9) Enthalpy

Question 1

Define enthalpy H

Answer 1

the heat content that is stored in a chemical system

Question 2

What does the law of the conservation of energy state?

Answer 2

energy cannot be created or destroyed

Question 3

Define exothermic reaction

Answer 3

a reaction in which the enthalpy of the products is smaller than the enthalpy of the reactants, resulting in heat loss to the surroundings (∆H is negative)

Question 4

Define endothermic reaction

Answer 4

a reaction in which the enthalpy of the products is greater than the enthalpy of the reactants, resulting in heat being taken in from the surroundings (∆H is positive)

Question 5

Standard pressure?

Answer 5

100 kPa

Question 6

Standard temperature?

Answer 6

298K / 25 degrees C

Question 7

Standard concentration?

Answer 7

1 mol per dm^3

Question 8

Standard state?

Answer 8

physical state of a substance under standard conditions

Question 9

Define activation energy Ea

Answer 9

the minimum energy required to start a reaction by the breaking of bonds

Question 10

Define standard enthalpy change of reaction ∆rH⊖

Answer 10

the enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products being in their standard states

Question 11

Define standard enthalpy change of formation ∆fH⊖

Answer 11

the enthalpy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions

Question 12

Define standard enthalpy change of combustion ∆cH⊖

Answer 12

the enthalpy change that takes place when 1 mole of a substance reacts completely with oxygen under standard conditions, all reactants and products being in their standard states

Question 13

Define standard enthalpy change of neutralisation ∆neutH⊖

Answer 13

the enthalpy change that accompanies the reaction of an acid by a base to form 1 mole of H2O (l) under standard conditions, all reactants and products being in their standard states

Question 14

What is special about ∆neutH⊖?

Answer 14

it is the same for all neutralisation reactions

Question 15

Give an equation to determine heat energy, q in Joules

Answer 15

q = mc∆T
m = mass of surroundings
c = specific heat capacity
∆T = temperature change of the surroundings (T(final) - T(intial))

Question 16

Define specific heat capacity

Answer 16

the energy required to raise the temperature of 1g of a substance by 1K

Question 17

Give a method (5) for the determination of enthalpy change of combustion

Answer 17

1. using a measuring cylinder, measure out 150cm3 of water into a beaker and record the initial temperature of the water
2. add methanol (or other fuel) to the spirit burner and weigh
3. place the burner under the beaker, light and burn the methanol whilst stirring the water with a thermometer
4. after about 3 mins, extinguish flame and record maximum temperature reached by the water
5. re-weigh the spirit burner
   Then use q = mc∆T with experimental data

Question 18

Name 4 factors of experimental determination of enthalpy change of combustion which compromise accuracy

Answer 18

heat loss to surroundings
incomplete combustion of methanol
evaporation of methanol from the wick
non-standard conditions
(all but the last would lead to a ∆cH⊖ value that is less exothermic than expected)

Question 19

How could heat loss to surroundings and incomplete combustion of methanol be minimised when determining enthalpy change of combustion?

Answer 19

use of draught screens

oxygen input

Question 20

How could evaporation of methanol from the wick be minimised when determining enthalpy change of combustion?

Answer 20

weigh burner as soon as possible after extinguishing the flame
some spirit burners have a cover

Question 21

Why is a plastic cup of polystyrene foam helpful to determine ∆rH?

Answer 21

light weight
cheap
waterproof
insulating

Question 22

How can a cooling curve be determined?

Answer 22

1. record temperature of solution every 30 seconds until constant
2. add an excess of solid powder, stirring and recording the temperature every 30 seconds until the temperature has fallen for several mins
3. plot a graph of temperature against time and extrapolate the cooling curve back to when the solid was added

Question 23

What is the purpose of a cooling curve?

Answer 23

gives a corrected / more accurate value for ∆T without heat loss

Question 24

Define average bond enthalpy

Answer 24

the average enthalpy change that takes place when breaking by homolytic fission 1 mol of a given type of bond in the molecules of a gaseous species

Question 25

What is a limitation of average bond enthalpies?

Answer 25

calculated from bond enthalpies in different chemical environments

Question 26

Energy is always required to _. Bond enthalpies are always _ and have _ enthalpy values.

Answer 26

break bonds
endothermic
positive

Question 27

Energy is _ therefore, _ is endothermic and ∆H is _

Answer 27

required to break bonds
bond breaking
positive

Question 28

Energy is _ therefore, _ is exothermic and ∆H is _

Answer 28

released when bonds form
bond making
negative

Question 29

Equation for ∆rH?

Answer 29

∆rH = ∑(bond enthalpies in reactants) - ∑(bond enthalpies in products)

Question 30

Define Hess’ Law

Answer 30

if a reaction can take place by more than 1 route and the initial and final conditions are the same, the total enthalpy change is the same for each route

Question 31

What does Hess’ Law enable?

Answer 31

enthalpy changes to be determined indirectly

Question 32

When using enthalpy changes of formation, ∆rH = ?

Answer 32

∑∆fH (products) - ∑∆fH (reactants)

Question 33

When using enthalpy changes of combustion, ∆rH = ?

Answer 33

∑∆fH (reactants) - ∑∆fH (products)