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CHEMISTRY NOTES ON ENERGY CHANGES IN A CHEMICAL REACTION

Definations for key terms

Activation energy: The minimum amount of energy for particles to collide with in order for a successful reaction to occur.

Endothermic reaction: An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases. In an endothermic reaction, the energy needed to break existing bonds is greater than the energy released from forming new bonds.

Exothermic reaction: An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases. In an exothermic reaction, the energy released from forming new bonds is greater than the energy needed to break existing bonds.

*Fuel cells: Fuel cells are supplied by an external source of fuel (eg hydrogen) and oxygen or air. The fuel is oxidised electrochemically within the fuel cell to produce a potential difference

Overall energy change of the reaction: The difference between the sum of the energy needed to break bonds in the reactants and the sum of the energy released when bonds in the products are formed.

Reaction profile: Reaction profiles can be used to show the relative energies of reactants and products, the activation energy and the overall energy change of a reaction.

*Rechargeable cells: Rechargeable cells and batteries can be recharged because the chemical reactions are reversed when an external electrical current is supplied.

INTRODUCTION

All chemical reactions react by gaining or losing energy. The overall energy change can be used to determine if a reaction is exothermic or endothermic.

A reaction that releases heat energy to the environment is called an exothermic reaction

A reaction that absorbs heat energy from its surrounding is called an endothermic reaction.

Energy in a chemical reaction is used up in bond breaking and released in bond formation.

Bond Breaking and Bond Forming

● When chemical reactions occur, energy is conserved.

o The amount of energy in the universe at the beginning is the same as at the end.

o this means if a reaction transfers energy to the surroundings, the product molecules must have less energy than the reactants, by the amount transferred

Exothermic reactions

● An exothermic reaction is one that transfers energy to the surroundings so the temperature of the surroundings increases.

 o Product molecules must have less energy than the reactants, by the amount transferred.

● Examples of exothermic reactions include; combustion, many oxidisation reactions and neutralisation.

● Everyday examples of exothermic reactions include; self-heating cans (e.g for coffee) and hand warmers.

 Endothermic reactions

● An endothermic reaction is one that takes in energy from the surroundings so the temperature of the surroundings decreases.

o product molecules must have more energy than reactants

● Examples of endothermic reactions are thermal decomposition and the reaction of citric acid and sodium hydrogencarbonate.

● Some sports injury packs are based on endothermic reactions.

Reaction profiles

● Chemical reactions can occur only when reacting particles collide with each other and with sufficient energy. o Activation energy = minimum amount of energy that particles must have to react

Endothermic and Exothermic reactions

  • Whether a reaction is endothermic or exothermic depends on the difference between the energy needed to break bonds and the energy released when the new bonds are formed.

Endothermic

  • If more energy is absorbed than is released, this reaction is endothermic.
  • More energy is required to break the bonds than that gained from making the new bonds.
  • The change in energy is positive since the products have more energy than the reactants.
  • The symbol ΔH (delta H) is used to show the change in heat energy. H is the symbol for enthalpy, which is a measure of the total heat of reaction of a chemical reaction.
  • Therefore an endothermic reaction has a positive ΔH

Breaking chemical bonds requires energy which is taken in from the surroundings in the form of heat

Exothermic

  • If more energy is released than is absorbed, then the reaction is exothermic.
  • 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 ΔH value.

Making new chemical bonds releases energy which radiates outwards from the reaction to the surroundings in the form of heat

Energy level diagrams

  • These are graphical representations of the heat changes in chemical reactions (see above).
  • The enthalpy of the reactants and products is displayed on the y-axis.
  • The reaction pathway is shown on the x-axis.
  • Arrows on the diagrams indicate whether the reaction is exothermic (downwards pointing) or endothermic (upwards pointing).

Enthalpy change during an exothermic reaction explanation:

  • During an exothermic reaction, energy is given out.
  • This means that the energy of the products will be lower than the energy of the reactants, so the change in enthalpy (ΔH) is negative.
  • This is represented on the energy-level diagram above with a downwards arrow as the energy of the products is lower than the reactants.

Enthalpy change during an endothermic reaction explanation:

  • During an endothermic reaction, energy is absorbed.
  • This means that the energy of the products will be higher than the energy of the reactants, so the change in enthalpy (ΔH) is positive.
  • This is represented on the energy-level diagram above with an upwards arrow as the energy of the products is higher than the reactants.

Calculating the Energy of a Reaction

Supplement:

  • Calculate the energy of a reaction using bond energies

Energy of reaction calculations

  • Each chemical bond has a specific bond energy associated with it.
  • This is the amount of energy required to break the bond or the amount of energy given out when the bond is formed.
  • This energy can be used to calculate how much heat would be released or absorbed in a reaction.
  • To do this it is necessary to know the bonds present in both the reactants and products.

Method

  • Add together all the bond energies for all the bonds in the reactants – this is the ‘energy in’.
  • Add together the bond energies for all the bonds in the products – this is the ‘energy out’.
  • Calculate the energy change: Energy change = energy in – energy out

Equation

Energy change     =     Energy needed in     –     Energy given  out

Example: An exothermic reaction

Hydrogen and chlorine react to form hydrogen chloride gas:

H2 + Cl2 → 2HCl

The table below shows the bond energies relevant to this reaction:

Energy In    =     436     +     243    =    679 KJ / Mole

Energy Out     =    2    x    432    =    864 KJ / Mole

Energy Change    =    679    –    864    =    -185 KJ / Mole

*The energy change is negative, showing that energy is released to the surroundings so it is an exothermic reaction.

Example: An Endothermic reaction

Hydrogen Bromide decomposes to form Hydrogen and Bromine:

2   x   ( H – Br )    →    H – H    +    Br – Br

The table below shows the bond energies relevant to this reaction:

Energy In    =     2     x     366    =    732 KJ / Mole

Energy Out     =    436    +    193    =    629 KJ / Mole

Energy Change    =    732    –    629    =    +103 KJ / Mole

*The energy change is positive, showing that energy is taken in from the surroundings so is an endothermic reaction

Exam Tip

  • For bond enthalpy questions, it is helpful to write down a displayed formula equation for the reaction before identifying the type and number of bonds, to avoid making mistakes.
  • The reaction thus becomes: H-H + Cl-Cl → H-Cl + H-Cl

Cells and batteries

● Cells contain chemicals which react to produce electricity

● The voltage produced by a cell is depended upon a number of factors

o E.g. type of electrode & electrolyte

● A simple cell can be made by connecting two different metals in contact with an electrolyte

● Batteries = two or more cells connected together in series to provide a greater voltage

● Non-rechargeable cells & batteries:

o Chemical reactions stop when one of the reactants has been used up

o Alkaline batteries are non-rechargeable

● Rechargeable cells & batteries:

o Can be recharged because the chemical reactions are reversed when an external electrical current is supplied Fuel cells

● Supplied by an external source of fuel (e.g hydrogen) and oxygen or air. the fuel is oxidised electrochemically within the fuel cell to produce a potential difference ● Overall reaction a hydrogen fuel cell involves the oxidation of hydrogen to produce water

● Hydrogen fuel cells offer a potential alternative to rechargeable cells & batteries: hydrogen fuel cell rechargeable cells and batteries fuel cells can be used constantly provided fuel keeps being put in can be recharged by reversing reaction, so fuel doesn’t need to keep being supplied hydrogen is a gas so needs to be stored at high pressure and so is harder to transport hard to dispose of- non-biodegradable only produces water when burnt will eventually stop working

● equations for each half cell:

○ At the cathode (negative electrode):

 H2 (g) –> 2e – +2H + (aq)

○ At the anode (positive electrode):

4H + (aq) + O2 (g) + 4e – → 2H2O(g)