Energetics Seneca

Question 1

Define enthalpy change.

Answer 1

• Measure of heat taken in or out of process.

Question 2

When objects are heated, what do they use the energy for?

Answer 2

• To expand.

Question 3

Why do we use enthalpy instead of energy?

Answer 3

• Energy relies on fixed volume.

Question 4

What condition must be constant to measure enthalpy?

Answer 4

• Pressure, which is constant in the atmosphere.

Question 5

Do exo and endothermic reactions take in or give out heat energy?

Answer 5

• Exo - give out.
• Endo - take in.

Question 6

Do exo and endothermic reactions have a positive or negative enthalpy change?

Answer 6

• Exo - negative.
• Endo - positive.

Question 7

Example of an exothermic reaction?

Answer 7

• Combustion.

Question 8

Example of endothermic reaction?

Answer 8

• Thermal decomposition.

Question 9

Endothermic making or breaking bonds?

Answer 9

• Breaking.

Question 10

Exothermic making or breaking bonds?

Answer 10

• Making.

Question 11

What is enthalpy change of a reaction the sum of?

Answer 11

• Individual bond enthalpies being broken and made during a reaction.

Question 12

What is the dominating term?

Answer 12

• Whether the reaction requires more energy to make or break bonds, giving the overall enthalpy.

Question 13

What is a memory tool to remember bond breaking and bond making?

Answer 13

-Bendo-Mexo.

Question 14

What is the symbol for enthalpy change?

Answer 14

• ΔH.

Question 15

What are standard conditions?

Answer 15

• 1 bar (100kPa).
• 298 K.

Question 16

What are the 2 standard enthalpy changes?

Answer 16

• Standard enthalpy of combustion (ΔcHθ).
• Standard enthalpy of formation (ΔfHθ).

Question 17

Standard enthalpy of combustion (ΔcHθ) meaning?

Answer 17

• Enthalpy change when one mole of substance in its standard state burns completely in oxygen under standard conditions.

Question 18

Standard enthalpy of formation (ΔfHθ) meaning?

Answer 18

• Enthalpy change when one mole of substance is formed in its standard state from the pure elements in their standard states under standard conditions.

Question 19

How does bomb calorimetry work?

Answer 19

• Burning a sample of compound in a sealed vessel + measuring temp change.
• Heating water jacket.

Question 20

Sources of inaccuracy in bomb calorimetry?

Answer 20

• Heat lost to surroundings.
• Incomplete combustion.
• Reactant may evaporate before it combusts.

Question 21

What is the equation to measure enthalpy change?

Answer 21

• q = m x c x ΔT.

Question 22

What do the letters represent in the enthalpy change equation?

Answer 22

• q - heat energy.
• m - mass of sample.
• c - specific heat capacity.
• ΔT - temp change.

Question 23

What are the units of q?

Answer 23

• Joules.

Question 24

How to gain accurate temperature changes?

Answer 24

• Measure temp loss after reaction is complete by extrapolating line.

Question 25

Why are the initial and final temperatures on a calorimeter inaccurate?

Answer 25

- Heat is always being lost from the calorimeter.

Question 26

What is the only case when we can assume that the reaction mixture has the same density or specific heat capacity as water?

Answer 26

- When the question states we can.

Question 27

What does Hess' Law mean?

Answer 27

- Enthalpy change of a reaction is independent of the pathway. - Enthalpy change is the same if its direct or indirect.

Question 28

What is Hess' Law useful for?

Answer 28

- Useful for obtaining enthalpy changes that aren't directly observable.

Question 29

What is the enthalpy of formation for any element?

Answer 29

- Zero.

Question 30

What is the meaning of bond enthalpy?

Answer 30

- Enthalpy change when one mole of bonds is broken in the gas phase.

Question 31

What do mean bond enthalpies mean in molecules that have identical bonds?

Answer 31

- The 2nd bond will require more energy to break than the 1st because the molecule will rearrange after the 1st bond is broken. - ∴ an average of bond enthalpy values is taken.

Question 32

What are the 2 main reasons that mean bond enthalpies are inaccurate?

Answer 32

- Bond enthalpies are only valid in gas phase. - Bond enthalpies depend on the particular molecule, however mean bond enthalpies are quoted generally.

Question 33

Is bond enthalpy positive or negative?

Answer 33

- Positive.

Question 34

In standard enthalpy of formation (ΔfHθ) where do the arrows point in Hess' triangle?

Answer 34

- From the elements in their standard states to the reactants + products.

Question 35

In standard enthalpy of combustion (ΔcHθ) where do the arrows point in Hess' triangle?

Answer 35

- Point from the reactants + products to the oxides.