Energy in reactions

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Energy

The amount of energy that’s released in a chemical reaction can be measured by measuring the starting temperature of the reactants (making sure that they’re the same), mixing them in a polystyrene cup and measuring the temperature of them after the reaction has taken place.

A problem with energy measurements is that a lot of energy can be lost to the surroundings. The effect of this can be reduced by putting the polystyrene cup into a beaker of cotton wool for added insulation. A lid could also be put onto the cup to reduce how much energy is lost by evaporation.

This method of measuring energy transfer works for reactions where solids are reacted with water, and for neutralisation reactions.

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Exothermic and endothermic reactions

Reactions can either be exothermic or endothermic.

An exothermic reaction is one that gives out energy to the surroundings, usually in the form of heat. Exothermic reactions are shown by a rise in temperature.

An endothermic reaction is one that takes in energy from the surroundings, usually in the form of heat. Endothermic reactions are shown by a drop in temperature.

In all chemical reactions old bonds are broken so that new bonds can be formed. Energy needs to be supplied to break bonds, which makes bond breaking endothermic. Energy is released when new bonds are formed, which makes bond formation exothermic.

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Calorimetry

Calorimetry is used to figure out how much energy is released when fuels are burnt. Its usually done in copper or glass containers.

Method:

  • Pour 50g of water into the can and record its starting temperature.
  • Weigh the spirit burner and its lid.
  • Place the burner underneath the can and light the wick. Making sure to stir the water constantly.
  • Keep this going until the water reaches the set temperature (50°).
  • Put out the flame and record the final temperature of the water.
  • Weigh the spirit burner and its lid again.
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Energy released per gram

To figure out the energy released per gram of meth's you need to:

  • Minus the final mass from the original mass to work out the mass of the meth's burnt.
  • Take the starting temp. from the final temp. to see the temperature change.
  • Then put the values into the equation:
  • Energy transferred (j) = mass of water (g) x specific heat capacity (4.2j) x temperature change (°)
  • You then need to divide the energy transferred by the mass of the meth's burned to work out how much energy is released per gram of meth’s.
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Bond energies

In Exothermic reactions the products have lower energy than the reactants, because energy is given out in the reaction.

  • The rise in the energy level diagram represents the reaction reaching its activation energy while breaking old bonds.
  • Exothermic reactions are –ve

In Endothermic reactions the products have higher energy than the reactants, because energy is taken in during the reaction.

  • The rise in the energy level diagram represents the reaction reaching its activation energy while breaking old bonds.
  • Exothermic reactions are +ve.
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Activation energy and catalysts

*Activation energy is the minimum energy needed by reacting particles to break their bonds.

The activation energy of a reaction can be lowered by catalysts. Catalysts are able to lower the activation energy needed for a reaction to take place, which makes the reaction happen faster.

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