Energetics

A level chemical energetics is the study of energy flow in chemical reactions. For example, you can:

• Determine the amount of energy given out by fuels
• Calculate the energy demands of an industrial process
• Deduce the theoretical energy of a chemical reaction knowing the amount of energy needed to break and make bonds

What is bond energy?

Reactions can be exothermic or endothermic. A bond is a force of attraction between two atoms. To overcome this force, energy is required, so bond breaking is endothermic and bond making is exothermic. The bonds in question are covalent bonds and the bond energy definition is:

‘The energy required to break 1 mole of bonds in gaseous covalent molecules under standard conditions’

Since the definition refers to bond breaking, the term is sometimes known as bond dissociation enthalpy.

So, for example, applying the definition to the bond dissociation enthalpy of hydrogen bonds, the equation is:

H₂ (g) → 2H (g)    ΔH^ϴ  = +436 kJ mol^-1

To break bonds energy is required from the surroundings and to make new bonds energy is released from the reaction to the surroundings

A reaction is exothermic or endothermic overall depending on the difference between the energy put in to break bonds and the energy released when bonds are formed. 

What are energy level diagrams?

Energy level diagrams are a way to visualise the flow of energy during a chemical reaction

The energy level diagram for exothermic reactions. ∆H is negative because the system (reactants) loses energy to the surroundings.

The transition state at the top of the energy hill represents the point at which all the bonds have been broken and so it is the maximum energy state of the molecule. Bonds have to be broken before they can recombine to form products, so this input of energy is the activation energy of the reaction.

• Find out more about energy level diagrams.

What are enthalpy changes?

When a chemical reaction occurs, changes in chemical energy take place. These changes are enthalpy changes. An enthalpy change is represented by the symbol ΔH (Δ= change; H = enthalpy) and the units are kilojoules per mol. An enthalpy change can be positive or negative.

Standard enthalpy changes are a way to make comparisons between data. They occur under the standard conditions of a pressure of 100 kPa and a temperature of 298 K (25 ^oC). Each substance involved in the reaction is in its normal physical state (solid, liquid or gas)

For example, some of the standard enthalpy changes are:

Calorimetry

The term calorimetry refers to the measurement of enthalpy changes in chemical reactions. To calculate the heat transferred in reaction you need to know three values, related in the following formula:

A simple calorimeter can be made from a polystyrene drinking cup, a vacuum flask or a metal can. You can find out more about how to carry out a calorimetry investigation in our revision notes on Calorimetry.

Hess' Law

Hess’s Law is an application of the first law of thermodynamics: energy can not be created or destroyed. In the case of chemical reactions, the total energy change is the same whichever route is taken. This can be expressed in the form of a cycle of diagram:

The enthalpy change from elements to products (direct route) is equal to the enthalpy change of elements forming reactants and then products (indirect route

Applications of Hess’ Law

You can use Hess’ Law to solve energetics problems, by applying a Hess’ cycle to given experimental data. This allows you to find the energy change for a reaction that cannot be measured directly. 

For example, the enthalpy of formation of ethane can be found using the enthalpies of combustion of ethane, graphite and hydrogen:

Further examples of Hess’ Law cycles and calculations can be found in our revision notes on Hess’ Law

What is bond enthalpy?

Calorimetry is a technique used to measure changes in the enthalpy of chemical reactions, but it doesn’t explain why energy is absorbed or released. Models and theories are used to explain energy changes on the basis of bonds breaking and being formed. 

Bond energies are affected by other atoms in the molecule, so average bond enthalpies are listed in data tables

What keyword definitions do I need to know for energetics? 

Some keyword definitions you need to know are:

• Enthalpy change - the heat change that takes place during chemical reactions at constant pressure
• Standard molar enthalpy of formation - the enthalpy change when 1 mole of a substance is formed from its elements under standard conditions, all reactants and products being in their standard states
• Standard molar enthalpy of combustion - the enthalpy change when 1 mole of a substance is completely burnt in oxygen under standard conditions, all reactants and products being in their standard states

This is a quick summary of some key concepts on energetics - remember to go through the full set of revision notes, which are tailored to your specification, to make sure you know everything you need for your exams!