Thermochemistry Section 17.1 - The flow of energy
What is Energy? Energy is the capacity for doing work or supplying heat Energy does not have mass or volume, and it can only be detected because of its effects Thermochemistry is the study of energy changes that occur during chemical reactions and changes in state When studying energy, a system is defined the part of the universe on which you focus your attention Surroundings are the part of the universe outside of the system
The law of conservation of energy In any chemical or physical process, energy is neither created nor destroyed If the energy of a system increases during this process, the energy of the surrounding must decrease by the same amount Every substance has energy stored inside it - the energy stored inside it is referred to as chemical potential energy The types of atoms and their arrangement determine the amount of chemical potential energy During a chemical reaction, the new substance(s) produced will have different amounts of chemical potential energy
Endothermic vs Exothermic processes Heat (q) refers to the energy that is transferred from one object to another because of a temperature difference between the two objects An increase in heat raises temperature Most chemical and physical processes involve either the absorption or release of heat The direction of heat flow is from the point of view of the system Heat flowing into the system from its surrounding - an endothermic process Generally flows from warm objects to colder objects q has a positive value Heat flowing out of the system into the surroundings - an exothermic process q has a negative value
Endothermic vs Exothermic
How do you measure heat flow You cannot use units for temperature to measure heat flow, even though they are related A calorie is the amount of heat needed to raise 1g of pure water 1 C Heat flow can be measure in calories or joules In food we use Calories with an uppercase C 1000 calories - 1 Calorie = 1 kilocalorie The Joule is the SI unit of energy One joule raises the temperature of 1g of water 0.2390 C 4.184 J = 1 cal
Why does it hurt to walk around a pool barefoot in summer, yet the water feels cool...
Heat Capacity Heat Capacity: the amount of heat needed to increase the temperature by exactly 1 C It will depend on both the mass and composition The greater the mass, the greater the heat capacity The higher the heat capacity of a substance, the less that the temperature will change Water has a high heat capacity
Specific heat The specific heat of a substance is the amount of heat it takes to raise the temperature of a 1g of the substance 1 C It can be expressed using either joules or calories
Specific heat of water Water has a high specific heat It takes a lot of heat to raise the temperature of water, and water also releases a lot of heat as it cools Water in lakes and oceans absorb heat from the air on warm days and release it back on cool days - providing a moderate climate in coastal areas
How do you calculate specific heat C = specific heat M = mass Delta T = change in temperature in Celsius Q = heat in joules of calories (which one is used will determine units of c)
Measuring and Expressing Enthalpy changes Section 17.2
How do you measure the amount of energy flow? Calorimetry is the measurement of the heat flow into or out of a system during a chemical or physical process Calorimetry measurements take place at a constant pressure The Enthalpy of a system is an expression of the heat flow of the system at constant pressure The heat absorbed or released by a reaction at constant pressure is the same as the change in enthalpy ( H) At constant pressure H = Q, heat and enthalpy mean the same thing
How do you measure heat flow? You use a calorimeter - there are two types, constant pressure and constant volume In constant pressure calorimetry, the reacting chemicals (the system) are dissolved in known amounts of water, measure initial temperature and then combine in a foam cup. When reaction is complete measure final temperature of the solution.
Key points about calorimetry The heat absorbed by the surroundings is equal to, but has the opposite sign of the heat released by the system qsys = -qsurr H is positive for an endothermic reaction H is negative for an exothermic reaction
Constant volume calorimetry These experiments are conducted in a bomb calorimeter The sample is placed in a constant-volume chamber (the mob) The sample is put under high pressure and and burned in the presence of oxygen The heat released warms water surrounding the chamber The temperature increase is measured, and used to calculate the amount of heat released
Thermochemical equations A chemical equation that includes the enthalpy change is a thermochemical equation Physical states of reactants and products must be included in the equation The Heat of Reaction is the enthalpy change for a chemical reaction exactly as it is written
Example - baking soda decomposition
The heat of combustion The heat of combustion is the heat of reaction for the complete burning of 1 mole of a substance With methane combustion it is an exothermic process Burning 1 mole of methane releases 890 kj of energy Heats of reaction are reported as enthalpy changes when reactions are carried out at standard temperature and pressures
Heat in Changes of state Section 17.3
What happens to the temperature of the water when ice melts?
The Heat of Fusion All solids absorb heat as they change from a solid to a liquid Whenever a change of state occurs, the temperature remains constant The heat absorbed by 1 mole of a solid substance as it melts to a liquid at a constant temperature is the molare heat of fusion ( H fus) Melting of ice to liquid, both at 0 C, requires the absorption of 6.01 kj of heat the molar heat of fusion of water The gain of heat causes a change of state instead of a change in temperature If excess heat is applied, the temperature of the water produced will rise above 0 C Melting/fusion is an endothermic process, therefore H fus values are positive
What about when something solidifies? The same is true, the temperature will not change when a substance changes from a solid to a liquid The molar heat of solidification ( H solid) is the heat released when 1 mole of a liquid substance solidifies at a constant temperature The amount of heat absorbed by a melting solid is equal to the amount of heat released when the liquid solidifies H fus = - H solid
What about turning from a liquid to a gas? Vaporization and Condensation are reverse processes that occur in a similar way to solidification and melting, but at the substances boiling point Molar heat of vaporization ( H vap ) - The heat absorbed when 1 mole of a liquid substance vaporizes at a constant temperature Molar heat of Condensation ( H cond ) - The heat released when 1 mole of a vapor condenses at its normal boiling points H vap = - H cond
Heat of a solution The molar heat of a solution ( Hsoln) is the enthalpy change that occurs when 1 moles of a substance dissolves This is what happens when hot packs are activated - CaCl2 mixing with water Heats of solution an be positive or negative depending on if it is an endothermic or exothermic process
Calculating heats of reaction Section 17.4
Hess s law Hess s law of summation- if you add two or more thermochemical equations to give a final equation then you can also add the heats of reaction to give the final heat of reaction Hess s law of heat summation allows you to use known heats of reaction to determine an unknown heat of reaction Can be useful if reaction occurs too slowly to measure, or if the reaction has a series of intermediate steps
How can Hess s law be used? When analysing the conversion of diamond to graphite this cannot be measured directly, but can be calculated using the enthalpy change of two specific reactions
Summating equations
Standard heats of formation Enthalpy changes during a process depend on the conditions during the process When comparing enthalpy changes, scientists specify a common set of conditions - referred to as a standard state The stable form of a substance at 25 Celsius and 101.3 kpa (1 atm) The standard heat of formation ( Hf ) of a compound is the change in enthalpy that accompanies the formation of 1 mole of a compound from its elements with all substances in their standard states
Standard heat of free elements Hf of a free elements in its standard state has been assigned as zero Any element that occurs as a diatomic molecule has a Example - H2, N2, O2, Cl2 Hf of zero
Standard heat of reactions Standard heats of formation provide another way to find a heat of reaction indirectly When a reaction occurs at standard conditions you can use standard heats of formation to calculate the heat of reaction - the enthalpy change is called the standard heat of reaction H = Hf (products) - Hf (reactants) Standard heats of formation of substances are known values, and all elements in their standard state Hf = 0 kj/mol
Common standard heats of formation