Unit 3 Chemistry - TSFX – The School for Excellence | Who …€¦ · · 2017-09-05© The School For Excellence 2017 Succeeding in the VCE – Unit 3 Chemistry ... 2017 Succeeding

Unit 3 Chemistry

succeeding in the vce, 2017

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© The School For Excellence 2017 Succeeding in the VCE – Unit 3 Chemistry Page 1

Potential Energy

Kinetic Energy

WHAT IS A FUEL?

The chemistry of fuels is an important area of study since: Society is dependent on the energy gained by fuels.

It is important to be able to compare the energy content and energy efficiency of

different fuels.

The combustion of fuels has significant environmental impacts.

ENTHALPY OF COMBUSTION Combustion reactions are energy releasing. This is because the energy content of the products is less than the energy content of the reactants. The difference in energy between the reactants and products is released as heat which increases the temperature of the surroundings. Chemical energy is stored inside every substance and results from the potential (stored) energy and kinetic (movement) energy of the system. Examples include: Attractions between protons and electrons. Repulsions between nuclei. Interactions between atoms (chemical bonds). Movement of electrons. Vibration of nuclei. Movement of atoms, ions and molecules.

Note: The greater the degree of motion, the higher the kinetic energy. The chemical energy of a substance is the sum of the potential and kinetic energies, and is referred to as the heat content or enthalpy (H). As different substances contain different combinations of atoms (and hence different types/strengths of bonding), it is highly unlikely that two different substances will have the same enthalpies. It is therefore reasonable to assume that reactions involve a change in enthalpy ( H ) or heat content.

Fuel: A chemical substance that can react via a chemical or nuclear reaction to produce useful energy.


The change in enthalpy as a system undergoes a chemical reaction is determined by calculating the difference between the enthalpies of the products and reactants.

H = Enthalpy of Products – Enthalpy of Reactants

-Products ReactantsH H H

When this difference is positive )( veH , the reaction is endothermic.

When this difference is negative )( veH , the reaction is exothermic.

Enthalpy values are usually measured in units of 1molkJ . If the energy of the products is less than the energy of the reactants, the reaction will be exothermic.

The combustion of fuels is always EXOTHERMIC energy releasing!


THERMOCHEMICAL EQUATIONS

A thermochemical equation shows the amount of energy released or absorbed (enthalpy change) during a chemical reaction. They are a useful tool for comparing the energy released by the combustion of fuels. An example of a thermochemical equation is:

molkJHOHCOOHC gggg /17786472 )(2)(2)(2)(62

This equation indicates that when 2 mole of 62HC reacts with 7 mole of 2O , 4 mole of 2CO

and 6 mole of OH2 are produced. In the process, 1778 kJ of energy is released. Important Notes: The unit molkJ / for H represents the energy released or absorbed in kJ per molar

amounts, as specified in the balanced chemical equation.

The amount of energy produced or released during a chemical process is directly proportional to the amount (in mole) of substance reacting. Therefore:

If the coefficients of a chemical reaction are trebled, the H value must also treble in value.

molkJHOHCOOHC gggg /)17783(1812216 )(2)(2)(2)(62

As endothermic and exothermic reactions are opposite processes, when an equation is

reversed, the H value changes in sign.

molkJHMgOOMg sgs /120422 )()(2)(

molkJHOMgMgO gss /120422 )(2)()(

Thermochemical equations must include:

The physical state of each species, as changes in state require different energy

changes. For example:

molkJHOHOH ls /6)(2)(2

molkJHOHOH gl /44)(2)(2

A positive or negative sign in front of the H value.

Unless otherwise specified:

H values are reported for the forward reaction.

Enthalpies are reported at 25°C.


QUESTION 1 The combustion of methane is described by the following equation:

molkJHOHCOOCH gggg /89022 )(2)(2)(2)(4

(a) How much energy would be released when 2.00 mole of methane is combusted? (b) What energy change would occur as the result of combusting 8.00 g of methane? Solution


CALCULATIONS INVOLVING THERMOCHEMICAL EQUATIONS Example: Find the energy released when mol500.4 of oxygen reacts completely with ethane.

molkJHOHCOOHC gggg /17786472 )(2)(2)(2)(62

7

1778

500.4

releasedE

Note: Always think carefully about the sign of your answer which will depend on the way the question was phrased.

You want to find the energy

released for 4.5 mole of oxygen.

From the balanced equation you know that

1778 kJ of energy is released for 7 mol of

oxygen.

kJ

releasedE

1143

50.47

1778


QUESTION 2 The equation for the combustion of octane is:

18 18( ) 2( ) 2( ) 2 ( )2 25 16 18 10,900l g g gC H O CO H O H kJmol

(a) What mass of oxygen would be needed to release 8516 kJ of heat? (b) Calculate the mass of 2CO that would be produced, and the associated energy

change, when 630 g of octane is burnt


ENERGY FROM FUELS The energy from fuels can be determined experimentally by combusting a known quantity of fuel and measuring the energy released. If the energy released is used to heat water, then the temperature change of the water will be proportional to the energy released by the fuel. Converting the temperature change of the water to the energy released by the fuel is done using the specific heat capacity of water.

THE SPECIFIC HEAT CAPACITY OF WATER The specific heat capacity of water is the amount of energy needed to increase one gram of water by one degree Celsius (or 1 Kelvin).

The specific heat capacity may have different units depending on the unit for energy, mass

and temperature. However, 11 CgJ is the most common unit used for SHC in VCE Chemistry. The energy gained or lost by a substance can be determined from its heat capacity, mass and temperature change.

E = Energy lost or gained (J) m = mass (g)

c = heat capacity of the substance being heated ( 11 CgJ )

= change in temperature ( C ) Note:

The mass of the substance being heated can be determined from its density (V

md ).

The density of water is 11 mlg .

Since solutions are mainly water, you may assume that their density is the same a water unless otherwise stated.

Water has a specific heat capacity of .

Therefore, of energy is required to raise the temperature of g of water by .

mcE


QUESTION 3 How much energy is required to raise the temperature of 100 ml of water by 10°C? Solution QUESTION 4 What change in temperature will result from 2.0 kg of water being supplied with 20.0 kJ of energy? Solution


EXPERIMENTAL DETERMINATION OF ENTHALPY OF COMBUSTION

A known amount of fuel is burnt.

The energy released is used to heat a known

amount of water.

The temperature change and specific heat capacity of the water is used to determine the energy that was transferred from the fuel to the water.

The energy of the fuel is calculated in 1molkJ , 1LkJ or 1gkJ .

Note: When the heat content of a fuel is calculated in this way, it will always be less than the true value since there is not a 100% transfer of chemical energy from the fuel into the water. Some heat will be lost to the environment and some will be used to heat up the container holding the water.


QUESTION 5 A propane burner place under a beaker containing 0.500 L of water and was used to combust 1.540 g of the gas. The temperature of the water increased by 30.00 K. (a) Calculate the molar heat of combustion of the fuel.

(b) The theoretical value for the molar heat of combustion for propane is 12217 molkJ . Explain why this value is different to the value calculated in part (a).


(c) What experimental procedures should be put in place to minimise energy loses from the combustion of the propane?

(d) A second experiment was completed, this time using butane. In order to compare the molar heat of combustion of the two fuels accurately, what experimental conditions must be kept constant?


QUESTION 6 The equations for the combustion of octane is shown below.

molkJHOHCOOHC lggl /900,101816252 )(2)(2)(2)(188

(a) If octane has a density of 1703.0 mlg , what would be the temperature change in 1.00 L of water when heated by the combustion of 5.00 ml of octane?

(Assume that 20.0% of the energy is not transferred into the water.)


(b) Assuming the same efficiency as in part (a), calculate the amount of octane needed to heat 1.000 L of water by 50°C.


SOURCING FUELS Non-renewable energy sources are fuels that are finite and will run out, as they are consumed at a rate faster than they can be produced. Examples include fossil fuels (coal, oil and gas) as well as nuclear materials.

Rate of Production ≤ Rate of Consumption Renewable sources can be replaced by natural processes within a relatively short period of time. Examples include: Wind, tides, waves, biofuels (from plant and animal matter), the sun.

Rate of Production ≥ Rate of Consumption As our main current energy sources are derived from non-renewable fuels, there is an urgent need for the development of alternative fuels to meet future energy requirements.

CARBON NEUTRAL FUELS The energy in carbon based fuel is released via combustion. The reaction produces carbon dioxide which inevitably ends up in the atmosphere.

Fuel + Oxygen Carbon Dioxide + Water Carbon neutrality means that producing and burning a carbon based fuel will not increase the carbon (as 2CO ) in the atmosphere. This is only possible if the amount of carbon released into the atmosphere when the fuel is burn is also extracted from the atmosphere in roughly the same time frame. From a scientific perspective, a fuel is deemed carbon neutral if: The use of that fuel to produce energy does not result in the emission of carbon

compounds into the atmosphere. For example, nuclear energy.

The time it takes to produce the fuel and recycle the carbon released into the atmosphere when that fuel is combusted is much shorter than the amount of time before global warming effects occur i.e. a few years.

For example, biofuels produced from plants are carbon neutral as the 2CO absorbed

from the air as a plant grows cancels out the 2CO emitted when it is burned and as this

process (complete recycling of carbon) takes place within a few years. Note: When fuels are described as carbon neutral, this often doesn’t take into account the carbon dioxide released in the production or transportation of the fuel.


QUESTION 7 The world faces an energy crisis because: A The demand for energy continues to increase B World oil production cannot continue to increase C Potential fuel shortage threaten economic and political stability D All of the above QUESTION 8 Which of the following fuels is both renewable and has relatively small carbon footprint? A Nuclear energy B Coal C Natural gas D Petroleum QUESTION 9 Fossil fuels are non-renewable since: A Once used up, they can never be replaced. B The planet is unable to generate enough organic material to replace them. C They increase the amount of carbon dioxide in the atmosphere. D They are used up at a faster rate than they can be replaced. QUESTION 10 Biofuels are considered carbon neutral since: A They release no carbon dioxide into the atmosphere. B They release no carbon into the atmosphere. C They can be replace at a rate greater or equal to their rate of consumption. D The amount of carbon dioxide absorbed to produce the biomass is equal to the amount

released when the fuel is burnt.


FOSSIL FUELS AND BIOFUELS Although there are many types of fuels used around the world, the highest proportion of useable energy comes from fossil fuels and biofuels. These two fuel sources will be the focus of Unit 3 Chemistry.

FOSSIL FUELS Fossil fuels are non-renewable sources of energy and are produced from the incomplete decay of animal and plant material. The main types fossil fuels include: Coal Crude Oil/ Petroleum Natural Gas (including Coal Seam Gas) Fossil fuels are found in deposits within the Earth (some which occur under the ocean floor) and are comprised of carbon rich and energy dense molecules. Coal mainly comprised of carbon. Crude oil and natural gas mainly a mixture of hydrocarbons. Coal seam gas mainly methane which is trapped in coal deposits.

ENVIRONMENTAL CONSIDERATIONS

Fossil fuels are NON – RENEWABLE since: The rate they are consumed > The rate they are formed.

Fossil fuels are NOT CARBON NEUTRAL since the rate of 2CO production > rate

2CO is removed from the atmosphere.

The burning of fossil fuels by humans is the largest source of emissions of carbon

dioxide, which is a greenhouse gases and a major contributor to global warming.


BIOFUELS As our main current energy sources are derived from non-renewable fuels, there is an urgent need for the development of alternative fuels to meet future energy requirements. Biochemical fuels (biofuels) are one of these alternatives, as they are renewable and are less polluting than coal or oil. Biochemical fuels are derived from organic materials, predominantly plants e.g. starch, wheat, sugar cane and vegetable oils. These fuels are generally formed via endothermic processes, so they have high levels of chemical potential energy which can be released as heat upon burning. There are three types of biochemical fuels (biofuels): Biomass

Biogas

Liquid fuels

RENEWABILITY To be renewable, an energy source has to be able to be replenished within the time frame that it is consumed. Biofuels are considered renewable since the plants from which they are derived can be replaced at the same rate that they are consumed to produce the biofuels.

CARBON NEUTRALITY In simplistic terms, biofuels can be considered to be carbon neutral since the carbon dioxide they produce is taken up by plants when new biofuel crops are grown. In this way there is no net increase in the amount of carbon dioxide in the atmosphere. Unfortunately, this does not take into account the energy consumed and carbon dioxide emitted when biofuel crops are grown, processes and transported. When these factors are taken into account, then biofuels are not carbon neutral. Fossil Fuel Inputs:

Biofuels Pesticides Fertilisers

Fuel for transport Fuel for machinery


BIOMASS

Biomass is a fuel produced directly or indirectly by biological resources. Biomass is the term used for all organic material originating from plants or animals. The energy contained in these fuels originates from the sun and has been trapped via photosynthesis.

Photosynthesis: )(2)(6126)(2)(2 666 gaqlightchlorophyl

lg OOHCOHCO

Plants process the glucose that is produced in plants into complex carbohydrates. It is the energy that is trapped within these compounds that is released upon combustion. Combustion of biomass transforms the chemical energy stored within the chemical bonds of the fuel to heat energy, which may then be used as an energy source. A simplified representation of the combustion reaction for biomass is:

HeatOHCOOBiomass ggg )(2)(2)(2

Biomass is the starting material for all biofuels. The original biomass crops get processed into biofuels such as biomethane, bioethanol and biodiesel.

Converted to complex carbohydrates

Biomass


QUESTION 11 Biomass can be converted into: A Solid fuel B Liquid fuel C Gaseous fuel D All of the above QUESTION 12 Using biomass as an energy source is considered more environmentally friendly than burning fossil fuels because: A Biomass produces no greenhouse gases. B Sourcing biomass has no environmental impacts. C There is enough naturally occurring biomass to meet demands. D The growth of biomass crops reduces the amount of carbon dioxide in the atmosphere. QUESTION 13 What is one disadvantage of bioethanol? A Bioethanol can be viewed as a carbon neutral fuel B Arable land is needed for bioethanol production C Bioethanol is renewable D Bioethanol production provides a use for excess sugar crops QUESTION 14 Which of the following are examples of how biomass can be converted into useful energy?

i. Burning animal dung to cook food. ii. Converting left over cooking oil to biodiesel. iii. Converting sugar cane into ethanol.

A i and ii B i and iii C ii and iii D i, ii and iii


ENVIRONMENTAL IMPACTS OF COMBUSTING FUELS: BIOFUELS AND FOSSIL FUELS

The burning of fuels has enormous environmental impact due to the variety of toxic chemicals that can be released into the environment. Some of these chemicals include: Carbon dioxide Nitrous oxides Sulfur dioxide Heavy metals Particulate matter

THE GREENHOUSE EFFECT Complete Combustion The complete combustion of fossil fuel is the most desirable reaction since it releases the largest amount of energy per amount of fuel.

Fuel + Oxygen Carbon Dioxide + Water + ENERGY The carbon dioxide and water produced in this reaction are greenhouse gases. This means they absorb infrared radiation which results in a gradual increase in the overall temperature of the earth's atmosphere. Since the amount of water vapour in the atmosphere is short lived (since it regularly falls as rain or snow), it is the additional carbon dioxide from combustion that has the greatest effect on global warming. The Greenhouse Effect involves: 1. Solar radiation from the Sun hits our atmosphere.

Some is reflected back into space. Some reaches the Earth’s surface and is absorbed by land and water.

2. The heat from the surface of the Earth, radiated back towards space. 3. Some of this heat is trapped which provides the warmth needed for life to exist.

4. The enhanced Greenhouse effect occurs due to the increased amounts of greenhouse

gases in the atmosphere due to human activities. This traps extra heat, causing the Earth’s temperature to rise.

The Earth’s average temperature has warmed by about 0.76°C over the past 100 years, with most of this warming occurring in the past 20 years. This warming has been scientifically proven to be a result of human activity such as burning fossil fuels. Although these temperature increases are small, the implications for the environment are huge.


What does a 0.76°C temperature rise mean? More hot days

More severe storms, floods, droughts and fire

Higher sea levels

More pressure on water supplies

Melting of the polar ice caps

Sea level rises

Reduced oxygen solubility in water

Increased acidity of the oceans Other principle greenhouse gases include, methane and nitrogen oxides. While most of these gases occur in the atmosphere naturally, levels have been increasing due to the widespread burning of fuels by growing human populations. It is predicted that average global temperatures are going to continue to increase by as much as 4°C by 2100 if greenhouse gas emissions cannot be controlled. This will radically change the world’s climate and place astronomical demands of food production and water security. Since 72% of emitted greenhouse gases is carbon dioxide, it is essential that environmentally friendly alternatives are found for electricity production or that the carbon dioxide produced is neutralised via schemes such as carbon sequestration or carbon trading. Carbon dioxide also contributes to acid rain since it can react with water to form a weak acid.

)(32)(2)(2 aqlg COHOHCO


QUESTION 15 The burning of which fuel would reduce the build-up of carbon dioxide in the atmosphere? A Ethanol B Diesel C Methane D Hydrogen QUESTION 16 The natural greenhouse effect is mainly caused by: A The trapping of solar energy as it enters the Earth’s atmosphere. B The trapping of energy as it irradiates from the Earth’s surface back into space. C The increase in carbon dioxide levels in the atmosphere due to human activity. D The deforestation of the planet. QUESTION 17 Which of the following is not a greenhouse gas? A Water vapour B Oxygen C Nitrous oxide D Carbon dioxide


QUESTION 18 Which of the following greenhouse gases is most abundant in the atmosphere? A Methane B Carbon dioxide C Water D Nitrous oxide QUESTION 19 The largest source of non-natural carbon dioxide emissions into the atmosphere comes from: A Energy supply B Transport C Agriculture D Wood fires


CALCULATIONS INVOLVING GASES When making fuel choices, there are a number of considerations to take into account. How much energy is stored in the fuel per mass, volume or mole?

What is the energy efficiency of the fuel?

How much greenhouse gas will be produced per unit of fuel?

How much energy is produced per unit of greenhouse gas?

How much room will be needed to store the fuel? Some of the answers to these questions can be obtained via stoichiometric calculations. These calculations will require a combination of the following concepts. Stoichiometric calculations using mole ratios from balanced equations via masses or

volumes.

Limiting reactant stoichiometry.

Molar volumes of gases.

Thermochemical Equations.

Molar Enthalpies or Molar Heat of Combustion.

The General Gas Equation.

Volume Ratios of Gases.

Density values.

The heat capacity of water.


THE GENERAL GAS EQUATION The general gas equation is developed from the combined gas equation by determining a value for k (the constant from combined relationship). It is given the symbol R and so the equation becomes:

Pressure ( )

Volume ( )

Amount (in mol) of gas ( )

Gas constant = 8.31

Temperature ( ) Useful Conversions: 760 = 1 atmosphere = 101,325 = 101.325

1 = 1000 = 1 1 = 1 Temperature (K)= Temperature ( C ) + 273 Note: The gas constant only has a value of 8.31 if the correct units are used as shown above.

P kPa

V L

n mol

R 11 molJK

T K

mmHg Pa kPa

3dm ml L3cm ml

PV=nRT


QUESTION 20 At what temperature, in degrees Celsius, will 0.0750 mol of CO2 occupy 2.75 L at 1.11 atm? Solution QUESTION 21 What amount, in mole, of O2 is present in a 0.50 L sample at 25°C and 1.09 atm? Solution


QUESTION 22 Ethanol ( 3 2CH CH OH ) is increasingly being used as an alternative to petrol, which mainly

consists of octane ( 8 18C H ). The density of ethanol is 1789.0 mlg .

2 5 ( ) 2( ) 2( ) 2 ( )3 2 3l g g lC H OH O CO H O 1364 /H kJ mol

A typical fuel tank holds 70.0 L of fuel. (a) What volume of ethanol would be present in a full tank of E10 petrol? (b) Calculate the volume of carbon dioxide produced from the ethanol in the tank given

conditions of 25°C and atmospheric pressure.

(c) Does burning 7.00 L of ethanol increase the net amount of carbon dioxide to the atmosphere by the amount calculated in part (b)? Explain.


QUESTION 23 In a laboratory experiment, 10.00 g of methanol was combusted with 9.000 L of pure oxygen. The temperature and pressure in the laboratory was 22°C and kPa104 . (Conditions were not held constant.) (a) Calculate the total mass of greenhouse gases produced. (b) What mass of greenhouse gas is produced per unit of energy?

State your answer in 1kJg .

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Unit 3 Chemistry - TSFX – The School for Excellence | Who …€¦ · · 2017-09-05© The School For Excellence 2017 Succeeding in the VCE – Unit 3 Chemistry ... 2017 Succeeding