AQA A Level Chemistry復習筆記1.6.3 Enthalpy Changes
Enthalpy Changes
- The total chemical energy inside a substance is called the?enthalpy?(or heat content)
- When chemical reactions take place, changes in chemical energy take place and therefore the enthalpy changes
- An?enthalpy change?is represented by the symbol ΔH?(Δ= change;?H?= enthalpy)
- An enthalpy change can be positive or negative
Exothermic reactions
- A reaction is exothermic when the products have less energy than the reactants
- Heat energy is?given off?by the reaction?to the?surroundings
- The?temperature?of the?environment increases?- this can be measured with a thermometer
- The?energy?of the?system decreases
- There is an enthalpy decrease during the reaction so ΔH?is negative
- Exothermic reactions are?thermodynamically?possible (because the enthalpy of the reactants is?higher?than that of the products)
- However, if the rate is too slow, the reaction may not occur
- In this case the reaction is?kinetically?controlled
The enthalpy change during an exothermic reaction
Endothermic reactions
- A reaction is endothermic when the products have more energy than the reactants
- Heat energy is?absorbed?by?the reaction?from?the?surroundings
- The?temperature?of the?environment decreases?- this can be measured with a thermometer
- The?energy?of the?system increases
- There is an enthalpy increase during the reaction so ΔH?is positive
The enthalpy change during an endothermic reaction
Exam Tip
It is important to specify the physical states of each species in an equation when dealing with enthalpy changes as any changes in state can cause very large changes of enthalpy.?For example:
NaCl (s) → Na+?(aq) + Cl-?(aq)?? ΔH?= +4 kJ mol-1
NaCl (g) → Na+?(g) + Cl-?(g)?? ΔH?= +500 kJ mol-1
Also, remember that the?system?is the substances?that are reacting?(i.e. the reaction itself) and the?surroundings is everything else?(e.g. the flask the reaction is taking place in).
Standard Enthalpy Changes
- To be able to compare the changes in enthalpy between reactions, all thermodynamic measurements are carried out under standard conditions
- These standard conditions are:
- A?pressure?of 100 kPa (you may see some older exam questions that use a figure of 101 kPa; the exact figure is 101 325 Pa, but it has been simplified in the current syllabus for problem-solving purposes)
- A?temperature?of 298 K (25?oC)
- Each substance involved in the reaction is in its?standard physical state?(solid, liquid or gas)
- To show that a reaction has been carried out under standard conditions, the symbol ? is used
- ΔH??= the standard enthalpy change
- There are a number of key definitions relating to enthalpy changes that you need to know
Enthalpy Definitions Table
Worked Example
Calculating the enthalpy change of reaction
One mole water is formed from hydrogen and oxygen, releasing 286 kJ of energy
H2?(g) + ?O2?(g)?→?H2O (I)? ? ? ΔHr??= -286 kJ mol-1
Calculate ΔHr??for the reaction below:
2H2?(g) + O2?(g)?→?2H2O (I)
Answer
-
- Since two moles of water molecules are formed in the question above, the energy released is simply:
-
- ΔHr??= 2 mol x (-286 kJ mol-1) = -572 kJ mol-1
Worked Example
Calculating the enthalpy change
Calculate ΔHf??for the reaction below, given that ΔHf??[Fe2O3(s)] = -824.2 kJ mol-1
4Fe(s) + 3 O2(g) → 2 Fe2O3(s)
Answer
-
- Since two moles of Fe2O3?(s) are formed the total change in enthalpy for the reaction above is:
- ΔHf??=? 2 x ( -824.2 kJ mol-1)?= - 1648 kJ
Worked Example
Calculating enthalpy changes
Identify each of the following as ΔHr?, ΔHf?, ΔHc??or ΔHneut?
- MgCO3?(s)?→?MgO (s) + CO2?(g)
- C (graphite) + O2?(g)?→?CO2?(g)
- HCl (aq) + NaOH (aq)?→?NaCl (aq) + H2O (I)
Answer
Answer 1:?ΔHr?
Answer 2:?ΔHf??as one mole of CO2?is formed from its elements in standard state?and?ΔHc??as one mole of carbon is burnt in oxygen
Answer 3:?ΔHneut??as one mole of water is formed from the reaction between an acid and an alkali
Exam Tip
The ΔHf??of an?element?in its standard state is zero.
For example, ΔHf??of O2(g) is 0 kJ mol-1
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