topic 5 - energy changes

• What are the two types of reactions based on energy changes?

  Exothermic and Endothermic reactions
• What happens to energy during chemical reactions?

  Energy is either absorbed or released
• What indicates that a reaction has absorbed energy from the surroundings?

  A fall in temperature
• What is an exothermic reaction?

  A reaction that transfers energy to the surroundings
• What is the best example of an exothermic reaction?

  Burning fuels (combustion)
• What type of reaction is neutralization?

  Exothermic reaction
• What happens when sodium is added to water?

  It releases energy, indicating an exothermic reaction
• What are some everyday uses of exothermic reactions?

  • Hand warmers using oxidation of iron
  • Self-heating cans of hot chocolate and coffee
• What is an endothermic reaction?

  A reaction that takes in energy from the surroundings
• What is a common example of an endothermic reaction?

  The reaction between citric acid and sodium hydrogencarbonate
• What occurs during thermal decomposition?

  Heating calcium carbonate causes it to decompose into calcium oxide and carbon dioxide
• How are endothermic reactions used in everyday life?

  • Used in sports injury packs to cool without a freezer
• Why is it important to measure energy changes in reactions?

  To know how much energy is absorbed or released
• What is the practical method to measure energy released in a reaction?

  Mix reagents in a polystyrene cup and measure the temperature change
• What is a major problem when measuring energy changes in reactions?

  The amount of energy lost to the surroundings
• How can you reduce energy loss during measurements?

  By insulating the polystyrene cup and covering it with a lid
• What types of reactions can the measurement method be used for?

  Neutralisation reactions or reactions between metals and acids
• How can you test the effect of acid concentration on energy released in a neutralisation reaction?

  1. Prepare different concentrations of hydrochloric acid.
  2. Mix with sodium hydroxide in a polystyrene cup.
  3. Measure temperature change over time.
• What do reaction profiles show?

  The relative energies of reactants and products and energy changes during the reaction
• In an exothermic reaction, where are the products in relation to the reactants on a reaction profile?

  The products are at a lower energy than the reactants
• What does the initial rise in energy on a reaction profile represent?

  The activation energy needed to start the reaction
• What is activation energy?

  The minimum amount of energy needed for reactants to collide and react
• In an endothermic reaction, where are the products in relation to the reactants on a reaction profile?

  The products are at a higher energy than the reactants
• What is bond energy?

  • The energy required to break a bond
  • Varies slightly depending on the compound
• What happens to energy when bonds are broken and formed?

  Energy is absorbed to break bonds and released when new bonds are formed
• In exothermic reactions, what is true about the energy of bonds?

  The energy released by forming bonds is greater than the energy used to break them
• In endothermic reactions, what is true about the energy of bonds?

  The energy used to break bonds is greater than the energy released by forming them
• How do you calculate the energy change for a reaction using bond energies?

  1. Sum the energies needed to break bonds in reactants.
  2. Subtract the energy released by forming bonds in products.
  3. Overall energy change = energy required - energy released.
• What is the energy change for the reaction between H₂ and Cl₂ forming HCl?

  • 184 kJ/mol
• Why can't you compare overall energy changes of reactions without knowing bond energy differences?

  Because the numerical differences in bond energies determine energy release
• What is an electrochemical cell?

  • A system made of two different electrodes
  • In contact with an electrolyte
  • Produces electricity through chemical reactions
• What materials are typically used for electrodes in an electrochemical cell?

  Metals
• What role does the electrolyte play in an electrochemical cell?

  It contains ions that react with the electrodes
• What happens when the electrodes in an electrochemical cell are connected by a wire?

  The charge flows and electricity is produced
• What can be used to measure the voltage of an electrochemical cell?

  A voltmeter
• What factors affect the voltage of a cell?

  Different metals reacting with the same electrolyte
• What is an electrochemical cell?

  An electrochemical cell is a system made up of two different electrodes in contact with an electrolyte.
• Why must the electrodes in an electrochemical cell be able to conduct electricity?

  Because they need to facilitate the flow of charge to produce electricity.
• What is the role of the electrolyte in an electrochemical cell?

  The electrolyte is a liquid that contains ions which react with the electrodes.
• How does a charge difference get set up in an electrochemical cell?

  The chemical reactions between the electrodes and the electrolyte create a charge difference.