Calorimetry

Calorimetry

The enthalpy change associated with a chemical reaction or process and the specific heat of a substance can be measured experimentally using calorimetry.The experimental measurement of

the amount of heat gained or lost Determined by measuring the

temperature change that occurs.

Calorimeter:An instrument used to measure the

heat gained or lost

CalorimetryTwo types of calorimetry are commonly

used:

Constant-pressure CalorimetrySpecific heatHrxn or Hsoln

Bomb Calorimetry (Constant Volume Calorimetry)Hcombustion

Calorimetry In constant-pressure calorimetry,

Measurements are made in an “open” container. Reactions occur at constant pressure

Calorimeter is insulated Assume that the amount of heat

gained from or lost to the surroundings is negligible any heat gained or lost during a chemical reaction or process comes from or goes into the solution being studied.

Calorimetry

In order to measure the specific heat of a substance, we need to know three values:qsubstance

massT

In order to measure a molar enthalpy change for a reaction, we need to know two values:qsubstance or qreactant

moles of substance or reactant

Cs = qsubst

mass x T

H = qsubst

moles subst.

Calorimetry

In both cases, the “unknown” that must be determined experimentally is the amount of heat gained or lost by the substance being studied (i.e. qsubstance).

In constant-pressure calorimetry, qsubstance is determined indirectly by measuring the heat gained or lost by the liquid present in the calorimetry (whose specific heat is known).

Calorimetry

We then apply the First Law of Thermodynamics: If heat is lost by the chemicals (or

object) during a reaction or process, then it must be gained by the solution (liquid) and vice versa.

The heat gained or lost by the reactants (or object) and the solution (liquid) are equal in magnitude but opposite in sign.

qobject = - qsoln or qrxn = -qsoln

Calorimetry Calorimetry Simulation:

Measuring the specific heat of a metal

How is the experiment performed?

What kind of data is collected?

How are the data used to determine specific heat?

http://group.chem.iastate.edu/Greenbowe/sections/projectfolder/animationsindex.htm

Calorimetry Determining specific heat or molar

heat capacity experimentally:Heat an object with a known mass to

95-100oC.Measure initial temperature of

object.Place known mass of water into

calorimeter and measure its temperature.

Add hot object to the waterMeasure the equilibrium

temperature (i.e. final temperature) of the water and object.

Calorimetry Types of data obtained from or used in a

calorimetry experiment often include:Sample Data

Mass of sample (object or compound)

Tinitial (sample) Tfinal (sample)

Solution Data Mass of solution Tinitial (solution) Tfinal (solution) Csoln

Often use specific heat of water if water is the solvent

Calorimetry Using the data to find specific heat:

Calculate the amount of heat gained by the water: qwater = Cwater x mwater x Twater

Calculate the amount of heat lost by the object qobj = - qwater

Calculate specific heat of object Cs = qobj

mobj x Tobj

CalorimetryExample: A 55.0 g piece of aluminum metal at 100.0oC was added to 51.3 g of water at 20.0oC. The equilibrium temperature of the system was 35.0oC. If the specific heat of water is 4.18 J/g.K, what is the specific heat of aluminum?

Calorimetry

Calorimetry This is the same procedure you will use

to calculate the specific heat of the metal sample you use in the lab.

Follow this procedure to do the calculations for Expt. 2.

You should expect a similar problem on your exam!

Calorimetry Calorimetry Simulation:

Measuring the molar heat of solution (Hsoln)

How is the experiment performed?

What kind of data is collected?

How are the data used to determine molar heat of solution?

Calorimetry Determining molar heat of solution

(Hsoln per mole of solute) from calorimetry data.Place known mass of water into

calorimeter.Measure the initial temperature of

the water.Add known mass of solute to water.Record equilibrium temperature of

the resulting solution.

Calorimetry Types of data collected when

measuring molar heat of solution:Mass of water Initial temperature of waterMass of soluteFinal temperature of solution

Calorimetry Determining Hsoln per mole of solute

from calorimetry data.

Assume Csoln = Cwater = 4.18 J/g-K unless told otherwise

Calculate the heat gained or lost by the entire solutionqsoln = Csoln x mass of solution x Tsoln

Mass of solution = mass of H2O + mass of solute

Calorimetry

Calculate heat gained or lost by the solute (salt)qsolute = - qsoln

Calculate molar heat of solution

Hsoln per mole = qsolute

mole solute

Calorimetry

Example: When a 2.125 g sample of solid ammonium nitrate dissolves in 30.000 g of water in a constant pressure calorimeter, the temperature drops from 22.0oC to 16.9oC. Calculate the molar Hsoln (in kJ/mole) for the dissolution process:

NH4NO3 (s) NH4+ (aq) + NO3

- (aq)

Assume that the specific heat of the solution is the same as pure water (4.18 J/g.K).

Calorimetry

Calorimetry You should expect a calorimetry

problem similar to this on your exam.

What would you actually observe when the dissolution process is exothermic?

What would you actually observe when the dissolution process is endothermic?

Calorimetry

For an exothermic process, the heat produced causes the temperature of the solution to increase. (Tsoln >0)

Chemical Chemical particleparticle

Heat released by chemical particle

q

q

qq

q

qq

Calorimetry

For an endothermic process, the heat gained comes from the reaction mixture and causes the temperature of the solution to decrease (Tsoln < 0)

Chemical Chemical particleparticle

Heat gained by chemical particle from the rxn mixture

q

q

qq

q

q

Calorimetry

Bomb CalorimetryConstant Volume

Calorimetry

Used to study combustion reactions

Measure Hcombustion

Calorimetry

As combustion occurs,Heat is releasedHeat is absorbed by the calorimeter

and its contentsTemperature of calorimeter &

contents increases.

The change in temperature of the calorimeter and its contents can be used to determine the heat of combustion.

Calorimetry To calculate the molar heat of combustion

from a calorimetry experiment: Calculate the heat absorbed by the

calorimeter and its contents using the heat capacity of the calorimeter:

qcal = Ccal x T (Notice that you do not need a mass term

because heat capacity has units of J/K)

Calculate the heat lost by the reactants:

qrxn = -qcal

Calorimetry Calculate molar heat of combustion

(Hcomb per mole reactant)

Hcomb = qrxn

mole reactant

Calorimetry

Example: When 2.00 g of methylhydrazine (CH6N2) is burned in a bomb calorimeter, the temperature of the calorimeter increases from 25.00oC to 32.25oC. If the heat capacity of the calorimeter is 7.794 kJ/oC, what is the molar heat of combustion for CH6N2?

2 CH6N2 (l) + 5 O2 (g) 2 N2 (g) + 2 CO2(g) + 6 H2O (g)

Calorimetry

Hess’s Law The heats of reaction (Hrxn) have been

measured and tabulated for many chemical reactions.

There are two approaches to determining the heat of reaction for a particular chemical reaction:CalorimetryUse tabulated Hrxn to calculate the

heat of reaction for another reaction of interest

Hess’s Law The enthalpy change for a reaction or

process is a state function:Depends only on the amount of

reactants and products used/formed and on their physical state

Does not depend on how the reaction was done. one step vs. multiple steps

Hess’s Law

Hess’s Law: If a reaction is

carried out in a series of steps, H for the overall (one-step) reaction is equal to the sum of the H’s for the individual steps.

Hess’s Law

IMPORTANT:When applying Hess’s Law, if you need

to multiply or divide the coefficients of an equation by a number then the H must also be multiplied by the same number H depends on amount of material

H2O (g) H2O (l) H = - 44 kJ

2 H2O (g) 2 H2O (l) H = 2 (- 44 kJ) = - 88 kJ

Hess’s Law IMPORTANT:

If a reaction has to be reversed, the magnitude of H stays the same but the sign must be reversed.

H2O (g) H2O (l) H = - 44 kJ

H2O (l) H2O (g) H = + 44 kJ

Hess’s Law

Example: Calculate the Hrxn for the incomplete combustion of C forming CO:

C (s) + ½ O2 (g) CO (g) H = ???

given the following reactions:

C (s) + O2 (g) CO2 (g) H = - 393.5 kJ

2 CO (g) + O2 (g) 2 CO2 (g) H = - 566.0 kJ

Hess’s Law

Hess’s Law

Example: Calculate Hrxn for

NO (g) + O (g) NO2 (g) H = ???

using the following thermochemical equations.

NO (g) + O3 (g) NO2 (g) + O2 (g) H = -198.9 kJO3 (g) 3/2 O2 (g) H = -142.3 kJO2 (g) 2 O (g) H = 495.0

kJ

Hess’s Law