Thermochemistry

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[Chung Ming Hong]

Dear all,
I have several questions regarding Thermochemistry.

1. According to the Form 5 Chemistry textbook, the amount of heat given out in an exothermic reaction is denoted as 'H'. In the examination, is it strictly and absolutely compulsory that students use the same symbol, 'H'? I have seen a number of online sources and some books using other symbols, such as 'Q' to denote heat liberated during an exothermic reaction. Will marks be deducted if students write 'Q' in examinations?
Example: Q = 50 g × 4.2 J g^-1 °C^-1 × 5.0 °C
                = 1050 J

2. Is it acceptable for students to omit units in calculations until the final answer?
Example: Q = 50  × 4.2  × 5.0
                = 1050 J

3. Which of the following calculations are acceptable, given the data below?

ΔH = -50.4 kJ mol^-1
Number of Moles = 0.05 mol

Answer 1:
   Heat Change
= ΔH × Number of Moles
= 50.4 kJ mol^-1 × 0.05 mol
= 2.52 kJ
= 2520 J

Answer 2:
   Heat Change
= ΔH × Number of Moles
= 50.4 kJ mol^-1 × 0.05 mol
= 2.52 kJ
= 2520 J
= 2520 J heat released

Answer 3:
   Heat Change
= ΔH × Number of Moles
= -50.4 kJ mol^-1 × 0.05 mol
= -2.52 kJ
= -2520 J
= 2520 J heat released

4. According to Form 5 Chemistry textbook, temperature change, θ is defined as highest temperature, T_f - initial temperature, T_i. However, if a student reverses the order in the examinations as shown below, will marks be deducted?

Initial Temperature, T_i = 25.0 °C
Highest Temperature, T_f = 30.0 °C

Answer 1
   Temperature Change, θ
= T_f - T_i
= 35.0 °C - 25.0 °C
= 5.0 °C

Answer 2
   Temperature Change, θ
= T_f - T_i
= 35.0 °C - 25.0 °C
= 5.0 °C
= Temperature Increases by 5.0 °C

Answer 3
   Temperature Change, θ
= T_i - T_f
= 25.0 °C - 35.0 °C
= -5.0 °C

Answer 4
   Temperature Change, θ
= T_i - T_f
= 25.0 °C - 35.0 °C
= -5.0 °C
= Temperature Increases by 5.0 °C

5. What are the differences between monoprotic acids, diprotic acids, monobasic acids and dibasic acids? What does the 'basicity of an acid' refer to?

Thank you.

[Ting Tin Chung]

Dear all,
I have several questions regarding Thermochemistry.

1. According to the Form 5 Chemistry textbook, the amount of heat given out in an exothermic reaction is denoted as 'H'. In the examination, is it strictly and absolutely compulsory that students use the same symbol, 'H'? I have seen a number of online sources and some books using other symbols, such as 'Q' to denote heat liberated during an exothermic reaction. Will marks be deducted if students write 'Q' in examinations?
Example: Q = 50 g × 4.2 J g^-1 °C^-1 × 5.0 °C
                = 1050 J

2. Is it acceptable for students to omit units in calculations until the final answer?
Example: Q = 50  × 4.2  × 5.0
                = 1050 J

3. Which of the following calculations are acceptable, given the data below?

ΔH = -50.4 kJ mol^-1
Number of Moles = 0.05 mol

Answer 1:
   Heat Change
= ΔH × Number of Moles
= 50.4 kJ mol^-1 × 0.05 mol
= 2.52 kJ
= 2520 J

Answer 2:
   Heat Change
= ΔH × Number of Moles
= 50.4 kJ mol^-1 × 0.05 mol
= 2.52 kJ
= 2520 J
= 2520 J heat released

Answer 3:
   Heat Change
= ΔH × Number of Moles
= -50.4 kJ mol^-1 × 0.05 mol
= -2.52 kJ
= -2520 J
= 2520 J heat released

4. According to Form 5 Chemistry textbook, temperature change, θ is defined as highest temperature, T_f - initial temperature, T_i. However, if a student reverses the order in the examinations as shown below, will marks be deducted?

Initial Temperature, T_i = 25.0 °C
Highest Temperature, T_f = 30.0 °C

Answer 1
   Temperature Change, θ
= T_f - T_i
= 35.0 °C - 25.0 °C
= 5.0 °C

Answer 2
   Temperature Change, θ
= T_f - T_i
= 35.0 °C - 25.0 °C
= 5.0 °C
= Temperature Increases by 5.0 °C

Answer 3
   Temperature Change, θ
= T_i - T_f
= 25.0 °C - 35.0 °C
= -5.0 °C

Answer 4
   Temperature Change, θ
= T_i - T_f
= 25.0 °C - 35.0 °C
= -5.0 °C
= Temperature Increases by 5.0 °C

5. What are the differences between monoprotic acids, diprotic acids, monobasic acids and dibasic acids? What does the 'basicity of an acid' refer to?

Thank you.

Dear Chung MH,
1. The symbol of heat change : E / Q / H all can be accepted. NO MARK will be deducted.... Dont worry about this.
2. Just put down unit at final answer.
3. Just use the first method. Dont use the rest even it is correct.
4. Always rememeber that  θ is refer to temperature changes : final temp - initial temp
    For Exothermic reaction (where final temp[35°C] higher than initial temp [30°C] ) = 35 - 30 = 5°C
    For Endothermic reaction (where final temp [25°C] lower than initial temp [28°C]) = 25 - 28 = -3°C , then during
    calculation dont bring the negative sign!!
5. monoprotic acid is the same as monobasic acid!!
   More detail is like this : Monoprotic acid means when the acid dissolve in water, it donates one proton (H⁺)     
                during dissociation.
               Monobasic acid means it only have one hydrogen ion can donate during dissociation.
                hydogen ion = proton....... so..... same

Hope this may help. Have a nice day

[Wong Hee Ting]

Mr Ting,

Simple and good explanation.

Just number 4,

4. Always rememeber that  θ is refer to temperature changes : final temp - initial temp
    For Exothermic reaction (where final temp[35°C] higher than initial temp [30°C] ) = 35 - 30 = 5°C
    For Endothermic reaction (where final temp [25°C] lower than initial temp [28°C]) = 25 - 28 = -3°C , then during
    calculation dont bring the negative sign!!

My opinion is the temperature change: highest temperature - initial temperature

Meaning that the answer would always be positive value because if in the endothermic reaction, the highest temperature would be the initial temperature, not the final temperature.

Dear Chung Ming Hong,

All the best to you!

[Ting Tin Chung]

Thank you Mr Wong..... 

[Jong Kah Yin]

Hi,
Just sharing of opinion

For essay question involving thermochemistry - emphasise the key word record the "highest temperature" for exothermic reaction and "lowest temperature" for endothermic reaction and not final temperature, as the final temperature will be room temperature again.

θ is just temperature change / temperature difference - should not have any +ve / -ve value.

In some books, monobasic acid is the same as monoprotic acid, but in local books, monoprotic / diprotic are generally used for acids whereas for alkali we use the term monobasic and dibasic for alkali
Another definition of monoprotic acid is an acid that produces 1 mole of hydrogen ions from 1 mole of acid when dissociate completely in water

Welcome comments / sharing, thank you

[Wong Hee Ting]

Hi,
Just sharing of opinion

For essay question involving thermochemistry - emphasise the key word record the "highest temperature" for exothermic reaction and "lowest temperature" for endothermic reaction and not final temperature, as the final temperature will be room temperature again.

θ is just temperature change / temperature difference - should not have any +ve / -ve value.

In some books, monobasic acid is the same as monoprotic acid, but in local books, monoprotic / diprotic are generally used for acids whereas for alkali we use the term monobasic and dibasic for alkali
Another definition of monoprotic acid is an acid that produces 1 mole of hydrogen ions from 1 mole of acid when dissociate completely in water

Welcome comments / sharing, thank you

Thanks Mr Jong.

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