HeatCauses of Change

Chapter 10

Chemistry 5.0

Energy Transfer

• Heat-energy that is transformed from one object to another due to a difference in temperature. (symbol for heat = q)

• Temperature = a measure of the average kinetic energy of the particles in a substance. Temperature is an intensive property, and heat is an extensive property.

• Thermochemistry – the study of heat changes in a chemical reaction.

Exothermic Reactions

• Release heat into the surroundings

• Heat is a product of the reaction

• Combustion reactions are exothermic

• C3H8 + 5O2 → 3CO2 + 4H2O + 2043kJ

Endothermic Reactions

• Heat is absorbed by the reactants and stored in the chemical bonds of the product.

• Heat acts as a reactant.

• C + H2O + 113kJ → CO + H2

Molar Heat Capacity

• The heat absorbed or released during a reaction depends on a difference in a quantity called enthalpy. (Total energy content of a sample.)

• The symbol for enthalpy is H.• When reactions take place at standard

temperature and pressure, q = H.• Stand. temp. = 25°C Stand. Pres. = 1 atm• Purest form of a substance = most stable form• Enthalpy change at STP denoted H°

Enthalpy Change ∆H = H products – H reactants

Type of reaction

∆H H products H reactants

Exothermic

Endothermic

-

+ > <

><

Exothermic Reaction

Endothermic Reaction

• What is the energy of the reactants?• What is the energy of the products?• Is the forward reaction exothermic or

endothermic?• What is the ΔH for the forward reaction?• What is the ΔH for the reverse reaction?

150 kJ

50 kJ

exothermic

-100 kJ

100 kJ

kJ

Calorimetry

• The study of heat flow and measurement.

• Calorimetry experiments determine the heats of reactions by making accurate measurements of temperature changes produced by a calorimeter.

Calorimeter

Calorimeter

Heat and Temperature

• Heat Capacity – amount of heat needed to raise the temperature of an object 1°C.

• Specific Heat – amount of heat needed to raise 1g of a substance 1°C.

-Symbol for specific heat is C.

Heat and Temperature

• Formula for heat absorbed for released:

q = C x m x ∆T

• Remember: Specific Heat of Water =

4.184 J/g· °C

Food as Fuels

• Carbohydrates typically have higher enthalpies; however, the products of their combustion, CO2 and H2O, have lower enthalpies.

• Therefore, the combustion of carbohydrates, especially fats, is exothermic.

• Sugars and Starches break down to glucose, which reacts with O2 in a combustion reaction.

Nutritional information on food labels can be gathered using a calorimeter.

Hess’s Law

• Hess’s Law states that if a series of reactions are added together, the enthalpy change of the net reaction will be the sum of the enthalpy changes of the individual steps.

Steps for using Hess’s Law

• Identify the compounds

• Locate the compounds on the periodic table

• Write a reaction from the table.

• Write the appropriate “sub equation.”– If needed, multiply equation and enthalpy change.– If you reverse the reaction, change sign of enthalpy

change.

• Add equations.

• Add enthalpy changes.