5.2 ENERGETICS

Question 1

Define an enthalpy change of a reaction

Answer 1

The heat energy it releases or absorbs per mole

Question 2

Is heat energy absorbed or released in an ENDOthermic reaction (and so what enthalphy sign is it)

Answer 2

Absorbed (+)

Question 3

Is heat energy absorbed or released in an EXOthermic reaction (and so what enthalphy sign is it)

Answer 3

Released (-)

Question 4

Units of enthalpy

Answer 4

kJ/mol

Question 5

How do you draw an energy level diagram for an ENDOthermic reaction

Answer 5

• Enthalpy scale on y-axis, and time/progression on x-axis
• Reactants drawn on line on bottom of diagram
• Vertical line up with an arrow (labelled with enthalpy)
• Products of horizontal line above line

Question 6

How do you draw an energy level diagram for an EXOthermic reaction

Answer 6

• Enthalpy scale on y-axis, and time/progression on x-axis
• Reactants drawn on line at top of diagram
• Vertical line down with an arrow (labelled with enthalpy)
• Products of horizontal line below

Question 7

When bonds are broken, what type of reaction is it

Answer 7

Endothermic - energy is absorbed

Question 8

When bonds are made, what type of reaction is it

Answer 8

Exothermic - energy is released

Question 9

Unit for bond energy

Answer 9

kJ/mol

Question 10

Equation for calculating with bond energies

Answer 10

Enthalpy change = Bonds broken - bonds made

Question 11

How to calculate a bond energy question

Answer 11

• Add up each individual bond energy for each bond
• Add them together for products and reactants
• Subtract products from reactants

Question 12

In an exothermic reaction, what is the ‘ratio’ of energy absorbed / released

Answer 12

Released > Absorbed

Question 13

In an endothermic reaction, what is the ‘ratio’ of energy absorbed/released

Answer 13

Absorbed > Released

Question 14

In an endothermic reaction, what is the ‘ratio’ of energy absorbed/released

Answer 14

Absorbed > Released

Question 15

What is the activation energy

Answer 15

The energy needed to break the bonds

Question 16

How do you make an energy level diagram, a reaction profile diagram

Answer 16

• There is a hump from the products to the reactants
• In the centre of the hump, and arrow up
• Labelled activation energy and the actual enthalpy (if avaliable)

Question 17

Q =

Answer 17

Q=mc 0t

Question 18

Enthalpy Change equation

Answer 18

-Q / moles

Question 19

Describe how to carry out a combustion calorimetry experiment

Answer 19

• Weigh 50g of water and plae in a can above a burner
• Measure inital mass of burner and intital temperature of water
• Ignite the burner, stir water for 30s then extinguish burner
• Measure final mass of burner, and final temperature of water

Question 20

What are the main sources of error in a combustion calorimetry experiment

Answer 20

• Heat loss - most of the thermal energy heats the air
• Incomplete combustion - releases less heat energy

Question 21

How do endothermic reactions get given a ‘+’ value for enthalpy change when being calculated

Answer 21

Q=mc0t will be negative and so -Q/mol (negative multipied by negative) will be positive

Question 22

Describe how to carry out a solution calorimetry experiment

Answer 22

• Pipette 25cm^3 of solution into polystyrene cup
• Measure intital temperature of solution
• Add 1g of metal and stir continuosly
• Mesure the maximum temperature of solution

Question 23

What are the sources of error in a solution calorimetry experiment

Answer 23

• Heat loss - thermal energy used to heat up air
• Incomplete reaction - not stired/left for long enough
• Solution is no longer pure water - won’t have same SHC

Question 24

Describe how to carry out a solution calorimetry experiment

Answer 24

• Pipette 25cm^3 of solution into polystyrene cup
• Measure intital temperature of solution
• Add 1g of metal and stir continuosly
• Mesure the maximum temperature of solution

Question 25

How to calculate enthalpy change from a combustion calorimetry experiment

Answer 25

* Q=mc0t (of water) * CONVERT TO KJ * Moles of fuel * Enthalpy = **-Q**/moles

Question 26

In an exothermic reaction, what is the 'ratio' of energy absorbed / released

Answer 26

Released > Absorbed

Question 27

When bonds are broken, what type of reaction is it

Answer 27

Endothermic