Ch. 15: Energy and Chemical Change

15.2 Heat

Objectives

• Describe how a calorimeter is used to measure energy absorbed or released.

• Explain the meaning of enthalpy and enthalpy change in chemical reactions and processes.

Measuring Heat

• Heat changes that occur during chemical and physical processes can be measured accurately and precisely using a calorimeter.

• A calorimeter is an insulated device used for measuring the amount of heat absorbed or released during a chemical or physical process.

Calorimetry

• A known mass of water is placed in the insulated chamber.

• The amount of energy that this water absorbs or releases can be calculated from the change in temperature that occurs.

Example

qw = mCwT

q = (50.0 g)(4.184 J/g 0C)(6.8 0C) = 1,400 J

Practice Problems

• A sample of metal is heated and put into a calorimeter containing 125 g of water at 25.6 0C. The final temperature of the water is 29.3 0C. How much heat in J is absorbed by the water?

• If 335 g of water at 65.5 0C lost 9750 J of heat, what was the final temperature of the water?

• The temperature of a sample of water increases from 20 0C to 46.6 0C as it absorbs 5650 J of heat. What is the mass of the sample?

Determining Specific Heat

Remember: we calculated the water absorbs 1,400 J. How much energy does the lead release?

Determining Specific Heat

• The lead releases 1, 420!! qPb = -1,420 J.• If we know the mass of lead (150 g) and the

change in temperature of the lead (28.8 0C - 100 0C = -71.2 C0), we can calculate the specific heat of lead.

qPb = mCPbTCPb = q/mT

C = -1,400 J/(150 g)(-71.2 0C)C = 0.13 J/g 0C

Specific Heat Practice Problems• To solve:

– Use Q = mwCwΔTw to determine the amount of heat the water absorbs.

– This is the same amount of heat the metal releases. (Just change the sign!)

– Use the heat, mass of metal (mm), and temperature change of metal (ΔTm) to find the specific heat of the metal (Cm).

– Determine the identity of the metal through its specific heat.

Chemical Energy and the Universe

• Every chemical reaction and change of physical state either releases heat (is exothermic) or absorbs heat (is endothermic).

Chemical Energy and the Universe

• Thermochemistry is the study of heat changes that accompany chemical reactions and phase changes.– The system is the specific part of the universe

that contains the reaction or process you wish to study.

– Everything else in the universe is considered the surroundings.

– The universe = system + surroundings

Chemical Energy and the Universe

• Consider heat flow in exothermic and endothermic reactions or processes

• In summary,

(EXOTHERMIC)

(ENDOTHERMIC)

Enthalpy and Enthalpy Changes

• The total amount of energy a substance contains is impossible to measure.

• But, for many reactions, the amount of energy lost or gained (CHANGE in energy) can be measured conveniently in a calorimeter.

• Reactions in a calorimeter or any lab take place at a constant atmospheric pressure.

Enthalpy and Enthalpy Changes

• Chemists use the term enthalpy (H) to represent the heat content of a system at constant pressure.

• Therefore, the change in enthalpy (H) is the heat absorbed or released in a chemical reaction at constant atmospheric pressure.

Hrxn is the difference between the enthalpy of the substances present at the end of a reaction and the enthalpy of the substances present at the beginning.

Enthalpy and Enthalpy Changes

• In other words, the difference between the heat contained in the products and the heat contained in the reactants is the enthalpy (heat) of reaction.

• Hrxn = Hproducts - Hreactants

Note

• Recall q was defined as the heat lost or gained in a chemical reaction or process.

• If the reaction takes place at constant pressure, q = Hrxn.

• (Therefore, Hrxn = mCT)

Enthalpy and Enthalpy Changes4Fe(s) + 3O2(g) 2Fe2O3(s) + 1625 kJ

• According to the equation, the reaction is exothermic - energy is written as a product.

• Therefore, Hreactants has to be greater than Hproduct and Hrxn has to have a negative sign.

• The enthalpy change for this reaction can then be indicated by the notation:

Hrxn = -1625 kJ

Enthalpy and Enthalpy Changes

Enthalpy and Enthalpy Changes

NH4NO3 + 27 kJ NH4+ (aq) + NO3

- (aq)

• This reaction is endothermic since energy is a reactant.

• Therefore, Hproducts has to be greater than Hreactant and Hrxn has to have a positive sign.

Hrxn = 27 kJ

Endothermic Enthalpy Change