Enthalpy changes (A level Chem)
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Enthalpy
Enthalpy - the driving force for life. We owe everything on this planet to the fact that we are exactly the correct distance away from the sun. All of our energy whether directly or indirectly comes from the sun.
Man has however become dependent on other energy sources.
* Fossil fuels, whose energy ultimately derives from the sun are burnt in huge quantities to fuel our means of transport,heat our homes etc. * Within our bodies energy is derived in a huge number of ways, from the tiny charges of electricity our nerves generate,to the energy derived from ATP in our muscles, a process known as hydrolysis.
With the exception of nuclear power, all these reactions are driven by chemistry. The relationship between chemical reactions and heat changes is often referred to as THERMOCHEMISTRY, which is part of a huge study area known as THERMODYNAMICS.
Enthalpy Change
The value of a heat change depends upon the conditions under which the change is measured.
The Heat Change measured at constant pressure is known as the enthalpy change and is given the symbol:
Standard Conditions
Internationally agreed convention under which enthalpy changes are measured. Under these conditions, an enthalpy change is called a standard enthalpy change.
- Temperature of 298K
- Pressure of 101,325 N m-2 (1 atmosphere).
Given the symbol:
Units
Heat is a form of energy and both are measured in the same units. The SI unit of energy is the Joule (J), although this is too small for most chemcial reactions, so the kiloJoule (kJ) is used.
Never use the term calorie, which is for food only, although some books may include calories in questions. 1 cal = 4.184 J.
Sign Convention
In terms of energy (or enthalpy changes), there are two types of chemical reaction:
Exothermic
Exothermic reactions do the following:
- Get hot to the touch
- Liberate heat energy to the surroundings
- Result in a general increase in the temperature of the surroundings
Endothermic
Endothermic Reactions do the following
- Get cold to the touch
- Absorb heat energy from the surroundings
- Result in a general decrease in the temperature of the surroundings
The sign convention states the following:
An enthalpy change can be represented as follows:
In reality carbon has different allotropes and as a result it is strictly speaking necessary to specify which of the allotropes are involved in the reaction. Carbon normally has either graphite (the standard form) C(s,graphite) or diamond C(s,diamond).
Enthalpy Diagrams
All Chemical Reactions involve the following
- Breaking of bonds
- Making of new bonds
- The reactants have a certain amount of enthalpy (internal energy).
- This is the same for products
The Enthalpy change of a reaction = difference in enthalpy between reactants and products.
This can be visualised on a graph for an exothermic/endothermic reaction.
An endothermic change would show the products higher in energy than the reactants. Delta H is calculated from this sort of data in the following way:
Enthalpy diagrams also indicate the relative stability of the reactants and products
- Lower enthalpy = more stability, so in an exothermic reaction the products are more stable than the reactants.
- As all substances wish to exist at a lower enthalpy (more stabilty), exothermic reactions are more common.
- For endothermic reactions the reverse is true.
Changes of State
Physical changes have enthalpy changes associated with them:
So the enthalpy of fusion of water would be the enthalpy change for the reaction:
It would be called fusion regardless of solid to liquid or liquid to solid. Remember:
1.From solid to liquid is endothermic (heat goes in) 2.From liquid to solid is exothermic (heat released)
Links
[1] Edweb - source of many things chemistry
Contributors
Julian Whitehead
