Embedded Files
The purpose of this module is to investigate factors that initiate and drive a chemical reaction. You will examine the relationship between enthalpy and entropy in calculating the Gibbs free energy. You will also examine the roles that enthalpy and entropy play in the spontaneity of reactions.
You will be provided with opportunities to understand that all chemical reactions involve the creation of new substances and associated energy transformations, which are commonly observable as changes in temperature of the surrounding environment and/or emission of light.
You will conduct investigations to measure the heat energy changes that occur in chemical reactions. You will describe reactions using terms such as endothermic and exothermic, and explain reactions in terms of the law of conservation of energy. You will use Hess’s Law to calculate enthalpy changes involved in the breaking and making of bonds.
There are three main areas of content addressed through the following questions:
MDQ1) What energy changes occur in chemical reactions? Enthalpy changes in chemical reactions
MDQ2) How much energy does it take to break bonds, and how much is released when bonds are formed? Enthalpy and Hess’ Law
MDQ3) How can enthalpy and entropy be used to explain reaction spontaneity? Entropy and Gibbs Free Energy
SYLLABUS OUTCOMES
11-11 analyses the energy considerations in the driving force for chemical reactions
11/12-1 develops and evaluates questions and hypotheses for scientific investigation
11/12-53 analyses and evaluates primary and secondary data and information
11/12-6 solves scientific problems using primary and secondary data, critical thinking skills and scientific processes [LOC2]
11/12-7 communicates scientific understanding using suitable language and terminology for a specific audience or purpose [LOC4]
MDQ1 What energy changes occur in chemical reactions?
MDQ1 What energy changes occur in chemical reactions?
Text 1 Reference Ch 14
Getting ready:
- Complete Tx2 WS4.1 Predicting products of reactions p160
1.1a conduct practical investigations to measure temperature changes in examples of endothermic and exothermic reactions, including:
- combustion.
In any chemical reaction, temperature is an important indicator. Whilst many of the reactions with which we are most familiar are exothermic, releasing heat to the surroundings, some are endothermic, absorbing heat from the surroundings.
Photosynthesis requires an input of light energy, hence it is an endothermic (net) reaction.
Respiration reeases energy in the form of energy-rich ATP molecules, hence it is an exothermic (net) reaction.
Chemical arithmetic:
Energy is locked up in chemical bonds. It is a form of chemical potential energy.
Energy must be added to break these bonds in reactant substances.
However, there are also bonds present in the product substances after the atoms have been
rearranged.
The mathematical difference between the total bond energy provides a net absorbance, or release of energy.
When the total energy of the products is lower than the total energy of the reactants, energy has been transferred from the system into the surroundings, often as heat. This is a description of an exothermic reaction.
When the total energy of the products is higher than the total energy of the reactants, energy has been transferred to the system from the surroundings, often again as heat. This is a description of an endothermic reaction.
There are two ways of writing a chemical equation to show these changes in energy:
Exothermic:
CH4(g) + 2O2(g) ® CO2(g) + 2H2O(g) + Energy
CH4(g) + 2O2(g) ® CO2(g) + 2H2O(g) ΔH = −ve
Endothermic:
CuCO3(s) + Energy ® CuO(s) + CO2(g)
CuCO3(s) ® CuO(s) + CO2(g) ΔH = +ve
PA 4.1 Tx2 p172 Exothermic and endothermic reactions
PA 4.2 Tx2 p175 Energy from different fuels
PA 4.3 Tx2 p178 Enthalpy changes in chemical reactions
View videos:
- DOR#1 Exothermic and endothermic reactions [6.16] https://www.youtube.com/watch?v=FwZYS_WlLlg&index=7&list=PLeFSFSJ9WqSA24lCCivXWB_Fruf_Jh6a8
TASK 1.1.1
1. Write equations to represent the complete combustion reactions of
a) methanol
b) ethanol
1.1b conduct practical investigations to measure temperature changes in examples of endothermic and exothermic reactions, including:
- dissociation of ionic substances in aqueous solution.
Enthalpy
The symbol H is used to denote heat content. The amount of thermal (heat) energy released or absorbed during a chemical reaction can also be referred to as its enthalpy (ΔH). NB This assumes constant pressure for the reaction, which we will do to keep things a little simpler at this stage.
Molar Heat of Solution
The molar heat of solution is a measure of the energy change which occurs when 1 mole of a substance dissolves in water.
- ΔH is negative if energy is released
- ΔH is positive if energy is absorbed
ΔH values are often given subscripts to identify the types of reactions being measured. So we can used ΔHsol to indicate a heat of solution.
We can dissolve a solute in a solvent in a beaker and measure the temperature changes, however it is better if we use a container with insulating properties (which means it exchanges less energy with the surroundings), such as a styrofoam cup.
A good example of an exothermic dissolution is the formation of a sodium hydroxide solution.
A good example of an endothermic dissolution is the formation of an ammonium chloride solution.
Exothermic Dissolution
Exothermic reactions release energy to the environment. They produce a net output of energy as the reaction proceeds.
Exothermic dissolution causes the temperature in the calorimeter to rise because energy is released into the water as the dissolution proceeds. (Exothermic chemical reactions also release heat to their surroundings as they proceed causing a temperature increase.) The energy quantity is negative in such cases as the total energy of the reaction system has decreased.
Eg freezing water, hydrochloric acid dissolution
Endothermic Dissolution
Endothermic reactions absorb energy from the environment. They require a net input of energy for the reaction to proceed.
Endothermic dissolution causes the temperature in the calorimeter to drop as energy is absorbed from the water to allow the dissolving to proceed. (Endothermic chemical reactions also absorb heat from the surroundings as they proceed causing a temperature drop.) The energy quantity is positive in such cases as the total energy of the reaction system has increased.
Eg when ice melts, energy flows from the liquid water to the solid in order to break hydrogen bonds. This reduces the total energy of the surrounding liquid and results in a decrease in temperature.
PA Which salts produce the greatest enthalpy change when added to water?
Design and carry out an investigation to measure the temperature change of water dissolution of a range of ionic substances.
View videos:
- DOR# Dissociation reactions [7.50] https://www.youtube.com/watch?v=l9SP_6Wcag0&index=6&list=PLeFSFSJ9WqSA24lCCivXWB_Fruf_Jh6a8
Task 1.1.2
1. Write an equation to represent the dissolution of the following, including the enthalpy change:
a) sodium hydroxide
b) ammonium nitrate
2. Draw a diagram to represent the flow of energy which is taking place during the two processes described in question 1
Task 1.1.3
Complete Tx1 p446 14.1 Review and Key Qs
1.2 construct energy profile diagrams to represent and analyse the enthalpy changes and activation energy associated with a chemical reaction
Enthalpy change (ΔH) is the change in energy between the products and the reactants in a chemical reaction. ΔH = Energy of Products - Energy of Reactants (EP – ER)
Activation Energy is the minimum energy required by the reactants to be able to produce the products. In other words, it is the energy needed to begin a chemical reaction.
Activation energy (EA) is often called the energy hill. Many chemical compounds do not react when they are mixed at SLC unless heat or a spark is added, eg hydrogen and oxygen. The reactant particles need sufficient kinetic energy during collisions to break the chemical bonds.
In the energy profile diagram below, molecules of nitrogen dioxide and carbon monoxide are mixed together but do not have sufficient energy to react spontaneously. They need the addition of energy to ‘climb the energy hill’. Enthapy graphs like the one below provide information about the Activation Energy and the Enthalpy Change for a particular chemical reaction. This also helps in identifying whether a reaction is exothermic or endothermic.
Figure 4 Activated complex and Activation Energy http://mhhe.com/physsci/chemistry/essentialchemistry/flash/activa2.swf
This graph also allows us to identify an activated complex. This occurs at the top of the activation energy hill and is a transition state between the reactants and products. It involves partial breaking of reactant bonds and partial formation of product bonds. Activated complexes only exist for a short time and depending on the energy of the system may go backwards towards the reactants or forwards towards the products.
Some chemical reactions are reversible. In this case, the EA will be higher for either the forward or reverse reaction, depending upon which one is exothermic and which is endothermic.
PA Use molecular models kits to model the progress of
a) endothermic
b) exothermic reactions against relevant energy profile diagrams.
View videos:
- Energy Profile Diagrams [9:00] https://www.youtube.com/watch?v=BWcLpXX4yf4
- DoR#4 Energy profile diagrams [6.15] https://www.youtube.com/watch?v=yAjY3COs9Zc&index=4&list=PLeFSFSJ9WqSA24lCCivXWB_Fruf_Jh6a8
TASK 1.2.1
Complete the Energy Diagram WS
11C4 1b Energy Diagrams WS.docxTASK 1.2.2
Complete online exercise https://www.sciencegeek.net/Chemistry/taters/energydiagram.htm
TASK 1.2.3
1. Draw and label an energy profile diagram for a reaction with a ΔH = + 50 kJ and an activation energy value of + 70 kJ.
TASK1.2.4
Complete Tx2 p161 WS2 Combustion of fuels Q1-3
TASK 1.2.5
Complete Tx1 p446 14.2 Review, Key Qs
1.3 investigate enthalpy changes in reactions using calorimetry and q=mcΔT (heat capacity formula) to calculate, analyse and compare experimental results with reliable secondary-sourced data, and to explain any differences
Calculating Specific Heat
When a substance absorbs heat, its temperature changes in proportion to the amount of heat is has absorbed (Tro, 2011, p. 75). The quantity of heat absorbed is symbolised by q and depends on the mass of the substance, the specific heat capacity of the substance and the change in its temperature.
The equation: q = mcΔT
is used to calculate heat energy (or enthalpy) changes during chemical reactions, where:
q = change in heat energy, measured in Joules (J)
m = the mass of the substance (water) in grams (g)
c = specific heat capacity (of water) in J/K/g or J/°C/g
ΔT = change in the temperature of the medium in K or °C.
If the temperature rises, ΔT >0, the heat energy change will be positive, ie heat will be absorbed from the reaction (exothermic).
If the temperature falls, ΔT <0, the heat energy change will be negative, ie heat will be absorbed by the reaction (endothermic).
Calorimetry
Figure 1: Calorimetry
A calorimeter is a device used to measure heat energy. Many reactions occur in an aqueous state (aq), ie, they occur in water. Due to water’s high specific heat capacity it is a good substance to use as a medium to absorb or release heat energy. By measuring the temperature of the water before, during and after the reaction, we can classify the reaction as endothermic or exothermic and calculate the enthalpy change.
The equation:
q = mcΔT
is used to calculate the enthalpy change by multiplying the change of temperature of the water by the mass of water then by the specific heat of water (assuming water is the medium).
q vs ∆H
q is used for the amount of heat absorbed or released when the temperature change occurs - as determined by q = mc∆T,
where c is the specific heat capacity of the material undergoing the temperature change.
We determine this quantity by experiment.
Then we need to make an (incorrect) assumption - that the amount of heat absorbed by the water (or other material) is equal to the amount of heat given out by the chemical reaction (∆H).
Because of heat loss to surroundings and incomplete combustion this assumption is invalid and we recognise this when assessing the validity of combustion experiments.
∆H is the heat energy change for the reaction and is per mole of reaction or mole of reactant consumed.
NB. qsystem always has the same sign as ∆H (of the system, by definition). qsurroundings therefore has the opposite sign to qsystem.
You might find it easier to perform your calculations by thinking in steps.
Step 1. Work out from practical the value of q.
Step 2. Make assumption that this is -∆H.
Step 3. Express the ∆H per mole of equation or per mole of fuel.
So to make the answer more simple, q is measured amount of heat energy; ∆H is theoretical (assumed) amount of heat energy change for a reaction as written in the equation.
Tx2 p175 PA 4.2 Energy from different fuels
View videos:
- DoR#3 Enthalpy change [6.07] https://www.youtube.com/watch?v=lehikcpTlXs&list=PLeFSFSJ9WqSA24lCCivXWB_Fruf_Jh6a8&index=5
TASK 1.3.1
1. Compare the molar masses of methanol and ethanol.
2. If you started with 100 g of methanol and 100 g of ethanol, how much energy would be released in each combustion reaction?
(NB. Molar heat of combustion for ethanol is 1367 kJ.mol-1 and molar heat of combustion for methanol is 726 kJ.mol-1) (Aylward & Findlay, 2008, p. 103)
3. Compare the values you obtained for methanol and ethanol with octane, by performing the same calculation on 100g of octane undergoing complete combustion. (Molar heat of combustion for octane is 5470 kJ.mol-1) (Aylward & Findlay, 2008, p. 95)
4. A student wanted to know how much heat energy is required to boil a cup of water to make coffee. The student had a cup of water (250 mL) and started from room temperature (25°C). The specific heat capacity of water is 4.18 J.g-1.K-1. Use the equation for heat energy absorption, q = mcΔT, to calculate the enthalpy change.
5. Calculate the amount of heat given out when 100 g water cools from 30°C to 25°C. (Hegarty, 2018, p. 50)
6. When 5.3 g of calcium chloride was dissolved in 250 mL of water at 20.5°C, the temperature rose to 24.5°C. Calculate the molar heat of solution of calcium chloride. Use the specific heat capacity of the final solution as 4.18 J g-1 K-1. (Hegarty, 2018, p. 51)
TASK1.3.2
Complete
- Tx2 p161 WS2 Combustion of fuels Q4
- complete Tx2 p163 WS 4.3 Investigating enthalpy change
TASK 1.3.3
Complete Tx1 Review, Key Qs
- p454 14.3
- p462 14.4
- p466 14.5
1.4 model and analyse the role of catalysts in reactions
A catalyst is a substance which increases the rate of a reaction without being changed by the reaction. The catalyst is the same at the end of the reaction as it was at the beginning. The catalyst helps break down bonds in one of the substances to help it re-bond to the other substance more readily. Catalysts lower the activation energy.
At a fixed temperature, the catalyst ensures that a higher proportion of particles possess energies greater than the lowered activation energy. This increases the rate of the reaction.
For a substance to be classified as a catalyst, it must:
- cause a reaction to proceed at a faster rate than a measured control.
- not be used up or changed by the end of the reaction (it may react but must reform at the end).
A special group of catalysts, called enzymes, are very important in the regulation of biologically important chemical reactions, including photosynthesis and respiration.
Three important industrial process which also require the use of a catalyst are the production of ammonia, called the Haber Process, the manufacture of nitric acid and the production of ethyl acetate.
PA Catalysis of a reaction between sodium thiosulfate and iron(III) nitrate solutions
View videos:
- Decomposition of hydrogen peroxide. https://www.youtube.com/watch?v=Ta4DomSDzF8 [4.00]
- DoR#5 Catalysts and activation energy [4.24] https://www.youtube.com/watch?v=4lnmIuW170o&index=3&list=PLeFSFSJ9WqSA24lCCivXWB_Fruf_Jh6a8
- video The Fuse School: What are catalysts? [3:00] https://www.youtube.com/watch?v=m_9bpZep1QM
TASK 1.4.1
Challenge Q: Write STTWS in response to:
- Does the type of catalyst matter in a chemical reaction?
TASK 1.4.2
1. Identify the catalyst in a catalytic converter. What is the purpose of a catalytic converter?
2. Name two biological catalysts, enzymes, which are found in the human body. Which reactions are they involved in catalysing? What is significant about these catalysts?
3. What types of materials are used to catalyse the industrial production of ammonia, nitric acid or ethyl acetate? Write an equation for the reaction. (Thickett, 2007, p. 330).
TASK 1.4.3
Complete Tx1 p473 14.6 Review, Key Qs
TASK 1.4.4
Complete
REVIEW
Complete under test conditions, then check answers with your notes:
- Tx1 p474-6 Chapter 14 Review
Google Sites
Report abuse