Chapter 6~Enthalpy Changes

Question 1

Definition of exothermic :

Answer 1

The term used to describe a reaction in which energy is lost to the surroundings : the enthalpy change is negative. The reactants have more energy than the products.

Question 2

The surroundings include 4 things

Answer 2

1. The solvent
2. The air around the test tube
3. The test tube itself
4. Anything dipping into the test tube e.g. A thermometer

Etc.

Question 3

4 examples of exothermic reactions

Answer 3

1. An acid ➕ a metal
2. Combustion 🔥 of fuels ⛽
3. Oxidation of carbohydrates in the bodies of animals 🐯🦍🐶🐑🐩🐺 and plants 🌿
4. the reaction of water 💦 with quicklime (calcium oxide)

Question 4

Definition of endothermic :

Answer 4

°a term used to describe a chemical reaction in which energy is absorbed for the surroundings : the enthalpy change is positive. The reactants have less energy than the products

Question 5

In an endothermic and exothermic reaction what happens to the temperature of the surroundings?

Answer 5

Exothermic = increases

Endothermic = decreases

Question 6

4 Examples of endothermic reactions

Answer 6

1. Hydrocarbonate reacts with aqueous citric acid
2. Decomposition of limestone by heating 🔥
3. Photosynthesis
4. Dissolving certain ammonium salts in water 💦 e.g. Ammonium chloride used in a cooling pack to treat injuries

Question 7

Definition of enthalpy change :

➕ formula

➕ unite

Answer 7

°The energy ⛮ transferred in a chemical reaction

🔺H= H products - H reactants

In words :

Enthalpy change = enthalpy of products minus enthalpy of reactants

Units : KJ. mol^-1

Question 8

Definition of enthalpy profile diagram :

Answer 8

°a diagram showing enthalpy change from reactants to products along the reaction pathway

Question 9

Enthalpy profile diagram :

For a exothermic reaction what always has a higher energy level?

Answer 9

The Reactants

Question 10

Enthalpy profile diagram:

What always has a higher energy level for an endothermic reaction?

Answer 10

The Products

Question 11

Definition of standard conditions :

Answer 11

° conditions of temperature and pressure that must be the same in order to compare moles of gases or enthalpy changes accurately. Standard conditions are pressure of 10^5 Pa (100kPa) and a temperature of 298 K (25 °C). As well as each substance involved in the reaction is in its normal physical state @ standard pressure and temperature

Question 12

What are the 3 standard conditions?

Answer 12

1. Pressure of 10^5 Pa (100kPa)~ approximately normal atmospheric pressure
2. Temperature of 298K (25°C)
3. Each substance involved in the reaction has to be in their normal physical state @100kpa and 298K

Question 13

Definition of Standard enthalpy change :

Answer 13

°an enthalpy change that takes place under standard conditions of pressure (10^5 Pa) and temperature (298K)

Question 14

Name the 6 varieties of enthalpy change :

They are a method of describing enthalpy changes according to the type of reaction that takes place.

Answer 14

Enthalpy change of :

1. Formation
2. Combustion
3. Neutralisation
4. Solution
5. Atomisation
6. Hydration

Question 15

In more general cases when we describe enthalpy changes of a chemical reaction, we refer to it as…

Answer 15

Enthalpy change of REACTION

Question 16

Definition of standard enthalpy change of reaction :

Answer 16

° the standard enthalpy change of reaction is the enthalpy change when the amounts of reactants shown in the equation react to give products under standard conditions. The reactants and products must be in their standard states.

Question 17

Definition of the standard enthalpy change of formation :

Answer 17

° the standard enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its elements under standard conditions. The reactants and products have to be in their standard states.

Question 18

Standard enthalpy change of reaction can be either:

a) exothermic
b) endothermic
c) both

Answer 18

c) both endothermic and exothermic

Question 19

Standard Enthalpy changes of formation are :

a) exothermic
b) endothermic
c) both

Answer 19

C) both endothermic and exothermic

Question 20

By definition, the standard enthalpy change of formation of any element in its standard state is =

Answer 20

Zero

Question 21

Definition of standard enthalpy change of combustion : 🔥

Answer 21

°the standard enthalpy change of combustion is the enthalpy change when one mole of a substance is burnt in excess oxygen under standard conditions. The reactants and products must be in their standard states.

Question 22

Are standard enthalpy changes of combustions :

a) exothermic
b) endothermic
c) both

Answer 22

A) exothermic

Question 23

What 2 enthalpy changes can be considered for :

S(s) +O2 ➡️SO2(g)

Answer 23

1. Combustion

2 formation

Question 24

Definition of enthalpy change of neutralisation :

Answer 24

° the standard enthalpy change of neutralisation is the enthalpy change when one mole of water💦 is formed by the reaction of an acid with an alkali under standard conditions.

Question 25

Are standard enthalpy changes of neutralisations :

a) exothermic
b) endothermic
c) both
d) neither

Answer 25

a) exothermic

Question 26

Definition of the enthalpy change of solution :

Answer 26

°the standard enthalpy change of solution is the enthalpy change when one mole of solute is dissolved in a solvent to form an infinitely dilute solution under standard conditions.

Question 27

Are standard enthalpy changes of solution :

a) exothermic
b) endothermic
c) both
d) neither

Answer 27

c) both

Question 28

Definition of standard enthalpy change of atomisation ⚛ :

Plus ➕ an example

Answer 28

°the standard enthalpy change of atomisation, is the enthalpy change when one mole of gaseous atoms is formed from its elements under standard conditions.

E. G. 1/2 H2 (g) ➡️ H (g)

Question 29

Are standard enthalpy changes of atomisation ⚛ :

a) exothermic
b) endothermic
c) both
d) neither

Answer 29

B) endothermic ➕

Question 30

Definition of standard enthalpy change of hydration of an anhydrous salt :

Answer 30

°the standard enthalpy change of hydration of an anhydrous salt is the enthalpy change when one mole of hydrated salt is formed from one mole of the anhydrous salt under standard conditions.

Question 31

According to an example in the book the standard enthalpy change of hydration of an anhydrous salt is

Assumption check with MrD

Answer 31

Exothermic

Question 32

State Hess’s law :

Answer 32

Hess’s law states that ‘the total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same’.

Question 33

The first law of thermodynamics is the Law of Conservation of energy that states…

Answer 33

‘energy cannot be created or destroyed’

Question 34

Definition of an enthalpy cycle

Answer 34

°a diagram showing alternative routes between reactants and products that allows the determination of one enthalpy change from other known enthalpy changes by using Hess’s law.

Question 35

Hess’s law tells us that the enthalpy change of reaction for the direct route is the same as for the…

Answer 35

Indirect route

Question 36

The equation that shows the relationship between direct route and indirect route :

Answer 36

🔼Hr =🔼H1 +🔼H2
NB!!!
The direction of the arrows on the enthalpy cycle matters it can change the sign to negative!!!!

Question 37

The 4 steps in the procedure of constructing an enthalpy cycle

Answer 37

1. Write balanced equation at the top 🔝
2. draw the cycle with elements (or enthalpy changes of combustion in some cases) at the bottom
3. Draw all arrows, making sure they go in the correct directions
4. Apply Hess’s law, taking into account the number of moles of each reactant and product

Question 38

For an enthalpy cycle for the enthalpy change of formation from enthalpy changes of combustion instead of elements being at the bottom, what are at the bottom

Answer 38

Combustion 🔥 products

Question 39

Bond breaking and bond making definition of exothermic chemical reaction :

Answer 39

If the energy required to break the bonds is less than the energy released when new bonds are formed, the reaction will release energy and is exothermic.

Question 40

Bond breaking and bond making definition of an endothermic chemical reaction :

Answer 40

If the energy required to break bonds is more than the energy released when new bonds are formed, the reaction absorbs energy and is endothermic.

Question 41

Definition of bond energy / bond enthalpy/bond dissociation energy ⛮ :

Answer 41

°the energy required to break one mole of a particular bond in one mole of gaseous molecules

Informal meaning : amount of energy needed to break a specific covalent bond. (can incorporate this into the formal meaning)