topic 5 - Energy changes

• exothermic reactions


  heat is let out transferring its energy into its surroundings making a rise in temperature

  ie burning fuels (combustion), hand warmers

  products are at a lower energy rate then the reactants

  Activation energy = inital rise in energy (energy needed to start reaction)
• endothermic reactions


  heat is taken out in causing the surroundings temperature to decrease

  ie thermal decomposition

  products are at a higher energy rate then reactants, the difference in height = overall energy change during the reaction (energy taken in)
• energy transfer can be measured by Work done = force × distance
• the work done when an object moves through a certain displacement against a constant resistance is equal to the product of the mass, acceleration and squared displacement.
• Why can't we measure a total amount of energy in a molecule?

  We can't measure from zero
• What do we measure instead of total energy in a molecule?

  Energy changes
• What is the chemical reaction for burning methane?

  • Reactants: Methane and oxygen
  • Products: Carbon dioxide and water
  • Equation: $CH_4 +$$2O_2 \rightarrow CO_2 +$$2H_2O$
• What is the activation energy in a chemical reaction?

  Energy required to start a reaction
• What does the energy profile diagram represent?

  Reaction progress and energy changes
• What happens to energy during the burning of methane?

  Energy is lost to surroundings
• What are the products of burning methane?

  Carbon dioxide and water
• What is the formula for the combustion of methane?

  $CH_4 +$$2O_2 \rightarrow CO_2 +$$2H_2O$
• What does the term "energy change" refer to in a chemical reaction?

  Difference in energy before and after
• How does the energy change during the combustion of methane affect the surroundings?

  It results in energy loss to surroundings
• What is the energy change in exothermic reactions?

  Molecules lose energy
• What happens to the surroundings during exothermic reactions?

  Surroundings gain energy and get hot
• What is the energy change in endothermic reactions?

  Molecules gain energy
• What happens to the surroundings during endothermic reactions?

  Surroundings lose energy and get cold
• What are the key differences between exothermic and endothermic reactions?

  Exothermic Reactions:

  • Molecules lose energy
  • Surroundings gain energy and get hot

  
  

  Endothermic Reactions:

  • Molecules gain energy
  • Surroundings lose energy and get cold
• measuring energy changes

  • temperature change
  • cotton wool
  • lid
  • polystyrene cup
• energy transferred= energy of products - energy of reactions
• what is the collision theory?

  the way particles collide together to make a chemical reaction occur

  if particles don’t hit hard enough they won’t cause a reaction

  head on collision causes a new substance

  glancing blows don’t transfer enough energy so no reactions happen
• measuring rate on a graph = vol/time