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KENT TRIPLE SCIENCE NETWORK From Kent Science Resource Centre
Developing Expertise in the Teaching of Energy Changes in Chemical Reactions
How comfortable are you with energy changes in chemistry? Personal knowledge? Delivery to the students? Practical aspects?
Problems … Students often lack secure understanding of basic chemical concepts. Many of the ideas involved are abstract and/or difficult to comprehend. The language & vocabulary used can cause problems. The mathematics and units involved.
Problems … Some students think of fuels as energy ‘stores’. Fuels are thought to ‘contain’ energy. Food is thought to ‘give’ energy. These ideas make the idea of fuel-oxygen system more difficult to accept and makes calculations of energy changes more problematic.
Problems … Energy is thought to be created (e.g in burning). Energy is thought to be used up (e.g a battery ‘runs out’). The language that we use may encourage students to think of energy as a ‘substance’ that can be made or used up. This makes the key idea of conservation of energy more difficult to grasp.
Problems … The idea that energy is released when bonds break is very common and partly arises from the picture of fuel as an ‘energy store’. It is similar to the idea that ‘cracking an egg releases its contents’.
Some spectacular energy changes in chemical reactions Demonstrations: Whoosh bottle DENERGY1 Igniting methane bubbles DENERGY2 Fat pan fire DENERGY3 Money to burn DENERGY4 Spontaneous exothermic reaction DENERGY5
Exothermic reactions – points to emphasise Energy is not made or lost. Energy is transferred from system to surroundings. Identify all of the reactants involved in the reaction. Refer to fuel – oxygen systems. Introduce the idea of system and surroundings.
Other exothermic reactions Extend the range of exothermic reactions beyond reactions that are obviously to do with fuels. Ask questions that will make explicit what is the system and what are the surroundings. Reinforce the idea of energy transfer. Identify exothermic and endothermic reactions EENERGY1
System and Surroundings What is the system and what are the surroundings? The temperature change of the surroundings is evidence that an energy change has taken place in the system.
Introduce endothermic reactions. Ask questions that will make explicit what is the system and what are the surroundings. Reinforce the idea of energy transfer. Summary – almost all reactions involve an energy change. A spectacular endothermic reactionDENERGY6
Measuring energy changes
How do we know that an energy change is taking place? We can feel the change in ‘hotness’. We can measure the change in temperature using a thermometer. Sometimes this is the only indication that a reaction has occurred.
Why do we measure energy changes? Eating different foods transfers different amounts of energy to us. Burning different fuels transfers different amounts of energy to the surroundings. We can compare foods or compare fuels This can be done through a comparison of ‘energy density’ See worksheets ARENERGY 4_1, 4_2 & 4_3.
Measuring the energy transferred when a fuel in burned Practical: EENERGY2 Helpsheet: ARENERGY3_1
Measuring the energy transferred when a fuel is burned q = mcΔT where q = energy transferred (in J), m = mass of water (in g) c = specific heat capacity of water (in J/[g°C])* ΔT = temp change (in °C or K). *or J g ‒ 1 K ‒ 1
Measuring the energy transferred when a fuel is burned Assume that 1 cm 3 of water has a mass of 1 g Assume that the specific heat capacity of water, c = 4.18 J/(g°C) or c = 4.18 J g -1 K -1 (i.e J are required for every 1 °C rise in temperature per g of water) Example: In burning 0.75g of ethanol the temperature of 100cm 3 of water rose 54.5 O C. q = 100 X 4.18 X 54.5 = J Scale this up for a mole of ethanol (M r = 46) Gives 22781/0.75 X46= 1400Kjmol -1
Why don’t we get ‘data book’ values? Uncertainties in measurements Limitations of the experimental method such as ………………………. ? Great practicals to evaluate!
Energy or Enthalpy? Enthalpy is a term used at A level. There are several different forms of energy such as heat, light and sound. In chemistry we are usually concerned with the transfer of ‘heat energy’. We use the term ‘Enthalpy’ and ‘enthalpy change’ (ΔH) to describe these changes. Through this presentation new will continue to use ‘Energy’ or ‘Heat Energy’.
Energy changes We can’t measure the absolute heat content of a system but we can measure changes when heat energy is transferred. So we measure ∆H, the heat energy change.
Energy changes Looking at energy transfer from the perspective of the system: In an exothermic reaction, heat is transferred from the system to the surroundings so ∆H is –ve In an endothermic reaction, heat is transferred from the surroundings to the system so ∆H is +ve
Exothermic and endothermic reactions Reactants Products Exothermic reaction Endothermic reaction Heat Energy H Reactants Products Energy taken in Heat Energy H ∆H is -ve ∆H is +ve Energy given out
Activation energy This should be related to common experiment with observations of the enthalpy of that reaction. The following demonstrations may offer a pathway: Burning magnesium in air – exothermic but requires heat to satisfy the activation energy. Mixing methane and oxygen. No reaction until a spark is applied. Potassium manganate (VII) with glycerol – a much lower activation energy requiring no heat input at room temp. Ammonium chloride with barium hydroxide – an extremely endothermic reaction that can reach -30 o C on a good day. Illustrate using diagrams. See later slides.
Representing energy Representing energy changes
H 2 (g) + ½ O 2 (g ) H 2 O(l) H 2 (g) + ½ O 2 (g ) H 2 O(l) Energy transferred from the system ∆H = kJ mol -1 Energy transferred to the system ∆H = kJ mol -1
Energy relationships Reaction∆H kJ mol -1 CaCO 3 (s) → CaO(s) + CO 2 (g)- 890 CaO(s) + CO 2 (g) → CaCO 3 (s) 2CaCO 3 (s) → 2CaO(s) + 2CO 2 (g) 10CaO(s) + 10CO 2 (g) → 10CaCO 3 (s) See ARENERGY8 – Transporting energy
Energy relationships Recap and re-emphasise Energy can’t be made Energy can’t be lost Energy is only transferred System Surroundings
ENERGY OF BONDING
Where does the energy come from? We need to think about all components of the system e.g. a fuel – oxygen system, not just the fuel And link the energy change to the reaction involved
Where does the energy come from? Existing bonds are broken – take in energy. New bonds are made – give out energy. The difference between these is the enthalpy change. Difficult ideas to get over – what models and analogies can you use?
Use models to reinforce bond breaking and making
Re-chargeable Hand Warmer
Reactants Products Bonds broken Bonds made Enthalpy change of reaction ∆H is -ve
Reactants Products Bonds broken Bonds made Enthalpy change of reaction ∆H is +ve
Bond Energies Each bond has a particular amount of energy associated with it. For example, to make or break an O-H bond in water, 464 Kj is involved per mole. Values for a type of bond vary slightly from compound to compound. Average bond energy values are published These enable us to estimate energy changes
Bond Energy example: H 2 + I 2 2HI Bonds broken: H-H436 I-I151 Total587 Bonds madeH-I298 H-I298 Total 596 Bond Energies Kjmol -1 H-H 436 I-I151 H-I298 Energy change = bonds broken – bonds made = 587 – 596 = - 9KJmol -1