4.5 - Energy changes - notes

4.5.1 Exothermic and endothermic reactions

4.5.1.1 Energy transfer during exothermic and endothermic reactions

• Chemicals store a certain amount of energy - and it varies with each chemical

  • If the product’s molecules of a reaction store more energy than the original reactants:

    • They must’ve take in the difference in energy from the surroundings

  • But if the products molecules store less:

    • then the excess energy was transferred to the surrounding during the reaction

  • Overall amount of energy doesn’t change (energy is conserved)

• An exothermic reaction is one which transfers energy to the surroundings, usually by heating. This is shown by a rise in temperature.

  • Example of Exothermic Reactions:

    • Burning fuels (combustion)

    • Neutralisation reactions (acid + alkali)

    • Oxidation reactions

  • Exothermic reactions have lots of everyday uses:

    • Some hand warmers use the exothermic oxidation of iron in the air with a salt solution catalyst) to release energy

    • Self heating cans of hot chocolate and coffee also rely on exothermic reactions

• An endothermic reaction is one which takes in energy from the surroundings. This is shown by a fall in temperature.

  • Examples of Endothermic reactions:

    • Reaction between citric acid and sodium hydrogencarbonate

    • Some sports injury packs

    • Thermal decomposition

    • Electrolysis

• Exothermic and Endothermic reaction r.p. flashcards on pmt

4.5.1.2 Reaction profiles

• For atoms or particles to react with each other in a chemical system they must first come into contact with each other in a collision

• Several factors come into play when analysing collisions such as:

  • Energy

  • Orientation

  • Number of collisions per second

• This exothermic reaction profile shows that the products are at a lower energy than the reactants

  • The difference in height represents the overall energy change in the reaction (the energy given out) per mole

  • The initial rise in the line represents the energy needed to break the old bonds and start the reaction

  • This is called the activation energy which = the minimum amount of energy the reactants need to collide with each other and react

  • The greater the activation energy, the more energy needed to start the reaction

• This endothermic reaction shows that the products are at a higher energy than the reactants

  • The difference in height represents the overall energy change in the reaction (the energy taken in) per mole

4.5.1.3 The energy change of reactions (HT only)

• During a chemical reaction old bonds are broken and new bonds are formed:

  • Energy must be supplied to break existing bonds - so this must be an Endothermic process

    • More energy is required to break the bonds than is released from making the new bonds

    • The change in energy is positive since the products have more energy than the reactants

    • Therefore an endothermic reaction has a positive change in energy

  • Energy must be released when new bonds are formed - so this must be an Exothermic process

    • More energy is released when new bonds are formed than energy required to break the bonds in the reactants

    • The change in energy is negative since the products have less energy than the reactants

    • Therefore an exothermic reaction has a negative change in energy

• Exothermic reactions is when the energy released forming the bonds is greater than the energy used to break them

• Endothermic reactions is when the energy used to break bonds is greater that the energy needed to form them

• Practice Bond energy calculations

4.5.2 Chemical cells and fuel cells

4.5.2.1 Cells and batteries

• An electrochemical cell = a basic system made up of 2 different electrodes in contact with an electrolyte

  • Electrolyte = a liquid that contains ions which react with the electrodes

  • The two electrodes of different reactivity are metals because they must conduct electricity

    • A common example is zinc and copper:

      • Zinc is the more reactive metal and forms ions more easily, readily releasing electrons

      • The electrons give the more reactive electrode a negative charge and sets up a charge difference between the electrodes

      • The electrons then flow around the circuit to the copper electrode which is now the more positive electrode

  • A wire connected to the electrodes enables charge to flow and electricity is produced

• The voltage of a cell depends on many factors:

  • Different metals will react differently with the same electrolyte

  • The bigger the difference in the reactivity of the electrodes the bigger the voltage of the cell

  • The electrolyte used in a cell will also affect the size of the voltage with different ions in solution will react differently with the electrodes used

• A battery is formed by connecting two or more cells together in series

  • The voltages of the cells in the battery are combined so there is a bigger voltage overall

• In Non-rechargeable batteries the reactants get used up

  • Over time the ions in the electrolyte and the metal ions on the electrode get used up and turn into products of the reaction

  • Once the limiting reactant is used up the reaction can’t happen so no electricity is produced

  • The products can’t be turned back into the reactants so the cell can’t be recharged

  • A good example -

    • Non-rechargeable batteries (alkaline batteries) are made up of cells which use irreversible reactions

• In a rechargeable cell, the reaction can be reversed by connecting it to an external electric current

4.5.2.2 Fuel cells

• A fuel cell is an electrical cell that’s supplied with a fuel and oxygen or air and uses energy from the reaction to produce electrical energy efficiently

  • When the fuel enters the cell it becomes oxidised and sets up a p.d within the cell

  • One type of fuel cell is the hydrogen-oxygen fuel cell

    • This fuel cell combines hydrogen and oxygen to produce nice clean water and energy

• Fuel cell vehicles don’t produce as many pollutants as other fuels

  • They’re only by-products are water and heat

• Electric vehicles don’t produce as much pollutants either - but their batteries are much more polluting

  • Batteries in ev cars are rechargeable but there’s a limit to how many times they can be recharged before they need replacing

  • Batteries are also more expensive to make than fuel cells

  • Batteries store less energy than fuel cells so would need to recharge more often - which can take more time

Advantages of hydrogen fuel cells

• They do not produce any pollution

• They produce more energy per kilogram than either petrol or diesel

• No power is lost in transmission as there are no moving parts, unlike an internal combustion engine

• No batteries to dispose of which is better for the environment

• Continuous process and will keep producing energy as long as fuel is supplied

Disadvantages of hydrogen fuel cells

• Materials used in producing fuel cells are expensive

• High pressure tanks are needed to store the oxygen and hydrogen in sufficient amounts which are dangerous and difficult to handle

• Fuel cells are affected by low temperatures, becoming less efficient

• Hydrogen is expensive to produce and store

• Electrolyte is often an acid

• Electrodes are often porous carbon with a catalyst

• At the negative electrode (the anode):

  • Hydrogen loses electrons to produce H2+ - this is oxidation as shown by the following equation:

• At the positive electrode (cathode):

  • These H+ ions in the electrolyte move to the cathode

  • Oxygen gains electrons from the cathode and reacts with h+ ions to form water. This is reduction as shown by the following equation:

• The overall reaction is hydrogen plus oxygen which gives water:

• There’s reduction at the cathode and oxidation at the anode which = a REDOX reaction