Thermochemistry

What is enthalpy change measured in

units of Kj/mol (kilojoules per mole)

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 it gets colder.

Enthalphy change between methane and chlorine CH4 (g) + Cl2(g) -> CH3Cl (g) + HCl (g)

Bonds need to be broken: 4 C-H bonds = 4 x (+413) = +1653kJ 1 Cl -Cl bonds = 1 x (+243) = +243kJ Total = +1895kJ Bonds need to be made: 3 C-H bonds = 3 x (-413) = -1239kJ 1 C -Cl bonds = 1 x (-345) = -346kJ 1 H-Cl bonds = 1 x (-432) = - 432kJ Total = - 2017kJ We can see that more energy is released when bonds are formed than required to break them, so the reaction releases energy over all, its exothermic. The overall change is +1895 + (-2017)kJ = -122kJ The negative sign of the answer shows that, overall, heat is given out as the bonds rearrange. You can show all this happening on an energy level diagram

Example of bond energy enthalpy change

Breaking chemical bonds requires energy. The stronger the bond, the more energy is needed to break it. Bond energies are measured in kJ/mol (kilojoules per mole). For example the Cl-C- bond energy is 243kJ/mol. This means that it will take 243kJ to break all the Cl-Cl bonds in 1 mole of chlorine gas. The bond energy are represents the amount of energy released when 1 mole of bonds form. The diagram shows that it needs an input of 243kJ of energy to break 1 mole of chlorine molecules into atoms. If the atoms recombine into their original molecules then the exactly same amount of energy will be released again. The H-Cl bond energy is 432kJ/mol. However, if you see 2HCl(g) in an equation, it takes 2 x 432kJ to break all the H-Cl bonds in the two moles of hydrogen chlorine gas. Or, 2 x 432 kJ will be released when 2HCL (g) is formed from H and Cl atoms.

What are the tests for water? there are 2 chemical and 1 physical

Chemical With a Bunsen burn hydrated copper sulphate to make it anhydrous copper sulphate, then add water Anhydrous copper sulphate, when added water should turn back to blue (hydrated copper sulphate) Chemical Add cobalt paper into the solution you are testing Cobalt paper turns from Blue - pink Physical Water boils at 100°C Should boil at 100°C

Practical - measuring enthalpy changes in combustion reactions - results

Combustion is an exothermic reaction so the temperature of the water goes up. As ethanol is burned the total mass of the burner and ethanol goes down. Using the above data we can determine how much heat is released when one mole of an alcohol is burned, that ism the molar enthalpy change of combustion you can use the equation: Q=mcΔT

Why do reactions either give out or absorb heat?

During chemical reactions binds in the reactants have to be broken and new ones formed to make the products. Breaking bonds need energy (endothermic) and making bonds release energy (exothermic)

Practical - describe reversible reactions such as the dehydration of hydrated copper(II) sulfate

Heat the test tube gently over a Bunsen burner. Observe what happens. 1. You had hydrated (contains water) copper sulfate -> CuSO4.5H2O Blue hydrated copper sulphate ⇌ white anhydrous copper sulfate + water - Heat is needed for this side of the reaction (endothermic) CuSO4.5H20 (s) ⇌ CuSO4 (s) + 5H2O (g) - The heat released is from this side of the reaction (exothermic) As you can see this is a reversible reaction because if you add heat (exothermic) it breaks the bonds from hydrated copper sulphate --> anhydrous copper sulphate and water. However, if you use endothermic processes, it will re-join to make hydrated copper sulfate from anhydrous copper sulphate and water. In simple terms: Blue copper sulfate is described as hydrated. This water is driven off when blue hydrated copper sulfate is heated, leaving white anhydrous copper sulfate. This reaction is reversible: hydrated copper sulfate ⇌ anhydrous copper sulfate + water CuSO4.5H2O(s) ⇌ CuSO4(s) + H2O(l) The forward reaction is endothermic and the reverse reaction is exothermic.

What happens if we change the pressure of a sealed equilibrium

If you increase the pressure the reaction will respond by reducing it. It can reduce the pressure by producing fewer gaseous molecules to hit the walls of the container, in this case by converting A + 2B (3 molecules) into C + D ( 2 molecules). Increasing the pressure will always cause the position of equilibrium to shift in the direction which produces the smaller number of gaseous molecules. However, if there is the same number of gaseous molecules on both sides of the equation, changing the pressure will make no difference to the position of equilibrium. Increasing or decreasing the pressure has no effect on the position of equilibrium. Increasing pressure - the position of equilibrium shifts to the side which has fewer gas molecules Decreasing pressure - the position of equilibrium shifts to the side which has more gas molecules.

Energy level diagrams for endothermic reaction explanation

In an endothermic reaction, the products have more (chemical) energy than the reactants. In order to supply the extra energy that is needed to convert the reactants ( lower energy) into the products ( higher energy) heat energy needs to be absorbed from the surroundings. This heat energy is converted 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.

Energy level diagram for exothermic reactions explanation

In an exothermic reaction, the products of the reaction have less (chemical) energy than the reactants. In the reaction, chemical energy (stored in the bonds of chemicals) is converted to heat energy, which is released to the surroundings. The temperature of the reaction mixture and its surroundings goes up.

Showing Exothermic reactions on energy level diagrams

In an exothermic reaction, the reactants have more (chemical) energy than the products; we say that the products are more stable than the reactants. As the reaction happens, energy is given out in the form of heat. That energy warms up both the reaction itself and its surroundings: The energy released being the ΔH, enthalpy change heat is evolved. From line to line.

Practical - measuring enthalpy changes in 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 into a 250cm3 glass beaker 2) Transfer 50cm3 of 0.200mol/dm3 copper (II) sulfate solution into the polystyrene cup using a measuring cylinder 3) Weigh 1.20g 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 temperature of the reaction mixture went up. The negative sign shows that this is an exothermic reaction; heat is released.

What is a reversible reaction

In some chemical reactions, the products of the reaction can react together to produce the original reactants. These reactions are called reversible reactions

What is dynamic equilibrium?

It is a dynamic in the sense that the reactions are still continuing, but the rate of the forward reaction is equal to the rate of the reverse reaction. (doesn't have to be 50%) It is an equilibrium in the sense that the total amounts or concentrations of the various things present (products and reactants) and constant.

Method - Practical - describe reversible reactions such as the dehydration of hydrated copper(II) sulfate

Method: 1) Take hydrated blue copper sulphate crystals 2) Take a Bunsen burner and heat the crystals 3) After the copper sulphate has turned grey let it cool for a bit 4) Add water 5) It should turn back to hydrated copper sulphate from anhydrous copper sulphate and water.

What are the 4 different Calorimetry experiments for determining the enthalpy changes of reactions?

Practical - measuring enthalpy changes in combustion reactions Practical - measuring enthalpy changes in displacement reactions Practical - measuring enthalpy changes when salts dissolve in water Practical - measuring enthalpy changes of neutralization between alkali and an acid equation introduced (Q=mc ΔT) to calculate the amount of heat released. Here the mass, the specific heat capacity and the temperature change are all referring to the substance heated. If we know how many moles of reactants are used in the reaction, we can then work out the molar enthalpy change, ΔH of the reaction in the unit kJ/mol.

What is the equation for specific heat capsity

The following equation can be used to calculate how much 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 = mc ΔT

Practical - measuring enthalpy changes of neutralization between alkali and an acid

The reaction between an alkali and an acid is essentially between OH- and H+ to form water OH- (aq) + H+ (aq) -> H2O (l) Lets look at the reaction between potassium hydroxide ( an alkali) and hydrochloric acid (an acid). Suppose you know the concentration of the potassium hydroxide but are not sure of the concentration of the dilute hydrochloric acid solution. The following method could be used to find out the concentration of the acid and how much heat released during the neutralization reaction. 1) Place a polystyrene cup in 250 cm3 glass beaker 2) Transfer 25cm3 of 2.00mol/dm3 potassium hydroxide into the polystyrene cup using a measuring cylinder 3) Record the initial temperature 4) Fill a burette with 50.00cm3 dilute hydrochloric acid 5) Use the burette to add 5.00cm3 of dilute hydrochloric acid to the potassium hydroxide 6) Stir vigousouly and record the maximum temperature reached 7) Continue adding further 5.00cm3 portions of dilute hydrochloric acid to the cup, stirring and recordning the maximum temperature each time, until a total volume of 50.00cm3 has been added.

Example of nitrogen dioxide and dinitrogen tetroxide with increasing and decreasing concentration

The system would shift to the side which uses it up. If you use more of 2NO2 you will get more of N2O4. same with the other side

How to predict the effect of changing conditions on the position of equilibrium:

Things we might try to do to influence the reaction include: - Temperature - Pressure (only in gas) - Concentration (only in solutions)

Specific heat capcity of water = 4.18 J/g/°C (joules per gram per degree Celsius). what does this mean?

This means that 4.18J of heat energy is needed if we want to increase the temperature 1g of water by 1°C. if you want the temperature of 1g of water to go up by 2°C, then 4.18 x 2 = 8.36J of energy would be needed to raise the temperature by 2°C.

What is the Haber process?

Used to make ammonia from hydrogen & nitrogen N + 3H2 --> 2NH3 (+heat) you want to try and get more ammonia, for more crop growth

Practical - measuring enthalpy changes in combustion reactions

Using calorimetry to measure the amount of heat given off when a number of small alcohols are burnt. You could use methanol, ethanol, propan-1-ol and butan-1-ol. The alcohols are burned in a small spirit burner, and the heat produced is used in heat some water in a copper can (the calorimeter) The following procedure could be used: - Measure 100cm3 of cold water using a measuring cylinder and transfer the water to a copper can - Take the initial temperature of the water - Weight 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 - Arrange the apparatus, shown in the diagram. So that the spirit-burner can be used to heat the water in the copper can. The apparatus is shielded as far as possible to prevent draughts. - Light the wick to heat the water. Stop heating when you have a reasonable temperature rise of water (say about 40.0°C). the flame can be extinguished by putting the lid back on the wick. - Stir the water thoroughly and measure the maximum temperature of the water - Weigh the spirit burner again with its lid on - The experiment can be repeated with the same alcohol to check for reliability, and then carried out again with whatever other alcohols are available.

catalysts used in equilibrium

What does the catalyst do? - No effect on how much product or reactant is made - It gives a faster rate (equilibrium is reached faster) - Increase cost but has a large impact on rate.

The reactions of metals with acids - Exothermic

When magnesium reacts with dilute sulfuric acid, for example, the mixture gets very warm Mg(s) + H2SO4 (aq) +H2 (g)

if the conditions in a equilibrium change what will happen?

When the conditions in a system in dynamic equilibrium, the reaction is always sets about counteracting and doing the opposite to any changes made. This leads to Le Chatelier's principle: When a reversable reaction is in equilibrium and you make a change, it will do what it can to oppose that change.

Example of nitrogen dioxide and dinitrogen tetroxide with increasing and decreasing temperature

When we change from nitrogen dioxide to dinitrogen tetroxide is exothermic. The negative sign △H shows that heat is given out by the forward reaction. If you decrease the temperature the position of the equilibrium will shift to produce more heat to counteract the change you have made. In other words, lowering the temperature causes the position of the equilibrium to shift in the exothermic direction and there will be more dinitrogen tetroxide in the equilibrium mixture. The colour of the reaction mixture will fade. If you increase the temperature, the position of equilibrium will shift to lower it again - the position of equilibrium shift in the reverse, endothermic direction. In other words, more nitrogen dioxide will be formed and the colour of the gas will darken.

What is the enthalpy change

You can measure the amount of heat energy taken in or released in a chemical reaction. It is called the enthalpy change of the reaction and is given the symbol ΔH. The enthalpy is the amount of heat energy taken in or given out in a chemical reaction. It is the difference between the energy of the products and the energy of the reactants.

You can only set up a dynamic equilibrium if the system is closed in a sealed container. What is a system? why is it sealed?

a place where the reaction takes place. eg: container, vessel etc has a lid on top so none of the reactant or products leave.

nitrogen and hydrogen into

ammonia

Examples of exothermic reactions

- Combustion of Fuels - The reactions of metals with acids - Neutralisation reactions (which an acid and a base react quantitatively with each other) - Displacement reactions

What are the two types of equilibrium?

- Static - no movement, still - Dynamic - movement, looks like it is still but its moving

Examples of endothermic reactions

- Thermal decomposition of metal carbonates. These are examples of endothermic reactions. You have to heat a carbonate constantly to make it decompose. for example, copper (II) carbonate (green) decomposes on heating to produces copper (II) oxide (black) CuCO3 (s) -> CuO (s) + CO2 Similarly, zinc carbonate decomposes to form Zinc oxide when heated ZnCO3 (s) -> ZnO (s) + CO2 (g)

How to approach bond breaking or making questions

1) split equation into bond making and breaking 2) from using the table calculate the amount of energy between making and breaking 3) see the difference

Reversable reactions in a sealed container:

A sealed container means that no substances are added to the reaction mixture and no substances escape from it. One the other hand, heat may be either given off of absorbed. A reversable reaction - in which the products can react with each other and go back to form reactants.

Neutralization reactions - Exothermic

About the only interesting thing that you can obverse happening here when sodium hydroxide solution reacts with dilute hydrochloric acid is that temperature rises: NaOH (aq) + HCl (aq) -> NaCl (aq) + H2O (l)

difference between ammonium and ammonia

Ammonium -> NH4 + (ion because has H+ ion) it is a solid Ammonia -> NH3 -> no charge, not an ion but is a gas

Exothermic reactions are reactions that

Any reaction that produces a flame is exothermic. Burning things produces heat energy. For instance, hydrogen burns in oxygen, producing water and lots of heat: 2H2 (g) + O2 (g) -> 2H20(l)

What is this an example of? +178 kJ/mol

Because the reactants are gaining energy, the enthalpy change of the reaction ΔH is given a positive sign. For example: CaCO3 (s) -> CaO(s) + CO2 (g) ΔH= +178 kJ/mol This means that 178kJ of heat energy must be absorbed to convert 1 mole of calcium carbonate into calcium oxide and carbon dioxide.

What is the definition of bond energy

Bond energy is defined as the amount of energy needed to break 1 mole of covalent bonds in gaseous molecules. Bond energies are measured in kJ/mol (kilojoules per mole).

Calculation of the heat released or absorbed during a reaction:

For example: If: energy needed to break all the bonds in a reaction = +1000kJ (endothermic) And: energy released when bonds are made in products = -1200kj (exothermic) Then: overall change = -200kJ (exothermic) The reaction would release 200kJ of energy

Showing 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. That extra energy has to come from somewhere, and it is taken from the surroundings. In the case of the thermal decomposition of carbonates in the laboratory, it comes from the Bunsen burner.

Practical - measuring enthalpy changes when salts dissolve in water - results

In this reaction temperature increases at first but then decreases. The reaction between the acid and the 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 adding cold acid to our warm solution ( there is no reaction because there is nothing for the acid to react with) and the temperature goes down. Two lines of best fit can be drawn. The point where the lines cross represents complete neutralization.

What happens if we change the temperature of a sealed equilibrium

Increasing the temperature will be exactly the opposite effect. The reaction equilibrium will change to remove the extra heat by absorbing it in an endothermic change. This time the reverse reaction is favoured and the position of equilibrium moves to the left. If you decrease the temperature, the positon of equilibrium will shift to produce more heat to conteract the change you have made. In other words, lowering the temperature causes the position of equilibrium To summarise: Increasing temperature: the position of equilibrium shifts in the endothermic reaction Decreasing temperature: the position of equilibrium shifts in the exothermic reaction

what does exothermic mean?

Some chemical reactions give out energy in the form of heat. A reaction that gives out heat to the surroundings is said to be exothermic reaction. If you are holding a test-tube in which an exothermic reaction is occurring, you will notice that the test-tube gets warmer.

how to get more ammonia

Temperature: Lowering the temperature. More exothermic reaction therefore more ammonia Pressure: Higher pressure. Side with more molecules will compact together making more ammonia. If we decrease the pressure the position of equilibrium shifts to the side which has more gas molecules, so there would be more H2 + N2. concentration: larger concentration of h2 +n2 so more ammonia is made

Example of bond energy including double bonds CH4 (g) + 2O2 (g) -> CO2 (g) + 2H2O (g)

The bond energy for O=O double bonds is 498kJ/mol. This is the energy required to break the whole of the double bond. Bond breaking: Endothermic 4 C-H = 4 x (+413) = +1652kJ 2 O=O = 2 x (+498) = +996kJ Total = + 2648kJ Bond making : Exothermic 2 C=O = 2 x (-743) = - 1486kJ 4 O-H = 4 x (-464) = -1856kJ Total = - 3342 Enthalpy change: +2648 + (-3342) = -694kJ/mol Releasing energy, therefore 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; the products are more stable. We can show this with a energy level diagram:

What is specific heat capacity?

The specific heat capacity tells us about how much energy has to be put in to increase the temperature of something. The specific heat capacity of a substance is defined as the amount of heat needed to raise the temperature of 1 gram of a substance by 1°C.

What is the sign for a reversable reaction?

The symbol ⇌ has two half arrowheads, one pointing in each direction. It is used in equations that show reversible reactions: · the forward reaction is the one that goes to the right · the backward reaction is the one that goes to the left The reaction mixture may contain reactants and products, and their proportions may be changed by altering the reaction conditions.

Displacement reaction- exothermic

The thermite reaction between powered aluminium and iron (III) oxide is a displacement reaction. This reaction releases a large amount of heat, which can be used in railway welding: 2A (s) + Fe2O3 (s) -> 2Fe (l) + Al2O3 (s)

What is this an example of? -466.9 Kj/mol

The ΔH written next to an equation represents the enthlahpy change of the reaction ie: 466.9kj of heat is given out when one mole of magnesium reacts in this way. You know heat has been given out because ΔH has a negative sign.

Practical - measuring enthalpy changes when salts dissolve in water

We can also use calorimetry experiments to work out the amount of heat given out/taken in when salts dissolve in water. The following procedure could be used: - Place a polystyrene cup in a 250cm3 glass beaker - Transfer 100cm3 of water into the polystyrene cup using a measuring cylinder - Record the initial temperature of the water - Weigh 5.20g of ammonium chloride using a weighing boat on a balance - Add the ammonium chloride to the water and stir the solution vigorously until all the ammonium chloride has dissolved. - Record the minimum temperature Note: the temperature of the water decreases, so the reaction is endothermic and heat is absorbed form the surroundings for the dissolving process to occur.

breaking bonds --> making bonds -->

endothermic (absorbs energy) exothermic ( releases energy)

For a reaction to take place it either has to be

endothermic or exothermic

what is ammonia used for?

fertilisers, cleaners, making compounds like nitric acid

what are the problems with lowering temperature and increasing temperature?

if we decrease the temperature the reaction will be slower. therefore a compromise temperature is used around 450 oC if we increase the pressure, it is really expensive and dangerous due to the thick pipes and high amounts of energy needed for high pressure. pressure needs to be compromised around 200 atm to help with the temperature, a catalyst is used to speed up the reaction. however, the catalyst has nothing to do with how much ammonia is produced. it only speeds up the reaction. iron can be used.

Practical - describe reversible reactions such as the effect of heat on ammonium

if you heat ammonium chloride, the white crystals disappear from the bottom of the tube and reappear further up. Heating ammonium chloride spilts it into the colourless gasses hydrogen chloride and ammonia. This reaction is called thermal decomposition. NH4Cl (s) -> NH3 (g) + HCl (g) White solid -> Colourless gases (Heat is needed for this reaction to occur, endothermic) These gases recombine further up the tube, where it to cooler, to form a white solid: NH3(g) + HCl -> NH4Cl (s) Colourless gasses -> white solid (Heat is released for this reaction to occur, exothermic) The reaction is reversable when conditions are changed from hot to cold! NH4Cl (s) ⇌ NH3 (g) + HCl (g) Ammonium chloride has been heated, reaction has shifted, where all the reactants have been broken apart and then re-joined again after the heat but in a different structure. For example ammonium chloride has the same elements as ammonia gas and hydrogen chloride but different structure. Smoke is formed called NH3 and HCl · Ammonia gas · Hydrogen chloride

Example of nitrogen dioxide and dinitrogen tetroxide with increasing and decreasing pressure

if you increase the pressure, the position of the equilibrium will shift to reduce it again by producing fewer gaseous molecules. In other words, the position of equilibrium will move to the right and the reaction will produce more dinitrogen tetroxide. If you lower the pressure, the position of equilibrium will shift to increase it again by producing more gaseous molecules, therefore, the position of equilibrium shifts to the left, and you will obtain a higher proportion of the brown nitrogen dioxide in the equilibrium mixture.

where is nitrogen and hydrogen produced?

nitrogen- air hydrogen - methane - cows

what is the bonding of ammona

simple molecular - covalent

What is the sign given for enthalpy change and what is put before them to show what kind of reaction it is

ΔH is given a minus or a plus sign 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 an exothermic reaction, ΔH is given as a negative number because the reactants are losing energy as heat. That heat is transferred to the surroundings, which then get warmer. ΔH is measured in units of Kj/mol (kilojoules per mole)