Chapter 34 Standard enthalpy change of reactions 34.1 Standard enthalpy changes of combustion, neutralization and formation 34.2 Determination of enthalpy changes using simple calorimetric methods Key terms Progress check Summary Concept map P. 1 / 66
34.1 Standard enthalpy changes of combustion, neutralization and formation Standard enthalpy change In order to compare enthalpy changes fairly, it is necessary to specify a set of standard conditions under which enthalpy changes are measured. P. 2 / 66
Standard conditions a pressure of one atmosphere (1 atm or 101 325 N m 2 ), a temperature of 25 C or 298 K, ( K is the unit for Kelvin temperature. Kelvin temperature = Celsius temperature + 273), a concentration of 1.0 mol dm 3 if the substance is present in solution, the substances involved are in their standard states i.e. the normal physical state of their most stable form at 25 C and 1 atm. 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 3 / 66
Standard states for some substances Standard state for carbon: graphite (not diamond) Standard state for phosphorus: red phosphorus (not white phosphorus) Standard state for oxygen: oxygen (not ozone) The enthalpy change of reaction measured under the standard conditions is regarded as standard enthalpy change of reaction. 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 4 / 66
Thermochemical equation Standard enthalpy change of reaction is often denoted by H. The symbol denotes that the reaction takes place under standard conditions. The units used for standard enthalpy changes are kj mol 1. 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 5 / 66
A thermochemical equation C(s) + O 2 (g) CO 2 (g) H = 393.5 kj mol 1 The above equation shows that, 393.5 kj of heat is released when 1 mole carbon atoms reacts with 1 mole oxygen molecules, forming 1 mole carbon dioxide molecules under standard conditions. Key point Standard enthalpy change of reaction is the enthalpy change when the numbers of moles of reactants represented by the thermochemical equation react completely under standard conditions. 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 6 / 66
Changing the physical state of a substance in thermochemical equation The physical states of the reactants and products should be specified in a thermochemical equation. Example 1 H 2 (g) + O 2 (g) H 2 O(g) H = 241.8 kj mol 2 1 1 H 2 (g) + O 2 (g) H 2 O(l) H = 285.8 kj mol 2 1 The above equations show that an additional amount of heat (44.0 kj mol 1 ) is released when steam condenses to liquid. 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 7 / 66
1 H 2 (g) + O 2 (g) 2 1 H 2 (g) + O 2 (g) 2 Enthalpy H = 241.8 kj mol 1 Enthalpy H = 285.8 kj mol 1 H 2 O(g) H 2 O(l) (a) (b) Figure 34.2 A diagram showing the relative enthalpies of the species involved 1 1 in (a) H 2 (g) + O 2 (g) H 2 O(g) and (b) H 2 (g) + O 2 (g) H 2 O(l). 2 2 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 8 / 66
Reversing a thermochemical equation The sign of H changes when a thermochemical equation is reversed. Example 1 H 2 (g) + O 2 (g) H 2 O(l) H = 285.8 kj mol 2 1 1 H 2 O(l) H 2 (g) + O 2 (g) H = +285.8 kj mol 2 1 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 9 / 66
1 H 2 (g) + O 2 (g) 2 Enthalpy H = 285.8 kj mol 1 (a) (b) H = +285.8 kj mol 1 H 2 O(l) Figure 34.3 A diagram showing the relative enthalpies of the species involved 1 1 in (a) H 2 (g) + O 2 (g) H 2 O(l) and (b) H 2 O(l) H 2 (g) + O 2 (g). 2 2 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 10 / 66
Multiplying or dividing a thermochemical equation When both sides of a thermochemical equation are multiplied or divided by a factor, H must be multiplied or divided by the same factor. Example 1 Ca(s) + O 2 (g) CaO(s) H = 635 kj 2 2Ca(s) + O 2 (g) 2CaO(s) H = 2 ( 635) kj = 1270 kj Class practice 34.1 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 11 / 66
Standard enthalpy change of combustion ( H c ) The enthalpy change of combustion measured under standard conditions is known as standard enthalpy change of combustion. Standard enthalpy change of combustion is often denoted by H c. Key point Standard enthalpy change of combustion of a substance is the enthalpy change when one mole of the substance is completely burnt in oxygen under standard conditions. 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 12 / 66
The standard enthalpy change of combustion of methanol is 715.0 kj mol 1. It means that when one mole of methanol is completely burnt in oxygen to give carbon dioxide and water, 715.0 kj of heat is released. The thermochemical equation is 3 CH 3 OH(l) + O 2 (g) CO 2 (g) + 2H 2 O(l) H c = 715.0 kj mol 1 2 Learning tip In the thermochemical equation representing the standard enthalpy change of combustion, the coefficient of the substance to be burnt must be 1. 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 13 / 66
Since combustion is an exothermic reaction, the standard enthalpy change of combustion is always negative. 3 CH 3 OH(l) + O 2 (g) 2 Enthalpy H c = 715.0 kj mol 1 CO 2 (g) + 2H 2 O(l) Figure 34.4 A diagram showing the relative enthalpies of the species involved in combustion of methanol. 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 14 / 66
Substance H c (kj mol 1 ) Substance H c (kj mol 1 ) H 2 (g) 285.8 C 3 H 8 (g) 2220 C(diamond) 395.4 C 4 H 10 (g) 2877 C(graphite) 393.5 C 8 H 18 (l) 5460 CO(g) 283.0 CH 3 OH(l) 715.0 CH 4 (g) 890.4 CH 3 CH 2 OH(l) 1371 C 2 H 6 (g) 1560 C 6 H 12 O 6 (s) 2816 Table 34.1 Standard enthalpy changes of combustion ( H c ) for some common substances. Most of the substances listed in the above table can be used as fuels. 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 15 / 66
Hydrogen (H 2 (g)) and methane (CH 4 (g)) are the main components of Hong Kong town gas. Propane (C 3 H 8 (g)) and butane (C 4 H 10 (g)) are found in LPG. Figure 34.5 Propane and butane are the major fuels in LPG. Which one has a higher standard enthalpy change of combustion? 2,2,4-trimethylpentane (C 8 H 18 (l)), also known as isooctane, is an important component of petrol for motor vehicles. When choosing a fuel for use, scientists will consider their standard enthalpy changes of combustion. 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 16 / 66 STSE connections 34.1 STSE connections 34.2
Standard enthalpy change of neutralization ( H n ) The enthalpy change of neutralization measured under standard conditions is known as standard enthalpy change of neutralization. Standard enthalpy change of neutralization is often denoted by H n. The standard enthalpy change of neutralization between dilute HNO 3 (aq) and NaOH(aq) is 57.3 kj mol 1. It means that 1 mole dilute HNO 3 (aq) reacts with 1 mole NaOH(aq) to produce 1 mole H 2 O(l), 57.3 kj of heat is released. 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 17 / 66
The thermochemical equation is HNO 3 (aq) + NaOH(aq) NaNO 3 (aq) + H 2 O(l) H n = 57.3 kj mol 1 Learning tip In the thermochemical equation representing the standard enthalpy change of neutralization, the coefficient of water must be 1. Key point Standard enthalpy change of neutralization is the enthalpy change when one mole of water is produced from neutralization between an acid and an alkali under standard conditions. 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 18 / 66
During neutralization, H + (aq) ions from the acid react with OH (aq) ions from the alkali to form water molecules. H + (aq) + OH (aq) + + H 2 O(l) Figure 34.6 H + (aq) ions and OH (aq) ions combine to form water molecules during neutralization. As nitrate ions and sodium ions are spectator ions, the reaction can be represented by the following ionic equation: H + (aq) + OH (aq) H 2 O(l) H n = 57.3 kj mol 1 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 19 / 66
As neutralization is an exothermic reaction, the standard enthalpy change of neutralization is always negative. H + (aq) + OH (aq) Enthalpy H n = 57.3 kj mol 1 H 2 O(l) Figure 34.7 A diagram showing the relative enthalpies of the species in the neutralization between a strong acid and a strong alkali. 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 20 / 66
Acid Alkali H n (kj mol 1 ) HNO 3 (aq) KOH(aq) 57.3 HNO 3 (aq) NaOH(aq) 57.3 H 2 SO 4 (aq) NaOH(aq) 57.3 HCl(aq) NaOH(aq) 57.3 CH 3 COOH(aq) NaOH(aq) 55.2 HCN(aq) NaOH(aq) 11.6 HCN(aq) NH 3 (aq) 5.4 CH 3 COOH(aq) NH 3 (aq) 51.5 HCl(aq) NH 3 (aq) 52.2 Table 34.2 Standard enthalpy changes of neutralization ( H n ) for different combinations of acids and alkalis. 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 21 / 66
All neutralization reactions between strong acids and strong alkalis are reactions between hydrogen ions and hydroxide ions. They release more or less the same amount of energy. The standard enthalpy changes for these neutralization reactions are about 57.3 kj mol 1. Example HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l) H n = 57.3 kj mol 1 1 1 H 2 SO 4 (aq) + NaOH(aq) Na 2 SO 4 (aq) + H 2 O(l) H n = 57.3 kj mol 2 1 2 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 22 / 66
Less heat will be released if a weak acid or a weak alkali is used. some energy has to be supplied for complete ionization of the weak acid or weak alkali. Example HCl(aq) + NH 3 (aq) NH 4 Cl(aq) H n = 52.2 kj mol 1 ammonia solution (a weak alkali) HCN(aq) + NaOH(aq) NaCN(aq) + H 2 O(l) H n = 11.6 kj mol 1 hydrocyanic acid (a weak acid) Think about Class practice 34.2 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 23 / 66
Standard enthalpy change of formation ( H f ) The enthalpy change of formation measured under standard conditions is known as the standard enthalpy change of formation. Standard enthalpy change of formation is often denoted by H f. The standard enthalpy change of formation of water is 285.8 kj mol 1. It means that 1 mole water is produced from the 1 reaction between 1 mole hydrogen and mole oxygen, 285.8 kj of heat is released. 2 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 24 / 66
The thermochemical equation is 1 H 2 (g) + O 2 (g) H 2 O(l) H f = 285.8 kj mol 1 2 Learning tip In the thermochemical equation representing the standard enthalpy change of formation, the coefficient of the substance that forms must be 1. Key point Standard enthalpy change of formation of a substance is the enthalpy change when one mole of the substance forms from its constituent elements in their standard states under standard conditions. 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 25 / 66 Think about
Standard enthalpy changes of formation can be positive or negative, depending on the heat content of the compound relative to that of its constituent elements. Examples The standard enthalpy change of formation of water is negative. The standard enthalpy change of formation of nitrogen dioxide is positive. The thermochemical equation is 1 N 2 (g) +O 2 (g) NO 2 (g) H f = +33.9 kj mol 1 2 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 26 / 66
1 H 2 (g) + O 2 (g) 2 Enthalpy H f = 285.8 kj mol 1 Enthalpy NO 2 (g) H 2 O(l) H f = +33.9 kj mol 1 1 N 2 (g) +O 2 (g) 2 (a) (b) Figure 34.8 Diagrams showing the relative enthalpies of the species involved in the formation of (a) H 2 O(l) and (b) NO 2 (g). 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 27 / 66
The standard enthalpy changes of formation of all elements in their standard states are zero. no heat change is involved when an element forms from itself. Examples H 2 (g) H 2 (g) H f = 0 kj mol 1 Br 2 (l) Br 2 (l) H f = 0 kj mol 1 Na(s) Na(s) H f = 0 kj mol 1 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 28 / 66
Elements Substance H f (kj mol 1 ) Substance H f (kj mol 1 ) C(graphite) 0 O 3 (g) +143.0 C(diamond) +1.9 Cl 2 (g) 0 H 2 (g) 0 Br 2 (l) 0 O 2 (g) 0 Na(s) 0 Ionic compounds Substance H f (kj mol 1 ) Substance H f (kj mol 1 ) NaCl(s) 411.0 CaO(s) 635.0 NaHCO 3 (s) 947.7 CaCl 2 (s) 795.0 Na 2 CO 3 (s) 1131 CaCO 3 (s) 1207 Na 2 SO 4 (s) 1385 (a) (b) 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 29 / 66 Think about
Covalent compounds Substance H f (kj mol 1 ) Substance H f (kj mol 1 ) H 2 O(l) 285.8 SO 2 (g) 297.0 H 2 O(g) 241.8 SO 3 (g) 395.5 HF(g) 268.8 C 6 H 12 O 6 (s) 1273 HCl(g) 92.3 C 12 H 22 O 11 (s) 2221 HBr(g) 36.2 CH 4 (g) 74.8 HI(g) +25.9 C 2 H 6 (g) 84.7 CO(g) 110.5 C 3 H 8 (g) 105.0 CO 2 (g) 393.5 CH 3 OH(l) 238.6 NO(g) +90.4 CH 3 CH 2 OH(l) 277.7 NO 2 (g) +33.9 (c) Table 34.3 Standard enthalpy changes of formation ( H f ) for some common (a) elements, (b) ionic compounds and (c) covalent compounds. 34.1 Standard enthalpy changes of combustion, neutralization and formation P. 30 / 66 Class practice 34.3
34.2 Determination of enthalpy changes using simple calorimetric methods Determination of enthalpy change of combustion The enthalpy change of combustion of a substance can be determined if the amount of heat released is found. This can be achieved by using simple calorimetric methods. A simple calorimeter can be used to determine the amount of heat released. P. 31 / 66
A metal can calorimeter (an example of a simple calorimeter) can be used to find the amount of heat released during combustion of ethanol. CH 3 CH 2 OH(l) + 3O 2 (g) 2CO 2 (g) + 3H 2 O(l) polystyrene board thermometer metal can water ethanol clamp stirrer spirit burner Figure 34.9 The experimental set-up for determining the amount of heat released by the combustion of ethanol. 34.2 Determination of enthalpy changes using simple calorimetric methods P. 32 / 66
Steps for determining the amount of heat released Step 1: Use a spirit burner to burn a certain mass of ethanol*. Step 2: Turn off the burner after 10 minutes. Step 3: Measure the maximum temperature rise of the water. Step 4: Calculate the amount of heat released during combustion. * The heat released in the combustion heats up the water in the metal can. This causes the temperature of water to rise. 34.2 Determination of enthalpy changes using simple calorimetric methods P. 33 / 66
Learning tip The specific heat capacity, c, of a solid or liquid is defined as the heat required to raise 1 g of the substance by 1 K (or 1 C). The specific heat capacity of water is 4.2 J g 1 K 1. Key point Heat released specific heat = mass of water capacity of water = m c T temperature change of water 34.2 Determination of enthalpy changes using simple calorimetric methods P. 34 / 66
The enthalpy change of combustion of ethanol can be determined by dividing the amount of heat released during combustion of ethanol by the number of moles of ethanol burnt. Key point Enthalpy change of combustion = Heat released during combustion Number of moles of the substance burnt Example 34.1 34.2 Determination of enthalpy changes using simple calorimetric methods P. 35 / 66
Calculate the heat released during the combustion Calculate the number of moles of the substance burnt Calculate the enthalpy change of combustion (kj mol 1 ) Figure 34.10 Steps for calculating the enthalpy change of combustion of a substance from experimental data. Experiment 34.1 Experiment 34.1 Class practice 34.4 34.2 Determination of enthalpy changes using simple calorimetric methods P. 36 / 66
Sources of error in determining the enthalpy change of combustion by simple calorimetric methods The experimental value of the enthalpy change of combustion of ethanol is 672 kj mol 1. The theoretical value of the standard enthalpy change of combustion of ethanol is 1370 kj mol 1 ). Why is there a difference? 34.2 Determination of enthalpy changes using simple calorimetric methods P. 37 / 66
Some sources of error 1. There was heat loss to the surroundings due to convection, conduction and evaporation. 2. The heat capacities of the metal can and the thermometer were not taken into account. Learning tip The heat capacity of a substance is the amount of heat required to raise the temperature of the substance by 1 K (or 1 C). 34.2 Determination of enthalpy changes using simple calorimetric methods P. 38 / 66
Some sources of error 3. Incomplete combustion occurred. In a limited supply of oxygen, some of the ethanol burnt incompletely to form soot (carbon) and carbon monoxide. 4. The theoretical value found in the data book is the standard enthalpy change of combustion. However, the experiment was not carried out under standard conditions. 34.2 Determination of enthalpy changes using simple calorimetric methods P. 39 / 66
The simple calorimeter is satisfactory for comparing H c values of a group of similar compounds because the errors for each experiment are similar. If an accurate value of the enthalpy change is to be found, a bomb calorimeter is used instead. Activity 34.1 34.2 Determination of enthalpy changes using simple calorimetric methods P. 40 / 66
Determination of enthalpy change of neutralization The enthalpy change of neutralization can be determined by directly measuring the temperature change of the reaction mixture. the heat released from the reaction = the heat absorbed by the reaction mixture. 34.2 Determination of enthalpy changes using simple calorimetric methods P. 41 / 66
Steps for determining the amount of heat released HCl(aq) + NaOH(aq) NaCl(aq) + H 2 O(l) Step 1: Mix equal volumes of 1.0 M dilute HCl(aq) and 1.0 M NaOH(aq) in an expanded polystyrene cup. SBA note Expanded polystyrene cups are good insulators of heat which can help minimize the heat loss from the reaction mixture to the surroundings. 34.2 Determination of enthalpy changes using simple calorimetric methods P. 42 / 66
thermometer stirrer expanded polystyrene cup HCl(aq) + NaOH(aq) lid beaker cotton wool (a) (b) Figure 34.11 (a) The apparatus used for constructing a simple calorimeter and (b) the simple calorimeter for determining the enthalpy change of neutralization. 34.2 Determination of enthalpy changes using simple calorimetric methods P. 43 / 66
Step 2: Stir the reaction mixture. Step 3: Measure the maximum temperature rise of the reaction mixture. Think about Step 4: Calculate the amount of heat released during the neutralization. 34.2 Determination of enthalpy changes using simple calorimetric methods P. 44 / 66
Key point Heat released = m c T where m = mass of the reaction mixture c = specific heat capacity of the reaction mixture (assuming that it is the same as that of water) T = temperature change of the reaction mixture 34.2 Determination of enthalpy changes using simple calorimetric methods P. 45 / 66
The enthalpy change of neutralization can be determined by dividing the heat released during neutralization by the number of moles of water formed. Key point Enthalpy change of neutralization = Heat released during neutralization Number of moles of water formed Learning tip As neutralization is an exothermic reaction, the value of the enthalpy change of neutralization should be negative. Example 34.2 34.2 Determination of enthalpy changes using simple calorimetric methods P. 46 / 66
Calculate the heat released during the neutralization Calculate the number of moles of water formed Calculate the enthalpy change of neutralization (kj mol 1 ) Figure 34.12 Steps for calculating the enthalpy change of neutralization from experimental data. Experiment 34.2 Experiment 34.2 Class practice 34.5 34.2 Determination of enthalpy changes using simple calorimetric methods P. 47 / 66
Sources of error in determining the enthalpy change of neutralization by simple calorimetric methods The experimental value of the enthalpy change of neutralization in Example 34.2 is 50.4 kj mol 1. The theoretical value of the standard enthalpy change of neutralization is 57.3 kj mol 1. Why is the experimental value less than the theoretical value? 34.2 Determination of enthalpy changes using simple calorimetric methods P. 48 / 66
Some sources of error 1. There was heat loss to the surroundings due to convection, conduction and evaporation. 2. The heat capacities of the expanded polystyrene cup and the thermometer were not taken into account. 3. The specific heat capacity of the reaction mixture was not the same as that of water. 4. The density of the reaction mixture was not the same as that of water. 5. The experiment was not carried out under standard conditions. 34.2 Determination of enthalpy changes using simple calorimetric methods P. 49 / 66
For more accurate work, the following items should be pre-determined. Heat capacity of the expanded polystyrene cup Heat capacity of the thermometer Specific heat capacity of the reaction mixture Density of the reaction mixture Example 34.3 34.2 Determination of enthalpy changes using simple calorimetric methods P. 50 / 66
Determination of enthalpy change of formation Direct determination of enthalpy changes of formation by simple calorimetric methods is difficult in most cases. The reasons are: some formation reactions are highly exothermic and cannot be safely carried out. the constituent elements are in different physical states and cannot be mixed well in a calorimeter. side reactions may lead to the formation of side products. 34.2 Determination of enthalpy changes using simple calorimetric methods P. 51 / 66
Key terms 1. calorimeter 量熱器 2. simple calorimetric method 簡單量熱法 3. standard conditions 標準條件 4. standard enthalpy change of combustion 標準燃燒焓變 5. standard enthalpy change of formation 標準生成焓變 6. standard enthalpy change of neutralization 標準中和焓變 P. 52 / 66
7. standard enthalpy change of reaction 標準反應焓變 8. standard state 標準態 9. thermochemical equation 熱化學方程式 Key terms P. 53 / 66
Progress check 1. What are the standard conditions for measuring standard enthalpy changes? 2. What is standard enthalpy change of reaction? 3. What is the unit of standard enthalpy change of reaction? 4. What is standard enthalpy change of combustion? 5. What is standard enthalpy change of neutralization? 6. What is the approximate value for standard enthalpy change of neutralization between a strong acid and a strong alkali? P. 54 / 66
7. Why is the value for standard enthalpy change of neutralization involving a weak acid or weak alkali smaller? 8. What is standard enthalpy change of formation? 9. How do we determine the standard enthalpy change of combustion by the simple calorimetric method? 10.How do we determine the standard enthalpy change of neutralization by the simple calorimetric method? 11.Why is the direct determination of enthalpy changes of formation difficult in most cases? Progress check P. 55 / 66
Summary 34.1 Standard enthalpy changes of combustion, neutralization and formation 1. The standard conditions adopted for thermochemical measurements are: a pressure of one atmosphere (1 atm or 101 325 N m 2 ), a temperature of 25 C or 298 K, a concentration of 1.0 mol dm 3 if the substance is present in solution, the substances involved are in their standard states. P. 56 / 66
2. Standard enthalpy change of a reaction is the enthalpy change when the numbers of moles of reactants represented by the thermochemical equation react completely under standard conditions. 3. Standard enthalpy change of combustion of a substance is the enthalpy change when one mole of the substance is completely burnt in oxygen under standard conditions. Summary P. 57 / 66
4. Standard enthalpy change of neutralization is the enthalpy change when one mole of water is produced from neutralization between an acid and an alkali under standard conditions. 5. Standard enthalpy change of formation of a substance is the enthalpy change when one mole of the substance forms from its constituent elements in their standard states under standard conditions. Summary P. 58 / 66
34.2 Determination of enthalpy changes using simple calorimetric methods 6. A calorimeter can be used to determine the amount of heat absorbed or released by the system in a process or reaction. 7. If the system of a combustion reaction is surrounded by water, the heat released during combustion is transferred to water. The heat transferred = mass of water specific heat of capacity of water temperature change of water Summary P. 59 / 66
8. The steps for calculating the enthalpy change of combustion of a substance are summarized below. Calculate the heat released during the combustion Calculate the number of moles of the substance burnt Calculate the enthalpy change of combustion (kj mol 1 ) Summary P. 60 / 66
9. In the determination of enthalpy change of combustion by simple calorimetric methods, there are some sources of error: (a) Heat is lost to the surroundings due to convection, conduction and evaporation. (b) The heat capacities of the metal can and the thermometer are not taken into account. (c) Incomplete combustion occurs. (d) The experiment is not carried out under standard conditions. Summary P. 61 / 66
10. A bomb calorimeter is a device for determining the enthalpy change of combustion of a substance accurately. 11. The enthalpy change of neutralization can be determined by directly measuring the temperature change of the reaction mixture. Summary P. 62 / 66
12. The steps for calculating the enthalpy change of neutralization are summarized below. Calculate the heat released during the neutralization Calculate the number of moles of water formed Calculate the enthalpy change of neutralization (kj mol 1 ) Summary P. 63 / 66
Concept map Standard enthalpy change Standard enthalpy change of combustion Standard enthalpy change of neutralization Standard enthalpy change of formation P. 64 / 66
Standard enthalpy change measured under Standard conditions by Simple calorimetric methods conducted in Calorimeter Concept map P. 65 / 66
Standard conditions Temperature = 298 K Pressure = 1 atm Substances in their standard states Concentration of solution = 1.0 mol dm 3 Concept map P. 66 / 66