ThermochemistryChapter 12

Thermochemistry

The study of the changes in the heat of chemical reactions.

Heat – the energy that is transferred from one object to another due to a difference in

temperature.

Bonds breaking requires energyBonds forming releases energy

Therefore…

Almost all chemical reactions absorb or release energy as heat.

Exothermic Reaction – Gives off more energy forming bonds than breaking bonds. (feels hot)

Endothermic Reaction – Requires more energy to break bonds than to form bonds. (feels cold)

Enthalpy

• The heat content of a chemical system.• Enthalpy = the amount of heat released or

absorbed when a chemical reaction takes place at constant pressure.

• Enthalpy depends on: temperature, physical state, and composition

Energy Vs. Enthalpy

• Very similar numerically.• Enthalpy is energy in a system at a constant

pressure.• Enthalpy is much more useful, most everyday

chemical rxns take place at constant pressure.

Enthalpy Change

H = HProducts - HReactants

• Measured in KJ• Comparisons are made at 1atm of pressure,

25°C (298K), & in its standard state of matter.

Standard State of an Element

• The most stable form of an element under standard conditions.

• Standard Enthalpy Change – change that is measured when reactants in their standard state change to products in their standard state - H°

Endothermic Vs Exothermic

• Energy is absorbed• Rxn Vessel becomes

cooler• Temp inside vessel

decreases• Energy of Reactants <

Energy of Products• + H

• Energy is released• Rxn Vessel becomes

warmer• Temp inside vessel

Increases• Energy of Reactants >

Energy of Products• - H

Writing Enthalpy Changes

N2(g) + 3H2(g) ----> 2NH3(g) H = - 92.4 kJ

Can Reverse Equation2NH3(g) -----> N2(g) + 3H2(g) H = + 92.4 kJ

_____________________________________________92.4 kJ of energy is released for every 1 mole of N2(g)

& 92.4 kJ of energy is released for every 3 moles of H2(g)

& 92.4 kJ of energy is released for every 2 moles of NH3(g)

produced.

1. How much energy is released if only 1 mole of ammonia (NH3) gas is produced?

2. How much energy is released if 10 moles of nitrogen (N2) gas and 30 moles of hydrogen (H2) gas is used in the reaction?

Hess’s Law

Hess’s Law states that the heat of a whole reaction is equivalent to the sum of it’s

steps.

Rules for Hess’s Law

1. If the coefficients of an equation are multiplied by a factor, the enthalpy change for the reaction is multiplied by the same factor.

2. If an equation is reversed, the sign of enthalpy change reverses also.

Examples

C + O2 CO2 Can occur as 2 steps

C + ½O2 CO H = – 110.5 kJCO + ½O2 CO2 H = – 283.0 KJC + CO + O2 CO + CO2 H = - 393.5 KJ

C2H4(g) + H2O(l) C2H5OH(l) Can Occur as 2 Steps

C2H4(g) + 3O2(g) 2CO2(g) + 2H2O(l)H=-1411.1 KJ

2CO2(g) + 3H2O(l) C2H5OH(l) + 3O2(g) H=+1367.1 KJ

C2H4(g) + H2O(l) C2H5OH(l) H= +44 KJ

More Practice @ http://www.saskschools.ca/curr_content/chem30/modules/module3/lesson5/hessmore.html

Calorimetry

• The study of heat flow and heat measurements

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

• Temperature change depends on the amount of heat released & the heat capacity of the surroundings.– Heat Capacity = Amt of heat necessary to raise the

temp by 1 °C• Heat capacity is effected by composition and mass

- Specific Heat(C) = Heat capacity of 1g of a substance

Specific Heat of Water

Water has one of the largest specific heats.

Specific Heat of H2O = 4.184J/g°C

4.184KJ/g°C = 1 calorie

calorieamt of heat needed to raise 1g of H2O, 1°C

Heat vs Temperature Change

A transfer of heat is detected by measuring a temperature change, however, a small

temperature change does not always signify a small transfer of heat.

Using Calorimeters

• q is used to denote heat measurements made in a calorimeter

• q of the surrounding is equal and opposite in magnitude to heat of the rxn : qrxn = -qsur

• To determine amt of heat absored by water qsur(water) = m x C x (Tf - Ti)