Chemistry Understanding Water V Name: Suite 403, 410 Elizabeth St, Surry Hills NSW 2010 (02) 9211 2610 info@keystoneeducation.com.au keystoneeducation.com.au
Water has a higher heat capacity than many other liquids Students learn to: Explain what is meant by the specific heat capacity of a substance Compare the specific heat capacity of water with a range of other solvents Explain and use the equation = Explain how water s ability to absorb heat is used to measure energy changes in chemical reactions Describe dissolutions which release heat as exothermic and give examples Describe dissolutions which absorb heat as endothermic and give examples Explain why water s ability to absorb heat is important to aquatic organisms and to life on earth generally Explain what is meant by thermal pollution and discuss the implications for life if a body of water is affected by thermal pollution Students: Choose resources and perform a first-hand investigation to measure the change in temperature when substances dissolve in water and calculate the molar heat of solution Process and present information from secondary sources to assess the limitations of calorimetry experiments and design modifications to equipment used Copyright Keystone Education 2015 2
Explain what is meant by the specific heat capacity of a substance Compare the specific heat capacity of water with a range of other solvents Specific heat capacity refers to the amount of energy required to raise one gram of a substance by 1 Kelvin or Celsius. It is usually measured in Joules/Kelvin/gram or J/K/g. It represents how much energy a substance can store. Water has a specific heat capacity of 4.18, which is relatively high compared to other solvents. Substance Water Ethanol Hexane Specific heat capacity (J/K/g) 4.18 2.44 2.26 Explain why the sand at the beach is hotter than the nearby water. Copyright Keystone Education 2015 3
Describe dissolutions which release heat as exothermic and give examples Describe dissolutions which absorb heat as endothermic and give examples Informally, enthalpy () is the sum of all energy in a system, given constant pressure. Enthalpy cannot be measured directly, but enthalpy change () for a reaction can be measured. A reaction which releases heat (energy) is called exothermic. The sign of is negative, because the energy in the system has been released. A reaction which absorbs heat is called endothermic. The sign of is negative, because the energy has been absorbed into the system. In a dissolution reaction, there are two stages: an endothermic stage and an exothermic stage. 1. In the endothermic stage, energy is used when the ionic bonds are broken and hydrogen bonds between water molecules is broken. 2. In the exothermic stage, energy is released when new bonds/forces are established. Whether a reaction is exothermic or endothermic overall depends on the energy absorbed and released in these two stages. Enthalpy changes are usually measured in kj/mol. For interested students Enthalpy is formally defined as a thermodynamic potential that consists of the internal energy of the system () plus the product of pressure ( ) and volume (). Copyright Keystone Education 2015 4
The dissolution of NaCl is given by: + NaCl s Na aq - + Cl aq ΔH = +4 kj/mol Is this reaction endothermic or exothermic? Explain the amount of energy absorbed and released in the stages of this reaction. The dissolution of NaOH has an enthalpy change of 44.5 kj/mol. Write down the chemical equation for the dissolution of NaOH. Is this reaction endothermic or exothermic? Explain the amount of energy absorbed and released in the stages of this reaction. Copyright Keystone Education 2015 5
Explain how water s ability to absorb heat is used to measure energy changes in chemical reactions Explain and use the equation = How do we actually measure enthalpy change? By using the idea of specific heat capacity and measuring temperature changes, we can convert this to an energy change. We do this using a device called a calorimeter. A simple calorimeter used in school laboratories might be made of a Styrofoam cup with the reaction occurring in the water inside. A thermometer is used to measure the temperature change, which is absorbed by the water. Once the temperature change is recorded, the following calorimetry equation is used: = = quantity of heat absorbed by the calorimeter (J) = mass of the material absorbing the heat (g) = the specific heat capacity of the material absorbing the heat (J/K/g) Δ = the change in temperature (K or C) Lastly, convert the energy change from J to kj/mol. Δ= Δ = enthalpy change per mole (kj/mol) = absolute enthalpy change converted to kj (kj) = number of moles of reactant Copyright Keystone Education 2015 6
What assumption is made with these calorimetric experiments? Explain in the words the equation =. Explain why it is necessary to convert the enthalpy change to a per mole measure. Keystone tips The reason there is a minus in is because when temperature increases, heat is released and energy is lost from the system. Many people like to remember minus m-cat to remember the formula because the Δ sign looks like an A. Some sources do not use and interchangeably use Δ for the absolute enthalpy change and the enthalpy change per mole. You may assume that 1.00mL = 1.00g of water. Copyright Keystone Education 2015 7
Example A calorimeter contains 50.0mL of water. 2.00g of ammonium chloride powder was added, and the mixture dropped in temperature from 25.1 C to 22.7 C. (a) Calculate the heat absorbed during this reaction. (b) Calculate the enthalpy change per mole of ammonium chloride. Keystone tip Copyright Keystone Education 2015 8
Enthalpy diagrams map over the time of the reaction. 1 Draw a sketch of an enthalpy diagram for an endothermic reaction. Draw a sketch of an enthalpy diagram for an exothermic reaction. 1 This is an introduction and will be covered in more depth in Energy. Copyright Keystone Education 2015 9
Choose resources and perform a first-hand investigation to measure the change in temperature when substances dissolve in water and calculate the molar heat of solution Process and present information from secondary sources to assess the limitations of calorimetry experiments and design modifications to equipment used Method 1. Use a burette to measure 50.0mL of water into a polystyrene cup. 2. Place a foam lid on the cup. 3. Insert a thermometer through the foam lid and record the temperature after 5 minutes. 4. Meanwhile, weigh out 1.00g of NaOH pellets. 5. Add pellets to the calorimeter and replace the lid. 6. Stir and record the temperature every minute over the next 5 minutes. 7. Calculate the molar heat of solution from the observed results. What is the aim of this first-hand investigation? Perform a risk assessment of this procedure. Identify Assess Control Copyright Keystone Education 2015 10
Justify the use of a polystyrene cup. Justify the measurement of the temperature every minute for 5 minutes. Brainstorm potential limitations of the above calculation and experiment. Copyright Keystone Education 2015 11
There are various limitations of the calorimetry experiments. Various factors needs to be taken into account including: Insulation Accuracy of temperature measurements Uniformity of mixture Completion of intended reaction Explain the above limitations and how each can be rectified. Copyright Keystone Education 2015 12
The molar heat of solution refers to the enthalpy change per mole of a substance that is being dissolved in a solvent (usually water). Below is some sample data from a student who performed this first-hand investigation. Minutes elapsed 0 1 2 3 4 5 Temperature ( C) 21.5 24.3 25.5 25.9 26.2 26.2 Calculate the enthalpy change of the reaction and the molar heat of solution of sodium hydroxide. Include a chemical equation in your answer. Copyright Keystone Education 2015 13
Problems 1. In a polystyrene calorimeter, 2.50g of NaOH is added to 100.0g of water. The temperature of the water rises from 25.0 C to 37.5 C. Calculate the molar heat of solution. Copyright Keystone Education 2015 14
2. In a polystyrene calorimeter, 1.25g of NaOH is added to 110.0g of water. The temperature of the water rises from 24.0 C to 29.4 C. Calculate the molar heat of solution. Copyright Keystone Education 2015 15
3. The dissolution of sodium chloride in water has an enthalpy change of Δ=+4 kj/mol. 3.00g of sodium chloride was dissolved in 100.0g of water with an initial temperature of 25.0 C. (a) Calculate the final temperature of the solution, assuming no heat loss. (b) Calculate the final temperature of the solution, assuming 50% heat loss. Copyright Keystone Education 2015 16
Explain why water s ability to absorb heat is important to aquatic organisms and to life on earth generally There are many reasons why water s high specific heat capacity is important to aquatic organisms and life in general: Regulating the temperature of water bodies Regulating the internal body temperature of animals Why is it important that the temperature of water bodies remain relatively constant? Why is it important that internal temperature of animals is maintained? Copyright Keystone Education 2015 17
Explain what is meant by thermal pollution and discuss the implications for life if a body of water is affected by thermal pollution Thermal pollution is defined as the degradation of water quality due to a change in water temperature (usually an increase). An increase in temperature of a water body decreases the solubility of oxygen and carbon dioxide. An increase in temperature increases metabolic rate of aquatic animals, resulting in these organisms consuming more food in a shorter amount of time. Identify some examples of thermal pollution. Explain the implications of a decrease in solubility of oxygen and carbon dioxide. Explain the implications of an increase in metabolic rate of aquatic animals. Copyright Keystone Education 2015 18