energetics I

Question 1

define enthalpy change

Answer 1

the heat energy change measured at a constant temperature

Question 2

what conditions is enthalpy change measured under

Answer 2

• 100kpa pressure
• usually 298 K temperature

Question 3

describe the transition of energy when making and breaking bonds

Answer 3

• to break bonds energy is taken in from the surroundings
• to make bonds energy is given out to the surroundings

Question 4

how to calculate overall enthalpy change ^H

Answer 4

^H= energy to break bonds (+ve) + energy to make bonds (-ve)

Question 5

describe an endothermic energy level diagram

Answer 5

• more energy needed to break bonds than make ones
• energy of reactants lower than energy of products

Question 6

describe an exothermic energy level diagram

Answer 6

• less energy is needed to break bonds than to make ones
• energy of reactants higher than energy of products

Question 7

define enthalpy change of reaction ^H°

Answer 7

the enthalpy change when quantities of substances in standard states react completely under standard conditions

Question 8

define enthalpy change of formation ^fH°

Answer 8

the enthalpy change when one mole of a substance is produced from its elements under standard conditions

Question 9

define enthalpy change of combustion ^cH°

Answer 9

the enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions

Question 10

define enthalpy change of neutralisation ^nH°

Answer 10

the enthalpy change when solutions of acid and alkali react together under standard conditions to produce one mole of water