Chapter 9

Question 1

What is enthalpy ?

Answer 1

A measure of the energy content of a substance, symbol H

Question 2

What is the enthalpy change of a reaction ?

Answer 2

The amount of energy released / taken in during a chemical reaction

Question 3

What is an exothermic reaction, what is the value of ΔH ?

Answer 3

Reactions that give out heat, release energy to surroundings
ΔH is negative

Question 4

What is an exothermic reaction, what is the value of ΔH ?

Answer 4

Reactions that take in heat, take in energy from surroundings
ΔH is positive

Question 5

What does the enthalpy profile of an exothermic reaction look like ?

Answer 5

Products lower than reactants
ΔH is negative so arrow points down

Question 6

What does the enthalpy profile of an endothermic reaction look like ?

Answer 6

Products higher than reactants
ΔH is negative so arrow points up

Question 7

What is the activation energy ( Ea ) of a reaction and how is it shown on an enthalpy profile diagram ?

Answer 7

The minimum amount of energy for a reaction to take place. It is the energy input required to break bonds.

Question 8

What is meant by the term standard conditions and standard states ?

Answer 8

Standard conditions : 100,000 Pa, 298K or 25 degrees celsius , all substances in their standard states (g/l/s), concentration of 1 mol/dm3
Standard states : the physical states under standard conditions

Question 9

What is meant by the standard enthalpy of reaction ( ΔrH° )?

Answer 9

It is the enthalpy change for a reaction as given, reaction do not fit a particular category
Solutions would be 1 mol/dm3 solutions

Question 10

What is meant by the standard enthalpy of formation ( ΔfH° )?

Answer 10

One mole of compounds formed from its elements

Question 11

What is the standard enthalpy of formation of an element ?

Answer 11

Zero

Question 12

What is meant by the standard enthalpy of combustion ( ΔcH° )?

Answer 12

( refers to complete combustion ) the heat released when one mole of substance is burned completely in excess oxygen under standard conditions.

Question 13

What is meant by the standard enthalpy of neutralisation ( ΔneutH° )?

Answer 13

ΔneutH° = 57kJ/mol ( constant value )
Enthalpy change associated with the formation of 1 mole of water from a neutralisation reaction between an acid and a base
H+ ( aq ) + OH- ( aq ) → H20 (l)

Question 14

What is the equation that links heat ( q ), mass of water ( m ), SHC ( C ) and change in temperature ?
What must the answer refer to ?

Answer 14

q = mc∆T
q = joules
m = g
c = 4.18 Jg-1k-1
∆T = kelvin

The answer will be in kilojoules ( convert to J if required )
The answer refers to 1 mole of substance

Question 15

What is the equation that links the heat change for an experiment ( q ), the number of moles of substance used ( n ), enthalpy change per mole ( ∆H )

Answer 15

∆H = q / n

Question 16

Why could there be a lack of accuracy in an experiment ( eg value calculated from experiment is less exothermic )

Answer 16

Heat loss to surroundings
Incomplete combustion of fuel
Not performed under standard conditions

Question 17

What is meant by the term “ average bond enthalpy “ ?

Answer 17

the energy required to break 1 mole of a specified type of bond in gaseous molecules.

Question 18

Why is breaking bonds an endothermic process ?

Answer 18

Requires energy

Question 19

Why is forming bonds an exothermic process ?

Answer 19

Releases energy

Question 20

What is meant by “ bond enthalpy “ ?

Answer 20

Average amount of energy required to break a mole of bonds in gaseous state.

Question 21

How to use bond enthalpies to calculate energy released in reaction ?

Answer 21

Draw strcutures
Identify bonds hroken and made
Calculate total energy
Add together to find ΔrH

ΔrH = total for breaking - total for making

Question 22

Describe Hess’ law for construction of enthalpy cycles

Answer 22

” the enthalpy change for any reaction is independent of the intermediate stages, so long as the inital and final stages are the same for each route “
Different routes give the same results, can be used for changes in enthalpy

Question 23

What is shown in Hess Cycle Type 1 ?

Answer 23

Uses combsution data, combustion products are written at the bottom of the diagram
Burning ( breaking bonds ) so arrows from reactants and products point to combustion products initially
Co2 and H20 produced in complete combustion

Question 24

What is shown in Hess Cycle Type 2 ?

Answer 24

Uses formation data, elements in their standard states are written at the bottom of the diagram
Forming / making bonds so arrows point from elements in their standard states to the reactants and products.

Question 25

Can you list and describe the techniques and procedures used to determine enthalpy changes indirectly?

Answer 25

To determine enthalpy changes directly, the primary technique used is calorimetry, which involves measuring the temperature change of a reaction mixture in a calorimeter, and then using the equation Q = mcΔT to calculate the heat energy transferred, thereby finding the enthalpy change; indirectly
Simple calorimetry : This involves mixing reactants in a polystyrene cup, measuring the temperature change of the solution, and using the Q = mcΔT equation to calculate the enthalpy change.
Combustion calorimetry : Used for combustion reactions, where a sample is burned in a bomb calorimeter, and the heat released is measured by the temperature change of the surrounding water.

Question 26

Can you list and describe the techniques and procedures used to determine enthalpy changes directly?

Answer 26

Hess’s Law is the key method, where enthalpy changes of known reactions are manipulated and combined to calculate the enthalpy change of a desired reaction that is difficult to measure directly
Constructing an enthalpy cycle: Draw a diagram representing the different reaction steps involved, where the overall enthalpy change is the sum of the enthalpy changes of individual steps.
Manipulating known equations: Chemical equations with known enthalpy changes can be reversed, multiplied by coefficients to align with the desired reaction, and then added together to calculate the target enthalpy change.