Unit 9 - Thermochemistry

Thermochemistry

the study of energy changes that occur during chemical reactions and changes in state

Energy

The ability to do work or cause change

energy

Every substance has a certain amount of \n _______ stored inside it

Chemical Potential Energy

the energy stored in the chemical bonds of a substance

What determines the amount of energy in a substance?

The kinds of atoms and the arrangement of atoms in a substance

System

Part of the universe in which the event of interest occurs (chemical reaction)

Energy changes occur as either

heat transfer or work, or a combination of both

Heat

energy that transfers from one object to another because of a temperature difference between the objects

q

Heat is represented by

Heat is measured in?

Joules (J) or calories (cal)

warmer, cooler

Heat flows from a \_______ object to a \_________ object

equal

Heat will continue to move from a warmer to a cooler object until they are \________

Heat Flow

Another term for heat transfer, the transfer of energy from a warmer object to a cooler object

Units For Measuring Heat Flow

1 J = 0.2390 cal

4\.184J = 1 cal

calories

• The quantity of heat needed to raise the temperature of 1 g of pure water by 1°C

• represented by cal

Law of Conservation of Energy

• In any chemical or physical process, energy is neither created nor destroyed

• It is conserved; can be transferred/changed into a different kind

Endothermic Process

• Heat flows into the system from the surroundings; Heat is absorbed

• Heat flowing into a system has a positive q value

Exothermic Process

• heat flows from the system to the surroundings; Heat is released

• Negative q value

Enthalpy

• Def: The heat absorbed or released by a reaction at constant pressure

• Accounts for the heat flow of the system

Things to Know About Enthalpy

• The terms heat and enthalpy change are used interchangeably

• symbolized as H

• q = ΔH

△H

change in enthalpy

Heat Capacity

The amount of heat needed to increase the temperature 1 g of a substance by exactly 1°C

Specific Heat

Amount of heat it takes to raise the temperature of 1 g of a by substance 1°C (same thing as Heat Capacity)

Calculating Specific Heat Formula

• q \= m x c x ΔT

Calculating Specific Heat Formula Explanation

• q is heat in joules or calories

• m is mass(g)

• ΔT is the change in temperature: ΔT = Tfinal - Tinitial

• c (specific heat) is J/(g·°C) or cal/(g·°C)

greater, greater

The \__________ the mass of the object, the \_________ its heat capacity

Metals

\_______ generally have low specific heats

Water

\_________ has a very high specific heat compared with the other substances

Calorimetry

Measurement of the heat flow into or out of a system for chemical and physical processes

reactant, product

In a chemical equation, the enthalpy change for the reaction can be written as either a \_________ or a \__________

Thermochemical Equation

a chemical equation that includes the enthalpy change

Thermochemical Equation Examples

• CaO(s) + H2O(l) → Ca(OH)2(s) + 65.2 kJ

| (Exothermic reaction, shows amount of energy released)

\
• 2NaHCO3(s) + 85 kJ → Na2CO3(s) + H2O(l) + CO2(g)

| (Endothermic reaction, shows amount of energy absorbed)

Heat of Reaction

the enthalpy change for the chemical equation exactly as it is written

Heat of Reaction Examples

• CaO(s) + H2O(l) → Ca(OH)2(s) ΔH = -65.2 kJ

\
• 2NaHCO3(s) → Na2CO3(s) + H2O(l) + CO2(g) ΔH = 85 kJ

\
(guess which one is which)

exothermic, higher

In \_______________ processes, the chemical potential energy of the reactants is \__________ than the chemical potential energy of the products

Heat of Combustion

Heat of reaction for the complete burning of one mole of a substance

Molar Heat of Vaporization

the amount of heat absorbed by one mole of a liquid as it vaporizes at a constant temperature

Molar Heat of Fusion

the amount of heat absorbed by one mole of a solid substance as it melts to a liquid at a constant temperature

Heat of Fusion

Amount of energy required to change a substance from the solid phase to the liquid phase.

Heat of Vaporization

The amount of energy required for the liquid at its boiling point to become a gas

boiling

A liquid that absorbs heat at its \___________ point becomes a vapor

vaporization, melting

\_______________ takes more energy than \_______________

Standard Heat of Formation

the change in enthalpy that accompanies the formation of one mole of a compound from its elements with all substances in their standard states at 25 degrees celsius

Standard State

Enthalpy changes generally depend on the conditions of the process. Scientists specify a common set of conditions as a reference point. These conditions, called the standard state, refer to the stable form of a substance at 25°C and 101.3 kPa

standard, difference

For a reaction that occurs at \____________ conditions, you can calculate the heat of reaction by using standard heats of formation. The standard heat of reaction is the \___________ between the standard heats of formation of all the reactants and products

Heating Curve

* a diagram that shows the temperature changes and changes of state of a substance as it is heated
* Describes enthalpy changes that take place during phase changes