Energetics
How can we make a calorimetry experiment more accurate
A bomb calorimeter can be used to obtain more accurate results for the enthalpy changes of combustion. The substance to be combusted is ignited electrically and burns in the presence of our oxygen. The heat transferred to the water and the calorimeter itself is determined to give a more accurate value of the enthalpy changes
What is specific heat capacity?
1. The specific heat capacity of a substance is defined as the amount of heat needed to raise the temperature of one gram of substance by one degrees celsius. The specific heat capacity tells us how much energy has to be put into something to increase its temperature. 2. For water the specific heat capacity is 4.18 J/g/°C (joules per gram per degree celsius). In other words, if you want the temperature of one gram of water to go up by two degrees celsius then 4.18 x 2 = 8.36 J of heat energy must be supplied. If you now have two grams of water then 8.36 x 2 J of energy would be required to raise the temperature by 2°C. The same philosophy applies to cooling a substance. 3. The amount of heat energy required is directly proportional to the mass and temperature change of the substance. The following equation can be used to calculate how much heat energy needs to be supplied to raise the temperature of mass (m) by ΔT °C heat energy change= mass x specific heat capacity x temperature change Q = m x c x ΔT
What are examples of endothermic reactions?
1. Thermal decomposition of metal carbonates The carbonate must be heated constantly to decompose since a great amount of energy is needed to break the chemical bonds. For example, copper (II) carbonate (green) decomposes on heating to produce copper (II) oxide (black) and carbon dioxide CuCO3 (s) -> CuO(s) + CO2 (g) Similarly zinc carbonate decomposes to form zinc oxide when heated ZnCO3 (s) -> ZnO (s) + CO2 (g) Not all endothermic reactions need to be heated. Sometimes they are just heated to speed up the reaction.
What is the enthalpy change of a reaction?
1. ΔH is the symbol for the enthalpy change of a reaction. "Δ" means "change in" and is produced "delta" and "H" means heat. It is not possible to measure how much enthalpy something has but only the change in enthalpy when it reacts. 2. The amount of heat energy released or taken in in a chemical reaction in in a chemical reaction is the enthalpy change. 3. It is the difference between the energy of the reactants and the products. ΔH is given a minus or a plus to show whether heat is being given out or absorbed by the reaction. You always look at it from the point of view of the reactants. For example, in an exothermic reaction, ΔH is given a negative number because the reactants are losing energy as heat which is then transferred to the surroundings, which then get warmer. 4. ΔH is measured in kJ/mol (kilojoules per mole) 5. For example, the enthalpy change of a reaction between one mole of magnesium and sulfuric acid could be ΔH= -466.9 kJ/mol since it is exothermic and the reactants lose energy.
What is an endothermic reaction?
A reaction that absorbs heat from the surroundings is said to be endothermic. If you hold a test-tube in which an endothermic reaction is occurring you will notice that it gets colder. In an endothermic reaction the products have more chemical energy than the reactants. This is because more energy is required to break the bonds than is released forming bonds. Therefore, the energy needed needed to convert the reactants into the products is absorbed as heat energy and converted into chemical energy (energy stored in the bonds of chemicals). The temperature of the reaction mixture and the surroundings goes down because heat energy has been converted into a different form of energy.
What is an exothermic reaction?
A reaction that gives out heat to the surroundings is said to be exothermic. If you are holding a test tube in which an exothermic reaction is occurring, the test tube gets warmer. In an exothermic reaction, the products of the reaction have less chemical energy than the reactants. This is because in the reaction chemical energy stored within the bonds of the chemicals is converted to heat energy, which is released to the surroundings. The energy needed to break the bonds is lesser than the energy released when bonds are formed. As a result, the temperature of the reaction mixture and the surroundings goes up.
Give an example of an exothermic reaction?
An example of an exothermic reaction is adding water to calcium oxide. If you add water to solid calcium oxide, the heat produced is enough to boil the water and produce steam. Calcium Hydroxide is produced CaO(s) +H2O(l) -> Ca(OH)2(s) Calcium oxide is known as quicklime and asking water to it is known as slaking it. The calcium hydroxide produced is known as slaked lime. It is less hazardous to use a lump of calcium oxide than a powder
What is a combustion reaction?
Any reaction that produces a flame is exothermic. Oxygen from the air reacting with a substance to form a full outer shell transfers thermal energy and light energy to the surroundings and is a combustion reaction. For example, hydrogen burns in oxygen, producing water and lots of heat.
What is bond energy?
Bond energy is defined as the amount of energy needed to break one mole of covalent bonds in gaseous molecules and is measured in kJ/mol Breaking chemical bonds requires energy. The stronger the bond, the more energy is needed to break it. For example, the bond energy for Cl-Cl is 243 kJ/mol. This means that it will take 243 kilojoules to break all the Cl-Cl bonds in one mole of chlorine gas. the bond energy also represents the amount of energy released when one mole of the bonds form. The H-Cl bond energy is 432 kJ/mol. If you see 2HCl (g) in an equation that means it will take 2 x 432 kJ to break all the covalent bonds in the two moles of hydrogen chloride gas. Or 2 x 432 kJ will be released when 2HCl is formed from H and Cl atoms Some bonds are much stronger than others, for example the bonds between iodine and hydrogen is about twice as strong as the bond between two iodine atoms. This can be used to calculate how much heat will be absorbed or released at certain points in the reaction and as a result whether it is exothermic or endothermic. Bond calculations only ever gives estimates of the amount of energy absorbed or released since the strength of a bond varies slightly depending on what is around it in the molecule.
Why do reactions give out and absorb heat?
During chemical reactions, bonds in the reactants have to be broken and new ones have to be formed to make the products. Breaking bonds needs energy (endothermic) and making bonds releases energy (exothermic) For example, when hydrogen burns in oxygen to make water, the bonds are broken in the hydrogen and the oxygen and new bonds are made in water. Energy has to be supplied to break the bonds in the hydrogen and oxygen molecules and energy is released when the new bonds are formed between the oxygen atoms and hydrogen atoms in the water molecules. This forms steam which then gives out heat when it condenses to form liquid water as intermolecular forces of attraction form between the molecules of water. In this reaction, the products are more stable than the reactants since they have less energy. This means that the total amount of energy released when covalent bonds are formed in the product (water) is more than is required to break the covalent bonds in the reactants (hydrogen and oxygen). Therefore, the reaction is exothermic. When you heat up a calcium carbonate, breaking up the original bonds in the compound uses more energy than released when new ones are made. The reactants are more stable than the products and therefore have more energy. This means the reaction was endothermic.
How can we calculate the ΔH of this displacement reaction using the results: Initial temperature of copper (II) sulfate solution/°C = 17.0 Maximum temperature of copper (II) sulfate solution/°C = 27.3
Heat given out in this reaction = 50 x 4.18 x (27.3 - 17.0) = 2.1527 kJ Here we assume the following: 1. the density of the copper sulfate solution is the same as that of water so 1cm3 has the mass of 1g 2. the specific heat capacity is the same as that of water which is a fairly reasonable assumption since the solution is mostly water In this experiment we have used excess zinc which means that there is more than enough to ensure that all the copper (II) sulfate reacts. If you calculate the number of moles of copper (II) sulfate present and the number of moles of zinc used in this procedure you could spot that the number of moles of zinc is more than the number of moles of copper (II) sulfate Number of moles (n) of zinc added = mass/relative atomic mass = 1.20/65 =0.0185 mol Number of moles (n) of copper (II) sulfate added = volume x concentration = 0.050 x 0.200 = 0.0100 mol Now we need to calculate how much heat is released when one moles of copper sulfate reacts with excess zinc Molar enthalpy change = heat energy change / number of moles of copper sulfate reacted 2.1527/0.0100 = 215 kJ/mol The amount of heat released in the displacement reaction when one mole of CuSO4 reacts with excess Zn is therefore ΔH = -215 kJ/mol
How can we evaluate these experimental results?
How accurate is the figure of ΔH = -1020 kJ/mol? The book value for the molar enthalpy change for the combustion of ethanol is actually ΔH = -1370 kJ/mol The value the experiment obtained is less exothermic than expected (it gave out less heat) This could be due to: 1. Large amounts of heat loss- the warm water gives out heat to the air, heat is lost from the flame, which goes straight to the air rather than the water, and heat is used to raise the temperature of the copper can and the thermometer rather than the water 2. Incomplete combustion of alcohol- this occurs when there isn't enough oxygen present. Incomplete combustion releases less heat than complete combustion. We can see this if the flame is yellow orange rather than blue and if there is soot produced at the bottom of the copper can However, that doesn't mean that these results can't be used to compare the results of repeating the experiment under similar conditions with other alcohols. You can use this to discover how the heat given out changes as the alcohol molecules get bigger. You should see that the combustion reaction gets more exothermic as the alcohol chain gets longer. In other words, longer alcohol chains give out more heat energy per mole when they burn than shorter ones. This is because the longer an alcohol chain, the more CH2 bonds there are and as a result the number of extra bonds broken and made increases. Therefore, the amount of energy released forming the bonds also increases in proportion to the amount of energy required to break the bonds. These results should really be plotted as bar graphs. Plotting it as a smooth graph isn't technically wrong but should only be used when there is a continuous independent variable which can be taken at any value. There is no such thing as an alcohol with 0.5 carbon atoms. Therefore, the number of carbon atoms in the molecule is a non-continuos variable since it can only take integer values.
How can we show an endothermic change on an energy level diagram?
In an endothermic change, the products have more energy than the reactants so we say that the products are less stable than the reactants. This extra energy is taken from the surroundings. In the example of thermal decomposition in a laboratory, it comes from the Bunsen burner. Because the reactants are gaining energy, the ΔH is positive. For example, in the thermal decomposition of calcium carbonate: CaCO3 (s) - CaO (s) + CO2 (g) ΔH= +178 kJ/mol This means that 178 kJ of heat energy must be absorbed to convert 1 mole of calcium carbonate into calcium oxide and carbon dioxide
How can you show an exothermic change on an energy level diagram?
In an exothermic reaction, the reactants have more chemical energy than the products so we say that the products are more stable than the reactants. The term stability is used to describe the relative energies of the reactants and the products in a chemical reaction. The more energy a chemical has, the less stable it is. As the reaction happens, energy is given out it the form of heat which warms up the reaction itself and the surroundings.
How can we measure enthalpy changes for displacement reactions?
In order to determine the enthalpy change of the reaction of zinc and copper (II) sulfate the following procedure could be used: 1. Place a polystyrene cup in a 250 cm3 glass beaker 2. Transfer 50cm3 of 0.200 mol/dm3 copper (II) sulfate solution into the polystyrene cup using a measuring cylinder 3. Weigh 1.20 g of zinc using a weighing boat on a balance 4. Record the initial temperature of the copper (II) sulfate solution 5. Add the zinc 6. Stir the solution as quickly as possible 7. Record the maximum temperature reached This experiment could be repeated with metals of different reactivities. The more reactive the metal is, the more heat should be released in the displacement reaction. Make sure you keep the number of moles of the metals, the size of the solid particles and the volume/concentration of the copper sulfate solution he same. Don't use metals more reactive than magnesium otherwise you'll be measuring the heat released when the metal reacts with the water instead.
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In this neutralization reaction the temperature increases at first but then decreases. The reaction between the acid and alkali is exothermic. At the beginning, the temperature goes up because the acid reacts with the alkali, giving out heat. But when all the alkali has been used up, we are just adding cold acid to our warm solution and no reaction is occurring because there is nothing left for the acid to react with. Therefore, the temperature goes down. Two lines of best fit can be drawn on the graph and the point where the lines cross represents complete neutralization. If the maximum temperature was 31.8 degrees celsius and 28.00 cm3 of acid had been used we can work out how much heat is released in the neutralization reaction. 1. The total volume of the solution at the neutralization point is 25 + 28 = 53 cm3 2. The heat given out = 53 x 4.18 x (31.8 - 19.3) = 2.7693 kJ (we have assumed that the specific heat capacity and density are the same as water) 3. Moles of potassium hydroxide = concentration x volume 2.00 x 25/1000 = 0.0500 mol Since this temperature was taken at the neutralization point the number of moles of hydrochloric acid will be the exact same as the number of moles of potassium hydroxide 4. Molar enthalpy change of neutralization = heat energy change / moles of KOH or Hal reacted = 2.7693 / 0.0500 = 55.4 kJ/mol The amount of heat released in the neutralization reaction when 1 mole of KOH reacts with 1 mole of HCl is therefore: = -55.4 kJ/mol We can also work out the unknown HCl concentration 1. We know that there was 0.0500 mole KOH originally present 28.00 cm3 of HCl was required for neutralisation therefore this contained 0.0500 mol of HCl Concentration = number of moles of HCl / volume of HCl used for neutralization = 0.0500 / 0.02800 = 1.79 mol/dm3
Calculate the energy released or absorbed in the reaction between methane and chlorine C-H = 413 Cl-Cl = 243 C-Cl = 346 H-Cl = 432
Methane reacts with chlorine gas in the presence of UV light to produce chloromethane and hydrogen chloride Firstly, the bonds in the methane and chlorine gas are broken 4 x 413 = +1652 kJ 1 x 243 = +243 kJ Total energy absorbed to break bonds = +1895 kJ Secondly, new bonds are formed to create chloromethane and hydrogen chloride 3 x 413 = -1239 kJ 1 x 346 = -346 kJ 1 x 432 = -432 kJ Total energy released = -2017 kJ Therefore, the reaction is exothermic because more energy was released when bonds were formed than taken in when bonds were broken The overall energy change = +1895 + (-2017) = -122 kJ
Other exothermic reactions?
Reactions of metals with acids: When magnesium reacts with dilute sulfuric the mixture gets very war Neutralisation reactions: When sodium hydroxide reacts with dilute hydrochloric acid the temperature rises Displacement reactions: The thermite reaction (reacting a metal oxide with aluminum) between powdered aluminum and iron (III) oxide is a displacement reaction. This reaction releases a large amount of heat when forming iron and aluminum oxide which can be used in railway welding
How to use the data from the calorimetry experiment to find the enthalpy change? Say the results of the calorimetry experiment are as follows: Volume of water/cm3 = 100 mass of burner + ethanol before experiment/g = 137.36 mass of burner + ethanol after experiment/g = 136.58 original temperature of water/°C = 21.5 final temperature of water/°C = 62.8
Say the results of the calorimetry experiment are as follows: Volume of water/cm3 = 100 mass of burner + ethanol before experiment/g = 137.36 mass of burner + ethanol after experiment/g = 136.58 original temperature of water/°C = 21.5 final temperature of water/°C = 62.8 Combustion is an exothermic reaction so the temperature of the water goes up and as ethanol is burned the total mass of the burner and ethanol goes down. The molar enthalpy change: 1. Enthalpy change = mass x specific heat capacity x change in temperature 2. Temperature change of water = 62.8 - 21.5 = 41.3 °C 3. Mass of water being heated = 100 g 4. Specific heat capacity of water = 4.18 J/g/°C 5. Heat gained by water = 100 x 4.18 x 41.3 = 17260 J 6. 17260/100 = 17.26 kJ This means that 17.26 kJ of energy is released by the combustion of the ethanol in this experiment. To find the amount of heat energy produced per mole of ethanol, we need to find out how many moles of ethanol are burned in the experiment. 1. Mass of ethanol burned = decrease in the mass of the burner = 137.36 - 136.58 = 0.78 g 2. Ethanol has the formula C2H5OH. If we add up the atomic masses ( (2 x 12) + (5 x 1) + 16 + 1 ) = 46 3. The mass/the relative molecular mass = the number of moles so 0.78/46 = 0.01696 mol 4. The molar enthalpy change of the combustion of ethanol = the change in heat energy / the number of moles of ethanol burned 17.26/0.01696 = 1020 kJ/mol Therefore the amount of heat energy released in the complete combustion of one mole of ethanol is: ΔH = -1020 kJ/mol
What is entropy?
The number of ways for the energy in a system to be distributed. It is greater in a more disordered system because the energy can be dispersed more flexibly. It follows that solids have a lower entropy than liquids, which in turn have a lower entropy than gases. If the entropy in a system increases during a reaction, it is more likely for that reaction to occur. When dissolving ammonium nitrate in water, although the process is endothermic, the NH4+ and the NO3 - ion parties in the giant ionic attic become separated from each other. The ions have more freedom of movement in water and the entropy of the system increases which outweighs the effect of enthalpy. This causes an endothermic reaction to occur spontaneously
Can you describe a practical measuring the enthalpy changes of neutralization between an alkali and an acid?
The reaction between an alkali and an acid is essentially between OH- and H+ ions to form water OH- (aq) + H+ (aq) -> H2O (l) The reaction we will be looking at is between potassium hydroxide (an alkali) and hydrochloric acid. If you know the concentration of the potassium hydroxide but not the concentration of the dilute hydrochloric acid solution, then the following method could be used to find out the concentration of the acid and how much heat is released during the neutralization reaction: 1. Place a polystyrene cup in a 250 cm3 glass beaker 2. Transfer 25cm3 of 2.00 mol/dm3 potassium hydroxide into the polystyrene cup using a measuring cylinder 3. Record the initial temperature 4. Fill a burette with 50.00 cm3 of dilute hydrochloric acid 5. Use the burette to add 5.00 cm3 of dilute hydrochloric acid to the potassium hydroxide 6. Stir vigorously and record the maximum temperature reached 7. Continue adding further 5.00cm3 portions of dilute hydrochloric acid to the cup, stirring and recording the maximum temperature each time until a total volume of 50.00 cm3 has been added We can plot a graph of the temperature of the mixture versus the volume of acid added
Calculate the enthalpy change for the combustion of methane using bond energies? C-H = 413 O=O = 498 C=O = 743 H-O = 464 The equation is: CH4 (g) +202 (g) -> CO2 (g) + 2H2O (g)
The reason water is described as a gas here is because bon energies can only be used for gaseous molecules. We could find out the enthalpy change using the liquid if we knew the amount of energy released in the process of converting the gaseous water to liquid water. Firstly, we would draw out the formula showing all the structural formulas and the bonds so we can make sure we get the number of bonds correct. 1. Bonds that need to be broken (endothermic) 4 C-H = 4 x 413 = +1652 kJ 2 O=O bonds = 2 x 498 = +996 kJ Total energy absorbed to break bonds = +2649 kJ 2. New bonds that need to be made (exothermic) 2 C=O = 2 x -743 = -1486 kJ 4 O-H = 4 x -464 = -1856 kJ Total energy released forming bonds = -3342 kJ Enthalpy change = +2648 + (-3342) = -694 kJ More energy is released when bonds are formed than is required to break them; the reaction is exothermic. Energy is given out when the reactants are converted to the products. Therefore, we know that the products must have less energy than the reactants so they are more stable
How can we calculate the molar enthalpy change of the ammonium salt dissolving in water if the sample results are: Initial temperature of water = 18.3 Minimum temperature of salt solution = 15.1
The temperature of the water decreases so the reaction is endothermic 1. Temperature change of the water = 18.3-15.1 =3.2 2. Heat lost= 100 x 4.18 x 3.2 = 1337.6 J / 100 =1.3376 kJ 2. We have assumed that: - The specific heat capacity of the diluted solution is the same as that of water which is reasonable because the mixture is mostly water - The mass of the solution is 100g since the mass of the ammonium chloride is very small and therefore ignored in the calculation. There are other major sources of error in the experiment such as heat absorbed from the surrounding air which makes a much greater difference 3. Moles of ammonium chloride: - Relative formula mass of ammonium chloride = 14 + 4 + 35.5 = 53.5 Number of moles = mass / relative formula mass = 5.2 / 53.5 = 0.0972 4. Molar enthalpy change of the solution ΔH= heat energy change / the number of moles of ammonium chloride dissolved ΔH= 1.3376 / 0.0972 The amount of heat absorbed when dissolving 1 mole of ammonium chloride in water is therefore: ΔH= 13.8 kJ/mol
Describe a calorimetry experiment for determining the enthalpy changes of reactions?
To measure the amount of heat absorbed or given out in several kinds of chemical reaction and physical changes, calorimetry is used. It is based on the idea that if we use heat from a reaction to heat another substance such as water, we can use the equation Q = m x c x ΔT to calculate the amount of heat released. In the equation, the mass and specific heat capacity and temperature change are all referring to the substance heated. If we know the moles of reactants used in the reaction, we can then workout the molar enthalpy change ΔH of the reaction in the units kJ/mol This can be done using the combustion of an alcohol to work out how much heat energy is released when one mole of the alcohol burns. The alcohols are burned in a small spirit burner and the heat produced is used to heat some water in a copper can (the calorimeter). The calorimeter is simply something we conduct the calorimetry experiment in, in this car a copper can. Method: Wear eye protection. Don't carry a lit spirit burner and don't fill one with a naked flame nearby 1. Measure 100 cm3 of cold water using a measuring cylinder and transfer the water into a copper can 2. Take the initial temperature of the water 3. Weigh a spirit burner containing ethanol with its lid on. the lid should be kept on when the wick is not lit to prevent the alcohol from evaporating 4. Arrange the apparatus so that the spirit burner can be used to heat the water in the copper can (see text book). The apparatus is shielded using a draught shield and an insulating card is put on top of the copper calorimeter 5. Light the wick to heat the water. Stop heating when you have a reasonable temperature rise. Extinguish the flame by putting the lid back on the wick. Make sure the temperature rise isn't too small as this will increase errors in reading the thermometer or in finding the mass change. 6. Stir the water thoroughly and measure the maximum temperature 7. The experiment can be repeated with the same alcohol to check reliability and then carried out again with whatever other alcohols are available
How can we measure enthalpy changes when salts dissolve in water?
We can also use calorimetry experiments to work out the amount of heat given out or taken in when salts dissolve in water Wear eye protection and avoid skin contact with the salts and their solutions Method: 1. Place a polystyrene cup in a 250 cm3 glass beaker 2. Transfer 100cm3 of water into the polystyrene cup using a measuring cylinder 3. Record the initial temperature of the water 4. Weigh 5.20g ammonium chloride using a weighing boat on a balance 5. Add the ammonium chloride to the water and stir the solution vigorously until all the ammonium has dissolved 6. Record the minimum temperature 7. Dissolving is a physical reaction rather than a chemical reaction so it could be either endothermic or exothermic
How can we calculate the energy released or absorbed during a reaction?
We can estimate the heat energy released or absorbed in a reaction by calculating how much energy would be needed to break the substance into individual atoms and then how much would be given out when the atoms recombine into new arrangements. For example, If: energy needed to break all the bonds into reactants is + 1000 kJ and the energy released when new bonds are made in produce = -1200 kJ This reaction would release 200 kJ of energy
