Chapter 6~Enthalpy Changes Flashcards
Definitions, key terms and formula
Definition of exothermic :
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.
The surroundings include 4 things
- The solvent
- The air around the test tube
- The test tube itself
- Anything dipping into the test tube e.g. A thermometer
Etc.
4 examples of exothermic reactions
- An acid ➕ a metal
- Combustion 🔥 of fuels ⛽
- Oxidation of carbohydrates in the bodies of animals 🐯🦍🐶🐑🐩🐺 and plants 🌿
- the reaction of water 💦 with quicklime (calcium oxide)
Definition of endothermic :
°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
In an endothermic and exothermic reaction what happens to the temperature of the surroundings?
Exothermic = increases
Endothermic = decreases
4 Examples of endothermic reactions
- Hydrocarbonate reacts with aqueous citric acid
- Decomposition of limestone by heating 🔥
- Photosynthesis
- Dissolving certain ammonium salts in water 💦 e.g. Ammonium chloride used in a cooling pack to treat injuries
Definition of enthalpy change :
➕ formula
➕ unite
°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
Definition of enthalpy profile diagram :
°a diagram showing enthalpy change from reactants to products along the reaction pathway
Enthalpy profile diagram :
For a exothermic reaction what always has a higher energy level?
The Reactants
Enthalpy profile diagram:
What always has a higher energy level for an endothermic reaction?
The Products
Definition of standard conditions :
° 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
What are the 3 standard conditions?
- Pressure of 10^5 Pa (100kPa)~ approximately normal atmospheric pressure
- Temperature of 298K (25°C)
- Each substance involved in the reaction has to be in their normal physical state @100kpa and 298K
Definition of Standard enthalpy change :
°an enthalpy change that takes place under standard conditions of pressure (10^5 Pa) and temperature (298K)
Name the 6 varieties of enthalpy change :
They are a method of describing enthalpy changes according to the type of reaction that takes place.
Enthalpy change of :
- Formation
- Combustion
- Neutralisation
- Solution
- Atomisation
- Hydration
In more general cases when we describe enthalpy changes of a chemical reaction, we refer to it as…
Enthalpy change of REACTION
Definition of standard enthalpy change of reaction :
° 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.
Definition of the standard enthalpy change of formation :
° 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.
Standard enthalpy change of reaction can be either:
a) exothermic
b) endothermic
c) both
c) both endothermic and exothermic
Standard Enthalpy changes of formation are :
a) exothermic
b) endothermic
c) both
C) both endothermic and exothermic
By definition, the standard enthalpy change of formation of any element in its standard state is =
Zero
Definition of standard enthalpy change of combustion : 🔥
°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.
Are standard enthalpy changes of combustions :
a) exothermic
b) endothermic
c) both
A) exothermic
What 2 enthalpy changes can be considered for :
S(s) +O2 ➡️SO2(g)
- Combustion
2 formation
Definition of enthalpy change of neutralisation :
° 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.
Are standard enthalpy changes of neutralisations :
a) exothermic
b) endothermic
c) both
d) neither
a) exothermic
Definition of the enthalpy change of solution :
°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.
Are standard enthalpy changes of solution :
a) exothermic
b) endothermic
c) both
d) neither
c) both
Definition of standard enthalpy change of atomisation ⚛ :
Plus ➕ an example
°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)
Are standard enthalpy changes of atomisation ⚛ :
a) exothermic
b) endothermic
c) both
d) neither
B) endothermic ➕
Definition of standard enthalpy change of hydration of an anhydrous salt :
°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.
According to an example in the book the standard enthalpy change of hydration of an anhydrous salt is
Assumption check with MrD
Exothermic
State Hess’s law :
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’.
The first law of thermodynamics is the Law of Conservation of energy that states…
‘energy cannot be created or destroyed’
Definition of an enthalpy cycle
°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.
Hess’s law tells us that the enthalpy change of reaction for the direct route is the same as for the…
Indirect route
The equation that shows the relationship between direct route and indirect route :
🔼Hr =🔼H1 +🔼H2
NB!!!
The direction of the arrows on the enthalpy cycle matters it can change the sign to negative!!!!
The 4 steps in the procedure of constructing an enthalpy cycle
- Write balanced equation at the top 🔝
- draw the cycle with elements (or enthalpy changes of combustion in some cases) at the bottom
- Draw all arrows, making sure they go in the correct directions
- Apply Hess’s law, taking into account the number of moles of each reactant and product
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
Combustion 🔥 products
Bond breaking and bond making definition of exothermic chemical reaction :
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.
Bond breaking and bond making definition of an endothermic chemical reaction :
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.
Definition of bond energy / bond enthalpy/bond dissociation energy ⛮ :
°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)
