topic 5 energetics and thermochemistry (9 hours)€¦ · topic 5 energetics and thermochemistry (9...

Topic 5 Energetics and Thermochemistry (9 hours)

1 © I.Study Learning Center Chemistry

5.1 Measuring energy changes

• Essential idea: The enthalpy changes from chemical reactions can be calculated from their effect on the temperature of their surroundings.

5.1 Measuring energy changes Nature of science: Fundamental principle—conservation of energy is a fundamental principle of science. (2.6) Making careful observations—measurable energy transfers between systems and surroundings. (3.1) Understandings:

• Heat is a form of energy. • Temperature is a measure of the average

kinetic energy of the particles. • Total energy is conserved in chemical

reactions. • Chemical reactions that involve transfer of

heat between the system and the surroundings are described as endothermic or exothermic.

• The enthalpy change (∆H) for chemical reactions is indicated in kJ mol-1.

• ∆H values are usually expressed under standard conditions, given by ∆Hᶱ, including standard states.

Applications and skills:

• Calculation of the heat change when the temperature of a pure substance is changed using q = mc∆T

• A calorimetry experiment for an enthalpy of reaction should be covered and the results evaluated.

Guidance:

• Enthalpy changes of combustion (∆Hcᶱ) and formation (∆Hfᶱ) should be covered.

• Consider reactions in aqueous solution and combustion reactions.

• Standard state refers to the normal, most pure stable state of a substance measured at 100 kPa. Temperature is not a part of the definition of standard state, but 298 K is commonly given as the temperature of interest.

• The specific heat capacity of water is provided in the data booklet in section 2.

• Students can assume the density and specific heat capacities of aqueous solutions are equal to those of water, but should be aware of this limitation.

• Heat losses to the environment and the heat capacity of the calorimeter in experiments should be considered, but the use of a bomb calorimeter is not required.

International-mindedness: • The SI unit of temperature is the Kelvin (K), but

the Celsius scale (°C), which has the same incremental scaling, is commonly used in most countries. The exception is the USA which continues to use the Fahrenheit scale (°F) for all non-scientific communication.

Theory of knowledge:

• What criteria do we use in judging discrepancies between experimental and theoretical values? Which ways of knowing do we use when assessing experimental limitations and theoretical assumptions?

Utilization:

• Determining energy content of important substances in food and fuels.

Syllabus and cross-curricular links: Topic 1.1—conservation of mass, changes of state Topic 1.2—the mole concept Aims:

• Aim 6: Experiments could include calculating enthalpy changes from given experimental data (energy content of food, enthalpy of melting of ice or the enthalpy change of simple reactions in aqueous solution).

• Aim 7: Use of databases to analyse the energy content of food.

• Aim 7: Use of data loggers to record temperature changes.



5.1.1 Thermochemistry terms Key principle:

• Every bond represents _______________________________. • Temperature represents the ____________________________ of the molecules

Temperature -

• Heat (q) – Note: System - Surroundings -

• Chemical reactions involve:

o

o Enthalpy change (∆H) - Chemical reactions that involve transfer of heat between the system and the surroundings are described as endothermic or exothermic.

• Exothermic -

• Endothermic - Note: ∆H = enthalpy of products - enthalpy of reactants

• Note that the enthalpy of the products is lower than that of the reactants:

• Products are energetically more favorable • Example:

• Note that the enthalpy of the products is greater than the enthalpy of the reactants:

• Products are energetically less stable

• Example:



5.1.2 Measuring enthalpy changes Specific heat capacity, c –

• Heat of a system is given by: Example 180.0 J of heat is transferred to a 100.0 g sample of iron, resulting in a temperature rise from 22.0°C to 26.0°C. Calculate the specific heat capacity of iron. Measuring enthalpy changes in aqueous solution

• General method for measuring enthalpy changes involving solutions in the laboratory requires measurement of

• Two types of examples that will be encountered:

o Enthalpy change of neutralization – this is the enthalpy change associated with the formation of one mole of water molecules when an acid (+H) reacts with an alkali (OH-) under standard conditions

o Enthalpy change of solution – enthalpy change when one mole of a solute is dissolved in excess solvent

• To solve problems:



Exercise 5.1

1. When a 1.15 g sample of anhydrous lithium chloride, LiCl was added to 25.0 g of water in a coffee-cup calorimeter, a temperature rise of 3.80 K was recorded. Calculate the enthalpy change of solution for 1 mol of lithium chloride

2. Consider the following experiment: 100.0 cm3 of 1.00 mol/dm3 potassium hydroxide solution are measured out and poured into a polystyrene cup and the temperature of the potassium hydroxide solution was measured. Then 120.0 cm3 of 1.00 mol/dm3 hydrochloric acid area measured out and the initial temperature was measured. The hydrochloric acid was then poured into the polystyrene cup and the mixture stirred rapidly. The maximum temperature was recorded. The results of the experiment: Initial temperature of potassium hydroxide solution = 19.7 C Initial temperature of hydrochloric acid = 19.7 C Maximum temperature reached = 25.9 C Use these data to determine the enthalpy change.



3. a. 100.0 cm3 of 1.00 mol dm-3 potassium hydroxide solution were reacted with 100.0 cm3 of

1.00 mol dm-3 hydrochloric acid. The temperature rise was 6.82 C. Calculate the enthalpy change.

b. The experiment in part a was repeated with 50.0 cm3 of 1.00 mol dm-3 potassium hydroxide solution and 50.0 cm3 of 1.00 mol dm-3 potassium hydroxide solution and 50.0 cm3 of 1.00 mol dm-3 hydrochloric acid. Calculate the temperature change of the reaction mixture.

4. Consider the following experiment: 100.0 cm3 of water are measured out and poured into a polystyrene cup and the temperature of the water was measured. Then 5.20 g of ammonium chloride are measured out. The ammonium chloride as added to the water and the solution stirred vigorously until all the ammonium chloride had dissolved. The minimum temperature was recorded. The results of the experiment: - Initial temperature of water = 18.3 C - Minimum temperature = 15.1 C Use the experimental data to determine the enthalpy change of the solution of ammonium chloride.



5. The following experiment may be used to determine the enthalpy change of reaction for: Zn(s) + CuSO4(aq) à ZnSO4(aq) + Cu(s) 50.0 cm3 of 0.200 mol dm-3 copper(II) sulfate solution are placed in a polystyrene cup. The temperature was recorded every 30s for 2 min. At 2 min 30s, 1.20g of powdered zinc are added. The mixture was stirred vigorously and the temperature recorded every half minute for several minutes. The results obtained were then plotted to give the following graph:

Use these data to determine the enthalpy change for this reaction

Standard enthalpy change

• Standard enthalpy change (∆Hᶱ) – enthalpy changes taken under standard conditions (Pressure = 100 kPa)

• Standard enthalpy change of formation (∆Hfᶱ) – the energy change upon the formation of 1 mol of a substance

• Standard enthalpy change of combustion (∆Hcᶱ) – the enthalpy of change when one mole of a substance is completely burnt in oxygen under standard conditions

o Always negative; always an exothermic process



Measuring an enthalpy change of combustion • Calorimetry – technique to measure enthalpy change of combustion

o Heat given out in a combustion reaction is used to heat another substance of known specific heat capacity (e.g. water)

• Error 1:

• Error 2: Exercise 5.2

1. Use the following experimental data to determine the enthalpy change of combustion of methanol (CH3OH):

2. Use the following experimental data to determine the enthalpy change of combustion of ethanol (C2H5OH) given that the specific heat capacity of water is 4.18 J/g°C • Mass of water = 150.00 g • Initial temperature of water = 19.5°C • Maximum temperature of water = 45.7°C • Initial mass of spirit burner = 12 • Initial mass of spirit burner = 121.67 g • Final mass of spirit burner = 120.62 g

topic 5 energetics and thermochemistry (9 hours)€¦ · topic 5 energetics and thermochemistry (9...

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