Enthalpy

Question 1

What is enthalpy?

Answer 1

H is a measure of the heat energy in a chemical system, which is the atoms, molecules or ions making up the chemicals.

Question 2

What is enthalpy change?

Answer 2

The difference in enthalpies.
ΔH = H (products) - H (reactants)
ΔH can be positive or negative, depending on whether the products contain more or less energy than the reactants.

Question 3

What is the Law of Conservation of Energy?

Answer 3

States that energy cannot be created or destroyed.
When a chemical reaction involving an enthalpy change takes place, heat energy is transferred between the system and the surroundings.
The system is the reactants and products.
The surrounding are the apparatus, lab, and anything not in the system.
The universe involves both.

Question 4

What is the direction of energy transfer?

Answer 4

From the system to the surroundings - exothermic change.
From the surroundings to the system - endothermic change.

Question 5

What is an exothermic energy transfer?

Answer 5

The chemical system releases heat energy to the surroundings.
Any energy loss by the system is balanced by the same energy gain by the surroundings.
ΔH is negative.
The temperature of the surroundings increases.

Question 6

What is the enthalpy profile diagram for an exothermic energy change?

Answer 6

The reactants are above the products.
The arrow showing ΔH points down to show it is negative.

Question 7

What is an endothermic energy transfer?

Answer 7

The chemical system takes in heat energy from the surroundings.
Any energy gain by the chemical system is balanced by the same energy loss by the surroundings.
ΔH is positive.
The temperature of the surroundings decrease as they lose energy.

Question 8

What is the enthalpy profile diagram for an endothermic energy change?

Answer 8

The reactants are below the products.
The arrow showing ΔH points up to show it is positive.

Question 9

What is activation energy?

Answer 9

The minimum energy required to break the bonds and for a reaction to take place.
Reactions with small activation energy take place very rapidly, because the energy needed to break the bonds is readily available from the surroundings.

Question 10

What are standard conditions?

Answer 10

Standard pressure is 100kPa.
Standard temperure is 298k or 25°C.
Standard concentration is 1 moldm^-3.
Standard state is the physical state of a substance under standard conditions.
ΔH° shows enthalpy change under standard conditions.

Question 11

What is the standard enthalpy change of reaction?

Answer 11

ΔrH° is the enthalpy change that accompanies a reaction in the molar quantities shown in the chemical equation under standard conditions, with all the reactants and products in their standard states.
E.g. Mg + 1/2 O2 –> MgO or 2Mg + O2 –> 2MgO (the kJ mol-1 value would be doubled for this).

Question 12

What is the standard enthalpy change of formation?

Answer 12

ΔfH° is the enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all the reactants and products in their standard states.
E.g. Mg + 1/2 O2 –> MgO
All elements have an enthalpy change of formation of 0 kJmol-1.

Question 13

What is the standard enthalpy change of combustion?

Answer 13

ΔcH° is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all the reactants and products in their standard states.
E.g. C4H10 + 6 1/2 O2 –> 4CO2 + 5H2O

Question 14

What is the standard enthalpy change of neutralisation?

Answer 14

ΔneutH° is the energy change that accompanies the reaction of an acid by a base to form one mole of H2O under standard conditions, with all the reactants and products in their standard states.
E.g. HCl + NaOH –> H2O + NaCl
The value is the same for all neutralisation reactions (-57kJmol-1)

Question 15

What is the equation for calculating energy change?

Answer 15

q = mcΔT
q = heat energy in j
m = mass of surroundings in g
c = specific heat capacity of water 4.18Jg-1K-1
ΔT = temperature change in k

Question 16

What is specific heat capacity?

Answer 16

The energy required to raise the temperature of 1g of substance by 1K.
Good conductors of heat - metals, have small c values.
Insulators - foam plastic, have large c values.
Mostly measuring for water, which is 4.18Jg-1K-1

Question 17

How do you calculate enthalpy change of combustion?

Answer 17

Calculate energy change q = mcΔT of the water in KJ (divide j by 1000).
Calculate the amount in mol of fuel burnt.
Calculate ΔcH in KJ mol-1, by energy change divided by mols, and a minus sign.

Question 18

Why are experiemental ΔcH values of methanol less exothermic than data book?

Answer 18

Heat loss to the surroundings other than water - air, beaker.
Incomplete combustion of methanol - see carbon as a black layer of soot on beaker.
Evaporation of methanol from the wick, so weigh the burner as soon as after extinguishing the flame.
Non-standard conditions.
Use draught screens and oxygen gas to minimise heat loss and incomplete combustion.

Question 19

What is method of using spirit burners?

Answer 19

Measure the water and pour into beaker, record intitial temperature of water.
Add methanol to spirit burner, weigh the spirit burner with methanol.
Place the spirit burner under the beaker, light the burner, and stir the water with the thermometer.
After 3 minutes extinguish the flame and immediately record temperature.
Re-weigh spirit burner containing methanol.

Question 20

What is the set up to calculate enthalpy change of reaction?

Answer 20

The enthalpy change of the reaction between two solutions can be determined using polystyrene foam cups.
These are cheap, waterproof, lightweight and have some insulation.

Question 21

How can you calculate enthalpy change of reaction?

Answer 21

The solution itself is the immediate surroundings.
The chemicals particles in the solution might react when they collide, and any energy transfer is between the chemical particles and water molecules.
A thermometer in the solution will record the temperature change so heat energy is calculated using q=mcΔT.

Question 22

What is average bond enthalpy?

Answer 22

Average enthalpy change when one mole of bonds of gaseous covalent bonds is broken.
Bond enthalpies are always endothermic, so always have a positive enthalpy value.

Question 23

What is bond breaking and making?

Answer 23

Energy is required to break bonds, so it is endothermic, ΔH is positive.
Energy is released when bonds form, so it is exothermic, ΔH is negative.

Question 24

What is the enthalpy profile diagram for exothermic reactions?

Answer 24

The energy released when making bonds is greater than the energy required when breaking bonds.
Small upwards arrow for breaking bonds, larger downwards arrow for making bonds.

Question 25

What is the enthalpy profile diagram for endothermic reactions?

Answer 25

The energy required when breaking bonds is greater than the energy released when making bonds.
Larger upwards arrow for breaking bonds, smaller downwards arrow for making bonds.

Question 26

What is the enthalpy change from average bond enthalpies?

Answer 26

ΔrH = ∑bond enthalpies in reactants - ∑bond enthalpies in products
The species must be gaseous, so the ΔrH is not standard.

Question 27

What is Hess’ law?

Answer 27

The total enthalpy change of a reaction is always the same, no matter which route is taken.

Question 28

How do you work out the enthalpy changes from ΔfH°?

Answer 28

• Construct the enthalpy cycle between reactants, products and their elements.
  The arrows point upwards from the elements to the reactants and products.
• Add ΔfH values and calculate the unknown enthalpy change.
  B+A =C or products - reactants
  Remember to check if the values are combustion or formation.

Question 29

How do you work out the enthalpy changes from ΔcH°?

Answer 29

-Construct the enthalpy cycle between the reactants, products and their common combustion products CO2, and H2O.
Both the reactants and products of the original reaction react to form the combustion products so point downwards.
-Add ΔcH values and calculate unknown enthalpy change.
A+C=B or reactants - products