≫3.1 - Energetics ✔

Question 1

In terms of enthalpy what is the system?

Answer 1

*The reaction itself.

Question 2

In terms of enthalpy what is the surroundings?

Answer 2

*Anything else, it’s what we can measure.

Question 3

What is an exothermic reaction?

Answer 3

*Energy is given out from the system into the surroundings causing a temperature increase of the surroundings.
*Products have less energy than reactants.

Question 4

What is an endothermic reaction?

Answer 4

*Energy is taken in from the surroundings into the system causing the temperature of the surroundings to decrease.
*Products have more energy than reactants.

Question 5

What is ΔH?

Answer 5

*Enthalpy change is the heat energy change measured under conditions of constant pressure.

Question 6

Sketch the reaction profile for a exothermic reaction:

Answer 6

Question 7

Sketch the reaction profile for a endothermic reaction:

Answer 7

Question 8

What is Ea?

Answer 8

*Activation energy - minimum energy required for reaction to occur.

Question 9

What does standard enthalpy change (ΔH⦵) refer to?

Answer 9

*Refers to standard conditions:
-101kPa pressure.
-Stated temperature (typically 298K)
-1 mole / 1moldm⁻³

Question 10

Define standard enthalpy of formation (ΔfH⦵):

Answer 10

*Enthalpy change when 1 mole of a compound is formed from its elements with all reactants and products in standard states under standard conditions.

Question 11

Define standard enthalpy of combustion (ΔcH⦵):

Answer 11

*Enthalpy change when 1 mole of a substance is burnt completely in excess oxygen with all reactants and products in their standard states under standard conditions.

Question 12

Give the equations for the standard enthalpy of formation for methane and water:

Answer 12

• C(s) + 2H₂(g) → CH₄(g)
• H₂(g) +0.5O₂(g) → H₂O(l)
  *Can use halves as it can only be 1 mole of water (or any product) due to the definition.

Question 13

Give the equation for the standard enthalpy of combustion for methane:

Answer 13

• CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)
  *Must always be 1 mole of the product your combusting.

Question 14

What is the equation for change in energy, q?

Answer 14

• q=mc∆T

*q=change in energy (J)
*m=mass (g)
*c=specific heat capacity (J°C⁻¹g⁻¹)
*∆T=change in temperature (°C or K)

Question 15

What is the equation for calculating energy change per mole?

Answer 15

• ∆H=q/n

*q=change in energy (kJ)

*n=moles used (mol)

*∆H=enthalpy change (kJmol⁻¹)

Question 16

If ∆H is negative what type of reaction is it?

Answer 16

*Exothermic.

Question 17

If ∆H is positive what type of reaction is it?

Answer 17

*Endothermic.

Question 18

Are breaking bonds endothermic or exothermic?

Answer 18

*Endothermic.

Question 19

Are making bonds endothermic or exothermic?

Answer 19

*Exothermic.

Question 20

Using bond enthalpies how can you calculate ∆H?

Answer 20

• (Total energy of breaking bonds)-(Total energy making bonds)

Question 21

Define mean bond enthalpy:

Answer 21

*The enthalpy change for one mole of a bond in a variety of different compounds averaged together.
*In gas state.

Question 22

What does Hess’s Law state?

Answer 22

*“The enthalpy change of a chemical reaction is independent of the route taken. It only depends on the initial and final states (assuming constant pressure.)”

Question 23

What are the two main types of calculations we do involving Hess’s law?

Answer 23

*Using ΔfH to find combustion enthalpy.
*Using ΔcH to find formation enthalpy.

Question 24

Hess’s Law:

When given enthalpies of formation where do the arrows point in the triangle?

(and sketch a brief model)

Answer 24

*Up.

Question 25

Hess’s Law:

When given enthalpies of combustion where do the arrows point in the triangle?

(and sketch a brief model)

Answer 25

*Down.

Question 26

Why do enthalpy values found using calorimetry differ from the accurate values?

Answer 26

*Energy is easily lost from the system through conduction or convection and inaccuracies in measuring temperatures.