Energetics - 5th

Question 1

what is the enthalpy change of a reaction?

Answer 1

• the heat energy it releases or absorbs per mole

Question 2

what is the symbol of enthalpy change?

Answer 2

• DELTA H
• has a sign, value and units

Question 3

what does this tell us:
N2 + 3H2 -> 2NH3
DELTA H = -92 kJ/mol

Answer 3

• this tells is that the reaction releases 92 kJ of heat energy every time 1 mole of N2 and 3 moles of H2 react to make 2 moles of NH3

Question 4

what sign tells us whether the reaction absorbs or releases energy?

Answer 4

+ = POSITIVE = ABSORBED = ENDOTHERMIC
- = NEGATIVE = RELEASED = EXOTHERMIC

Question 5

what tells us how much heat energy is absorbed or released?

Answer 5

• the value tells us how much heat energy is absorbed or released
• the units are always kJ / mol so that different reactions can be compared easily

Question 6

what is the meaning of this reaction and enthalpy change:
BaCO3 + heat
DELTA H = +245 kJ/mol

Answer 6

• absorbs a lot of heat energy

Question 7

what is the meaning of this reaction and enthalpy change:
NaCl + H2O
DELTA H = +35 kJ/mol

Answer 7

• absorbs a small amount of heat energy

Question 8

what is the meaning of this reaction and enthalpy change:
CH4 + O2
DELTA H = -890 kJ/mol

Answer 8

• releases a lot of heat energy

Question 9

what is the meaning of this reaction and enthalpy change:
C8H18 + O2
DELTA H = -5330 kJ/mol

Answer 9

• releases a huge amount of heat energy

Question 10

what does how much heat energy a reaction absorbs or releases depend on?

Answer 10

• the balance between breaking bonds (which absorbs energy) and making bonds (which releases energy)