module 3: 9.1 enthalpy changes

Question 1

define the term enthalpy

Answer 1

enthalpy (H) is a measure of the heat content or energy in a chemical system. The chemical system refers to the atoms, molecules or ions making up the chemicals.

Question 2

what is an exothermic change

Answer 2

• from the system to the surroundings, it releases heat energy to the surroundings.
• Delta H is negative
• The temperature of the surroundings increase as they gain energy (temp increase)
• Reactants is higher than products

Question 3

what is an endothermic change

Answer 3

• from the surroundings to the system, it takes in heat energy from the surroundings
• Delta H is positive
• The temperature of the surroundings decrease as they lost energy (temp decrease)
• Reactants is lower than products

Question 4

define the term activation energy

Answer 4

the energy required to break bonds acts as an energy to the reaction, known as activation energy E (subscript) a
It is the minimum energy needed for the reaction to take place

Question 5

what are the standard conditions

Answer 5

• standard pressure - 100 kPa
• standard temperature - 25 degrees C or 298 K
• standard concentration - 1 mol dm-3
  standard state - physical state under standard conditions

Question 6

define the standard enthalpy change of reaction

Answer 6

the standard enthalpy change of reaction (ΔrH^⦵) is the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and produces in their standard states

Question 7

define the enthalpy change of formation

Answer 7

the standard enthalpy change of formation (ΔfH^⦵) is the enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states

Question 8

define enthalpy change of combustion

Answer 8

the standard enthalpy change of combustion (ΔcH^⦵) is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states.

Question 9

define enthalpy change of neutralisation

Answer 9

the standard enthalpy change of neutralisation (ΔneutH^⦵) is the energy change that accompanies the reaction of an acid by a base to form one mole of H20 (l), under standard conditions, with all reactants and products in their standard states.

Question 10

formula for energy (Q)

Answer 10

Q = mcΔT
q - energy (J)
m - mass of water
c  - 4.18 (J/g/ degrees C or K)
ΔT - Change in temperature (degrees C)