C7 - Energy Changes
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define exothermic reaction
A reaction that transfers energy to the surroundings
define endothermic reaction
A reaction that takes in energy from the surroundings
give 2 examples of exothermic reactions
• respiration • combustion • neutralisation • oxidation
give 2 examples of endothermic reactions
• photosynthesis • thermal decomposition • reduction
What is an energy change represented by?
ΔH
Endothermic or Exothermic? 1. breaking bonds 2. making bonds
• breaking bonds is endothermic (energy is needed to break bonds) • making bonds is exothermic
Endothermic or Exothermic? 1. positive ΔH 2. negative ΔH ΔH - energy change
• endothermic because more energy is stored in the products • exothermic because less energy is stored in the products
define activation energy
the minimum energy required for a reaction to start
What diagram is used to show if a reaction is exothermic or endothermic?
Reaction profile - potential energy against time
When do reactions occur in terms of particles?
Chemical reactions occur when reacting particles collide and have sufficient kinetic energy.
steps for bond energy calculations
• balance the equation • draw the displayed formula for all the molecules • calculate the energy required to break bonds in the reactants, ER • calculate the energy required to make bonds in the products, EP • ΔH = ER - EP
What type of reaction occurs in an electrical cell?
A redox reaction (oxidation reduction) - displacement
Write a half equation for the reaction occurring at the negatively charged electrode in a hydrogen-oxygen fuel cell higher
2H2 + 4OH- → 4H2O + 4e-
Write a half equation for the reaction occurring at the positively charged electrode in a hydrogen-oxygen fuel cell higher
O2 + 2H2O + 4e- → 4OH- opposite electrolysis of OH-
What does a fuel cell use?
A constant supply of fuels to produce a voltage
What does a chemical cell use?
2 metal electrodes and a salt bridge to produce a voltage
What are the advantages of hydrogen-oxygen fuel cells?
• No pollutants produced • No need to be electrically recharged
What are the disadvantages of hydrogen-oxygen fuel cells?
• Hydrogen is highly flammable • Hydrogen is difficult to store
What is the product of hydrogen-oxygen fuel cells?
Water