4.3 Measuring Enthalpy Changes

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the standard molar enthalpy of combustion: The standard enthalpy change of combustion is the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions with all reactants and products in their standard states. 4.3 Measuring Enthalpy changes Example: This shows the standard enthalpy change for the complete combustion of the hydrocarbon ethane. 2 Key points: 1. we know that this shows complete combustion as we have made carbon dioxide and water. 2. All chemicals are in their standard states for 298k and 100kpa pressure 3. Equation is balanced. 4. The C2H6 will always be 1 mole so balance it accordingly. C₂H6 + 3.502 (g) (g) Ø ACH 2CO2 + 3H₂O (g) (1) ▾ How do we know that complete combustion has occured? The products should be carbon dioxide and water ▾ Give an example of an equation which represents standard enthalpy of combustion? E.g. C (s) + O₂(g) → CO₂ (g) 2 ▾ How do we calculate the standard enthalpy change of combustion? 4.3 Measuring Enthalpy changes Mass of water = 400 g Temperature change of water = +36.5 °C Mass of fuel that combusted = 3.75 g 1. First we need to calculate the thermal energy that transferred into the water. and we use this equation 3 q = m ↑ ↑ energy change of the water (J) energy change of the water (J) 2. Putting our numbers into the equation gives us this value. q specific heat capacity of water J g-¹K-1 mass of water (g) mass of water (g) 4.3 Measuring Enthalpy changes q = 61028 J q = c AT C temperature change of the water (°C) = 400 x 4.18 x 36.5 ↑ specific heat capacity of water J g-¹K-1 temperature change of the water (°C) 61.028 kJ 3. Now we can use this to calculate the standard enthalpy change of combustion of the ethanol. 4. The units of standard enthalpy change of combustion are kJ Mol-1 and the per mole refers to the amount in moles of fuel that was combusted. 5. From our results, we can divide the mass by the molar mass. 4 Mass of fuel combusted = 3.75 g molar mass of ethanol = 46.0 g mol-1 number of moles = number of moles 6. We then calculate the standard enthalpy change of combustion: ACH ACH ACH e || 4.3 Measuring Enthalpy changes 3.75 g 46.0 g mol-1 中 ACH = 0.0815 mol thermal energy (kJ) number of moles 61.028 0.0815 7. Because this reactions exothermic we need to give this reaction a negative sign. 748.8 kJ mol-1 -748.8 kJ mol-1 5 https://www.youtube.com/watch?v=-KKQqfDRALs&list=PL9louNCPbCxWNOATfXgGk_ISkKaRL-7tR &index=4 ▾ Steps for working out the enthalpy change of combustion You need to be able to calculate the enthalpy change of combustion from experimental results. Combustion is a reaction involving oxygen to for oxides. Determination of AH, for a liquid fuel is one of the easiest enthalpy experiments to carry out. Burn a known mass of the substance in air: to heat a known mass of water (remember that the density of water is 1 g cm³ and the specific heat capacity of water is 4.18 Jg¹K¹); and measure the temperature change of the water. . ENTHALPY CHANGE OF COMBUSTION 250g Example calculation During combustion 1.50 g of propan-1-ol, CH3CH₂CH₂OH, heated 250 cm³ of water by 45°C. Find the enthalpy change of combustion of propan-1-ol. Step 1- Find the heat exchange. Heat exchange is calculated using the equation Q = mcAT. 250 cm³ of water has a mass of 250 g :: m = 250 AT = +45.0°C Water has a specific heat capacity of 4.18 J g¹ K¹. c = 4.18 Heat gained (temperature increase) by surroundings, Q = mcAT :: Q = 250 x 4.18 x (+45.0) = +47025 J Combustion reactions are exothermic (exothermic reactions have a negative AH) :: AH = -47025 J Step 2 - Find out the amount, in mol, that reacted. Molar mass (M) of CH3CH₂CH₂OH F 60.0 g mol-¹ Use the equation n = m M m = 1.50 g Ar of this n=1,50 = 0.025 mol 60.0 Step 3-Work out the heat loss in kJ mol-¹. Divide the heat loss by the amount, in mol, of fuel that was burnt. always kJ 4.3 Measuring Enthalpy changes AH, for 1 mol CH3CH₂CH₂OH = -1881000 J = -1881 kJ To convert J to kJ, just divide by 1000 mass Molar Mass 1. add 2. Then Always negatie its, exother me (combustia) as For 0,025 mol CH3CH₂CH₂OH, AH = -47025 J. For 1 mol divide AH by the number of moles CH3CH₂CH₂OH = -47025 J 0.025 Enthalptly change per Mole Step 4 - Write down the equation together with the enthalpy change in kJ mol¹¹ CH₂CH₂CH₂OH(1) 41O2(g) → 3CO2(g) + 4H₂O(1) AH, -1881 kJ mol-¹ B If you always follow this method, and get loads of practice before your exams, you will be fine! ▾ The standard enthalpy change of neutralisation ▾ State the definition of the standard enthalpy change of neutralisation. AN = Q 3 moje O2 Đ co to Balance it 6 This is the enthalpy change when one mole of water is formed in a neutralisation reaction under standard reactions. Example: The equation of the reaction between hydrochloric acid and sodium hydroxide solution. 1. In this reaction, the key parts are the hydrogen ion from the acid and the hydroxide ion from the alkali. 2. This means that the standard enthalpy change of neutralisation is always the same between any strong acid and any strong alkali. HC/ + (aq) I H* (aq) from acid NaOH OH (aq) (aq) 4.3 Measuring Enthalpy changes from alkali neut ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ ▾ The standard enthalpy change of formation ▾ State the definition for the standard enthalpy change of formation NaC/ + H₂O (aq) (1) ▾ What is the standard enthalpy change of neutralisation between any strong acid and any strong alkali? -57 kJ mol-1. This reaction is exothermic H₂O (1) The standard enthalpy change of formation is the enthalpy change when 1 mole of a compound is formed in its elements in their standard states under standard conditions. Examples: Key points: • Always be accurate with the elements, always put elements. H2 instead of H. • Standard enthalpy change of formation is for one mole of product, so the product should have nothing to do with balancing, no big numbers on the fron 7 4C + 5H₂ (g) A₁H ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ ▾ Give an example of an equation which represents standard enthalpy of formation? ▾ What is the standard enthalpy change of formation for ammonia? 0.5N₂ + (g) e There are many e.g. H₂ (g) + ½ O₂ (g) → H₂O (1) N₂ + 3H₂ 2 (g) (g) 4.3 Measuring Enthalpy changes C4H10 (g) 1.5H2 2NH3 ΔΗ (g) ⇒ NH3 (g) ▼ The standard enthalpy change of reaction ▼ State the definition for the standard enthalpy change of reaction. The standard enthalpy change of reaction is the enthalpy change when a reaction occurs in the molar quantites shown in the chemical equation, under standard conditions with all reactants and products in their standard states. ▾ What is standard enthalpy change of reaction used for? 8 The standard enthalpy change of reaction is used for any chemical reaction which does not fit into the previous three categories. Example: C₂H4 (g) 7tR&index=5 + ▾ What instrument can be used to measure heat: • There are no instruments that measure heat directly ▾ How can enthalpy change be measured: • Transferring the heat into a mass of a substance, often water . Then you need to know: o Mass of the substance being heated up or cooled down, Kg o Temperature change, AT o Specific heat capacity of the substance, c C/₂ (g) A Level Chemistry Revision "How to Determine the Standard Enthalpy Change of Reaction" In this video, we look at how to determine the standard enthalpy change of a reaction. First I take you through the experimental procedure. I then explain to you how to correct the results to take into account heat losses from the experimental setup. Finally, I show you how to carry out the calculation. G https://www.youtube.com/watch?v=7OW_DM8ZXbg&list=PL9louNCPbCxWNOATfXgGk_ISKKARL- Zn + (s) 6.54 g ▾ How do we calculate the standard enthalpy change of reaction? Example Question: 4.3 Measuring Enthalpy changes e A₁H ΔΗ 2Ag(NO3)2 (aq) 200 cm³ 1 mol dm-³ C2H4C/2 (1) How to Determine the Standard Enthalpy Change of Reaction Chemistry A Level AQA/OCR / Edexcel 6.54g of zinc was added to 200cm3 of silver nitrate solution with a concentration of 1 mol dm-3, correcting for cooling, the temperature increase by 42 degrees celsius. Calculate the standard enthalpy change of reaction. Zn(NO3)2 + Ag (aq) (s) 9 1. To answer this you need to know that the silver nitrate is the excess reagent. 2. First we calculate heat energy released by the reaction and we use this equation: q = m ↑ ↑ mass of water (g) energy change of the water (J) energy change of the water (J) specific heat capacity of water J g-¹K-1 3. We used 200cm3 of solution, because this is an aqueous solution, we can assume that the density ofthis solution is close the density of water, so the mass of our solution is 200g. 4. Because its aqueous we can also use the specific heat capacity of water which is 4.18 J g-1 K-1. 5. The temperature increase was 42 degrees celsius. 6. Then we get this mass of water (g) 7. Then we divide by 1000 to get this: 4.3 Measuring Enthalpy changes c AT C ↑ q = 35112 J temperature change of the water (°C) q=200 x 4.18 x 42 ↑ temperature change of the water (°C) specific heat capacity of water J g-¹K-1 10 9 = 35.112 kJ 8. At the start of the question we were told that the silver nitrate was in excess and zinc was the limiting reagent, so to work out standard enthalpy change of reaction, we need to divide the heat energy of the number of moles of zinc. 9. We have 6.54g of zinc and the molar mass of zinc is 65.4 g mol-1 10. This is gives us 0.1 moles of zinc in our reaction Zn + (s) 6.54 g 65.4 gmol-1 0.1 mol 2Ag(NO3)2 (aq) excess 11. Now we can divide the heat energy by the number of moles. 4.3 Measuring Enthalpy changes Standard enthalpy change of reaction Ө A₁H Ө A-H Zn(NO3)2 + Ag (aq) (s) = 35.112 kJ 0.1 mol = 351.12 kJ mol-1 12. Because its exothermic, we need to give this a negative number. 11 Standard enthalpy change of reaction 35.112 kJ 0.1 mol ▾ What does per mole refer to? ΔΙΗ 5.4 g of aluminium was reacted with unlimited chlorine under standard conditions. 70.6 kJ of heat energy was released. Calculate A,H* A₁H = -351.12 kJ mol-1 2AI + 3C/₂ (s) ▾ Heat and temperature ΔΗ Ө 4.3 Measuring Enthalpy changes = = (g) The per mole refers to the whole balanced chemical equation, so what that means if that if one mole of this entire balanced chemical equation takes place, in other words 2 moles of aluminium react with 3 moles of chlorine to make 2 molecule of aluminium choride, then 706 kJ of heat energy is released. What is the difference between heat and temperature The simple calorimeter or q=mcAT — 2AIC/3 (s) -70.6 kJ X 10 -706 kJ mol-1 Heat is the sum of all the particles energy, therefore it is affected by the amount of substance. Temperature is releated to the mean knietic energy od the particels in a system, so is independent of the number of particles present. 12 ▾ What is the use of this experiment? We're going to determine the standard enthalpy change of combustion of a liquid fuel for example ethanol. In order to do this, we will combust the fuel and use the thermal energy released, to heat a known mass of water. By measuring the temperature change of water, we can determine the standard enthalpy change of combustion. ▼ What does a calorimeter measure? • Enthapy change ▾ How can you calculate enthalpy change from experinmental data? • I Use the equation Q = mcAT, M- mass of substance • C-specific heat capacity of that substance • delta T- the change in the temperature ▾ Draw a simple calorimeter and label the apparatus -thermometer 4.3 Measuring Enthalpy changes -metal calorimeter water spirit burner ethanol 1. We have ethanol in a spirit burner. 2. Above this, we have a metal can containig a fixed mass of water, scientists call this metal can a calorimeter 3. We use thermometer to measure the temperature of the water. 13 ● Thermometer to measure change in temperature of water Beaker containing known mass of water Gauze Tripod Spirit burner containing substance to be tested (change in mass used to calculate moles burned) ▼ What can the a simple calorimeter be used for and why: To compare the ACH values of a series of similar compounds because the errors will be similar for every experiment (since the compounds are similar) 4.3 Measuring Enthalpy changes Heatproof mat Worked example: Working out the enthalpy change The calorimeter in Figure I was used to measure the enthalpy change of combustion of methanol. CH₂OH(1) + 10₂(g) → CO₂(g) + 2H₂O(l) 0.32 g (0.01 mol) of methanol was burnt and the temperature of the 200.0g of water rose by 4.0 K. Heat change = q = mx cx AT = 200.0 x 4.2 x 4.0 = 3360J 0.01 mol gives 3360J So 1 mol would give 336000J or 336 kJ AH = -340 kJ mol-¹ (negative because heat is given out) 14 200 g water spirit burner ethanol Figure 1 A simple calorimeter 4.3 Measuring Enthalpy changes ▾ What must be known when using a calorimeter: • The mass of water and the starting temperature • Specific heat capacity of water ▾ What assumption is made when using a calorimeter, and why is it not so in practice: • You assume all the heat from the fuel being burnt goes to the water directly, which is never the case since heat with always be absorbed by particles in the air ▾ How do we carry this experiment out? 1. First, we use a balance to measure the starting mass of the spirit burner and the fuel 15 4 4.3 Measuring Enthalpy changes 164.65 g 2. We then use the thermometer to measure the start temperature of the water. 3. Next we remove the cap from the spirit burner and immeditaley light the wick. IT IS VERY IMPORTANT THAT WE DONT LEAVE THE BURNER UNCAPPED IF IT IS NOT LIT. THAT IS BECAUSE FUEL WILL EVAPORATE AND MAKE OUR FINAL RESULTS LESS ACCURATE. thermometer -metal calorimeter water spirit burner ethanol 16 4. The thermal energy released now causes the temperature of the water to increase. WE NEED TO STIR THE WATER WITH THE THERMOMETER TO MAKE SURE THAT THE THERMAL ENERGY IS DISTRIBUTED. 5. After several minutes, we extinguish the flame by replacing the cap on the burner and use the thermometer to read the temperature of the water. thermometer metal calorimeter 4.3 Measuring Enthalpy changes water spirit burner ethanol 6. We then use the balance to measure the final mass of the spirit burner and the fuel. 160.90 g 7. by subtracting the final mass from the starting mass, we can calcualte the mass of fuel that combusted. 17 Here are the results: Mass of water = 400 g Temperature change of water = +36.5 °C Mass of fuel that combusted = 3.75 g 8. We can now calculate the standard enthalpy change of combustion. ▾ How can the calorimeter be improved: . By placing a lid ontop, mineral wool around the beaker and enclose the flame with a draught screen to minimise heat loss resources-tuition-courses What is a flame calorimeter; how does it differ to a simple calorimeter? Reduces heat lost to the surrounding to give more accurate results: has a spiral chimney made of copper, an enclosed flame and the fuel is burnt in pure oxygen, not air. Why might the published value of a fuel be a lot higher than the value calculated from the experiment? experiment value of ethanol: -748.8 kJ Mol-1 Published value of ethanol: -1367 KJ Mol-1 1. First, if we leave the unlit spirit burner uncapped then the fuel will evaporate. This will make it appear that we burned more fuel than we actually did, and because of this our results will show the reaction to be less exothermic than it actually was. 2. Secondly, a lot of the heat energy released by the fuel does not pass directly into the water, some of the heat energy is transferred to the metal calorimeter, and a great deal of heat energy is simply transferred to the air. 3. It is also possible that not all of the fuel underwent complete combustion. If incomplete combustion occurs then this releases less thermal energy than complete combustion. 4. Our Experiment may not have been carried out under standard conditions. 4.3 Measuring Enthalpy changes 18 ▾ What is the specific heat capacity of dilute solutions usually taken as: 4.2Jg ¹K-¹, which the the same as that of water -1 ● Practice Questions: 4.3 Measuring Enthalpy changes 19