3.1.4 - Energetics

Question 1

What is enthalpy

Answer 1

Heat energy

Question 2

What is enthalpy change

Answer 2

Heat energy change measured under conditions of constant pressure

Question 3

Calculation for enthalpy change

Answer 3

Enthalpy of products - enthalpy of reactants

Question 4

What is a exothermic reaction

Answer 4

Gives out heat energy to surroundings

Question 5

Explain an exothermic reaction and show it’s graph

Answer 5

• loss of enthalpy
• enthalpy change = negative
• temperature goes down

|___
| |
| |______
|__________

Question 6

What is endothermic reaction

Answer 6

Takes energy from surroundings

Question 7

Explain endothermic reaction and it’s graph

Answer 7

• gain of enthalpy
• enthalpy change is positive
• temperature goes down

______
| |
|____|
|_________

Question 8

What are standard conditions

Answer 8

Pressure of 100kPa

And a stated temperature

Question 9

Define standard enthalpy change of formation

Answer 9

The enthalpy change when 1 mol of a compound is formed from its elements under standard conditions, all reactants and products being in standard states

Question 10

What is enthalpy of formation of an element

Answer 10

Zero

Question 11

Define standard enthalpy of combustion

Answer 11

The enthalpy change when 1 mol of a substance is completely burned in excess oxygen under standard conditions, all reactants and products being on standard conditions

Question 12

What type of reaction is standard enthalpy of combustion

Answer 12

Always exothermic so is always negative

Question 13

Define heat capacity

Answer 13

Amount of heat required to raise temperature by 1 K

Question 14

Equation for heat capacity

Answer 14

q= C 🔺t

Question 15

Define specific heat capacity

Answer 15

Amount of heat required to raise 1kg of a substance by 1K

Question 16

Equation for specific heat capacity

Answer 16

q = m c 🔺t

Question 17

Equation for enthalpy change

Answer 17

🔺H = -q/n

Question 18

What does it mean if enthalpy change is positive and what does it mean if it is negative

Answer 18

• is exothermic

+ is endothermic

Question 19

Assumptions made in calorimetry

Answer 19

All energy is transferred
Heat is not escaping to surroundings
All heat is absorbed

Question 20

State hess’s law

Answer 20

The overall enthalpy change is independent of route take

Question 21

Using standard enthalpies of formation

Answer 21

🔺rH 
Reactant ----->products 
     \                       / 
      \🔺fH            / 🔺fH 
       \                   / 
   Elements in standard
               States

Question 22

Calculating standard enthalpies of formation

Answer 22

Elements 
Stand states ---> compound 
       \                           /
        \ 🔺cH              / 🔺cH
         \                       / 
            Combustion products

Question 23

Define mean bond enthalpies

Answer 23

The energy needed to break 1 mol of particular gaseous covalent bond averages over several compounds where the bond exists

Question 24

What is symbol for mean bond enthalpies

Answer 24

-

E

Question 25

Define dissociation energy

Answer 25

Refers to specific bond in a specific comping

Question 26

What are the only molecules with same bond enthalpies

Answer 26

Diatomic molecules

Question 27

How to use mean bond enthalpies

Answer 27

1) draw the molecule
2) write and equation using hess’s law
3) bond broken - bonds formed