Introducing Enthalpy and Enthalpy Change

Question 1

what does the first law of thermodynamics state

Answer 1

• that during a reaction energy cannot be created or destroyed
• however, it can be converted from one form of energy to another

Question 2

what are the two most important forms of energy for a chemist

Answer 2

chemical and heat energy

Question 3

what are the two components chemical energy is made up of

Answer 3

kinetic and potential energy

Question 4

what is kinetic and potential energy

Answer 4

• kinetic energy is the measure of the motion of particles in a substance
• potential energy is the measure of how strongly these particles interact with one another

Question 5

what is heat energy

Answer 5

• the portion of the kinetic and potential energy of substance
• that is responsible for the temperature of the substance

Question 6

what is the heat energy of a substance directly proportional to

Answer 6

• its absolute temperature

- the temperature measured in kelvin

Question 7

what is enthalpy

Answer 7

the measure of the total energy of a system

Question 8

if we cant directly measure the enthalpy of a system, what do we measure in stead

Answer 8

the enthalpy change that takes place during a physical or chemical change

Question 9

what is the symbol for enthalpy change

Answer 9

delta H

Question 10

what is the enthalpy change of a process

Answer 10

• the heat energy that is transferred between the system and the surroundings
• at a constant temperature

Question 11

if enthalpy is the total measurement of the energy in a system, what formula can be derived from the relationship between enthalpy and internal energy and why

Answer 11

• enthalpy = internal energy + (pressure x volume)
• H = U + PV
• because enthalpy includes the internal energy

Question 12

what is internal energy and why are pressure and volume included in the formula

Answer 12

• internal energy is the energy required to create a system
• but enthalpy also includes the amount of energy required to make room for it
• by displacing its environment and establishing its volume and temperature
• so enthalpy would be the sum of the internal energy and the product of the pressure and volume

Question 13

what is an exothermic reaction

Answer 13

where heat energy is transferred from the system to its surroundings

Question 14

what is an endothermic reaction

Answer 14

where heat energy is transferred from the surroundings into the system

Question 15

are processes such as freezing water and the combustion of petrol exo or endo and why

Answer 15

• they are exothermic
• because the energy level of the products is less than that of the reactants
• meaning heat energy has been lost from the system and has been transferred to the surroundings

Question 16

are process such as melting ice an photosynthesis exo or endo and why

Answer 16

• they are endothermic
• because the energy level of the product(s) is higher than the reactants
• meaning heat energy was absorbed from the surroundings and transferred into the system

Question 17

the reaction HCl + NaOH = NaCl + H20 is exothermic and has an energy change with a magnitude of 57.1 kJmol-1. how would you write this to accurately show the enthalpy change and why

Answer 17

• the delta H would = -57.1kJmol-1
• because if it is exothermic, heat energy has been lost to the surroundings
• meaning the whole system has actually lost 57.1kJmol-1 of energy
• so a minus sign is used to show this

Question 18

what sign would you then use to show the enthalpy change of an endothermic reaction and is it even necessary

Answer 18

• delta H = +x kJmol-1

- the plus sign isnt really needed as the number would be positive anyway

Question 19

how can you know a reaction is exothermic through observtion

Answer 19

• exothermic reactions result in an immediate increase in temperature
• so the temperature of the reaction mixture increases
• which can be felt or measured with a thermometer

Question 20

how can you know a reaction is endothermic through observation

Answer 20

• endothermic reactions result in a decrease of temperature of the reaction mixture
• which can be felt or measured with a thermometer

Question 21

what is any reaction that has to be heated in order for it to take place called

Answer 21

an endothermic reaction

Question 22

what is an example of a type of reaction that is usually endothermic with a real example

Answer 22

• thermal decomposition
• like CaCO3 = CaO + CO2
• which has a delta H of 178kJmol-1

Question 23

what are the standard conditions that enthalpy changes should usually be quoted in

Answer 23

• a pressure of 100kPa

- with a temperature of 298K

Question 24

what is the enthalpy change measured under these conditions called and what is its symbol

Answer 24

• the standard enthalpy change
• given the symbol deltaH- with a subscript of 298K
• or just deltaH-
• the - has a circle drawn around it

Question 25

what is the symbol for the standard enthalpy change of reaction

Answer 25

delta rH-

Question 26

what do you need to remember about what the standard enthalpy change for reaction is determined by

Answer 26

• the reaction as written, not necessarily the reaction with whole number ratios
• so a reaction can be written as N2 + 3H2 = 2NH3
• giving a delta rH- of -92kJmol-1
• but for the reaction when written it is 1/2N2 + 3/2H2 = NH3
• giving you a delta rH- of -46kJmol-1

Question 27

what does the ‘per mole’ in the enthalpy change mean in each case

Answer 27

• it refers to one mole of the equation

- and not to one mole of any reactant or product