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  • Created by: helena
  • Created on: 29-05-13 18:26


Enthalpy, H,is the heat content that is stored in a chemical system

An enthalpy change ΔH is 

  • the heat exchange with the surroundings during a chemical reaction, at constant pressure
  • ΔH = ΔHproducts – ΔHreactants
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Exothermic reactions

  • The enthalpy of the products is smaller than the enthalpy of the reactants
  • There is a heat loss from the chemical system to the surroundings
  • ΔH has a negative sign
  • combustion and respiration are good examples of exothermic reactions


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Endothermic Reactions

The enthalpy of the products is greater than the enthalpy of the reactants

there is a heat gain to the chemical system from the surroundings

ΔH is positive

Photosynthesis is a good example of an endothermic reaction


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Enthalpy profile diagrams - catalyst (exo)


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Enthalpy profile diagrams - catalyst (endo)


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Standard Enthalpy Changes

Standard conditions are:

  • a pressure of 1 atm (100 kPa)
  • temperature of 25oC (298K)
  • a concentration of 1 moldm-3 

For a standard enthalpy change any substance must be in its standard state

The Standard Enthalpy change of reaction ΔHr θ

The Standard Enthalpy change of combustion ΔHc θ

The Standard Enthalpy change of Formation ΔHf θ

The enthalpy change of formation of an element is defined as 0 kJmol-1

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Determination of Enthalpy Changes


  • m = mass of surroundings involved in reaction
  • c = the specific heat capacity
  • ΔT = the temperature change

ΔT = Tfinal - Tinitial

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Enthalpy Change of Combustion

If we compare the values, the experimental value of ΔHc θ is different to the theoretical value. This could be because:

  • There may have been incomplete combustion
  • There may have been heat loss to the surroundings
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Bond Enthalpies

Bond Enthalpy is the enthalpy change that takes place when breaking by homolytic fission one mole of a given bond in the molecule of a gaseous species

MEXOBENDO - making bonds is exothermic, breaking bonds is endothermic

In an exothermic reaction the bonds that are formed are stronger than the bonds that are broken

In an endothermic reaction the bonds that are formed are weaker than the bonds made

ΔH = Σ(bond enthalpies bonds broken) - Σ(bond enthalpies bonds made)

Hess' Law states that, if a reaction can take place by mroe than one routs and the initial and final conditions are the same, the total enthalpy change is the same for each route

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Enthalpy changes from ΔH f θ


ΔH[reaction] = ΔH[products] - ΔH[reactants]

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Enthalpy changes from ΔH c θ


ΔH[reaction] = ΔH[reactants] - ΔH[products]

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Rates of Reaction - Collision Theory

The rate of reaction is defined as the change in concentration of a reactant or a product in a given time

rate = change in concentration / time

If you increase the temperature particles will have more energy, meaning they are more likely to collide

If you increase the pressure the particles will be closer together meaning they are more likely to collide

If you increase concentration there will be a greater probability of collisions

A smaller surface area increases rate of reaction

Adding a catalyst speeds up rate by decreasing the Activation Energy

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A catalyst increases the rate of a chemical reaction without being used up. The catalyst may form an intermediate, but will be regenerated at the end

A catalyst lowers the activation energy by providing an alternative route, with a lower energy

Both start and end at the same point on the enthalpy profile diagram

If the Ea is lowered then less energy is required for the molecule to react. This saves energy cost

With less energy required, less fossil fuels are burnt, and less carbon dioxide will be released into the atmosphere

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Economic Importance of Catalysts

Products can be made more quickly and easily, requiring less energy

Biocatalysis is any process in which the catalyst is an enzyme

Heterogeneous catalysis is catalysis of a reaction in which the catalyst is in a different state to the reactants

Homogeneous catalysis is catalysis of a reaction in which the catalyst is in the same state as the reactants

Industrial use of enzymes has many benefits:

  • Lower temperature and pressure can be used, saving energy and costs
  • Enzymes often allow a reaction to take place which forms pure products, with no side reactions. Theyt often remove the need for complex separation techniques, reducing costs
  • conventional catalyst are often poisonout and can pose disposal probleems at the end of their industrial life. Enzymes are biodegradable
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Boltzmann distribution


  • The boltzmann distribution is the distribution of energies of molecules at a particular temperature
  • The area under the curve = number of molecules in a sample. This doesn't change
  • Onlly the molecules with energies higher than Ea are able to react
  • The curve never touches the x-axis
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Effect of temperature on the Boltzmann graph


At a higher temperature the peak moves to a higher energy with a lower height

A greater proportion of molecules exceeds Ea, rate of reaction increases

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Affects of a catalyst to the Boltzmann graph


A greater proportion of molecules exceeds the lower Ea, rate increases

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Chemical Equilibrium

Dynamic Equilibrium is the equilibrium that exists in a closed system when the rate of the forward reaction is equal to the rate of the reverse reaction. A system is in dynamic equilibrium when the concentrations of the products and reactants remain the same

The position of equilibrium can be altered by changes in pressure, concentration or temperature

Le Chatelier's Principle states that when a system in dynamic equilibrium is subjected to change, the position of equilibrium will shift to minimise change

Increase the pressure and the position of equilibrium will shift to the side with the least moles

Increase the concentration of the reactants and the system will try and remove it by making more products

Increasing the temperature will favour the endothermic direction

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