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Enthalpy and reaction rates

Enthalpy changes and reaction profiles

Enthalpy changes and reaction profiles

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Pearson Edexcel

OCR AAQAPearson Edexcel

Modern analytical techniques I


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Tutor: Sophie

Summary

​​Enthalpy changes and reaction profiles

In a nutshell

Enthalpy changes are key to understanding the transfer of heat energy during a reaction. In this summary, you will learn about exothermic and endothermic reactions, and how these relate to enthalpy. Reaction profile diagrams allow you to determine if a reaction is exothermic or endothermic and reveal key parts of the reaction.



Enthalpy

​​Enthalpy is a thermodynamic measure of the heat energy in a system and the surroundings. The more enthalpy (energy) a substance has, the more unstable it is.


Enthalpy change

Enthalpy change, ΔH\varDelta H refers to the heat energy change from breaking and making of bonds during a reaction. This occurs at constant pressure and the units are kJ mol1kJ\ mol^{-1}.


Standard conditions of enthalpy change

Standard enthalpy change, ΔHθ\varDelta H^\theta refers to the enthalpy change of a reaction measured with elements in the standard states, at standard pressure and a specific temperature. The standard pressure is 100 kPa100\ kPa​ and the specified temperature is typically measured at 298 K298\ K


Note: Standard pressure can be referred to in units of 100 kPa100\ kPa or the equivalent of 1 atm1\ atm


​These standard conditions are important to use as both temperature and pressure impact enthalpy changes. Temperature can be converted from oC^oC to KK

by adding 273273.


Example

To convert the boiling point of water from oC^o C to KK​:​

100 oC+273=373 K100\ ^oC +273= 373\ K

​​


Exothermic and endothermic reactions

In both endothermic and exothermic reactions, energy must be absorbed by the reactants to break the bonds in each molecule. Then, in the formation of new bonds by the products, energy is released. 


If the released energy is greater than the one absorbed, ΔH\varDelta H is negative and the temperature of the surroundings increases.


Example

The combustion of coal is exothermic as heat is released from burning a fuel.

C(s)+O2 (g)CO2 (g)+heatC_{(s)} + O_2\ _{(g)} \rightarrow CO_2\ _{(g)} +heat


If the absorbed energy is greater than the one released, ΔH\varDelta H is positive and the temperature of the surroundings decreases.

Example

The melting of ice is endothermic as heat is absorbed.

H2O(s)+heatH2O(l)H_2O _{(s)} + heat \rightarrow H_2O _{(l)}​​


Properties of exothermic and endothermic reactions

PROPERTY

EXOTHERMIC

ENDOTHERMIC

energy, EE​​
released
absorbed
temperature, TT​​
increases
decreases
enthalpy change, ΔH\varDelta H​​
negative
positive



Reaction profile diagrams

Reaction profile diagrams show the enthalpy change, ΔH\varDelta H in a reaction by displaying the relative enthalpies of the reactants and products. 


Activation energy, EaE_a refers to the amount of energy required to overcome the energy barrier to start the chemical reaction. This is the energy required to break the chemical bonds of the reactants.  


The reaction profile diagram of an exothermic reaction can be seen below.

Chemistry; Energetics I; KS5 Year 12; Enthalpy changes and reaction profiles


Exothermic reactions start with unstable (higher enthalpy) reactants and forms stable (lower enthalpy) products resulting in a negative ΔH\varDelta H. Exothermic reactions have a small EaE_a as less energy is needed to  overcome the energy barrier of the higher enthalpy reactants to form lower enthalpy products.


The reaction profile diagram of an endothermic reaction can be seen below.


Chemistry; Energetics I; KS5 Year 12; Enthalpy changes and reaction profiles


Endothermic reactions start with stable (lower enthalpy) reactants and forms unstable (higher enthalpy) products, resulting in a positive ΔH\varDelta H. Endothermic reactions have a large EaE_a as more energy is needed to overcome the energy barrier of the lower enthalpy reactants to form higher enthalpy products. 


Note: ΔH-\varDelta H arrows point downwards for exothermic reactions and +ΔH+\varDelta H point upwards for endothermic reactions.

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Exothermic and endothermic reactions and catalysts

Exothermic and endothermic reactions and catalysts

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Enthalpy changes and reaction profiles

Enthalpy changes and reaction profiles

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FAQs - Frequently Asked Questions

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