C7 - energy changes

• What is involved when a chemical reaction takes place?


  Energy is involved
• What happens during an exothermic reaction?


  Energy is transferred to the surroundings
• What would you observe during a practical exothermic reaction?


  An increase in temperature
• Name two examples of exothermic reactions.


  Combustion and respiration
• What is an everyday example of an exothermic reaction?


  Hand warmers
• What characterizes an endothermic reaction?


  Energy is absorbed from the surroundings
• Give an example of an endothermic reaction.


  Thermal decomposition of calcium carbonate
• What happens during a practical endothermic reaction?

  A decrease in temperature is observed
• What is the energy level of reactants in an exothermic reaction?

  Higher than the products
• What occurs to the surroundings' temperature in an exothermic reaction?

  The temperature increases
• What is the energy level of reactants in an endothermic reaction?

  Lower than the products
• What happens to the surroundings' temperature in an endothermic reaction?

  The temperature decreases
• What is activation energy?

  The minimum energy required for a reaction
• What do catalysts do in a chemical reaction?

  They increase the reaction rate
• How do catalysts affect activation energy?

  They lower the activation energy
• In an endothermic reaction, what is the sign of ΔH?

  Positive
• In an exothermic reaction, what is the sign of ΔH?

  Negative
• What are bond energies measured in?

  kJ/mol
• How do you calculate the change in energy for a reaction?

  Using bond energies of broken and made bonds
• What is the first step in calculating energy change for the reaction 2H₂O₂ → 2H₂O + O₂?

  Write the symbol equation
• How many O-H bonds are broken in the reaction 2H₂O₂ → 2H₂O + O₂?

  Two O-H bonds
• What is the total energy of bonds broken in the reaction 2H₂O₂ → 2H₂O + O₂?

  2148 kJ/mol
• What is the total energy of bonds made in the reaction 2H₂O₂ → 2H₂O + O₂?

  2354 kJ/mol
• What is ΔH for the reaction 2H₂O₂ → 2H₂O + O₂?

  -206 kJ/mol
• What does a negative ΔH indicate about a reaction?

  The reaction is exothermic
• What is the aim of the required practical investigation?

  To investigate temperature changes in reactions
• What equipment is used in the required practical investigation?

  Polythene cup and thermometer
• What is the first step in the method for the practical investigation?

  Gather the equipment
• What type of chemical energy does a chemical cell convert?

  Chemical energy into electrical energy
• What is a battery?

  More than one cell connected in series
• What is the difference between rechargeable and non-rechargeable cells?

  Rechargeable cells can be reused
• What happens when a non-rechargeable cell is used up?

  It can be recycled
• How do rechargeable cells work?

  Electrical current reverses chemical reactions
• What affects the voltage of a cell?

  The combination of metals used
• How does the reactivity of metals affect voltage?

  Bigger difference increases voltage
• What is a fuel cell?

  A cell that needs fuel and oxygen
• How do fuel cells produce voltage?

  By oxidizing hydrogen electrochemically
• What is produced as a byproduct of hydrogen oxygen fuel cells?

  Water
• What is the ionic equation for the reaction in a fuel cell?

  2H₂ + O₂ → 2H₂O
• What happens to hydrogen and oxygen in a fuel cell?

  Hydrogen is oxidized, oxygen is reduced