C7 ENERGY CHANGES (9-1 AQA GCSE)

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ENDOTHERMIC & EXOTHERMIC REACTIONS

ENERGY IS CONSERVED IN REACTIONS

energy before a reaction = energy after a reaction

ENDOTHERMIC- takes in energy ('in'do)

EXOTHERIMC- gives out energy ('exit'o)

In an exothermic reaction, energy is given out. The overall temperature of the surroundings increases, (the thermomiter shows an increase in temp). In an endothermic reaction, energy is taken in. The overall temperature of the surroundings decreases, (the thermomiter shows an decrease in temp).

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REACTION PROFILES

(http://en.citizendium.org/images/f/fb/ExoEndo_Reax.png)I know that the first diagram is for an exothermic reaction because the products are at a lower energy level than the reactants, therefore energy was given out. The energy change will be equal to the difference in energy between reactants & products. I know that the second diagram is for an endothermic reaction because the products are at a higher energy level than the reactants, therefore energy was taken in. The energy change will be equal to the difference in energy between reactants & products.

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BOND ENERGY CALCULATIONS

the energy change = energy in – energy out

(remember negative enery change = exothermic) and (positive energy change = endothermic) e.g

bond              H-H      C-H       O=O    O-H       C=O

bond energy  436       413        496      463       743

Calculate the energy transfer when methane undegoes combustion:

CH4  +  2O2  --  CO2  +  2H2O

(C-H)4  +  (O=O)2 -- (C=O)2 + (O-H)4

1625+992 --  1486+1852

2644 - 3338

=  -694 KJ/MOL

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CHEMICAL CELLS AND BATTERIES

  • CELLS= A cell is a system that contains chemicals which react and produce energy
  • a cell contains two different electrodes in contact with an electrolyte
  • the two electrodes must be able to conduct electicity and are usually metals
  • the electrolyte is a liquid that conducts electricity and contains ions which react with electrodes
  • the voltage of a cell will depend on the type of electrode and the type of metal
  • BATTERIES= are formed by connecting two or more cells together in a series
  • the voltage of a battery is greater than any of the individual cells

NON RECHARABLE AND RECHARGABLE BATTERIES:

Over time the reacting particles- the ions in the electrolyte and the metal ions on the elctrode- get used up and turned into the products of the reaction. once any one of the reactants are used up, the reaction can't happen and so no electricty can be produced. the products can be turned into the reactants, so the cell cant be charged. in a rechargable cell, the reaction can be reversed by connecting it to an axternal electric current

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FUEL CELLS

A fuel cell is a cell thats supplied with fuel and oxygen and efficiently transfers the energy release by the reaction between them as electricity.

Fuel cells are fed with an external source of fuel (e.g., hydrogen), plus oxygen or air. The fuel is oxidised electrochemically within the fuel cell to produce a voltage. The electricity provided by the fuel cell can be used to run an electric vehicle or to power other equipment.

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Comments

elenaluisa

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Really useful resource :)

sulaimanmd

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John765

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it doesn't let me add it to my favourites :(

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