Thermochemistry Example Problems


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Example Number One

This problem is the equivalent of area number 3 on the time-temperature graph.

calculate the heat necessary to raise 27.0 g of water from 10.0 °C to 90.0 °C

The important factor about this problem is that a temperature change is involved. Therefore, the equation to use is:

q = (mass) (Δt) (Cp)

This summarizes the information needed:

Δt = 80.0 °C
The mass = 27.0 g
Cp = 4.184 Joules per gram-degree Celsius

Only one calculation is needed and it is:

q = (27.0 g) (80.0 °C) (4.184 Joules per gram-degree Celsius)

Multiply it out and round off to the proper number of sig figs.

By the way, you might want to think about how I knew to use the specific heat value for liquid water!!


Example Number Two

This problem is the equivalent of area number 3 and number 4 on the time-temperature graph.

How many kJ are required to heat 45.0 g of H2O at 25.0 °C and then boil it all away?

We must do two calculations and then sum the answers.

Calculation Number One uses this equation:

q = (mass) (Δt) (Cp)

This summarizes the information needed:

Δt = 75.0 °C
The mass = 45.0 g
Cp = 4.184 Joules per gram-degree Celsius

We then have:

q = (45.0 g) (75.0 °C) (4.184 Joules per gram-degree Celsius)

This gives an answer of 14121 J

Calculation Number Two uses this equation:

q = (moles of water) (ΔHvap)

This summarizes the information needed:

ΔHvap = 40.7 kJ/mol
The mass = 45.0 g
The molar mass of H2O = 18.0 gram/mol

Substituting, we obtain:

q = (45.0 g / 18.0 g mol¯1) (40.7 kJ/mol)

This gives 101.75 kJ. Adding 14.121 kJ and rounding off gives 115.9 kJ.


Example Number Three: 33.3 grams of ice at 0.00 °C has heat added to it until steam at 150.0 °C results. Calculate the total energy expended. (Hint: 4 calculations are needed - melt, raise, boil, raise.) Good luck. Go to answer.

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