Introduction to Enthalpy | A-level Chemistry | OCR, AQA, Edexcel

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The key points covered in this video include:
1. Conservation of Energy
2. What is Enthalpy?
3. Standard Conditions
4. System and Surroundings
5. Enthalpy Change
6. Enthalpy Profile Diagrams
7. Activation Energy
8. Enthalpy Terms - Enthalpy
Change of:
a) Reaction
b) Combustion
c) Formation
d) Neutralisation
Conservation of Energy
Chemical bonds are the forces of attraction that bind atoms together. Chemical energy lies within these chemical bonds, It is a form of potential energy. In chemical reactions, energy is changed from one form to another. E.g. Chemical Energy may change to thermal energy. No energy is lost. It is converted from one form to another.
What is Enthalpy?
Enthalpy, H, is the thermal energy that is stored in a system. We can’t measure the directly enthalpy of products and reactants. Instead, we can measure the amount of energy that is absorbed or released to the surroundings. The method in which this is done can vary. You can measure the change in energy by looking at the change in thermal energy. Temperature Increase. Heat gain to surroundings, Heat loss in a chemical system. Temperature Decrease. Heat loss to surroundings, Heat gain in a chemical system. Enthalpy change, ΔH, is the heat energy change at a constant pressure.
Standard Conditions
Δ - Indicates a change. H - Enthalpy. Θ - Indicates standard conditions. Pressure: 100 KPa (100,000 Pa), Temperature: 298K. Standard states are the states which substances are in under standard conditions. For example, the standard state of water is liquid and the standard state of magnesium is solid.
System and Surroundings
These are part of the terminology used to discuss components of chemical reactions. System. The chemical reaction. Atoms, Bonds. Surroundings. Everything else!
Enthalpy Changes
In general, the enthalpy change is the difference between the enthalpy of the products and the reactants. From the overall enthalpy change, we can classify reactions as either: Exothermic: Release Heat. Endothermic: Absorb Heat.
Enthalpy Profile Diagrams
Exothermic Reactions. The Enthalpy of the products is smaller than the enthalpy of the reactants, The chemical reaction releases heat, There is a heat loss from the system to the surroundings, ΔH is negative.
Activation Energy
The activation energy is the minimum energy required to start a reaction. It is like rolling a ball to the top of the hill, In order to allow the ball to roll down the other side. Exothermic Reactions. The products have a lower energy than the reactants, Nevertheless, an input of energy is reqyured to break the initial bonds and start the reaction, Once the activation energy has been overcome, the energy output of the reaction provides enough energy to sustain the reaction. The reaction becomes self-sustaining. Endothermic Reactions. The products have a higher energy than the reactants.
Enthalpy Change of Reaction
The energy change associated with a given reaction.
Enthalpy Change of Formation
The energy change that takes place when 1 mole of a compound is formed from its constituent elements in their standard state under standard conditions.
Enthalpy Change of Combustion
The energy change that takes place when 1 mole of a substance is completely combusted.
Enthalpy Change of Neutralisation
The energy change associated with the formation of 1 mole of water from a neutralisation reaction under standard conditions.

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[@anugyasubedi3562]

In the last question shouldn't it be 173 - 9? Why did she take it as positive?

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[@oluomaonugha387]

Thank you so much I was really struggling with this concept

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Pls if u can explain Extractions of elements

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[@aishaabbasi1544]

when calculating the activation energy for that reverse reaction, why do you use only 9 and not -9, as it says in the question

[@eatinsomtin9984]

This is as its the reverse reaction sister, meaning that you would have to add the 9 as you are trying to see what the total energy was before the reaction took place.

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Do we have to do full book for our exams ? or some specific topics?

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[@lifelyrics5659]

Isn't enthalpy change of combustion burnt is excess O2?

[@MB-gk1ou]

Yeah it’s the same as saying it’s completely combusted- if it was burnt in not in excess of oxygen, then it would be partial combustion

[@GLB20]

Does breaking bonds needs energy or releases energy?

[@sophiegeorge9601]

Breaking bonds needs energy, it is endothermic

[@ankitdoddamane391]

@@sophiegeorge9601 You can't call it endothermic, because exothermic and endothermic is a term for the overall reaction, not just a part of it

[@Meekwinner]

Energy taken in

[@zarakkhan1134]

@@sophiegeorge9601 energy is stored in the initial bonds or old bond of the reactant when we add energy the bond absorbs energy and as a result bonds get break....,,after bonds is brokenwhere does the initial energy and the thermal energy which we provided went ?????

[@tomrobingray]

"The total enthalpy, H, of a system cannot be measured directly." - wikipedia
"Enthalpy is then a precisely measurable state variable" - hyperphysics
?????

[@damnman5113]

Dancing Tom Enthalpy of the system cannot be measure directly, I think the enthalpy change can be measured directly but not Enthalpy of the system. That,s why we need born haber cycle to somewhat measure the enthalpy of the reaction.

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