Topic 5 - Energy Changes

• What is an endothermic reaction?


  A chemical reaction that takes in heat energy from the surroundings causing the surrounding temperature to decrease
• What is an exothermic reaction?


  A chemical reaction that transfers heat energy to the surroundings so the surrounding temperature increases.
• Examples of an endothermic reaction:


  Sports injury packs

  Citric acid + Sodium Carbonate reaction

  Thermal decomposition reactions (when a compound broken down by heat)
• Examples of an exothermic reaction:


  combustion (or any oxidation)

  hand warmers (for skiing)

  neutralisation (when acids react with alkali (bases))

  self-heating cans
• What is activation energy?

  Activation energy is the minimum amount of energy required for a chemical reaction to occur.
• What are reaction profiles?


  Reaction profiles show the energies of reactants and products in chemical reactions
• What does an endothermic reaction profile look like

  Since the products have more energy than the reactants it looks like this:

  
• What does an exothermic reaction profile look like?


  Since the reactants have more energy than the products it looks like this:

  
• Is bond breaking an exothermic or endothermic process?

  Endothermic as it takes energy to break bonds
• Is bond making an exothermic or an endothermic process?

  Exothermic as energy is released when bonds are made
• Bond energies for exothermic processes:

  The energy released from forming new bonds is greater than the energy needed to break existing bonds.
• Bond energies for endothermic processes:

  The energy needed to break existing bonds is greater than the energy released from forming new bonds
• Experiment set-up and method for investigating temperature change in reactions

  1. Place polystyrene cup in glass beaker, add 30 ml hydrochloric acid

  2. Measure and record temperature of acid

  3. Add 5 ml sodium hydroxide, stir and record max temperature when thermometer stops changing

  4. Repeat adding 5 ml sodium hydroxide each time, recording the max temperature, until 40 ml total is reached

  5. Wash equipment and repeat experiment for second trial
• Endothermic reactions


  Absorb energy from the surroundings
• Exothermic reactions


  Transfer heat into the surroundings
• Results and analysis

  1. Calculate mean max temperature for each volume of sodium hydroxide

  2. Plot graph and draw straight line of best fit

  3. Read off max temperature change from graph
• The bigger the temperature change in the reaction, the more energy is absorbed or released
• What is an electrolyte?

  A solution that can conduct electricity due to the free moving dissolved ions within it.
• Why do cells run out?

  The chemicals in the cell run out so the reaction stops and electricity stops being produced.
• How is voltage in a cell affected by the choice of metals used as electrodes?

  The greater the difference between the reactivity of the metals, the greater the voltage.
• What is electron flow in a cell like?

  Electrons flow from the more reactive metal so it is oxidised.

  Electrons flow to the less reactive metal so it is reduced.
• If Oxidation and Reduction both occur, what is this known as?

  A redox reaction
• What is a battery?

  A battery is when two or more cells are connected together in series.

  This gives a greater voltage than the lone cell.
• How do rechargeable batteries work?

  The chemical reactions can be reversed when an external electrical current is applied - so when reactants are used up to make products, the products can then be turned back into reactants so the reaction can reoccur. ----> REVERSIBLE REACTION
• What is the type of energy that fuel cells convert?

  Chemical energy -----> Electrical energy
• In Hydrogen-Oxygen fuel cells, what is the anode and what is the cathode and why is this special?

  Anode is negative - Hydrogen - and Cathode - Oxygen - is positive - opposite to electrolysis
• What are the electrodes in fuel cells made of?

  Porous Carbon
• How do the Hydrogen ions move in a Hydrogen-Oxygen fuel cell?

  They movee through the electrolyte to the cathode
• What happens to the Hydrogen and what happens to the Oxygen?

  Hydrogen is oxidised

  Oxygen is reduced
• Where do the electrons move through and where do they go?

  Move through the wire to the cathode
• How is the fuel obtained?

  Steam reformation of Methane

  Electrolysis of Water
• Why is Steam Reformation of Methane bad?

  Uses natural gas (methane) as a raw material - non-renewable

  Requires very high temperatures

  Takes a lot of energy to make
• Why is Electrolysis of Water better but still with downsides?

  Uses water as a raw material - readily available

  Requires vast amounts of electrical energy which typically come from non-renewable resources
• Advantages of using Hydrogen-Oxygen fuel cells

  Only require Hydrogen and Oxygen

  Water is the only product so no contribution to climate change

  Small in size
• Disadvantages of using Hydrogen-Oxygen fuel cells

  Hydrogen is a flammable gas making it difficult to store

  Expensive to make due to lots of energy being required to obtain Hydrogen

  Need a constant supply of Hydrogen fuel
• Where does Hydrogen get oxidised - give half equation - and what then happens?

  In the anode

  Ions then move through electrolyte to cathode

  (2)H2 -> 4H+ + 4e-
• Where does Oxygen get reduced and give half equation?

  In the cathode

  O2 + 4H+ + 4e- -> 2H2O
• What is the overall reaction equation in a Hydrogen-Oxygen fuel cell?

  O2 + 2H2 --> 2H2O
• What is a pure substance?

  A single element or compound - not mixed with any other substance.

  Each pure substance has its own specific boiling and melting point.

  e.g. pure water boils at 100C and freezes at 0C
• How to identify a pure substance?

  Melts and boils at a specific point whilst impure substances have a broader temperature range.