

6. Shift left, K decreases

7. Shift right, K increases

When given a LaChatleir problem, what is the first step?

Write in heat depending on if its endo (reactant) or exothermic (product)

What is the relationship between K and temperature?

Endothermic: Proportional

Exothermic: Inversely proportional

True or false, all solids dissolve to some extent?

True, insoluble salts dissolve a small amount

What is ksp?

The solubility product constant, used to calculate the solubility of an ionic compound

When you’re comparing molar solubilities (ksp), what do you do?

If they break up into the same number of ions, you can simply compare the molar solubility (whats given). If they don’t have the same ions, you must calculate the ksp value

What does a higher ksp value signify in terms of solubility?

Higher solubility

What is Q called in Ksp problems?

Ion product, but its the same

How do you know if a precipitate forms when comparing Ksp?

When you double the volume, what happens to the concentration?

It halves

When does q = ΔH

When pressure is constant

Can specific heat be negative?

no

What is heat capacity ( c) ?

The heat absorbed/increase in temperature in Kj. So, usually, heat capacity times temp