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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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