l11 chapt7-1 web

Chemistry 5

Chapter-7 Thermochemistry Part-1 16 October 2002

Themodynamics–Introduction ƒ What is thermodynamics? natural gas + O2 Æ CO2 + H2O

A study of the energy associated with change. ƒ Why should you care? To understand why things happen: • Why does water evaporate? • Why do hot objects cool? • Why do some molecules react and others do not? • Why do some reactions give off heat and others not? The answers to these questions are related to energy!

We need to understand energy to understand chemistry and how reactions happen?

Thermodynamic Systems ƒ A beaker with liquid:

The beaker contains a liquid that is a chemical system, and the beaker is open to the environment.

ƒ How can this system be classified? Key Points: • System– liquid and beaker • Surroundings– the environment • System is open to Surroundings

Classification of Systems ƒ Three Cases:

Open

Closed

Isolated

Universe as a System?

ƒ How should it be classified?

The Universe is an Isolated System [Convention: system + surroundings = universe]

However, we are still interested in what happens within this isolated system and other systems; i.e., understand

ENERGY

Other Key Concepts ƒ Energy is the capacity to do work! • wound up spring releases energy as it unwinds • energy used to move a weight against gravity • Unit of energy is joule, J; 1J = 1kg.m2.s-2

ƒ Work is done when a force acts through as distance • raising a weight • moving a car • Work = force x distance {= [m(kg)xa(m.s-2)]xd(m) = kg.m2.s-2}

ƒ kinetic energy • is the energy of a moving object • k.e. = 1/2m.u2

ƒ potential energy • is the stored energy • tightly wound spring has high potential energy

ƒ Thermal energy • is the energy associated with random molecular motions

Heat ƒ Energy and Heat: Heat is the transfer of energy from regions of high temperature to regions of low temperature; for example, between system and surroundings. ƒ Heat transferred and temperature changes? The heat (q) for a given temperature change depends on • magnitude of the temperature change • quantity of substance • nature of substance

The amount of heat required to raise the temperature of 1 g water 1oC is 1 calorie; 1 cal = 4.184 J ƒ Demonstration:

Specific Heat ƒ Heat Capacity • The quantity heat required to change the temperature of a system by one degree is called the heat capacity. • On a per weight basis this is referred to as the specific heat capacity or specific heat. • For H2O, specific heat = 1 cal/g.K = 4.18 J/g.K ƒ Specific heat and quantity of heat A difference in temperature leads to energy transfer, with the heat (q) lost related to • sample mass, m • change in T, ∆T • specific heat capacity Specific heat = q/m.∆T or q = m x specific heat x ∆T ƒ Specific Heats of Substances:

Substance H2 O Al glass

Spec. Heat (J/g•K) 4.184 0.902 0.84

ƒ Conservation of Energy

qsys + qsurr = 0 or qsys = -qsurr

Heats of Chemical Reaction ƒ Demonstrations: • CaCl2 + H2O

Gives off heat to surroundings

• NH4Cl + H2O Takes heat from surroundings ƒ Heat of Reaction The heat of reaction, qrxn, is the quantity of heat transferred between system and surroundings during a chemical reaction. Heat reaction reflects changes in internal energies of reactants and products. ƒ Exothermic and Endothermic Reactions Exothermic– gives off heat to surroundings Endothermic-- takes heat from surroundings ƒ Calorimeters: Determining heats of chemical reactions. • Treat system– the cup and contents– as an isolated system • qrxn = -qcalorim

Heats of Chemical Reaction-2 ƒ 100 ml solutions of 1.00 M NaCl and 1.00 M AgNO3 at 22.4 oC are mixed in coffee cup calorimeter and the resulting temperature rises to 30.2 oC. What is chemical reaction and what is the heat per mole of product? 1. AgNO3(aq) + NaCl(aq) Æ AgCl(s) + Na+(aq) + NO3-(aq) 2. Determine heat of reaction:

qrxn = -qcalorim = - m x specific heat x ∆T Mass, m = volume solution x density (ca. d(H2O)) Specific heat = value for H2O = 4.18 J/g-K ∆T = Tfinal – Tinitial 3. To calculate heat/mole, determine moles of AgCl(s) produced: 0.100 L x 1.00 mol/L = 0.1 mol AgNO3 = 0.1 mol NaCl by reaction stoichiometry => 0.1 mol AgCl(s)