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  • Created on: 31-05-14 13:19

Chemical energy and enthalpy

Chemical energy is a from of potential energy locked in chemical bonds.

Enthalpy is the heat content stored in a chemical system. It cannot be measured directly; instead we observe the heat changes during a chemical reaction.

The law of conservation of energy says that no heat energy is ever lost, it is simply transferred into the environment, or taken in from the environment.

An enthalpy change is when the products hold a different amount of chemical energy than the reactants. There is a heat exchange with the surroundings.

An exothermic reaction- the products hold less energy. An endothermic reaction- the products hold more energy than the reactants.

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Exothermic and exothermic reactions

Exothermic

Oxidation of fuels e.g. petrol

C8H18 +12.5O2 >> 8C02 +9H20

Respiration

Overall equation = C6H12O6 +602 >> 6CO2 + 6H20

Endothermic

Thermal decomposition of limestone

CaCO3 >>> CO2 + CaO

Photosynthesis in plants

Sugars are made from co2 and h20; light from the sun provides the energy.

6CO2 +6H2O >>> C6H12O2 + 6O2 (reverse of resouration- including energy! = 2801 kjmol)

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Enthalpy profile diagrams

  • X AXES- PROGRESS OF REACTION
  • Y AXES- ENTHALPY

Activation energy can act as a barrier to many chemical reactions taking place. It is required to break the bonds in reactants.

In exothermic reactions, an initial spark or heat is required, but the energy then produced makes the reaction self sustaining. Endothermic reactions- the activation energy is larger than energy released by making bonds.

(http://en.citizendium.org/images/f/fb/ExoEndo_Reax.png)

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Standard enthalpy change

The standard enthalpy change of reaction is the enthalpy change that accompanies a reaction in the molar quantities in a chemical equation, all reactants and products in their standard state.

Standard conditions = 25centigrade/ 298k and 1atm/100kpa

Standard state= the physical state of a substance in the conditions above.

Enthalpy of combustion-

The enthalpy change when one mol of a substance is burnt in excess oxygen to form its products, all in their standard state.

Enthalpy of formation-

The enthalpy change when one mole of a susbtance is fromed from its consitutent elemst, all in their stantard state.

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Determining enthalpy changes

You can only observe heat exchnage with the surroundings.

Specific heat capacity- the energy required raise the temperature of 1 gram of a substnace by one degree celcius.

Heat exchange with the surroundings uses the equation:

Q= mcAT Joules

You can do this usinga calorimeter.

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Enthalpy change of combustion

When writing the equation, make sure you only burn ONE mole of the substance e.g.

Al + 3/4 O2 >>> 1/2Al2O3

Experimentally:

  • measure a volume of water into a beaker
  • weigh container with fuel
  • take initial temperature of the water
  • heat water a reasonable amount, and take this temperature
  • extinguish flame, and reweigh the fuel container.

Q = m (mass of water) x c (capacity of water 4.18) x heat change (how many degrees water was heated by)

You usually find differences between experimental values and the real enthalpy. Why:

  • incomplete combustion
  • energy absorbed by equipment
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Bond enthalpies

Bond enthalpy is the enthalpy change that takes place when breaking one mole of a bond of a gaseous species, by homolytic fission.

Average bond enthalpy is the average enthlpy change of the above (over various conditions/chemical species.)

Breaking bonds requires energy, and making bonds releases energy.

  • An exothermic reaction, the bonds in products are stronger than those broken (and since bond making releases energy, more energy is released)
  • in endothermic, the bonds in products are weaker than those broken (and since breaking bonds requires energy, this means more energy is absorbed.)
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Hess' law

The enthalpy change of a reaction is independent of the route taken.

Why cant we measure the enthalpy directly sometimes?

  • high activation energy
  • slow reaction rate
  • more than one reaction taking place

So we can use bond enthalpies of combustion or formation for each individual substance, and add/minus them using a hesses cycle.

Combustion- BURN your house DOWN (arrows point down.)

Formation- BUILD it back UP (arrows point up.)

Don't forget; Elements taht exist in diatomic molecules dont have an enthlpy change of formation, so just ignore it when making an equation.

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Rates of Reaction + Collision Theory

The rate of reaction in the change of concentration of a reactant or product in a given time.

rate = change in concentration/ time

The arte can be affected by 5 things: temperature, pressure (for gases), surface area, a catalyst and concentration.

When two molecules meet, they MIGHT react; they must collide with sufficent energy to overcome the activation energy of a reaction. They must also be in the correct orientation.

Conecbtration increase means the reactants are more likely to collide, as they arecloser together, The are more frequent collisons, and therefore mire frequent successful collisions.

Increase of pressure (in gases) has the same effect of increasing concentration. Same number of moelcules occupy a smaller space, and are closer together= more collisions= more frequent collisions= higher rate of reaction.

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Catalysts

A catalyst is a particle that speeds up the rate of reaction without being used up in the process. It does this by lowering the activation energy and providing an alternatuve route for the reaction

Heterogeneous catalysts: different physical state to the reactants.

Homogenous catalusts: same physical state as the reactants.

What are catalysts used for?

  • reduce costs by lowering the amount of energy needed to begin the reaction.
  • reduced CO2 release- as less energy is needed for reactions.
  • Improving percentage yield of a reaction
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Economic importance of catalysts

Products can be made more quickly with less energy, cutting costs and  waste.

The Haber process (making ammonia) uses an iron catalyst to weaken N=N bond. The Zeigler-Natta process forpolymerisation uses TiCl3 catalysts. They are also found in catakystuc converters in cars, redcuing toxic and harmful watse products from reaching the air.

Biological Catalysts- enzymes

Biocatalysis involves enzymes, which are particularly useful for their specificty. This means less waste products and a higher atom economy. They also generally work in mid-conditions, as they can denature at extremes.

Enzymes in industry

benefits

  • biodegradable (oppossed to traditional catalysts which are often poisonous or toxic.)
  • specific allowing a purer product to be made
  • lower tempertaures save energy and costs.

examples: Ibuprofen, alcoholic drinks, detergents and cleaning products.

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Boltzmann disribution

The Boltzmann distribution is the distribution of energies of molecules at a particular temperature.

  • ON X AXES = ENERGY
  • ON Y AXES = NUMBER OF MOLECULES WTH GIVEN ENERGY

All samples of gas and liquids contain molecules with low kinetic energy, high kinetic energy and average kinetic energy.

Key features

  • The area under the curve is the total number of molecules in the sample.
  • There are NO  molecules with 0 energy- line starts at origin
  • There is NO maximum energy for a molecule- line does not touch the x axes
  • Only molecules with energy GREATER than the Ea can react.
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Factors on distribution

The effect of temperature

At higher temperatures, the average energy of all the particles in higher (and the peak is lower and moves to the right.) Total number of particles stays the same.

Molecules have more kineticenergy, and are more likely to make successful collisons. This means more are reaching the activation energy, and tehrefore the rate of reaction increases.

The effect of a catalyst

Catalysts effectively reduce the activation energy (move it to the left.) This means many more particles can react, even with lower kinetic energy. There will be more successful collisions in a certain amount of time.

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Dynamic Equilibrium

  • The reaction must be reversible.
  • The reactants and products are in equal concentrations.
  • The rate of the forwarda nd reverse reaction is equal.
  • The system must be isolated, with no chnage in materials or external conditions.

Le Chateliers principle: When a system in dynamic equilibrium is subject to change, the position of the equilibrium will shift so as to minimise the change.

Effect of concentration

If the concentration of reactants increases, the equilibrium will favour the forward reaction, so as to remove more reactants, and minimise the change. The opposite happens if more products are added.

Effect of pressure

If the pressure is increased, the equilibrium will favour the side with fewer gaseous mols, as these extert less pressure. If the pressure of the system in decreased, the equilibrium will favour the side with more gaseous mols, as these extert higher pressure.

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Equilibrium in industry

The effect of temperature

If the forward reaction of a equilibrium is exothermic, then lowering the temperature will cause th equilibrium to favour the forward reaction/the right. This is because the exothermic forward reaction opposes the decrease in temperature by releasing heat energy.

  • Increase in temperaure; equilibrium favours endothermic direction.
  • Decrease in temperature: equilibirum favours exothermic direction.

The effect of a catalyst

  • A catalayst speeds up the rate of the forward and reverse reaction equally, so does not cause the equilibrium to change.
  • However, it does help the equilibrium be established faster.
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Equilibrium in industry- the Haber Process

Chemists stive to make the highest ield of a desired product as quickly and cheaply as possible.

The Haber process uses this reaction to make ammonia:

N2 + 3H2 <> 2NH3 enthalpy change = -92 kjmol-1

Nitrogen is obtained from air by fractional distillation, and Hydrogen by reacting Methane with water. Ammonia is amde by the forward reaction

  • The forward reaction is exothermic
  • and has fewer moles of gas

Therefore, a low temperature and high pressure would cause the equilibrium to favour the forward reaction.

PROBLEMS:

  • low temperature = low rate of reaction would mean the yield was actually quite low.
  • high pressure= expensive and potentially dangerous
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Compromise in the Haber process

Temperature

Must be high enough for a reasonable rate of reaction, but not so high the equilibrium shifts to the left. 400-500 degrees is often used.

Pressure-

A high pressure is still used, but not so high its endangers the work force. 200atm is often used.

Catalyst-

an iron catalyst is used to increase the rate of reaction, allowing lower temperatures to be used. This also reduces cost and environmental impact.

Unreacted hydrogen and nitrogen are passed through the reactor agian untill all has been reacted (Ammonia is liquefied and removed.)

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Enthalpy change of reaction

Bond broken's enthalpies minue bonds made enthalpies

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