17.1-Bond Energies

Energy must always be supplied to break bonds

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

• Energy must be supplied to break existing bonds, so bond breaking is an endothermic process

• Energy is released when new bonds are formed, so bond formation is an exothermic process

• In exothermic reactions the energy released by forming bonds is greater than the energy to break them

• In endothermic reactions the energy used to break bonds is greater than the energy released by forming them

Bond energy calculations-need to be practised

• Every chemical bond has a particular bond energy associated with it

• This bond energy varies slightly depending on the compound the bond occurs in

• You can use these known bond energies to calculate the overall energy changes for a reaction

• The overall energy change is the sum of the energies needed to break bonds in the reactants minus the energy released when the new bonds are formed in the products

• You can’t compare the overall energy changes of reactions unless you know the numerical differences in the bond energies

17.2-Cells and Batteries

Simple Cells

• A simple cell is a source of electrical energy

• The simplest design consists of two electrodes made from metals of different reactivity immersed in an electrolyte and connected to an external voltmeter by wire, creating a complete circuit

• 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

• The difference in the ability of the electrodes to release electrons causes a voltage to be produced

• The greater the difference in the metals reactivity then the greater the voltage produced

  • Voltage = an electromotive force or potential difference expressed in volts.

• The electrolyte used also affects the voltage as different ions react with the electrodes in different ways

17.3-Fuel Cells

What are fuel cells?

• Chemical cells use chemical reactions to convert and transfer energy to electrical energy.

• They will produce a voltage only up until one of the reactants has been used up (we say the battery has "gone flat").

• Fuel cells will produce a voltage continuously, provided they have a constant supply of fuel and oxygen (from the air).

Hydrogen-oxygen fuel cells

• In this type of fuel cell, hydrogen and oxygen are used to produce a voltage.

• The only product from this reaction is water.

• A hydrogen-oxygen fuel cell and electric motor are much quieter, and need less maintenance, than a petrol or diesel engine, but the hydrogen still needs to be stored in a container - like a tank.

Uses of Fuel Cells

• Hydrogen, diesel and petrol are all highly flammable fuels. Fuel cells have their advantages and disadvantages depending on the use.

Fuel cells in spacecraft

Fuel cells in vehicles

Practice Questions:

• How do cells produce electricity?

  • Cells contain chemicals in the form of a solid metal (electrode) and an ionic solution (the electrolyte), which react and generate electricity by releasing electrons. The voltage produced by a cell depends on the type of electrode and electrolyte used.

• Weakness of fuel cells

  • Hydrogen is difficult to store

• What are fuel cells?

  • Chemical cells use chemical reactions to convert and transfer energy to electrical energy.

• What are simple cells?

  • A source of electrical energy