unit 9

Question 1

What does ΔH stand for?

Answer 1

change in enthalpy

Question 2

What does it mean if ΔH is negative?

Answer 2

reaction is exothermic; less energy than the start

Question 3

What does it mean if ΔH is positive?

Answer 3

reaction in endothermic; more energy than the start

Question 4

Are all combustion reactions endo or exo thermic?

Answer 4

exothermic (heat is given off; think of an explosion, it is not taking in heat, it is giving it off)

Question 5

Definition of specific heat?

Answer 5

amount of heat required to raise one gram of the substance 1 degree Celsius

Question 6

What is the symbol for specific heat?

Answer 6

c (joules/ grams C)

Question 7

What is the symbol for energy/ heat?

Answer 7

q

Question 8

What does it mean if q is positive?

Answer 8

energy is absorbed endothermic

Question 9

What is another meaning for bond strength?

Answer 9

bond energy (use the bond energy equation)

Question 10

If a reaction was positive enthalpy (+𐤃H or endothermic) and positive entropy (+𐤃S or more disorder), what is 𐤃G?

Answer 10

at high temperatures it will be thermodynamically favorable at low temperatures it will be thermodynamically unfavorable

Question 11

If a reaction was negative enthalpy (-𐤃H or exothermic) and negative entropy (-𐤃S or more order), what is 𐤃G?

Answer 11

at low temperatures it will be thermodynamically favorable;at high temperatures it will be thermodynamically unfavorable

Question 12

When a solid melts into a liquid is that an increase or decrease in entropy?

Answer 12

increase (more motion possible in a liquid)

Question 13

When a solid is formed when two solutions are added together is that an increase or decrease in entropy?

Answer 13

decrease (a solid allows less motion/disorder of particles) (the opposite of this would be a solid dissolving/dissociating in a solution)

Question 14

What does it mean if q is negative?

Answer 14

energy is lost (exothermic)

Question 15

What is the enthalpy of formation for any element in its natural state?

Answer 15

0 (zero)

Question 16

What does positive ΔS mean?

Answer 16

more disorder in the system

Question 17

What does entropy mean?

Answer 17

randomness or disorder of a system

Question 18

What does ΔS stand for?

Answer 18

entropy

Question 19

What does negative ΔS mean?

Answer 19

less disorder in the system; more order

Question 20

The enthalpy (𐤃H) for any element in its natural state is…

Answer 20

ZERO (ex. O2 has an enthalpy of zero when it is a gas)

Question 21

What is the equation for bond energy?

Answer 21

sum of the energies of reactants - sum of energy for products

Question 22

The entropy of a pure solid at absolute zero is…

Answer 22

Zero (you will rarely see this because we typically work in standard conditions)

Question 23

Entropy is measured in…

Answer 23

Joules (mole-1) (Kelvin-1)

Question 24

If 𐤃G is greater than zero what does it mean about the reaction?

Answer 24

It is NOT thermodynamically favorable (you can force the reaction but the products will turn right back into the reactants)

Question 25

If 𐤃G is less than zero what does it mean about the reaction?

Answer 25

It IS thermodynamically favorable (the reaction will proceed in the forward direction)

Question 26

What does it mean if 𐤃G is zero?

Answer 26

Reaction at equilibrium.

Question 27

If a reaction was negative enthalpy (-𐤃H or exothermic) and positive entropy (+𐤃S or more disorder), what is 𐤃G?

Answer 27

NEGATIVE; the reaction will be thermodynamically favorable no matter what the tempurature

Question 28

If a reaction was positive enthalpy (+𐤃H or endothermic) and negative entropy (-𐤃S or more order), what is 𐤃G?

Answer 28

POSITIVE; the reaction with be thermodynamically unfavorable no matter what the tempurature

Question 29

When a solid or liquid becomes a gas is that an increase or decrease in entropy?

Answer 29

increase (more motion allowed in a gas than a solid or liquid)

Question 30

When a gaseous chemical reaction produces more molecules is that an increase or decrease in entropy?

Answer 30

increase (there are even more randomly moving particles)

Question 31

When the temperature of a substance decreases is that an increase or a decrease in entropy?

Answer 31

decrease (as energy decreases, the particles slow down and become more ordered) (the opposite of this is when temperature increases)