Enthalpy changes AS
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The specification states the following learning outcomes.
3.2.1 Enthalpy changes
(a) explanation that some chemical reactions are accompanied by enthalpy changes that are exothermic (ΔH, negative) or endothermic (ΔH, positive)
(b) construction of enthalpy profile diagrams to show the difference in the enthalpy of reactants compared with products
(c) qualitative explanation of the term activation energy, including use of enthalpy profile diagrams
(d) explanation and use of the terms:
(i) standard conditions and standard states (physical states under standard conditions)
(ii) enthalpy change of reaction (enthalpy change associated with a stated equation, ΔrH)
(iii) enthalpy change of formation (formation of 1 mol of a compound from its elements, ΔfH)
(iv) enthalpy change of combustion (complete combustion of 1 mol of a substance, ΔcH)
(v) enthalpy change of neutralisation (formation of 1 mol of water from neutralisation, ΔnH)
(e) determination of enthalpy changes directly from appropriate experimental results, including use of the relationship: q = mcΔT
(f) (i) explanation of the term average bond enthalpy (as the breaking of 1 mol of bonds in gaseous molecules)
(ii) explanation of exothermic and endothermic reactions in terms of enthalpy changes associated with the breaking and making of chemical bonds
(iii) use of average bond enthalpies to calculate enthalpy changes and related quantities (see also 2.2.2 f)
(g) Hess’ law for construction of enthalpy cycles and calculations to determine indirectly:
(i) an enthalpy change of reaction from enthalpy changes of combustion
(ii) an enthalpy change of reaction from enthalpy changes of formation
(iii) an enthalpy change from unfamiliar enthalpy cycles
(h) the techniques and procedures used to determine enthalpy changes directly and indirectly.
EXOTHERMIC AND ENDOTHERMIC REACTIONS
Learners will have met the concept of exothermic and endothermic reactions at GCSE, and used reaction profile diagrams. They will likely have carried out simple reactions that give measurable temperature changes allowing them to classify reactions. They may have carried out calorimetry experiments to determine the enthalpy of combustion of a range of fuels, commonly alcohols.
Approaches to teaching the content
Before recapping exothermic/endothermic, it may be worth ensuring learners are distinguishing carefully between ‘heat’ and ‘temperature’. Heating is the process whereby thermal energy (measured in joules, J) is transferred from a hotter object to a cooler object. The thermal energy of an object is due to the movement of the particles in that object – translational, rotational and vibrational. The direction of energy transfer is determined by the temperature of the objects – measured in kelvin (K, absolute temperature) or degrees Celsius (oC)
Further discussion can be found at:
and a simple experiment:
Time should also be spent on ensuring learners have a firm understanding of the conservation of energy. A full discussion of misconceptions, and ideas about how to tackle them, is discussed in Chapter 7 and 11 of this document:
Learners also need a solid understanding of the terms ‘system’ and ‘surroundings’, which may have been met at GCSE. The system is defined as those substances directly involved in a chemical reaction. The surroundings are the rest of the universe. Energy will transfer either from the system to the surrounding or vice-versa. The direction of energy transfer defines the reaction as exothermic/endothermic.
Exothermic reactions are those that have an energy transfer from the substances involved in the reaction (system) to the surroundings, causing a temperature rise. Breaking down the term into exo- meaning outside and thermic meaning heat may help, especially when linked to other terms prefixed with exo- e.g. exoskeleton.
Endothermic reactions are those that have an energy transfer from the surroundings into the substances involved in the reaction (system), causing a temperature drop.
REACTION PROFILE DIAGRAMS
Reaction profiles provide a visual representation of the enthalpy changes of reactions. Learners will need to appreciate the importance of both bond breaking and making during a reaction, and the relative enthalpies of the substances involved
Approaches to teaching the content
This section requires the uses of molecular models such as Molymods to demonstrate the concepts. Learners should have a secure understanding of balancing equations, conservation of mass and energy. Use of the Atoms and Equations delivery guide may be useful:
A simple model of a reaction is considered: the reaction between methane and oxygen – a reaction well known by learners from their use of Bunsen burners. Learners will be well aware that this is an exothermic reaction!
Learners should produce a model of a methane molecule (CH4), and two of the oxygen molecule (2 × O2). They then rearrange the atoms of the reactants into those of the products, one carbon dioxide (CO2) and two water molecules (2 × H2O), therefore modelling the reaction:
CH4 + 2O2 → CO2 + 2H20
This demonstrates to the learner that the bonds of the reactant molecules need to be broken before the bonds of the products are made.
Reaction profile diagrams should now be constructed, possibly using mini-whiteboards or large pieces of poster paper. Groups could collaborate so that models of the reactants (left hand side of the diagram), separated atoms (top of the curve) and products (right hand side) are placed on the diagram, and photos taken for their notes.
Neat versions of exothermic and endothermic enthalpy profile diagrams should form part of learners notes, with particular attention paid to:
- labelling of the axes (y = enthalpy; x = reaction progress)
- labelling of reactant and product lines
- relative height of the reactant and product lines (product lower than reactant for exothermic reactions)
- curved line connecting the reactant and product lines.
- activation energy arrow drawn between the reactant line and the top of the curve
Enthalpy changes of reaction are given the symbol ΔH (Δ is the symbol for change, H is the symbol for enthalpy). Enthalpy changes are usually measured in J mol-1. There are various types of reaction that have standard definitions, which are distinguished by the addition of a subscript letter between the Δ and the H.
Enthalpy change of reaction (ΔrH) – the enthalpy change associated with a stated reaction.
Enthalpy change of formation (ΔfH) – enthalpy change associated with the formation of 1 mol of a compound from its elements, at standard conditions and standard states.
Enthalpy change of combustion (ΔcH) – enthalpy change associated with the complete combustion of 1 mol of a substance, at standard conditions and standard states.
Enthalpy change of neutralisation (ΔneutH) – enthalpy change associated with formation of 1 mol of water from neutralisation, at standard conditions and standard states.
Standard conditions and standard states are taken as:
- pressure – 100 kPa
- temperature – 298 K
- concentration – 1.00 mol dm–3
- states state – the state of a substance under standard conditions.
A card sort activity (Teacher Resource 1 and Learner Resource 1) can be used to consolidate these ideas. Some incorrect definitions are included that will allow probing of learner misconceptions.
MEASUREMENT OF ENTHALPY CHANGES DIRECTLY BY EXPERIMENTATION
A common confusion is the assumption that an increase in temperature in a reaction means the enthalpy must be positive because ‘the reaction has more energy’. Careful delineation of the system and surroundings helps here. For example, in the reaction between zinc and copper sulfate solution, an exothermic reaction, the products, zinc sulfate solution and copper, are more energetically stable - ΔH is negative. The energy is transferred to the surroundings – mostly the solvent water – which thus increases in temperature. Noting the concentration of water (about 55 mol dm–3) can be helpful in students understanding the temperature reading is dominated by the energy of the water.
Repeating the practical they will likely have carried out at GCSE may be useful for making the links. Example practicals are:
The YouTube video explains how a bomb calorimeter works.
Rise in temperature of the water is recorded and the total energy is then calculated.
This can be replicated in the lab by carrying out a simple experiment and using the equation in Learner Resource 2 to calculate the energy given out per mole of chemical.
(b) Solid ammonium thiocyanate, NH4SCN, reacts with solid barium hydroxide, Ba(OH)2, as shown in the equation below.
2NH4SCN(s) + Ba(OH)2(s) → Ba(SCN)2(s) + 2H2O(l) + 2NH3(g)
A research chemist carries out an experiment to determine the enthalpy change of this reaction.
In the experiment, 15.22g of NH4SCN is reacted with the slight excess of Ba(OH)2. The reaction absorbs energy, cooling the 50.0g of water from 21.9°C to 10.9°C.
(i) Calculate the energy absorbed, in kJ, during this reaction. The specific heat capacity of the water = 4.2Jg-1K-1.
energy = ............................................kJ (2)
Question taken from OCR Chemistry A F322 Jan 11 question paper - question 3b
Answer Q=50 x 4.2 x 11.0 = 2.3 (kJ)
(a) A student investigates the reaction between magnesium and dilute hydrochloric acid.
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
The student determines the enthalpy change for this reaction.
In her experiment, she reacts 0.486g of magnesium with 50.0cm3 of 2.00mol dm-3 HCl(aq). The HCl(aq) is in the excess.
(i) Calculate the energy released, in kJ, during this reaction.
The specific heat capacity of the water = 4.18Jg-1K-1.
The density of the solution in 1.00g cm-3
energy = ............................................kJ (2)
(ii) Calculate the amount in moles, of magnesium used by the student.
amount = ............................................mol (1)
(iii) Calculate the enthalpy change of the reaction. Give your answer to three significant figures.
Question taken from OCR Chemistry A F322 Jan 12 question paper - question 3a
Answers: i) 2.68; ii) 0.02; -134
There are many examples in everyday life where both endothermic and exothermic reactions happen. For example, self-heating cans for expedition food as used by outdoors enthusiasts provide an example of exothermic reactions.
The cooling packs used in sports injuries are examples of endothermic reactions where two chemicals mix to produce an endothermic reaction to reduce the swelling.
Following the activity in the conceptual chapter on reaction profile diagrams, students should be made aware at this point that all reactions begin as endothermic processes as the diagrams show as energy is required to push the reaction over the activation energy boundary.
This could be related to the initial idea that methane gas will burn in oxygen. However, this reaction will not proceed unless either a flame or a spark is present.
This could be related to the misconception that mobile phones should not be used when filling up your car with petrol or diesel. However, you should not smoke whilst filling up your car. Should the clothing you wear also be taken into account?
Ref: Chem. Educ. Res. Prac., 2004, 5, 301-25
In this section the indirect measurement of enthalpy changes will be considered. This is done by considering both the use of average bond enthalpy calculations and Hess’s Law to calculate enthalpies of formations and combustion.
In the teaching of using bond enthalpies it is very important that the students know and keep referring back to the definition of a bond enthalpy:
Bond enthalpy: the average enthalpy change that takes place when breaking 1 mole of a given bond in the molecule of a gaseous species.
Learners should take careful note that this enthalpy is for gaseous molecules.
Students should also be made aware that they will not need to remember the actual values but should be aware of the magnitude of these values.
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© OCR 2015 - This resource may be freely copied and distributed, as long as the OCR logo and this message remain intact and OCR is acknowledged as the originator of this work.
OCR acknowledges the use of the following content (usage does not imply endorsement): Teacher Resource 1 and Learner Resource 1: Background image Viktoriya/Shutterstock.com
Teacher Resource 1 and Learner Resource 2: Bomb calorimeter by courtesy of Encyclopaedia Britannica, Inc., copyright 1997; used with permission
Teacher Resource 4 and Learner Resource 4: Enthalpy of formulation calculations adapted from Calculations for A Level Chemistry by E Ramsden (Nelson Thornes, 2001), reprinted by permission of the publishers, Oxford University Press.
Teacher Resource 5 and Learner Resource 5: Adapted from Mathematics for A Level Chemistry: A Course Companion by S Doyle, reprinted by permission of the publishers, Illuminate Publishing