Topic 5 - Energy changes

• What must an energy change be accompanied with?

  an absorption or release of energy from the system
• What is the conservation of energy?

  Energy cannot be created or destroyed, only transferred from one form to another.
• What is an exothermic reaction?

  A chemical reaction that releases heat to its surroundings.
• What is the energy store of reactants and products in exothermic reactions?

  The energy stored by the reactants is more than the energy stored by the products
• What is the overall energy change in exothermic reactions?

  Negative
• Examples of exothermic reactions

  • combustion
  • neutralisation
  • Oxidisations
• Uses of exothermic reactions

  • hand warmers
  • Self heating cans
• What is an endothermic reaction?

  a chemical reaction that absorbs heat from its surroundings, resulting in a decrease in temperature.
• What is the energy of reactants and products in endothermic reactions?

  Reactants have lower energy than products.
• Examples of endothermic reactions

  • citric acid and sodium hydrocarbonate
  • Thermal decomposition
• Uses of endothermic reactions

  sports injury packs
• What is the overall energy change of an endothermic reaction?

  Positive
• What is the activation energy?

  The minimum energy required for a chemical reaction to occur.
• How is overall energy change displayed on a reaction profile?

  the difference in height between the energy of reactants and energy of products
• Summary of method of temperature changes practical

  • measure 30cm3 dilute hydrochloric acid in cup
  • Measure temp and place in beaker
  • Add 5cm3 sodium hydroxide and measure final temp
  • Repeat
• What is being continuously added in the temperature changes practical?

  5cm3 sodium hydroxide
• What does the highest temperature signify in temperature changes practical?

  The volume of sodium hydroxide that was needed to fully neutralise the hydrochloric acid
• What is sodium hydroxide and hydrochloric acid an example of?

  A neutralisation (exothermic reaction)
• Why does the temperature start to fall in temperature changes practical?

  Because the reaction is complete and no more heat energy is being released, therefore it will decrease naturally and dissipate to surroundings
• Hazard of temperature changes practical?

  Hydrochloric acid and sodium hydroxide are both irritants
• Safety precaution of temperature changes practical?

  wear safety goggles and wash hands immediately after contact to minimise contact with solution
• Control variables of temperature changes practical?

  Energy dissipated (insulation), volume of hydrochloric acid, volume added of sodium hydroxide, starting temperature when repeating
• Independent variable of temperature changes practical

  overall volume of sodium hydroxide
• Dependant variable of temperature changes practical

  Temperature of solution
• Why is solution contained in insulated cup in temperature changes practical?

  to minimise heat energy dissipation
• What further steps could be taken to make experiment more accurate?

  An insulated lid
• What kind of error would not be eliminated by mean average in temperature changes practical?

  Systematic error
• What process is bond breaking?

  Endothermic
• What process is bond making?

  Exothermic
• What happens to energy is bond breaking?

  Energy is absorbed from surroundings
• What happens to energy is bond making?

  Energy is transferred to surroundings
• What is a bond energy?

  Amount of energy required to break of form a bond
• Equation for overall energy change?

  Overall energy change = total energy of bonds broken - total energy of bonds made
• What type of reaction has low energy being used to break bonds and high energy being used to make bonds?

  Exothermic
• What type of reaction has high energy being used to break bonds and low amount of energy being used to make bonds?

  Endothermic
• What is ‘energy in’?

  Total amount of bond energies for reactants
• What is ‘energy out’?

  Total amount of bond energies for products