Enthalpy Changes

Question 1

what is an exothermic reaction?

Answer 1

• releases energy to the environment
• results in an overall negative energy change (delta H)
• the reaction temperature decreases

Question 2

what is an endothermic reaction?

Answer 2

• absorbs energy from the environment
• results in an overall positive energy change
• the reaction temperature increases

Question 3

what are standard condition of an enthalpy change reaction?
(Delta Hr (symbol))

Answer 3

1- pressure of 100KPa (1atm)
- 298K temperature
- standard states (its physical state at room temp/pressure)

Question 4

what is the enthalpy change of combustion?
(delta Hc (symbol))

Answer 4

the enthalpy change when 1 mole of a substance is completely burnt in oxygen under standard conditions

Question 5

what is the enthalpy change of formation?
(delta Hf (symbol)

Answer 5

the enthalpy change when 1 mole of a compound is formed directly from its elements under standard condition

Question 6

what is the enthalpy change of neutralisation?
(delta Hneut (symbol))

Answer 6

the enthalpy change when 1 mole of water is formed directly from neutralisation under standard conditions

Question 7

when balancing equations surrounding the enthalpy changes of combustion / neutralisation / formation, what do they all have in common?

Answer 7

they all refer to 1 mole of something
- in the equation, there cannot be more than one mole of it
==> if cannot balance keeping it at one mole, balance normally and then divide through to make 1 mole