Chemistry- Energy Calculations

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Energy

Fuels are used to provide us with energy

Larger the rise in temperature....the more energy released

SHC= specific heat capacity (amount of energy to raise 1g of water by 1 degree)

Q= mc^T

  • Burning fuel raised temp of 50cm3 of water from 16-41 degrees.
  • How much energy was released by the fuel?
  • mass water = 50g     SHC= 4.2J
  • Rise in temp = 41-16 - 25
  • Q= 50X4.2X25
  • 5250J

 

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Energy transfers in solutions

Temperature change - measured using a CALORIMETER

Can use polystyrene cup and thermometer to act as calorimeter

A+B - C

60cm3 of solution containing 0.1 moles of A is mixed with 40cm3 of a solution containing 0.1moles of B. Temp before mix= 19.6 Temp after = 26.1 (Q=mc^T)

  • Temp change = 26.1- 19.6 = 6.5
  • mass = 60+ 40 = 100g
  • Q= 6.5x100x4.2 = 2.73 J = 0.1 moles
  • 2.73x10 = 27.3 - 1 mole :)

 

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Energy level diagrams

Show us the relative amount of energy contained in the reactants and products

Exothermic reactions = products are at lower energy level than the reactants.

  • when reactants form products - energy is RELEASED!
  • Temp of surrondings increases, surrondings get hotter!

Endothermic reactions = products at higher energy level than reactants.

  • energy is absorbed from the surrondings
  • Temp of surronding decreases, surrondings get colder!!

 

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Energy level diagrams

Show us the relative amount of energy contained in the reactants and products

Exothermic reactions = products are at lower energy level than the reactants.

  • when reactants form products - energy is RELEASED!
  • Temp of surrondings increases, surrondings get hotter!

Endothermic reactions = products at higher energy level than reactants.

  • energy is absorbed from the surrondings
  • Temp of surronding decreases, surrondings get colder!!

 

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Activation Energy and Catalysis

Minimum amount of energy needed to start a reaction = ACTIVATION ENERGY!

Catalysts = increase the rate of reaction

  • provide an alternative pathway to the products
  • Means higher proportion of reactant particles can now react.

 

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Bond breaking & Bond Making

Breaking bonds is an endothermic process, as energy needs to be supplied - taken from the surrondings

When bonds formed....= energy is released - EXOTHERMIC REACTION

Energy released when new bonds form(as products made) is MORE than energy needed to break bonds in reactants.

  • transfers energy to surrondings = exothermic!

Energy needed to break bonds in reactants is MORE than energy released when new bonds form

  • transfers energy to reaction = ENDOTHERMIC!!

 

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Bond Energy

Energy needed to break bond between two atoms

Measured in KJ/mol (work out energy change ^H)

ENERGY CHANGE = energy released in new bonds - (minus) energy needed to break bonds.

If positive figure.....energy released is EXOTHERMIC

 

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Fuel Issues

Carbon dioxide gas, become major cause of global warming.

Scientists developing hydrogen as fuel, as produces NO pollutants

  • waste products = water

Efficient use of energy = oxidising hydrogen in fuel cell.

  • energy released transferred to eletrical energy

 

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