5.1 Measuring energy changes

Question 1

Heat

Answer 1

Transfer of energy between objects of different temperatures

Question 2

2nd law of thermodynamics

Answer 2

Heat will spontaneously flow from an object of higher temperature to an object of lower temperature

Question 3

Thermal equilibrium

Answer 3

When no energy is transferred as objects reach the same temperature

Question 4

Enthalpy

Answer 4

Heat released or absorbed by a system at constant pressure

Question 5

Can total enthalpy be measured?

Answer 5

No but the changes in enthalpy can

Question 6

What are the standard conditions?

Answer 6

100kPa and 25C/298K

Question 7

Temperature

Answer 7

Average kinetic energy of particles in a substance

Question 8

Absolute zero

Answer 8

Temperature at which particles have least amount of kinetic energy 0K or -273C

Question 9

Difference between open and closed systems

Answer 9

Matter and energy move freely in open systems however only energy can move in closed

Question 10

Exothermic and endothermic

Answer 10

Exo is negative while endo is positive. Heat is released in exothermic while heat is absorbed in endothermic

Question 11

Example of exothermic reactions

Answer 11

Combustion and neutralization reactions

Question 12

Describe energy profile of exothermic reactions

Answer 12

Products release more energy then products so delta H is negative

Question 13

Example of endothermic reactions

Answer 13

Thermal decomposition

Question 14

Is exothermic and endothermic bond breaking or bond forming?

Answer 14

Bond forming is exothermic while bond breaking is endothermic

Question 15

Equation for enthalpy change

Answer 15

q= mcT where:
q--> energy absorbed or released (J) 
m--> mass (g) 
c--> Specific heat capacity (J/gC) 
T--> Change in temp. (C)

Question 16

Specific heat capacity

Answer 16

Amount of heat required to raise temp. of one g of a substance by 1C or 1K

Question 17

Do metals have high or low specific heat capacity?

Answer 17

They have low SHC sp they heat up and lose heat quickly

Question 18

Enthalpy change of combustion

Answer 18

Change of energy when one mole of a substance is completely burned in the presence of oxygen under standard conditions

Question 19

Enthalpy change of formation

Answer 19

Energy change upon formation of 1 mole of a substance from its constituent elements in standard states

Question 20

Enthalpy change of formation of an element

Answer 20

0kJ/mol as they can’t be formed from anything

Question 21

Standard enthalpy change of a reaction

Answer 21

Energy change that takes place when molar amounts of reactants as shown in stoichiometric equations react together to form products under standard conditions

Question 22

What are the two ways to calculate standard enthalpy change of a reaction?

Answer 22

1. Bond energy values: Heat absorbed - Heat released

2. Sum of enthalpy of products - Sum of enthalpy of reactants

Question 23

Enthalpy change of neutralization

Answer 23

Energy change when an acid and base react together to form one mole of water

Question 24

What apparatus can be used to find molar enthalpy of combustion and how is it used?

Answer 24

Calorimeters and spirit burners. Known mass of alcohol is burned and temperature increase of water in calorimeter is measured for a certain period. Calculate molar EOC of alcohol by looking at mass of alcohol burned and temp change of water

Question 25

Experimental error

Answer 25

(Experimental value- theoretical value)/theoretical value x 100

Question 26

Possible reasons for experimental error

Answer 26

• Heat loss to surroundings
• Heat absorbed by calorimeter
• Incomplete combustion of alcohol
• Assuming density is same as water
• Assuming SHC is 4.18 J/gC