UNIT 2 CHEMISTRY INDEX BOOK 1

UNIT 2 CHEMISTRY INDEX BOOK 1 What Makes Water Such a Unique Chemical? Page 1 AoS 1: How Do Substances Interact With Water? AoS 2: How Are Substance...
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UNIT 2 CHEMISTRY INDEX BOOK 1 What Makes Water Such a Unique Chemical?

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AoS 1: How Do Substances Interact With Water? AoS 2: How Are Substances in Water Measured and Analysed? AoS 3: Practical Investigation

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The Importance of Water

Page 3

Structure of Water Intermolecular Bonding

Page 3 Page 4

The Unique Properties of Water

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High Melting and Boiling Point High Latent Heat Values of Water Specific Heat Capacity

Page 7 Page 9 Page 15

The Significance of Water’s Properties

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Water as a Solvent

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Solubility of Ionic Compounds Solubility of Molecular Substances in Water Solubility of Gases in Water The Importance of the Solvent Properties of Water

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Solution Concentration

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Molarity Other Calculations Using Molarity Calculating the Number of Particles in Solution

Page 33 Page 38 Page 40

Other Concentration Units

Page 41

Percentage by Mass (% w/w) Percentage by Volume (% v/v) Percentage by Mass/Volume (% w/v) Parts Per Million (ppm) and Parts Per Billion (ppb)

Page 41 Page 42 Page 43 Page 44

Other Means of Expressing Concentration

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Mass of Solute Per Litre of Solution Mass of Solute Per Gram of Solution Density

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Converting Concentration Units

Page 48

Converting Between Concentration Units

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Dilutions

Page 56

Dilution Using Molarity

Page 60

Chemical Reactions in Water

Page 63

Precipitation Reactions

Page 64

Net Ionic Equations

Page 67

Writing Ionic Equations

Page 67

Acids and Bases

Page 70

Writing Acids and Bases Reactions Common Acids and Bases Properties of Acids and Bases

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Acids and Bases in Water

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Ionisation of Acids Dissociation of Bases

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Acid-Base Reactions

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Conjugate Pairs

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Reactions Involving Acids

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Amphoteric Substances

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The Self-Ionisation of Water

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Polyprotic Acids

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Acid and Base Strength

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The Difference Between Acid Strength and Concentration

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Acidic, Basic and Neutral Solutions

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Self-Ionisation of Water Neutral Solutions Acidic Solutions Basic Solutions

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The pH Scale

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pH Calculations at 25°C

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Calculating the pH of an Acidic Solution Calculating the pH of an Alkaline Solution

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Understanding the pH Scale

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Acid Deposition

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Redox Chemistry

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Redox Reactions Oxidants Reductants Oxidation Numbers Finding the Oxidation Number of an Element in a Compound Identifying Redox Reactions Other Ways of Recognising Redox Reactions Writing Redox Equations Balancing Redox Half Equations Combining Half Equations

Page 117 Page 117 Page 117 Page 121 Page 122 Page 128 Page 134 Page 135 Page 135 Page 137

Electrochemical Cells

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Example of a Galvanic Cell: The Daniell Cell

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Half Cells The External Circuit Internal Circuit

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The Electrochemical Series

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Uses of the Electrochemical Series

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Predicting Reaction Spontaneity

Page 149

Writing Redox Reactions

Page 152

Corrosion

Page 162

Corrosion Protection Surface Protection Alloying Electroplating Electrochemical Protection Impressed Current Sacrificial Anode

Page 165 Page 165 Page 165 Page 165 Page 165 Page 165 Page 165

Solutions

UNIT 2 CHEMISTRY WHAT MAKES WATER SUCH A UNIQUE CHEMICAL? Unit 1 has three Areas of Study (AoS):

AoS 1: HOW DO SUBSTANCES INTERACT WITH WATER? In this area of study students focus on the properties of water and the reactions that take place in water including acid-base and redox reactions. Students relate the properties of water to the water molecule’s structure, polarity and bonding. They also explore the significance of water’s high specific heat capacity and latent heat of vaporisation for living systems and water supplies. Students investigate issues associated with the solubility of substances in water. Precipitation, acid-base and redox reactions that occur in water are explored and represented by the writing of balanced equations. Students compare acids with bases and learn to distinguish between acid strength and acid concentration. The pH scale is examined and students calculate the expected pH of strong acids and strong bases of known concentration. Source: VCAA Chemistry Study Design 2016 – 2020

AoS 2: HOW ARE SUBSTANCES IN WATER MEASURED AND ANALYSED? In this area of study students focus on the use of analytical techniques, both in the laboratory and in the field, to measure the solubility and concentrations of solutes in water, and to analyse water samples for various solutes including chemical contaminants. Students examine the origin and chemical nature of substances that may be present in a water supply, including contaminants, and outline sampling techniques used to assess water quality. They measure the solubility of substances in water, explore the relationship between solubility and temperature using solubility curves and learn to predict when a solute will dissolve or crystallise out of solution. The concept of molarity is introduced and students measure concentrations of solutions using a variety of commonly used units. Students apply the principles of stoichiometry to gravimetric and volumetric analyses of aqueous solutions and water samples. Instrumental techniques include the use of colorimetry and/or UV-visible spectroscopy to estimate the concentrations of coloured species in solution, atomic absorption spectroscopy data to determine the concentration of metal ions in solution and high performance liquid chromatography data to calculate the concentration of organic compounds in solution. Source: VCAA Chemistry Study Design 2016 – 2020

© The School For Excellence 2016

Master Classes – Unit 2 Chemistry – Book 1

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AoS 3: PRACTICAL INVESTIGATION Substances that are dissolved in water supplies may be beneficial or harmful, and sometimes toxic, to humans and other living organisms. They may also form coatings on, or corrode, water pipes. In this area of study students design and conduct a practical investigation into an aspect of water quality. The investigation relates to knowledge and skills developed in Area of Study 1 and/or Area of Study 2 and is conducted by the student through laboratory work and/or fieldwork. The investigation requires the student to develop a question, plan a course of action that attempts to answer the question, undertake an investigation to collect the appropriate primary qualitative and/or quantitative data (which may including collecting water samples), organise and interpret the data and reach a conclusion in response to the question. Source: VCAA Chemistry Study Design 2016 – 2020

© The School For Excellence 2016

Master Classes – Unit 2 Chemistry – Book 1

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THE IMPORTANCE OF WATER

In our everyday lives, we give very little consideration to the importance of water and its unique properties. However, there are few aspects of life on Earth, that aren’t affected by water in some way.

Source: Study.com

STRUCTURE OF WATER Water is a molecular compound with the molecular formula H 2 O . The central oxygen atom has two lone pairs of electrons but also forms two single covalent bonds with two hydrogen atoms. The bonding and non-bonding pairs of electrons like to arrange themselves as far from each other as possible in order to minimise the amount of repulsion between them. Usually, four pairs of electrons around a central atom would result in a tetrahedral shape with an angle of 109.5° between them. However, since non-bonding pairs (lone pairs of electrons) exert a stronger repulsion force than bonding pairs, the covalent bonds are pushed closer together resulting in a distorted tetrahedral shape in which the H − O − H angle is 104.5°.

The shape of the atoms and lone pairs is a distorted tetrahedral.

Source: www.chem1.com

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Master Classes – Unit 2 Chemistry – Book 1

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To properly describe the shape of a water molecules, the lone pairs are ignored. It is the relative positions of the atoms that determines its shape. The position of the atoms makes the shape of the molecule bent or V-shaped.

There are few molecules that are more stable or difficult to decompose than water.

INTERMOLECULAR BONDING •

Dispersion forces

Dispersion Force

The electrons in water molecules are in constant motion so at any moment in time, electrons can accumulate at one side of the molecule. This makes one end of the molecule slightly negative and the other side slightly positive. This separation of charge is called an instantaneous dipole. The presence of this dipole distorts the electrons of a neighbouring molecule, producing an induced dipole. A weak electrostatic attraction then forms between the molecules which is called a dispersion force. Dispersion forces are relatively weak and become significant only when the molecules are very close. •

Hydrogen bonding Hydrogen bonding is a special case of dipole-dipole bonding. It occurs between molecules when a particularly strong dipole exists within the molecules. In water, the high electronegativity of the oxygen means that electrons are permanently more attracted to it. Because of this, the oxygen end of the molecule becomes slightly negative and the hydrogen ends become slightly positive. This separation of charge causes a permanent dipole to form. The positive end of one water molecule is then attracted to the negative end of a neighbouring molecule. This type of intermolecular force of attraction is called hydrogen bonding.

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Master Classes – Unit 2 Chemistry – Book 1

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Hydrogen bonding is the strongest type of intermolecular bonding and is the reason behind many of the unique properties of water.

Source: study.com

QUESTION 1 The labelled diagram that best represents the bonding within and between water molecules is: A

B

C

D

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Master Classes – Unit 2 Chemistry – Book 1

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QUESTION 2 Which property of liquid water arises directly from the polar nature of its molecules? A B C D

Its ability to dissolve oxygen. Its ability to conduct an electric current to a small extent. Its ability to dissolve many polar and ionic substances. Its ability to flow.

QUESTION 3 Which property of water arises directly from the strength of the covalent bonding in the water molecule? A B C D

Its relatively high melting point and boiling point for its mass. The energy required to break water molecules up into separate atoms. Its specific heat capacity. Is relatively high surface tension.

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Master Classes – Unit 2 Chemistry – Book 1

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THE UNIQUE PROPERTIES OF WATER Many of the properties of water are unusual when they are compared with those of other similar compounds of low molecular mass. In particular, water has: • •

An unusually high melting and boiling point. A higher specific heat capacity and latent heat than almost any other liquid.

HIGH MELTING AND BOILING POINT The graph below shows the boiling points of the Group 16 hydrides.

The boiling point of water is clearly much higher than that of any of the other Group 16 hydrides. This can be explained by the existence of hydrogen bonding. Water is the only Group 16 hydride which has hydrogen bonding between its molecules in addition to dipoledipole bonding and dispersion forces. Molecules of H 2 S , H 2 Se and H 2Te are only attracted via dispersion forces and dipole-dipole bonding which are weaker forms of intermolecular bonding than hydrogen bonding. Therefore the strength of intermolecular bonding between water molecules is stronger than that between the molecules of any of the other hydrides and hence water requires more energy to change from a liquid to a gaseous state.

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Master Classes – Unit 2 Chemistry – Book 1

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The significant impact of hydrogen bonding on the boiling point of water is illustrated by extrapolating the boiling points of H 2 S , H 2 Se and H 2Te (shown below). This allows the boiling point of water to be predicted in the absence of hydrogen bonding.

If hydrogen bonding was absent, the predicted boiling point of water would be -90°C. A similar pattern is reflected in the melting points of the Group 16 hydrides. Just as hydrogen bonding causes the boiling point of water to be higher than that of other Group 16 hydrides, it also explains its usually high melting point. Since the intermolecular forces of attraction are greater in water than in any of the other Group 16 hydride, it will require more energy to turn from a solid to a liquid.

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Master Classes – Unit 2 Chemistry – Book 1

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HIGH LATENT HEAT VALUES OF WATER Latent heat is the amount of energy absorbed or released during a change of state. To understand this concept, a clear understanding of the molecular changes occurring during the heating or cooling of a substance is needed.

Phase Changes of Water

Molecular changes during the heating of water Temperature change: -40°C → 0°C Below zero degrees Celsius, water exists as a solid. The water molecules are in fixed positions in a molecular lattice. As the temperature is increased from -40°C → 0°C, the added energy makes the molecules vibrate more vigorously. This increase in kinetic energy is reflected in the increasing temperature of the water.

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Temperature: 0°C Once the temperature reaches 0°C, the water will start to melt. At this point, the vibrations of the water molecules are large enough that the attractive forces holding the lattice together are no longer strong enough to hold the water molecules in fixed positions. During this time, the energy added to the system is absorbed by the intermolecular bonding and so the temperature of the system does not increase. Energy will continue to be absorbed by the intermolecular bonding until it has been weakened to such an extent that individual particles are free to move as a liquid. At this point, the water has melted. The heat absorbed during this transition period is called the latent heat of fusion. Specific Latent Heat of Fusion for Water: L f ( H 2 O) = 334 kJ .kg −1 Note: The specific latent heat of solidification is the amount of energy released as a substance changes from a liquid to a solid. For water, the specific latent heat of solidification is still 334 kJ kg-1. The sign does not change since the definition of specific latent heat of solidification is the amount of energy released, which will be a positive value. Temperature change: 0°C → 100°C Between 0 and100°C, water exists as a liquid. The water molecules move freely around each other, however the intermolecular forces of attraction are still strong enough for some attraction to occur between them. As the temperature is increased from 0°C → 100°C, the molecules gain kinetic energy and move more vigorously. This increase in kinetic energy is reflected in the increasing temperature of the water. Temperature: 100°C Once the temperature reaches 100°C, the water will start to boil. At this point, the movement of the water molecules are energetic enough that the attractive forces between them are totally overcome. During this time, the energy added to the system is absorbed by the intermolecular bonding and so the temperature of the system does not increase. Energy will continue to be absorbed by the intermolecular bonding until it has been weakened to such an extent that individual particles are free to move independently from each other as a gas. At this point, the water has boiled. The heat absorbed during this transition period is called the latent heat of vapourisation. Specific Latent Heat of Vapourisation for Water: Lv ( H 2 O ) = 2265 kJ .kg −1

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Master Classes – Unit 2 Chemistry – Book 1

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Note: The specific latent heat of condensation is the amount of energy released as a substance changes from a gas to a liquid. For water, the specific latent heat of condensation is still 2265 kJ kg-1. The sign does not change since the definition of specific latent heat of condensation is the amount of energy released, which will be a positive value. Temperature change: 100°C → Above 100°C, water exists as a gas. The water molecules move in a totally independent and disorganised manner. The intermolecular forces of attraction are totally overcome so that there is no effective attraction between the water molecules. As the temperature is increased above 100°C, the molecules will continue to gain kinetic energy and move more vigorously. This increase in kinetic energy is reflected in the increasing temperature of the steam. Summary: •

Once water reaches 0°C, every kilogram of water will require 334 kilojoules of energy in order to make the transition from a solid state to a liquid state.



Once water reaches 100°C, every kilogram of water will require 2265 kilojoules of energy in order to make the transition from a liquid state to a gaseous state.



During these phase changes, the temperature of the system will not change.

Comparing Specific Latent Heat of Fusion and Vaporisation Substance

Latent of Heat Fusion (kJ/kg)

Latent Heat of Vaporisation (kJ/kg)

Helium

5.23

20.9

Nitrogen

25.5

201

Ethanol

104

213

Water

334

2265

Compared to other molecular substances of similar sizes, water has a high specific latent heat of fusion and vaporisation. This is due to the hydrogen bonding that exists between water molecules. The small size of the water molecules allows the hydrogen bonding to be very effective in attracting the molecules to each other and therefore larger amounts of energy are needed to cause changes in state.

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Master Classes – Unit 2 Chemistry – Book 1

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QUESTION 4 Amount of energy required to change liquid to gas without any change in temperature is termed as A B C D

Latent Heat of Fusion Latent Heat of Vaporization Heat Capacity Specific Heat Capacity

QUESTION 5 Heat is added to a substance, but its temperature does not rise. Which one of the following statements provides the best explanation for this observation? A B C D

The substance must be a gas. The substance must be cooler than its environment. The substance undergoes a change of phase. The substance is a solid.

QUESTION 6 The condensation of water will: A B C D

Warm the surroundings. Cools the surroundings. Sometimes warms and sometimes cools the surroundings, depending on the relative humidity at the time. Has no effect on the temperature of the surrounding.

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Master Classes – Unit 2 Chemistry – Book 1

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QUESTION 7 As a solid undergoes a phase change to a liquid state, it: A B C D

Releases heat while remaining at a constant temperature. Absorbs heat while remaining at a constant temperature. Releases heat as the temperature decreases. Absorbs heat as the temperature increases.

QUESTION 8 A student graphed the temperature of gaseous naphthalene against time while it was cooling. The following three regions are observed: • • •

During the first 2 minutes, the temperature dropped from 90°C to 80°C. In the next 6 minutes, the temperature remained at 80°C. In the final 4 minutes, the temperature fell from 80°C to 60°C.

The melting point of naphthalene is? A B C D

Above 90°C 90°C 80°C 60°C or below

QUESTION 9 What mass of water would melt if 2004 J of energy was applied when two blocks of ice are rubbed together? A B C D

3g 6g 12 g 18 g

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Master Classes – Unit 2 Chemistry – Book 1

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QUESTION 10 Why are burns from steam more severe than burns from boiling water? Solution

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SPECIFIC HEAT CAPACITY Heating a substance requires energy. This energy increases the internal energy of the substance by increasing the kinetic energy of its molecules and so the temperature of the substance rises. The amount of heat energy needed to change the temperature of a substance depends on: • • •

What the substance is. How much of it is being heated. The change in temperature of the substance.

The amount of energy needed to increase the temperature of a substance is related to the bonding within it. The stronger the bonding, the larger the amount of energy needed in order to make the particles vibrate more quickly and hence increase its temperature. Therefore, different substances will require different amounts of energy in order to undergo the same temperature change. A useful way of comparing the amount of energy needed to increase the temperature of different substances is to compare their specific heat capacities. The specific heat capacity of a substance is the amount of energy needed to change the temperature of 1 g of the substance by 1°C. The higher the specific heat capacity of a substance, the greater the amount of energy that needs to be added in order to increase 1 g of that substance by 1°C. For example: Water and Ethanol •

Water has a specific heat capacity of 4.184 J / g / °C . Therefore, 4.184 J of energy is needed to raise the temperature of 1 gram of water by 1°C.



Ethanol has a specific heat capacity of 1.413 J / g / °C . Therefore 1.413 J of energy is needed to raise the temperature of 1 gram of ethanol by 1°C.

The differences in the heat capacities of water and ethanol clearly shows that water requires more energy per gram to increase its temperature. It also means that if the same amount of energy was applied to one gram of water and one gram of ethanol, there would be a greater increase in the temperature of the ethanol. When a set amount of energy is added to different materials, their temperatures will increase by different amounts. Substances with higher heat capacities will: • • •

Require more energy to heat up. Will take longer to cool down. Will store energy more effectively.

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Master Classes – Unit 2 Chemistry – Book 1

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The heat energy gained or lost by a substance during a temperature change can be calculated using the formula:

Q = mcΔT Where Q = Energy ( J ) c = Specific Heat Capacity ( J / g / °C ) m = Mass of substance heated/cooled ( g ) ΔT = Temperature Change ( °C or K )

EXAMPLE 1 What amount of energy would need to be added to 250 ml H 2 O in order to heat it from 25.0°C to its boiling point? Solution

Q = mcΔT = 250 × 4.18 × (100 − 20) = 83680 J = 83.7 kJ EXAMPLE 2 Given that the specific heat capacity of copper is 0.38 J / g / °C , what would be the change in temperature of a 10 g block of lead if 100 J of energy is applied to it? Solution

Q = mcΔT Q mc 100 = 10 × 0.38 = 26 °C

ΔT =

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Master Classes – Unit 2 Chemistry – Book 1

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Specific Heat Capacities of Common Materials Specific Heat Capacity

Substance

( J / g / °C )

Ammonia ( NH 3 )

2.06

Ethanol ( CH 3CH 2 OH )

2.41

Water

4.18

Copper

0.381

Iron

0.438

Compared to other molecular substances of similar sizes, water has a high specific heat capacity. This is due to the hydrogen bonding that exists between water molecules. The small size of the water molecules allows the hydrogen bonding to be very effective in attracting the molecules to each other. The strength of these hydrogen bonds means that large amounts of energy need to be absorbed before the hydrogen bonds have weakened enough for the water molecules to increase their kinetic energy (movement). It is only then that the temperature will increase. QUESTION 11 Why is the specific heat capacity of water greater than that of ethanol? (Hint: Consider intermolecular bonding, and how many molecules of each would be present in a 1.0 g sample.) Solution

QUESTION 12 Ethanols specific heat capacity is about half of that of water. Equal amounts of energy are added to equal masses of both liquids in separate insulated containers. The water increases in temperature by 25°C. The temperature of the ethanol will rise to: A B C D

12.5 °C 25°C 50°C It depends on the rate of heating

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Master Classes – Unit 2 Chemistry – Book 1

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QUESTION 13 Which of the following properties of water is not explained by the presence of hydrogen bonding between the water molecules? A B C D

High latent heat of vaporisation. Colourless and odourless. Relatively high melting and boiling points. Expansion on freezing.

QUESTION 14 The specific heat of copper is about 0.4 J/g/°C. How much heat is needed to change the temperature of a 30 gram sample of copper from 20.0°C to 60.0°C? A B C D

1000 J 720 J 480 J 240 J

QUESTION 15 100.0 g of water at 25.00°C absorbs 100 J of heat. What is its final temperature? A B C D

0.24°C 24.76°C 25.00°C 25.24°C

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Master Classes – Unit 2 Chemistry – Book 1

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QUESTION 16 Find the amount of heat energy (J) required to change the temperature of 1.0 kg of water by 5.0°C. Solution

QUESTION 17 100.0 g of water is cooled from 30.10°C to 25.05°C. How much heat energy is released? Solution

QUESTION 18 Using your knowledge of the properties of water, explain why it is cooler by a lake (or any body of water) in the summer and warmer by a lake in the winter. Solution

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