Exam 3

Question 1

When do you use “Products - Reactants”?

Answer 1

Any of these three cases:
When you have a table of delta H standard formation values.
When you have a table of delta G standard formation values.
When you have a table of standard S values.

Question 2

Enthalpy has what symbol?

Answer 2

H

Question 3

Entropy has what symbol?

Answer 3

S

Question 4

Gibbs energy has what symbol?

Answer 4

G

Question 5

What variable relates to heat?

Answer 5

It is actually q (lower case!), but for this class that’s the same as delta H.

Question 6

What variable relates to randomness?

Answer 6

S

Question 7

What must be true if given the current conditions a reaction will shift to form products?

Answer 7

Q < K and delta G < 0

Question 8

What must be true if given the current conditions, a reaction will shift to form reactants?

Answer 8

Q > K and delta G > 0

Question 9

What must be true when a reaction is at equilibrium?

Answer 9

Q = K and delta G = 0

Question 10

What must be true if a reaction generally favors products?

Answer 10

K > 1 and delta G standard < 0

Question 11

What must be true if a reaction generally favors reactants

Answer 11

K < 1 and delta G standard > 0

Question 12

When should you use “Reactants - Products”

Answer 12

When you are given a table of bond enthalpies. It’s the problems where you need to draw Lewis structures.

Question 13

What is the freezing point and boiling point of water?

Answer 13

Freezing: 0 degrees Celsius. Boiling 100 degrees Celsius

Question 14

You are doing a calorimetry problem where one object is changing its temperature. What equation do you start with?

Answer 14

Delta H = m C (Tf - Ti)

Question 15

You are doing a calorimetry problem where two substances are changing their temperature. What equation do you start with?

Answer 15

m C (Tf - Ti) + m C (Tf - Ti) = 0

Question 16

How do you find out which chemical is the limiting reagent?

Answer 16

Convert everything to moles and then use the mole to mole ratio in the balanced reaction to figure out which reactant makes fewer moles of product.

Question 17

You are doing a calorimetry problem where a chemical reaction is performed in water. What equation(s) do you use?

Answer 17

Delta H process + m C (Tf - Ti) = 0
and also use “the ratio”

Question 18

You are doing a calorimetry problem where a chemical reaction is performed in a calorimeter and that calorimeter is put in water. What equation(s) do you use?

Answer 18

Ccal (Tf - Ti) + Delta H process + m C (Tf - Ti) = 0
and also use “the ratio”

Question 19

When it comes to units, what two common mistakes should you be on the lookout for in your work?

Answer 19

Make sure you don’t use kJ and J in a single equation.
Make sure all temperatures are in Kelvin (temperature CHANGES can stay in Celsius)

Question 20

When do you use “Products OVER Reactants”

Answer 20

When you are calculating Q or you are calculating K from current pressures or molarities.
(Don’t forget your exponents, and watch out for solids and liquids)

Question 21

For an entropy dominated reaction, raising the temperature will shift the reaction to which side?

Answer 21

It will shift towards the more random side
The side with more gas. Or if there is no gas at all, then the side with more liquid/aqueous)

Question 22

When you flip a reaction, what do you do to its delta H value?

Answer 22

Flip the sign. This also what you do for its delta S value and its delta G value.

Question 23

When you multiply a reaction by a constant, what do you do to its delta H value?

Answer 23

You multiply the delta H by that constant. This also what you do for its delta S value and its delta G value.

Question 24

When you sum up reactions, what do you do with their delta H values.

Answer 24

You also sum up their delta H values. This also what you do for its delta S value and its delta G value.

Question 25

How do you go about working an ice, water, steam problem?

Answer 25

Break it up into steps. Some steps will be mc(Tf - Ti) steps. Some steps will be ratio steps with the delta H fusion or delta H vaporization.

Question 26

What are the three laws of thermodynamcis?

Answer 26

1) Energy cannot be created or destroyed
2) For a process to occur the total entropy of the universe must increase or remain the same
3) The entropy of a perfect crystal of a pure substance at absolute zero is defined as zero.

Question 27

Which value of R are you going to use the most on this test?

Answer 27

8.314 J / (mol K)
(Only use the other 0.08206 L atm / (mol K) one if you are using PV = nRT)

Question 28

If the temperature went up by 26 deg C, how much did it go up by in Kelvin?

Answer 28

26 K

Question 29

What constant do you need to calculate the enthalpy change associated with melting?

Answer 29

You’ll need to set up a ratio with the delta H fusion (which for H2O is 6.02 kJ/mol)

Question 30

What constant do you need to calculate the enthalpy change associated with boiling?

Answer 30

You need to set up a ratio with the delta H vaporization (which for H2O is 40.7 kJ/mo)

Question 31

You have delta H of some particular process. But the question asks how much heat was given off. What do you do?

Answer 31

No no no, “cry and curse your evil professor” is NOT the correct answer! The correct answer is to simply “flip the sign” of your delta H.

Question 32

The question wants you to solve for an equilibrium constant, but doesn’t give you any molarities or pressures. What equation do you use?

Answer 32

Delta G standard = - R T Ln (K)