Presentation on theme: "Title: Lesson 2 Measuring Enthalpy Changes Learning Objectives: – Understand the technique of calorimetry, including the assumptions underpinning it –"— Presentation transcript:
Title: Lesson 2 Measuring Enthalpy Changes Learning Objectives: – Understand the technique of calorimetry, including the assumptions underpinning it – Calculate enthalpy changes from experimental data – Complete a calorimetry experiment
Main Menu Refresh Which is true for a chemical reaction in which the products have a higher enthalpy than the reactants? Reaction∆H A.endothermicpositive B.endothermicnegative C.exothermicpositive D.exothermicnegative
True or False? 1) Exothermic reactions have a negative ΔH 2) Endothermic reactions have a negative ΔH 3) Breaking bonds is an exothermic reaction 4) In endothermic reactions, the products have less energy than the reactants 5) Standard temperature is 275K 6) Standard pressure is 100kPa 7) Standard enthalpy of combustion only involves fuels 8) Photosynthesis is an exothermic reaction 9) Endothermic reactions absorb energy 10) In E = mcΔT, c is the number of moles of compound used T T T T F F F F F F 7.3 MEASURING ENTHALPY CHANGES
HEAT & TEMPERATURE - What is the difference? Temperature Average kinetic energy of the particles Independent of the number of particles Heat Total energy of all particles Energy of every particle is included Dependent on the number of particles Heat always flows from high low temp Q: What has more heat, luke warm swimming pool or a red hot nail? Why? 7.3 MEASURING ENTHALPY CHANGES
Main Menu Heat changes can be calculated from temperature changes If the same amount of heat is added to two different objects, the Δ T will not be the same, as the average kinetic energy of the particles will not increase by the same amount. Smaller number of particles will experience a larger temperature increase. The increase in temperature when an object is heated will depend on: - Mass of object - Heat added - Nature of substance
Main Menu Specific heat capacity, C The specific heat capacity of a substance is the amount of energy required to raise the temperature of one gram (g) by one Kelvin (K). Specific heat capacity is different for different substances: Substance Specific Heat Capacity J g -1 K -1 Water4.18 Ethanol2.44 Air1.00 Iron0.450 Copper0.385
DIFFERENT TYPES OF ΔH Standard enthalpy change of reaction (ΔH r ) Enthalpy change when a reaction occurs in the molar quantities shown in the balanced chemical equation under standard conditions Standard enthalpy change of formation (ΔH f ) Enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions e.g. 2C (s) + 3H 2 (g) + ½O 2 (g) C 2 H 5 OH (l) Standard enthalpy change of combustion (ΔH c ) Enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions in standard states 7.3 MEASURING ENTHALPY CHANGES
USING THE EQUATION q = mcΔT No instrument measures heat directly To measure enthalpy change we transfer the heat to a known mass of another substance or material, usually water q (or ∆H) = mcΔT 7.3 MEASURING ENTHALPY CHANGES q = enthalpy or heat change (J) m = mass (in g) of solution in calorimeter (either directly or indirectly heated) this is also same as volume in cm 3 ΔT = change in temp of the solution (K) c = specific heat capacity of solution (4.18 Jg -1 K -1 ) ∆H can easily be converted to a molar value by dividing it by the number of moles of reactant (Jmol -1 ) Assumptions: m is just the total mass of water used This is valid as the mass of water used is much greater than the mass of any of the other substances C is just the specific heat capacity of water, ignoring the reactants This is valid as the specific heat capacity of water is much higher than most other substances, so they absorb very little of the heat
Main Menu Relationship between different objects Use heat capacity (c) Defined as the heat needed to increase the temperature of an object by 1K. A swimming pool has a larger heat capacity than a kettle. Note the different between specific heat capacity and heat capacity!
Main Menu Enthalpy changes and the direction of change An exothermic reaction can be compared to a person falling off a ladder. Both changes lead to a decrease in stored energy. Products are more stable than the reactants. However, stability is relative. E.g. Hydrogen peroxide is stable compared to its elements but unstable compared to its decomposition products water and oxygen…
Main Menu Falling up the ladder… We do not expect a person to fall up a ladder but endothermic reactions do occur… Endothermic reactions are less common and occur when there is an increase in disorder of the system, e.g. owing to a formation of a gas…
HOW CAN WE CALCULATE ENTHALPY CHANGES? 7.3 MEASURING ENTHALPY CHANGES Simple calorimeters Bomb calorimeter There are two ways of measuring enthalpy changes: Measuring enthalpy changes of combustion Measuring enthalpy changes of reaction
KEY IDEAS Specific heat capacity (c) – the amount of energy needed to raise the temperature of 1g of substance by 1K. For water and solutions it is 4.2 Jg -1 K -1 KEY IDEAS Two step calculations. 1)Work out the energy transferred 2)Work out the energy supplied per mole. 7.3 MEASURING ENTHALPY CHANGES
Main Menu Calorimetry – Enthalpy of combustion Calorimetry is used to measure the amount of heat released/absorbed in a reaction. The reaction is used to heat some water, and the temperature change measured If we know the mass of water used, the specific heat capacity of the water and the temperature change, we can calculate the heat change. If we are being accurate, we should also take into account the heat capacity of the calorimeter itself as this also heats up.
CALCULATING ENTHALPY CHANGE OF COMBUSTION STEP 1: Calculate the heat lost or gained using q = mcΔT Convert Joules to kilojoules STEP 2: Calculate number of moles of one of fuel that caused the enthalpy change, from the mass that reacted use n = mass ÷ Molar mass STEP 3: Calculate ΔH c (in kJmol -1 ) using actual heat change (q in kJ) and number of moles of fuel that burned (n) Use equation: ΔH c = q ÷ n
CALCULATING ENTHALPY CHANGE OF COMBUSTION E.g. 1.16g of a fuel is burned in oxygen raising the temp of 100g of water from 295.3K to 357.8K. M r of the fuel is 58. Calculate ΔH c Step 1: q = mcΔT = 100 x 4.18 x (357.8 – 295.3) = 26125J = 26.125kJ Step 2: n = mass ÷ M r = 1.16 ÷ 58 = 0.02 moles of fuel Step 3: ΔH c = q ÷ n = 26.125 ÷ 0.02 = -1306 kJmol -1
Main Menu The IB data booklet value is -1367 kJ/mol… Can you think of 3 reasons why the experimental value is less than the literature value?
Main Menu The difference between the values can be accounted for by any of the following factors: Not all the heat produced by the combustion reaction is transferred to the water. Some is needed to heat the calorimeter and some has passed to the surroundings. The combustion is unlikely to be complete due to the limited oxygen available. The experiment was not performed under standard conditions.
Enthalpy changes are determined by simple calorimetry experiments. Known volumes, masses (m) and concentrations are used. The initial (pre-reaction) temperature is taken (t 1 O C). The reaction is allowed to proceed and then the final temperature is recorded (t 2 O C). Overall temperature change ΔT is t 2 O C- t 1 O C ENTHALPY CHANGES OF REACTION
CALCULATING ENTHALPY CHANGE OF REACTION 7.3 MEASURING ENTHALPY CHANGES STEP 1: Calculate the heat lost or gained using q = mcΔT Convert Joules to kilojoules STEP 2: Calculate number of moles (n) of one of the reactants that caused the enthalpy change, from the mass that reacted use n = mass ÷ molar mass OR n = (conc n x volume) ÷ 1000 STEP 3: Calculate ΔH r (in kJmol -1 ) using actual heat change (q in kJ) and number of moles (n) Use equation: ΔH r = (q ÷ n) x number of moles reacting in balanced equation
CALCULATING ENTHALPY CHANGE OF REACTION 7.3 MEASURING ENTHALPY CHANGES E.g. 30g of ammonium chloride (NH 4 Cl) is dissolved in water. Temperature decreases from 298K 296K. Total mass of the solution was 980g. Calculate the standard enthalpy change of reaction Balanced equation: NH 4 Cl (s) NH 4 + (aq) + Cl - (aq) Molar mass, M, of NH 4 Cl = 14 + (4 x 1) + 35.5 = 53.5 gmol -1
CALCULATING ENTHALPY CHANGE OF REACTION 7.3 MEASURING ENTHALPY CHANGES Step 1: q = mcΔT = 980 x 4.18 x (298-296) = 8192.8J = 8.1928kJ Step 2: n = mass ÷ molar mass = 30g ÷ 53.5gmol -1 = 0.5607 moles of NH 4 Cl Step 3: The balanced reaction involves 1 mole of NH 4 Cl, so: ΔH r = (q ÷ n) x 1 = 8.1928 ÷ 0.5607 = 14.6 kJmol -1 Remember: IF temp increases need to add a minus to q value!
Main Menu Another example When 200 cm 3 of 1.00 mol dm -3 sodium hydroxide solution was added to 200 cm 3 of a 0.400 mol dm -3 solution of sulphuric acid, the temperature rose from 24.5 o C to 30.0 o C and a neutral solution was obtained. Determine the enthalpy change when one mole of sulphuric acid is fully neutralised by sodium hydroxide. Determine ∆ H: ∆ H = -m.C ∆. T ∆ H = -(200+200) x 4.18 x (30.0-24.5) = -9,196 J Determine n(H 2 SO 4 ): n(H 2 SO 4 ) = conc. x vol. = 0.400 x (200/1000) = 0.0800 mol Determine Molar ∆ H: Molar ∆ H = ∆ H / n(H 2 SO 4 ) = -9196 / 0.0800 = 114950 J = -115 kJ mol -1
Main Menu Sources of error and assumptions for measuring enthalpy in a polystyrene cup Heat is lost from the system as soon as the temperature rises above the temperature of the surroundings (around 20 o C) Maximum recorded temperature is lower than the true value obtained in a perfectly insulated system. Excess zinc powder is added to a calorimeter of a known volume of copper sulphate solution We can make allowances for heat loss by extrapolating the cooling line back to when the reaction started…
Main Menu To proceed with our calculations we have made the following assumptions… 1. No heat loss from the system 2. All heat goes from reaction to the water 3. The solution is dilute: V(CuSO 4 ) = V(H 2 O) 4. Water has a density of 1.00gcm -3
SUMMARY QUESTIONS 7.3 MEASURING ENTHALPY CHANGES 1. 0.05 mol of a compound dissolves in water, causing the temperature to increase from 298K to 301K. Total mass is 220g. Calculate the enthalpy change in kJmol -1. Assume c = 4.18 Jg -1 K -1 2. A calorimeter, containing 200g of water, was used to calculate ΔH c of pentane (C 5 H 12 ) (M r = 72). 0.5 g of pentane was burnt, increasing the water temp by 29K. a) calculate ΔH c ; b) Suggest reasons why your value may be different to the value given in a data book 3. ΔH c of octane is -5512 kJmol -1, its M r is 114. Some octane was burnt in a calorimeter with 300g of water. The temperature increase was 55K. Calculate the mass of propane burnt.
SELF ASSESS 7.3 MEASURING ENTHALPY CHANGES 1. 0.05 mol of a compound dissolves in water, causing the temperature to increase from 298K to 301K. Total mass is 220g. Calculate the enthalpy change in kJmol -1. Assume c = 4.18 Jg -1 K -1 q = mcΔT = 220 x 4.18 x (301 – 298) = 2758.8 J = 2.7588 KJ ΔH = q ÷ n = 2.7588 ÷ 0.05 = -55.2 kJmol -1 REMEMBER: the minus symbol as temp went up!
SELF ASSESS 7.3 MEASURING ENTHALPY CHANGES 2. A calorimeter, containing 200g of water, was used to calculate ΔH c of pentane (C 5 H 12 ) (M r = 72). 0.5 g of pentane was burnt, increasing the water temp by 29K. a) calculate ΔH c ; b) Suggest reasons why your value may be different to the value given in a data book q = mcΔT = 200 x 4.18 x 29 = 24244 J = 24.244 KJ n = m ÷ M r = 0.5g ÷ 72 = 0.00694 moles of fuel ΔH = q ÷ n = 24.244 ÷ 0.00694 = -3490 kJmol-1 b) Some heat from combustion will be transferred to the surroundings and not the water. May not have been complete combustion. May not have been standard conditions. Inaccuracies in the measuring equipment
SELF ASSESS 7.3 MEASURING ENTHALPY CHANGES 3. ΔH c of octane is -5512 kJmol -1, its M r is 114. Some octane was burnt in a calorimeter with 300g of water. The temperature increase was 55K. Calculate the mass of propane burnt. q = mcΔT = 300 x 4.18 x 55 = 68970 J = 68.97 KJ ΔH = q ÷ n rearranges to n = q ÷ ΔH = 68.97 ÷ 5512 = 0.0125 mol n = mass ÷ M r rearranges to mass = n x M r = 0.0125 x 114 = 1.43g octane
Main Menu Calorimetry in Practice In this experiment you will determine the enthalpy change for the reaction of magnesium with sulphuric acid. To do this accurately, you will first need to determine the heat capacity of the calorimeter. Follow the instructions hereinstructions here
Main Menu Key Points ∆H = -m.C.∆T Units: Joules, J Assumptions: Specific heat capacity of solutions is the same as that of water Total mass is the same as the volume of solution used