3.5 - Enthalpy changes

Question 1

define enthalpy

Answer 1

measure of heat energy in a chemical system

Question 2

define chemical system

Answer 2

atoms, molecules or ions that make up chemicals

Question 3

define exothermic

Answer 3

system releases heat energy to surroundings
delta H = negative

Question 4

define endothermic

Answer 4

system takes in heat energy from surroundings
delta H = positive

Question 5

draw energy profiles for exothermic and endothermic reactions

Question 6

define activation energy

Answer 6

minimum energy required for a reaction to take place, to break chemical bonds

Question 7

what are the standard units?

Answer 7

pressure = 100kPa
temperature = 298K
concentration = 1moldm-3

Question 8

define standard state

Answer 8

physical state of a substance under standard conditions

Question 9

define average bond enthalpy

Answer 9

enthalpy change when one mole of gaseous covalent bonds are broken

Question 10

define enthalpy change of reaction

Answer 10

enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions

Question 11

define enthalpy change of combustion

Answer 11

enthalpy change when one mole of a substance reacts completely with oxygen, with all reactants and products in their standard states under standard conditions

Question 12

define enthalpy change of neutralisation

Answer 12

enthalpy change for the formation of one mole of water from the reaction of an acid with a base

Question 13

define enthalpy change of formation

Answer 13

enthalpy change when one 1 mole of a substance is formed from its constituent elements, with all reactants and products in their standard states under standard conditions

Question 14

what is the enthalpy change of formation for all elements?

Answer 14

0 kJmol-1

Question 15

describe a method of how to determine enthalpy change of combustion experimentally

Answer 15

• measure volume of water, pour into beaker, record initial temperature
• add fuel to spirit burner and weigh
• place spirit burner under beaker, light it and stir water with thermometer
• extinguish flame after 3 mins and immediately record water temperature
• re-weigh spirit burner

Question 16

list the reasons why experimental enthalpy change of combustion might not be accurate

Answer 16

heat released to surroundings, incomplete combustion, non-standard conditions

Question 17

describe a method of how to determine enthalpy change of reaction experimentally

Answer 17

• polystyrene cup with thermometer clamped
• add one reactant into cup and take temperature every 30s, ensuring it remains constant
• add other reactant and stir, measure temp every 30s until it has fallen for several mins
• plot graph of temp against time
• extrapolate cooling curve section back to when second reactant added

Question 18

how could experiment for enthalpy change of reaction be adjusted for neutralisation?

Answer 18

use total volume of both reactants and ensure moles of reactant not in excess

Question 19

equation for enthalpy change

Answer 19

q = mc ∆ t

q = enthalpy change (J)
m = mass of water/solution (g)
c = 4.18 (specific heat capacity)
∆ t = change in temperature (K)

Question 20

equations that use reactants - products

Answer 20

• average bond enthalpy
• enthalpy change of combustion
• enthalpy change of reaction

Question 21

equations that use products - reactants

Answer 21

• enthalpy change of formation

Question 22

rules to remember for calculations

Answer 22

if ASKED TO CALCULATE (not values given!):
- formation = all arrows up
- combustion = all arrows down
- reaction = reactants down, products up

for q = mc ∆ t remember if calculating reaction to multiply final number by coefficient in equation e.g 2AgNo3 is x2

Question 23

note to remember when calculating formation from average bond enthalpies

Answer 23

bond breaking is endo (+)
bond making is exo (-)
so ensure the signs you use match this