Exothermic and endothermic reactions

• Explain how a covalent bond holds two atoms together.

  electrostatic force of attraction between shared pair of negatively charged electrons and both positively charged nuclei
• Explain how a catalyst increases the rate of the reaction.

  because a catalyst provides an alternative pathway that has lower activation energy
• Explain why the incomplete combustion of methane is dangerous.

  Carbon monoxide is made this combine with the blood
• Explain why, in terms of the energy involved in bond breaking and bond making, the combustion of methane is exothermic.

  Energy is taken in and energy is given out when bonds are made. the energy given out is greater than the energy taken in
• State the law of conservation of energy.

  Energy cannot be created or destroyed, only transferred
• How does the law of conservation of energy apply to chemistry?

  In all chemical reactions, energy is either transferred to the surroundings or from the surroundings.
• What is an exothermic reaction? 


  A reaction where energy is transferred to the surroundings
• Give three examples of exothermic reactions.

  Combustion, oxidation and neutralisation.
• What happens to the temperature of the surroundings during an exothermic reaction?

  It increases.
• What is an endothermic reaction?

  A reaction where energy is absorbed from the surroundings.
• Give two examples of endothermic reactions.

  Thermal decomposition reactions, citric acid reacting with sodium hydrogencarbonate.
• What happens to the temperature of the surroundings during an endothermic reaction?

  It decreases.
• State two uses of exothermic reactions.

  Self-heating cans, hand warmers.
• State one use of endothermic reactions.

  Some cooling sports injury packs.
• Define activation energy.

  Minimum energy particles need to collide and react successfully.
• What label goes on the y-axis of a reaction profile?

  Energy.
• For an exothermic reaction profile, explain which is higher in energy,

  reactants or products?

  Reactants because they lose energy to surroundings.
• For an endothermic reaction profile, explain which is higher in energy,

  reactants or products?

  Products because they gain energy from surroundings.
• Explain why activation energy arrows always start in line with reactants and point up to the top of the peak.

  Because energy is being absorbed by reactants.
• Explain why the enthalpy arrow points down from reactants to products in an exothermic profile.

  Overall, reactants lose energy to surroundings.
• Explain why the enthalpy arrow points up from reactants to products in an endothermic profile.

  Overall, reactants gain energy from surroundings.
• What difference would you see in reaction profiles if a catalyst is used?

  A lower peak and lower activation energy.
• Explain how a catalyst works. 


  Allow reactions to proceed at a lower activation energy using a different reaction pathway.
• What type of energy change is bond breaking in reactants?

  Endothermic.
• What type of energy change is bond making in products?

  Exothermic.
• What is the template we can use to simplify the bond energy explanation for exothermic changes?

  Energy absorbed breaking bonds in reactants is less than energy released making bonds in products.
• What is the template we can use to simplify the bond energy explanation for endothermic changes?

  Energy absorbed breaking bonds in reactants is more than energy released making bonds in products.
• What is the equation for calculating energy changes?

  Enthalpy = bond energy reactants - bond energy products
• How do you get the bond energy of reactants in an energy change

  calculation?

  Add up all the bond energies of all the bonds in the reactants
• Explain why an exothermic energy change is a negative value.

  Overall, energy is lost to surroundings.