Gen Chem 3 Exam 3

Question 1

In a calorimetry problem with one object changing its temperature, what equation should you start with?

Answer 1

Delta H = m C (Delta T)

Question 2

In a calorimetry problem with a hot substance touching a cold substance, what equation should you start with?

Answer 2

m C (Delta T) + m C (Delta T) = 0

Question 3

In a calorimetry problem with a reaction done in water, what equation(s) should you start with?

Answer 3

(Delta H Process) + m C (Delta T) = 0

You will also need “the ratio”

Question 4

In a calorimetry problem with a reaction done in a calorimeter in water, what equation(s) should you start with?

Answer 4

Ccal (Delta T) + (Delta H Process) + m C (Delta T) = 0

You will also need “the ratio”

If details about the water are not provided in the problem, you should leave out the m C (Delta T) term, because in that case the water has already been accounted for by the Ccal constant.

Question 5

In an ice, water, steam problem what is the first thing you should think about?

Answer 5

Plan out how many steps the problem has. Don’t forget the phase change steps.

Question 6

The temperature of something went up by 15 degrees Celsius. What is the temperature change in Kelvin?

Answer 6

Still 15 Kelvin. A temperature change in Celsius is the same as a temperature change in Kelvin

Question 7

Name 3 very simple unit-related carless mistakes that you should watch out for.

Answer 7

Beware if a problem has kilojoules one place and Joules in another place.

Remember to use the 8.314 J / (mol K) version of R, not the 0.08206 (L atm) / (mol K) version of R for all energy calculations.

Remember that temperatures should be in Kelvin. (Temperature CHANGES are fine to keep in Celsius)

Question 8

When should you use “Products - Reactants?”

Answer 8

Whenever you are presented a table of:
Standard Delta H formation values
Standard Delta G formation values
or
Standard S values

Question 9

When should you use “Reactants - Products?”

Answer 9

When you are given a table of BOND enthalpies (or bond energies)

Question 10

A combustion reaction has what chemicals in it?

Answer 10

Chemical of Interest + Diatomic Oxygen –> Carbon Dioxide + Water

Question 11

When you flip a reaction, how does it change the reaction’s Delta H value?

Answer 11

It switches the sign of the delta H value.

The same is true for a reaction’s Delta G value and for its Delta S value

Question 12

When you multiply an entire reaction by a constant, how does its Delta H value change?

Answer 12

The Delta H value gets multiplied by that constant.

The same is true for a reaction’s Delta G value and for its Delta S value

Question 13

When I refer to “the ratio” in calorimetry problems, what is that formula

Answer 13

Delta H of Written Reaction / Limiting Reagent Coefficient = Delta H of Process / Moles of Limiting Reagent Actually used

You ARE allowed to switch both denominators to be in terms of grams instead of moles, but you got to switch both of them not just one.

Question 14

How can you look at a balanced reaction and determine the sign of delta S for the reaction?

Answer 14

Count the gasses. If it has more gas products than gas reactants, then delta S is positive. Ignore the things that are not gasses.

If there are no gasses at all present, then count the liquids/aqueous chemicals. If the products side has more of those, then delta S is positive.

Question 15

For an entropy-dominated reaction, and increase in temperature will shift the reaction which way?

Answer 15

It will shift it to the more random side (the side with more gas.)

Question 16

A positive value of delta G standard for a reaction means what?

Answer 16

This reaction favors reactants. The equilibrium location lies towards the reactants side.

Question 17

A negative value of delta G standard for a reaction means what?

Answer 17

This reaction favors products. The equilibrium location lies towards the products side.

Question 18

A positive value of delta G for a reaction means what?

Answer 18

Given the current conditions, this reaction needs to shift to form reactants in order to reach equilibrium. The reaction is not spontaneous as written under these conditions.

Question 19

A negative value of delta G for a reaction means what?

Answer 19

Given the current conditions, this reaction needs to shift to form products in order to reach equilibrium. The reaction is spontaneous as written, given the current conditions.

Question 20

You are given CURRENT pressures (or molarities) of various chemicals. What should you be looking for on your equations sheet?

Answer 20

You will be looking for an equation that has “Q” in it, because that is Products / Reactants with current amounts. You will be looking for the equation:
Delta G = Delta G Standard + RT Ln (Q)

Question 21

You are given EQUILIBRIUM pressures (or molarities) of various chemicals. What should you be looking for on your equation sheet?

Answer 21

You will be looking for an equation that has “K” in it, because that is Products / Reactants at equilibrium. You will be looking for the equation:
Delta G Standard = - R T Ln( K )

Question 22

If I give you amounts of two different reactants in a chemical reaction, how do you go about finding which one is the limiting reactant?

Answer 22

Pick a comparison chemical (it’s easiest to understand if you choose a product chemical) Convert your initial amounts of each chemical to moles, and then use the mole-to-mole ratio to find out how many moles of product you could make with each chemical. Whichever one made less of the comparison chemical is the limiting reactant.

Question 23

When carbon is in a Lewis structure, and has a formal charge of zero, how many bonds does it have?

Answer 23

It either has:
A triple bond and a single bond
A double bond and two single bonds
Four single bonds.

Question 24

When a halogen is in a Lewis structure, and it has a formal charge of zero, how many bonds does it have?

Answer 24

It has one single bond

Question 25

When hydrogen is in a Lewis structure, and it has a formal charge of zero, how many bonds does it have?

Answer 25

One single bond.

Question 26

When oxygen is in a Lewis structure, and it has a formal charge of zero, how many bonds does it have?

Answer 26

It either has:
Two single bonds or
One double bond

Question 27

When nitrogen is in a Lewis structure, and it has a formal charge of zero, how many bonds does it have?

Answer 27

It either has
A triple bond
A double bond and a single bond
Three single bonds

Question 28

If you know the delta H of several reactions, and you sum up the reactions, what should you do with their delta H values?

Answer 28

Sum up their delta H values.

The same is true for the delta G values of reactions and for delta S values of reactions.

Question 29

Let’s say you know the math rules for flipping, multiplying, and summing up reactions. But what are some STRATEGY tips for getting things in the right place?

Answer 29

(1) Only focus on one chemical at a time. Get the correct of that chemical in the correct place. Then move on to a different chemical.

(2) Skip over difficult chemicals that show up in multiple places. Hopefully they will work themselves out.

(3) Once you are done manipulating one of your given reactions, cross it off the list and don’t use it again.

(4) If you have too MUCH of one chemical somewhere, you can cancel out some of it if you can find a way to get that same chemical on the OTHER side of the reaction as well.

(5) If all else fails, instead of focusing on the chemicals you NEED, try focusing on canceling out the chemicals you DON’T need.

Question 30

When setting up a Q expression or a K expression, what things should you keep in mind in order to avoid careless mistakes?

Answer 30

Use the correct unit (molarity of Kc and Qc pressures for Kp and Qp )

It’s Products / Reactants, not Reactants / Products

Don’t forget to turn coefficients from the reaction into exponents

Don’t include solids or liquids.

There are no plus signs in these formulas. Everything gets multiplied or divided.

Question 31

In a calorimetry problem, how do you know when to set things equal to zero or when not to?

Answer 31

If you just have ONE substance, don’t set it equal to zero.

Every other time in a calorimetry problem, you should be summing everything up and setting it equal to zero.
Failure to do it this way can often result in the “Finding Nemo” forgotten negative sign.

Question 32

What is the density of water?

Answer 32

In reality it varies a tiny bit, but for this class I told you to always assume it is 1 g / mL

Question 33

What is the freezing/melting temperature of water, and what is the boiling/condensation temperature?

Answer 33

Water melts/freezes at zero degrees Celsius

Water boils/condenses at 100 degrees Celsius

Question 34

What constant should you be using when melting water?

Answer 34

You should be using the delta H fusion in your “ratio” calculation

Question 35

What constant should you be using when water is switching from a liquid to a gas?

Answer 35

You should be using the delta H vaporization in your “ratio” calculation. I’ll give you this value on the test, but it is 40.7 kJ/mol