3.1.4 Energetics

Question 1

Bond breaking is…

Answer 1

• Endothermic.

Question 2

Bond making is…

Answer 2

• Exothermic.

Question 3

How do we determine whether a reaction is endo or exothermic?

Answer 3

• Sum of energy in vs energy out.

Question 4

Transition state?

Answer 4

• State at which chemical bonds are partially formed + broken.

Question 5

Activation energy meaning?

Answer 5

• Minimum energy needed for reactant molecules to have a successful collision + start the reaction.

Question 6

Do exothermic or endothermic reactions have a higher activation energy?

Answer 6

• Endothermic.

Question 7

Is enthalpy change +ve or -ve in exothermic?

Answer 7

• -ve.

Question 8

Is enthalpy change +ve or -ve in endothermic?

Answer 8

• +ve.

Question 9

Where does the activation energy occur?

Answer 9

• Between reactants + transition state.

Question 10

Where does enthalpy change occur?

Answer 10

• From reactants to products.

Question 11

What is enthalpy?

Answer 11

• Total chemical energy inside a substance.

Question 12

What symbols represent enthalpy change?

Answer 12

• ΔH.

Question 13

Exothermic reactions are thermodynamically possible because…

Answer 13

• Enthalpy of the reactants is higher than products.

Question 14

What are 2 examples of exothermic reactions?

Answer 14

• Combustion.
• Neutralisation.

Question 15

What is an example of an endothermic reaction?

Answer 15

• Thermal decompostion.

Question 16

What is enthalpy change?

Answer 16

• Energy change of system at constant pressure.

Question 17

What are standard conditions?

Answer 17

• 1 atm / 100kPa.
• 25℃.

Question 18

℃ to K?

Answer 18

• ℃ + 273 = K.

Question 19

What is standard enthalpy of formation?

Answer 19

• ΔH of 1 mole of compound is formed from its constituent elements in standard conditions w/ reactants + products in standard state.

Question 20

What is standard enthalpy of combustion?

Answer 20

• ΔH of 1 mole of substance is burned completely in oxygen in standard conditions w/ reactants + products in standard states.

Question 21

What is heat?

Answer 21

• Sum of all particle’s energy ∴ affected by amount of substance.

Question 22

What is temperature?

Answer 22

• Related to mean kinetic energy of particles in a system ∴ independent of particles present.

Question 23

How can you calculate enthalpy change from experimental data?

Answer 23

• Q=mcΔT.

Question 24

How does a flame calorimeter differ from a simple calorimeter?

Answer 24

• Reduces heat loss to surroundings to obtain more accurate results.
• Spiral chimney of copper.
• Enclosed flame.
• Burnt in pure oxygen as opposed to air.

Question 25

How to measure enthalpy change for reaction occurring in (aq)?

Answer 25

- Expanded polystyrene cup as calorimeter. - Measure temp change of solution. - Heat capacity of solution --> 4.18. - Density --> 1g/cm3.

Question 26

How can experimental determination of enthalpy change be more accurate?

Answer 26

- Cooling curves.

Question 27

What is Hess' law?

Answer 27

- ΔH of reaction is same regardless of route taken.

Question 28

What is enthalpy of an element?

Answer 28

- 0.

Question 29

What is **bond dissociation enthalpy**?

Answer 29

- ΔH required to break a covalent bond w/ all species in gaseous state. - Differs for same bond in different molecules.

Question 30

What is **mean bond enthalpy**?

Answer 30

- Average value for bond dissociation enthalpy of a given bond.

Question 31

Why may experimental methods for enthalpy determination not be very accurate?

Answer 31

- Heat lost to surroundings. - Not in standard conditions. - Reaction may not be complete.

Question 32

Why is using bond enthalpies not as accurate as using standard enthalpy of combustion/formation?

Answer 32

- Bond enthalpies are a mean across diff molecules. - ΔH of combustion/formation apply to just that molecule ∴ more accurate.

Question 33

What is underneath the equation in enthalpy of combustion?

Answer 33

- The oxides of the elements.

Question 34

In enthalpy of combustion do the arrows point towards or away from the middle?

Answer 34

- Towards.

Question 35

What is underneath the equation in enthalpy of formation?

Answer 35

- The elements in their standard states.

Question 36

In enthalpy of formation do the arrows point towards or away from the middle?

Answer 36

- Away.