Thermochemistry

Nature of Energy

Kinetic Energy E

k = ½mv2

m=mass (kg) v=velocity (m/s)

Potential Energy E

p = mgh

g = gravity (9.8 m/s2) h = height (m)

Chemical Energy

Units of Energy

Joule (J) – 1 kg-m2/s2

Small amount of energy so we use kJ Calorie (cal) – amount of energy needed to

raise the temperature of 1 gram of water by 1 degree Normally uses Cal = 1000 cal = 1kcal

1 cal = 4.184 J How many calories are there in 15 joules?

Transfer of Energy

To understand energy transfer we must define two quantities: System

Closed vs Open Surroundings

Energy transferred in two ways Work (w) – causes motion

W = F x d Heat (q) – causes temperature change

Energy is the ability to do work or transfer heat.

First Law of Thermodynamics

1st Law = Energy is conserved. Internal energy of a system =

sum of all kinetic and potential energy of the components of the system Internal Energy = E We measure ΔE = Efinal – Einitial

ΔE has two parts Magnitude = number, unit Direction = + or -

For a chemical reaction: What is the initial state? What is the final state?

Heat and Work

ΔE = q + w q > 0 when heat is

added to the system w > 0 when work is

done on the system Endothermic

q > 0 Exothermic

q < 0

Heat, Work and Internal Energy

Energy Practice

If hydrogen and oxygen gases are trapped in a cylinder and ignited water is produced. As the reaction occurs, the system loses 1150J of heat to the surroundings. The reaction also causes a piston to rise as the hot gases expand. The expanding gas does 480J of work on the surroundings as it pushes against the atmosphere. What is the change in internal energy?

Energy Practice

Calculate the change in the internal energy of the system for a process in which the system absorbs 140J of heat from the surroundings and does 85J of work on the surroundings.

Calculate ΔE and determine where the process is exo- or endothermic for the following cases. A system releases 133kJ of heat to the surroundings

and does 39 kJ of work on the surroundings The system absorbs 77.5kJ of heat while doing

63.5kJ of work on the surroundings

State Functions

State function = a function whose value only depends on its present condition not on the history of the sample

State Functions

ΔE = q + w

Enthalpy

Usually the only kind of work done by a chemical reaction is mechanical work Reactions normally

done at constant P

Zn(s)

+ 2H+(aq)

→ Zn2 +

(aq) + H

2(g)

W = -PΔV

Constant Volume vs. Pressure

ΔE = q + w W = -PΔV

qv= ΔE

qp = ΔE + PΔV

Enthalpy

H = enthalpy Enthalpy measures the heat flow in chemical

changes occurring at constant pressure H = E + PV ΔH = ΔE + PΔV ΔH = q

p

When is ΔH positive and when is it negative?

Exo- and Endothermic

Enthalpy Practice

Indicate the sign of the enthalpy change, ΔH, in each of the following processes carried out under atmospheric pressure and indicate whether the process is exo- or endothermic.

a) an Ice cube melts

b) 1g of butane (C4H

10) of combusted in sufficient

oxygen to give complete combustion to CO2 and

H2O

c) a Bowling is dropped from a height of 8 ft into a bucket sand

Enthalpy Practice

Suppose we confine 1g of butane and sufficient oxygen to completely combust it in a cylinder. The cylinder is perfectly insulating, so no hear can escape to the surrounding. A spark initiates combustion of the butane, which forms carbon dioxide and water vapor. If we used this apparatus to measure enthalpy change in the reaction, would the piston rise, fall, or stay the same?

Hint write a balanced reaction

Enthalpies of Reaction

ΔH = Hfinal

– Hinitial

Enthalpy of products – enthalpy of reactants

Heat of Reaction (ΔHrxn

) = Enthalpy change of the reaction 2H

2(g) + O

2(g) → 2H

2O(g) ΔH = -483.6 kJ

Thermochemical equation Enthalpy Diagram

Enthalpy Guidelines

Enthalpy is an extensive property CH

4(g) + 2O

2(g) → CO

2(g) + 2H

2O(l) ΔH = -890kJ

1 mol of CH4 w/ 2 mol of O

2 releases 890kJ

2 mol of CH4 w/ 4 mol of O

2 releases 1780kJ

How much heat is released when 4.50g of methane is burned in a constant pressure system?

Concept Practice

Hydrogen peroxide can decompose to water and oxygen by the following reaction: 2H

2O

2(l) → 2H

2O(l) + O

2(g) ΔH = -196kJ

Calculate the value of q when 5.00g of H2O

2(l)

decomposes at constant pressure.

Enthalpy Guidelines

The enthalpy change for a reaction is equal in magnitude but opposite in sign to ΔH for the reverse reaction.

The enthalpy change for a reaction depends on the state of the reactants and products.

Concept Practice

Consider the following reaction:

2Mg(s) + O2(g) → 2MgO(s) ΔH = -1204kJ

a) Is the reaction exo- or endothermic?

b) Calculate the amount of heat transferred when 2.4g of Mg(s) reacts at constant pressure.

c) How many grams of MgO are produced during an enthalpy change of -96.0kJ?

d) How many kJ of heat are absorbed when 7.50g of MgO(s) are decomposed into Mg(s) and O

2(g) at

constant pressure?

Homework Set 1

12, 17, 20, 21, 22, 25, 29, 34, 35, 41