Enthalpy changes

Question 1

What is the definition of enthalpy?

Answer 1

a value that represents the heat content of a system.

Question 2

What is the symbol for enthalpy?

Answer 2

H

Question 3

What is the analogy for breaking and forming bonds?

Answer 3

Breaking bonds = pulling apart magnets (takes effort therefore absorbs energy)

Forming bonds = snapping magnets together (releases energy)

Question 4

Describe an exothermic reaction

Answer 4

A reaction in which energy is transferred from the system to the surroundings. The temperature of the surroundings increase however, the energy of the system decreases therefore (ΔH is negative).
More energy is released by bond making than is used in bond breaking.

Question 5

How can you tell that the reaction is exothermic or endothermic from just looking at the ΔH (enthalpy change)

Answer 5

If ΔH is negative = exothermic

If ΔH is positive = endothermic

Question 6

Describe an endothermic reaction

Answer 6

A reaction in which energy is transferred from the surroundings to the system. Therefore the temperature of the surroundings decrease however, the energy of the system increases (ΔH is positive).
More energy is required to break bonds than is released by making bonds.

Question 7

In an exothermic reaction, why is ΔH negative?

Answer 7

Because the products have a lower enthalpy than the reactants, energy is released to the surroundings, so ΔH is negative.

Question 8

Why are exothermic reactions thermodynamically favourable?

Answer 8

Exothermic reactions are thermodynamically favourable because they go to a lower energy state, but whether they actually occur depends on the activation energy and rate, so they may be kinetically controlled.

Question 9

In an endothermic reaction, why is ΔH positive?

Answer 9

Because the products have a higher enthalpy than the reactants, energy is absorbed from the surroundings, so ΔH is positive.

Question 10

Describe an energy profile diagram for an exothermic reaction

What is on the x and y axis?

Answer 10

An energy profile diagram for an exothermic reaction will show that the products are at a lower enthalpy than the reactants. Activation energy and enthalpy change labelled.

X = progress of reaction
y = energy (enthalpy)

Question 11

Describe an energy profile reaction diagram for an endothermic reaction

What is on the x and y axis?

Answer 11

An energy profile diagram for a endothermic reaction will show that the products are at a higher enthalpy than the reactants. Activation energy and enthalpy change labelled.

x = progress of reaction
y = energy (enthalpy)

Question 12

What is the definition of activation energy? (Ea)

Answer 12

The minimum amount of energy needed for reactant molecules to have a successful collision and start the reaction

Question 13

What must happen for comparison in enthalpy between reactions?

Answer 13

All thermodynamic measurements must be carried out in standard conditions.

These conditions are:
A pressure of 100 kPa
Each substance involved in the reaction is in its standard physical state (solid, liquid or gas)
A temperature of 298 K (25 degrees)

Question 14

What is the symbol used to show that a reaction has been carried out under standard conditions?

Answer 14

ΔH⊖ = the standard enthalpy change

Question 15

What is the definition for a standard enthalpy change of reaction?

What is the symbol for a standard enthalpy change of reaction?

Answer 15

Standard enthalpy change of reaction, ΔHθr

A standard enthalpy change is the enthalpy change when reactants in their standard states react under standard conditions (298 K, 100 kPa, 1 mol dm⁻³).

Can be both exothermic and endothermic

Question 16

What is the definition for standard enthalpy change of formation?

What is the symbol for a standard enthalpy change of formation?

What is the standard enthalpy change of formation for O2?

Answer 16

Standard enthalpy change of formation, ΔHθf

The enthalpy change when one mole of a compound is formed from its elements in their standard states

Can be both exothermic and endothermic

The ΔHf⊖ of an element in its standard state is zero.

For example, ΔHf⊖ of O2(g) is 0 kJ mol-1

Question 17

What is the definition for standard enthalpy change of combustion?

What is the symbol for a standard enthalpy change of combustion?

Answer 17

Standard enthalpy change of combustion, ΔHθc

The enthalpy change when one mole of a substance, in its standard state, is burnt in excess oxygen

It is an exothermic change

Question 18

What is the definition for standard enthalpy change of neutralisation?

What is the symbol for a standard enthalpy change of neutralisation?

Answer 18

Standard enthalpy change of neutralisation, ΔHθneut

The enthalpy change when one mole of water is formed by reacting an acid and an alkali

It is an exothermic change

Question 19

What is the definition of specific heat capacity?

Answer 19

Specific heat capacity is the energy required to raise the temperature of 1g of a substance by 1 °C without a change of state.

Question 20

What is calorimetry?

How is a simple calorimeter made?

Answer 20

The measurement of enthalpy changes in a chemical reaction.

A simple calorimeter is made from a polystyrene drinking cup, a vacuum flask or metal can

Question 21

What is the specific heat capacity of water?

This is given in the datasheet.

Answer 21

4.18 J g-1 K-1 or sometimes 4.18 J g-1 °C-1

Question 22

What is the equation to calculate heat energy?

Answer 22

q = m x c x ΔT

q = heat transferred in J
m = mass of water in g
c = the specific heat capacity
ΔT = the temperature change

Question 23

When calculating the energy transferred of aqueous solutions of acid, alkalis and salts, what should the values be for m and c, unless specified?

Answer 23

Aqueous solutions of acid, alkalis and salts are assumed to be largely water so you can just use the m and c values of water when calculating the energy transferred.

Question 24

What is the equation to calculate any changes in enthalpy per mole of a reactant or product.

What is the expected value for an exothermic and endothermic reaction?

Answer 24

ΔH = q / n
or
(m x c x ΔT / n)

ΔH for exothermic = expected to be a negative value e.g. combustion
ΔH for endothermic = expected to be a positive value.

Question 25

What is the definition of bond dissociation energy (bond energy / enthalpy)

Answer 25

The energy required to break one mole of a specific bond in the gaseous state, forming gaseous atoms or radicals

Question 26

What is the definition of average bond enthalpy? What is the difference between bond dissociation energy and average bond enthalpy?

Answer 26

The average enthalpy change required to break one mole of a covalent bond in the gaseous state. Actual bond enthalpies may differ from the average as the average bond enthalpy considers the mean value for a particular bond in a range of molecules whereas the bond dissociation enthalpy refers to breaking a specific bond in a specific molecule.

Question 27

What is the equation for the standard bond enthalpy of a reaction?

Answer 27

enthalpy change for bonds broken - enthalpy change for bonds formed. as energy is required to break bonds, bond breaking is endothermic, ΔH is positive as energy is released making new bonds, bond forming is exothermic, ΔH is negative

Question 28

What is Hess's Law for enthalpy cycles?

Answer 28

Hess’s Law states that the total enthalpy change for a reaction is independent of the route taken, as long as the initial and final conditions are the same.

Question 29

What is the use of Hess's law?

Answer 29

To calculate enthalpy changes which cant be found experimentally using calorimetry.

Question 30

Why cant the enthalpy change of formation for propane be found experimentally using calorimetry?

Answer 30

Because C and H dont react under the same conditions 3C (s) + 4H2 (g) -> C3H8 (g)

Question 31

What are the 2 rules which help with the calculations of enthalpy changes of formation and combustion because the enthalpy change cant be calculated experimentally using calorimetry?

Answer 31

ΔHr = ∑ΔHf(products) - ∑ΔHf(reactants) ΔHr = ∑ΔHc(reactants) - ∑ΔHc(products)