Unit 13 Thermochemistry

Energy The ability to do work or cause a change Often measured in joules (J) Law of Conservation of Energy – energy is neither created nor destroyed It is often transferred & can change form

Enthalpy (H) Total energy content in a sample under a constant pressure The change in enthalpy (H) is often used interchangeably with heat (q) Often measured in joules

Units of Energy Energy can also be measured in calories calorie is amount of heat to change 1 g of water by 1 C Food Calories are kilocalories 1 Calorie = 1 kilocalorie = 1000 calories 1 cal = 4.184 J 1 Cal = 4184 J

Heat in phase changes Remember, the temperature does not change during a phase change

Latent heat Heat absorbed or released by a substance during a process that does not involve a change in temperature Solid  Liquid = Latent Heat of fusion (Lf) Liquid  Gas = Latent Heat of vaporization (Lv) Varies for substances based on IMF & mass

Calculating latent heat Q=m Lf OR Q=m Lv How much energy does it take to melt 16 g of ice at it’s melting point? The Lf of ice is 80 cal/g Q = m Lf Q = 16g * 80 cal/g Q = 1280 calories OR 1.280 Calories

Molar heat of fusion ( H fus) Heat absorbed by 1 mole of a solid as it melts to a liquid at a constant temperature Molar heat of solidification ( H solid) Heat lost when 1 mole of a liquid becomes a solid ( H fus) = ( H solid) but opposite sign

Using molar heat of fusion How many grams of ice at 0 deg C melts if 2.25 kJ heat added? The molar heat of fusion of water is 6.01 kJ/mol. 2.25 kJ g mol 1 6.01 mol kJ 18 1

Molar heat of vaporization Amount of heat to vaporize 1 mole of a substance Equal but opposite to the molar heat of condensation

UNDERSTANDING CHECK The heat of vaporization of water is 40.79 kJ/mol. How much heat is required to turn 14 g of water into vapor at the boiling point?

Heat (q) Heat – energy transfer between objects at different temperatures Goes from warmer object to cooler object and not vice-versa Temperature (T) – measure of how hot or cold OR measure of average kinetic energy Higher temp = faster movement

Heat vs. Temperature Heat is an EXTENSIVE PROPERTY Physical property that depends on the amount of sample present Match vs campfire Temperature is an INTENSIVE PROPERTY Physical property that does not depend on the amount of sample

Exothermic Reaction Surroundings System Energy Change in matter where E is released, often as heat Surroundings get hotter q or H of the system is negative System Surroundings Energy

Exothermic Reactions Joules of energy created are written with the products of the balanced reaction On a graph, the products are lower in energy than the reactants Makes sense because energy is RELEASED during the reaction

C + O2 ® CO2 + 395 kJ Energy Reactants Products ® C + O2 -395kJ CO2

Endothermic Surroundings System Energy Change in matter in which energy is absorbed, often as heat Surroundings get cooler q or H of the system is positive System Surroundings Energy

Endothermic Joules of energy absorbed are written with the reactants of the balanced reaction On a graph, the products are higher in energy than the reactants Makes sense because energy is ABSORBED during the reaction

CaCO3 + 176 kJ ® CaO + CO2 CaCO3 ® CaO + CO2 Energy Reactants Products

Heat capacity (c) How much heat it takes to heat an object by 1C Affected by identity of the substance (sand vs. water) & amt present (ocean vs. pan)

Heat capacity Specific heat capacity (c) is the amount needed to heat 1 g of a substance by 1C Affected by identity of the substance The higher the specific heat, the more energy it takes to change its temperature Formula q = m c T For water, c = 1 cal/gºC or 4.186 J/gºC

How much heat is needed to change the temperature of 12 g of silver with a specific heat of 0.057 cal/gC from 25C to 83 C? Q = m c T m = 12 g c = 0.057 cal/gC T = 58 C (no need to change to Kelvin)

If you put 6500 J of heat into a 15 g piece of Al at 25 C , what will the final temperature be? ( c = 0.90 J/gC ) Q = m c T m = 15 g c = 0.90 J/gC Q = 6500 J

Applying specific heat Equal masses of copper and water are heated until the temperature is increased by 10 degrees. Why did the copper’s temperature increase more quickly? Water has a very HIGH heat capacity and therefore changes temp SLOWLY Walking on water or sand on a hot day?

1st law of thermodynamics Law of Conservation of Energy – The change in the internal energy of a system is equal to the amount of energy added by heating minus the amount lost by the work done by the system Eating a cheeseburger gives you the amount of energy equal to what’s in the burger minus the energy used to chew & digest

Calorimetry Measures heat using a device called a calorimeter Measure temp before and after reaction Since the change in enthalpy (H) can be used interchangeably with heat (q), we can use the equation q = m c T Water c = 1 calorie/gram * degree Celsius c = 4.184 Joule/gram * degree Celsius

Bomb calorimeter

Coffee Cup Calorimeter

Example A chemical reaction is carried out in a coffee cup calorimeter. There are 75.8 g of water in the cup, and the temperature rises from 16.8 ºC to 34.3 ºC. How much heat was released?