Enthalpy Changes

Question 1

What is an enthalpy change?

Answer 1

The amount of heat released (or absorbed) by a chemical reaction carried out at a constant pressure. Units: kJ per mol

Question 2

What is an exothermic reaction?

Answer 2

A reaction where heat energy is released to the surroundings (the enthalpy change is negative)

Question 3

What is an endothermic reaction?

Answer 3

A reaction where heat energy is absorbed from the surroundings (the enthalpy change is positive)

Question 4

What is average bond enthalpy?

Answer 4

The average enthalpy when 1 mole of gaseous bonds is broken by homolytic fission

Question 5

What are standard conditions?

Answer 5

Temperature = 25 degrees celcius
Pressure = 1 atm/100kPa
Solutions must have a concentration of 1.00mol per dm cubed

Question 6

Define the standard enthalpy change of reaction

Answer 6

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

Question 7

Define the standard enthalpy change of neutralisation

Answer 7

The enthalpy change that accompanies the formation of 1 mole of H20(l) from neutralisation, under standard conditions

Question 8

Define the standard enthalpy change of formation

Answer 8

The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions

Question 9

Define the standard enthalpy change of combustion

Answer 9

The enthalpy change for complete combustion of 1 mole of a substance under standard conditions, all reactants and products being in their standard states

Question 10

Define Hess’ Law

Answer 10

Hess’ Law states that the enthalpy change of a reaction depends on only the initial and final states and is independent of the route taken

Question 11

What are all 10 types of energies?

Answer 11

1 - Heat 
2 - Solar/light
3 - Electrical
4 - Chemical potential
5 - Gravitational potential
6 - Elastic potential
7 - Sound
8 - Nuclear
9 - Kinetic
10 - Ionisation

Question 12

What does The First Law of Thermodynamics state?

Answer 12

‘Energy can be converted from one form to another and cannot be created of destroyed’

Question 13

On enthalpy change graph, which reaction has the product with more energy and which reaction has the reactants with more energy?

Answer 13

Exothermic - reactants have more energy than products

Endothermic - products have more energy than reactants

Question 14

What is the equation for the enthalpy change of a reaction

Answer 14

Enthalpy of products - Enthalpy of reactants

Question 15

Give two examples of endothermic reactions

Answer 15

• Thermal decomposition

- Photosynthesis

Question 16

Name the two types of bond fission

Answer 16

• Homolytic fission

- Heterolytic fission

Question 17

Describe homolytic fission and draw an equation of the homolytic bond breaking of chlorine

Answer 17

• Bond breaks equally
• One electron goes to each atom
• This is shown by half arrows (not needed in equation)
• Two radicals are the products of homolytic fission
• A radical is a species with an unpaired electron

In the equation the reactant should be Cl2 with two Cl products each with a dot to show the presence of an electron (making them both radicals)

Question 18

Describe heterolytic fission and draw an equation of the heterolytic bond breaking of chlorine

Answer 18

• Bond breaks unequally
• Both electrons go to one atom
• Shown by full arrow
• Makes one positive and one negative ion

In the equation the reactant should be Cl2 with two Cl products. Each Cl product should have squared brackets around it. One should have two dots for electrons and this should have a negative charge. The other should then have a positive charge

Question 19

Is energy required or released when breaking bonds?

Answer 19

Required (Endothermic)

Question 20

Is energy required or released when making bonds?

Answer 20

Released (Exothermic)

Question 21

How do you calculate the enthalpy change of a reaction

Answer 21

Equation:

Total heat energy = mass x specific heat capacity x temperature change

• Specific heat capacity is 4.18 unless stated otherwise
• Mass of the species that changes temperature (usually water)
• To remember the equation think Q = MCAT