C5 Energy Changes

Question 1

describe exothermic reactions+ examples

Answer 1

• heat energy is released - temp of surroundings increases
• reactants at a higher energy level than products
• energy released through the formation of bonds is greater than the energy needed to break bonds
• combustion, oxidation, neutralisation
• self-heating cans, hand warmers

Question 2

describe endothermic reactions+ examples

Answer 2

• heat energy is taken in - temp of surroundings decreases
• reactants at a lower energy level than products
• energy released through the formation of bonds is less than the energy needed to break bonds
• thermal decomposition, citric acid+ hydrogen carbonate
• cold packs

Question 3

state a method used to find temp changes in reacting solutions (exo/endo thermic reaction) to measure energy transfer practical

Answer 3

• measure a volume of two reactants (25cm3)
• place one liquid in a polystyrene cup, then place an insulating lid on top with a thermometer through it
• record the starting temp of the solution
• add the second solution and record the temp
• wait 30secs and record the temp again
• repeat until there is no change in temp
  -> if the temp increases the reaction is exothermic
  -> insulating lid+ polystyrene limit dissipation of energy to surroundings
  -> other variables that affect temp change: concentration of a reactant, reactivity of metal being used

Question 4

describe collision theory and activation energy

Answer 4

• collision theory: reactant particles must collide with each other
  -> providing particles have sufficient energy to start a reaction
• a collision that produces a reaction is a
  successful collision
• activation energy - minimum amount of
  energy needed for a collision to be successful
• activation energy is diff for different reactions

Question 5

describe bond energies

Answer 5

• when a bond is formed energy is released (exothermic, - kJ/mol)
• to break a bond energy needs to be supplied (endothermic, + kJ/mol)
• bond energies are different for different bonds
• energy required to make a bond = energy required to break the same bond

Question 6

state the equation for calculating energy changes

Answer 6

energy to break bonds - energy to form bonds
total bond energy of reactants - total bond energy of products
measured in kJ/mol