Thermochemical Equations


This article was written by Henry Ja a current VCE Chemistry and Legal Studies tutor. Henry is currently accepting students, so if you’re interested in his services, you can see his profile here

In Year 11 you would have learnt how to balance a chemical equation. In many cases, it’s a trial and error process when attempting to balance.

In Unit 3 you will have learnt about ‘thermochemical equation’, but what are they?

In simple terms, a thermochemical equation is a balanced chemical equation with ∆H value.

Before we look further into thermochemical equations we first need to understand what ∆H is.

  • ∆H (delta H) is known as the enthalpy change. ∆H represents the energy change of a chemical reaction.

Reactions can either release (exothermic reactions) or absorb (endothermic reactions) energy, and ∆H allows us to determine if a reaction is either one of the two.

  • ∆H values contain both a sign and number.

In exothermic reactions, the ∆H has a negative sign, whilst endothermic reactions the ∆H has a positive sign.

Now that we have established what ∆H represents, let’s see if we can apply that to an example.

The reaction above is a combustion reaction of methane. You should know from your readings that a combustion reaction releases energy. But other than from your general knowledge how else can we tell this reaction releases energy, and hence is an exothermic reaction?

  • The ∆H value!
  • ∆H value here is –890 kJ/mol which tells us that for every mol of methane combusted 890 kJ of energy is released.

***The reverse reaction would be an endothermic reaction. The ∆H value would have the same numerical value but a different sign.

Key points to remember:

  • A thermochemical equation is made up of a balanced chemical equation & ∆H value. (VERY IMPORTANT)
  • The ∆H value can tell us whether a reaction is exothermic or endothermic.
  • ∆H is SPECIFIC for a SPECIFIC chemical equation.
    • Changing the states of the reactants and products will result in a different ∆H value.
    • If the coefficients in the chemical equation are changed, then the ∆H value will also change.
  • kJ/mol is JUST a unit. It does not always mean for every mol of a substance ‘x’ amount of energy is released. You need to look at the equation you are given and go from there.

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Thermochemical Equations

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