Energy changes

• What is the conservation of energy principle?

  Energy is conserved in chemical reactions, the amount of energy in the universe at the end of the chemical reaction is the same as before the reaction takes place
• What is an exothermic reaction?


  A reaction where energy it transfered INTO the surroundings so that the surroundings temperature increases ( GIVES OUT ) eg: combustion, oxidation reactions and neutralisation reactions (negative sign of energy change, negative value)
• WHat is an endothermic reaction?

  A reaction where energy is TAKEN IN from the surroundings so the surrounding temperature decreases - thermal decomoposition, reaction of citric acid and sodium hydrogencarbonate (positive sign of change, positive value)
• What is activation energy?

  Minimum amount of energy that particles need to react when they collide(bump on reaction profile) - which when drawn needs to have no gaps between, arrow must meet the highest point
• What is a reaction profile?

  A reaction profile is a graph which shows the relative energies of reactants and products alongside the activation energy of the reaction
• How to structure question explaining HOW you can tell if a reaction is exothermic/endothermic?


  -the products have LESS/MORE energy than the reactants

  -meaning energy is being GIVEN OUT/TAKEN IN to the surroundings

  -this is an EXOTHERMIC/ENDOTHERMIC reaction

  -the enthalpy change is negative for an exothermic and positive for an endothermic reaction
• What occurs in a chemical reaction in terms of bond energies?

  Energy is supplied to break bonds and energy is released when bonds are made
• Describe an exothermic reaction in the terms of bond breaking/forming

  energy released from forming the bonds is greater than that needed to break the bonds.
• Describe an endothermic reaction in the terms of bonds breaking

  Energy needed to break the bonds is greater than the energy released when forming them.
• What is the euqation to find enthalpy change in terms of bond energies?

  energy of reaction = sums of bonds broken - sum of bonds made

  (left - right)
• What are some things to remeber when working out the enthalpy change?


  -look carefully at diagram, if not provided draw it!

  -circle the diagram and check for double/triple bonds

  -set it out on sperate sides (break) and (form)

  -WRITE THEM OUT INDIVIDUALLY

  -remeber units kJ
• How would you calculate the bond energy for a specific mole value?


  -using mols= mass/mr

  -calculate the Mr of the molecule

  -the calculate the total mols using the mass given in the question

  -multiply the worked out value by the correct bond energy provided in the table

  -remeber units!
• What is a cell?


  A cell is composed of two electrodes dipped in an electrolyte solutionm producing electricity as a result of a chemical reaction

  (eg: copper rod and zinc rod, in salt solution)
• What is a battery?

  A battery consists of two or more cells connected in series
• What determines the voltage obtained from a cell?


  Identities of metals used as electrodes and the identity and concentration of an electrolyte
• State the advantages and disadvantages of using cells and batteries

  PROS : more or less cheap, some are rechargble, a convenient source of electrical energy

  CONS : harmful chemicals
• Describe rechargeable and non-rechargeable cells

  Rechargeable - chemical reactions are reversed when an external current is suppled

  Non-rechargeable - reactants are used up, cannot be recharged
• What is a fuel cell?

  Fuel cells are supplied by fuel and oxygen to oxidise the fuel to generate electricity
• What is the overall reaction in a hydrogen fuel cell?

  2H^2 + O^2 = 2H^O
• What happens to the hydrogen in the anode?

  Oxidation of the hydrogen (looses electrons)

  • 2H^2 = 4H+ + 4e-
• What is in the centre of a hydrogen-oxygen fuel cell?

  Electrolyte - a solution ions can move through (potassium hydroxide)
• What is special about the electrodes in a fuel cell?


  The names for them are different for example:

  • the negative electrode is the ANODE
  • the positive electrode is the CATHODE

  this is because it is completely opposite to electrolysis where the ANODE is POSITIVE and the CATHODE is negative

  • they are also both made of porous carbon containing tiny holes and catalysts to speed up the reaction
• Which fuels enter which inlet and compartment?

  Hydrogen - enters the LEFT inlet into the ANODE compartment

  Oxygen - enters the RIGHT inlet in the CATHODE compartment#
• What are the two by-products of the fuel cell?

  -WATER

  -HEAT
• What happens in the anode compartment?

  Hydrogen enters the compartment, it is here oxidised by the anode, meaning the hydrogen atoms become hydrogen ions as they are loosing electrons. (becoming h+ ions)

  Giving us the half equation [ H^2 = 2h+ + 2e-]
• What routes to the electrons and hydrogen ions take?

  Hydrogen ions move through the electrolyte

  Electrons move through the wire

  BOTH TOWARDS THE CATHODE
• What happens in the cathode?


  The hydrogen ions and electrons bond with the oxygen already stored in the compartment, bonding together to create water. Remeber 02 is a diatomic molecule so we need two sets of hydrogen (4 total)

  Giving us the half equation [02 + 4h+ + 4e- = 2h20]
• Where does the electrical energy come from this reaction?


  The electrical energy in this reaction comes from the movement of electrons through the wires, any device connected here would be poweered by the chemical reaction
• In terms of potential difference, what happens once fuel enters the cells?

  As hydrogen enters the cell, it becomes oxidised, loosing electrons.

  This sets up a potential difference between the electrodes, meaning their simply is a difference of charge between the two

  This potential difference drives the electrons around the circuit in order to generate electricity
• What are some pros and cons of hydrogen fuel cells?

  PROS: only waste product is H20, only require hydrogen and oxygen to fuel (which are both abundant), simple devices lasting longer than normal batteries and are greener to dispose of

  CONS: takes up large amounts of space to store, is dangerous to store as it is explosive, to get the hydrogen needed fossil fuels are needed to generate electricity to extract pure hydrogen, which in most cases is being extracted from a greenhouse gas anyway (methane)
• What is the general rule for voltage production?


  The GREATER the DIFFERENCE in reactivity between the two metals used, the HIGHER the voltage is produced.
• When asking for which metal is LEAST reactive in a table of data, which value should you look for and why?


  The less reactive metal will be the metal with the highest value on the table as it has the highest difference in voltage between the metal being compared and itself.
• What are some control variables that should be done so a practical done to measure voltage would end up as a fair test?

  -Concentration of electrolyte

  -Temperature of electrolyte
• For the practical of measuring energy change in a neutralisation reaction, what are the variables?

  INDEPENDENT : volume of sodium hydroxide (what your change)

  DEPENDENT : maximum temperature reached (what you measure)

  CONTROL : volume of hydrochloric acid, concentration of hydrochloric acid and concentration of sodium hydroxide (what you keep the same)
• What are the first five steps of the practical?

  Place polystyrene cup into breaker (insulation and support)

  Measure 30cm^3 of hydrochloric acid with a measuring cylinder

  Transfer measured out acid into polystyrene cup

  Using a clamp, place thermometer inside liquid, ensuring the bulb is fully immersed

  Measure the initial temperature of acid, record in a table, set aside
• What are the last step of the practical?

  Measure 5cm^3 of sodium hyroxide solution using measuring cylidner

  Transfer this into the polystrene cup

  Place a plastic lid once added, placing the thermometer through the hole in lid, ensure the bulb is fully immersed

  Using thermometer stir gently,measure new temperatures and log

  Repeat the last steps until 40cm^3 of NaOH are added
• What should you do once the practical is complete?

  Repeat the whole process again twice, so that you are left with three sets of data in order to take an accurate mean from all findings

  Also plot graph to show the correlation between the increase in sodium hydroxide and the increase of the maximum temperature!
• How do catalysts increase reaction rates?

  By lowering the activation energy, increasing the likelihood of a successful collision
• What kind of reaction is bond making?

  Exothermic - releases energy
• What kind of reaction is bond breaking?

  Endothermic - as it requires energy