Module 13

Question 1

In a chemical reaction, where is the potential energy? Where is the kinetic energy?

Answer 1

The potential energy is in the bonds of the molecules in the reaction. Kinetic energy is the heat that is either absorbed or released in the reaction.

Question 2

If a chemical reaction has a positive AH, will the beaker that contains the reaction feel hot or cold once the reaction is finished?

Answer 2

Cold

Question 3

Why is Hess’s law a more exact way of determining AH than the technique that uses bond energies?

Answer 3

It is more exact because it also takes into account the phases of each substance in the reaction.

Question 4

What is a state function? Give 2 examples

Answer 4

It is a function whose final value is independent of it’s path. Enthalpy and Gibbs free energy

Question 5

Which of the following substances will have a AHf* of 0?

NaOH (aq) Na+ (aq) O2 (g) O (g) Cl2 (g) H2 (l)

Answer 5

O2 (g) Cl2 (g)

Question 6

1) What is a defining property of an endothermic graph?
2) What is a defining property of an exothermic graph?
3) How do you figure out the AH of a reaction from a graph?
4) What do you draw to signify a large activation energy?
5) What do you draw to signify a low activation energy?

Answer 6

1) For an endothermic graph the second half of the graph (the product) is higher than the first half (the reactant)
2) For an exothermic graph the second half of the graph is lower than the first half.
3) You minus the number value of the reactant line from the number value of the product line (# P - # R).
4) A large hump,
5) A small hump

Question 7

If you have 2 50.0 kg blocks of copper, and one is at a temperature of 50C while the other is at a temperature of 25C, which has more entropy?

Answer 7

The warmer block has the higher entropy

Question 8

If a chemical reaction is exothermic but has a negative AS, what could you do to the temperature to make it possible to run the reaction?

Answer 8

Lower the temperature to make the TAS smaller.

Question 9

Hess’s Law:

Answer 9

Enthalpy is a state function

Question 10

The Enthalpy of formation is

Answer 10

(🔺Hf) The 🔺H of a formation reaction.

Question 11

The equation of Hess’s Law:

Answer 11

🔺H* = (sum of) 🔺Hf* (products) - (sum of) 🔺Hf* (reactants)

Question 12

What is significant of the standard enthalpy of formation of an element.

Answer 12

The standard enthalpy of formation of an element in its ELEMENTAL FORM is 0.

Question 13

What is energy considered in an exothermic reaction?

What is energy considered in an endothermic reaction?

Answer 13

exo - energy is considered a product

endo - energy is considered a reactant

Question 14

What is the Change in Enthalpy

Answer 14

(AH) The energy change that accompanies a chemical reaction

Question 15

What is the AH sign for exothermic and endothermic reactions? What happens to the Heat?

Answer 15

endo = + Heat is absorbed
exo = - Heat is released

Question 16

What is Standard conditions?

Answer 16

It is the temperature and pressure that most chemical reactions or done in. Room temperature (25*C) and Normal atmospheres (1.00 atm)

Question 17

What is an energy diagram? And what does it convey? What is its X and Y names?

Answer 17

It is a picture of a chemical reaction. The AH and the activation energy. (the intermediate state is at the top of the hump aka activation energy) x- reaction coordinate y- Energy (kJ)

Question 18

What is activation energy?

Answer 18

It is the energy necessary to start a chemical reaction.

Question 19

What is Entropy?

Answer 19

It is a measure of the disorder that exists in any system

Question 20

The Second Law of Thermodynamics:

Answer 20

The entropy of the universe must always either increase or remain the same. It can never decrease.
(AS universe >= 0)

Question 21

Entropy and phases. Entropy and Temperature. Entropy and matter.

Answer 21

It increases from solids to gases. It increases with increasing temperature. Entropy increases with increasing matter.

Question 22

What has absolute entropy?

Answer 22

All substances even elements

Question 23

Gibbs free energy equation is

Answer 23

AG = AH - T x AS

Question 24

When is a reaction spontaneous. What does this mean?

Answer 24

When AG < 0. It is consistent with the second law

Question 25

1) When there is more entropy on the reactant side the sign is
2) When AH is negative it goes on the _______ side
3) When there is more entropy on the product side the sign is
4) When the AH is positive it goes on the ______ side

Answer 25

1) Negative
2) Product
3) Positive PRODUCT POSITIVE
4) Reactant

Question 26

How do you find the activation energy?

Answer 26

The intermediate point - the reactants

Question 27

AH stands for:

a. The potential energy associated with a substance
b. the enthalpy of a molecule
c. The potential energy change associated with a reaction
d. None of the above

Answer 27

C. the potential energy change associated with a reaction.