Chem: Unit 4 Chemical Thermodynamics

enthalpy of combustion

-burning a substance completely with oxygen -combistion reaction -enthalpy called ^

Antoine Lavoisier

1783- proposed that caloric moved from hot objects to cold objects

net enthalpy

= sume of enthalpies from inital combustion and the change of state

enthalpyy

Most chemical reactions occur at constant pressure and volume, so in chemistry—and in this course—we will refer to enthalpy as it relates to the thermal energy of the system (not work). Enthalpy cannot be measured directly.

heat capacity

substances vary in ability to store thermal energy -indication of how fast an object heats or cools -amt of heat needed to raise or lower an object's temperature

heat trasfer in ice water

-Ice molecules (-20°C) move more slowly than water molecules (21°C). -Fast-moving water molecules collide with the ice crystals and transfer thermal energy to them. -The ice crystal molecules vibrate faster and the ice temperature increases. -The water molecules move more slowly and the water temperature decreases. -As the ice crystals vibrate faster and faster, they eventually break apart and melt into liquid water. -The melted ice and the surrounding liquid reach some equal temperature that is between 0°C and 21°C.

thermochemical equation guidlines

-Like other chemical equations, thermochemical equations must be balanced. -The states of all reactants and products are noted in parentheses. -The change in enthalpy, or enthalpy of reaction (ΔH), is written at the right. (ΔH can be written in units of kJ or kJ/mol.) -Endothermic reactions have positive ΔH. -Exothermic reactions have negative ΔH.

measure of heat exchange

-calorie (cal) : the amount of heat it takes to raise one gram of water by one degree Celsius -joule (J) : one calorie is equal to 4.186 J -heat 1 cup of water to boiling you must add 18.7 kcal

hand warms

-cellulose bag, iron powder, water, vermiculite to absorb the water -water is contained in a thin inner bag surrounded by vermiculite0 iron powder -when you break the inner bag the water and oxygen from the air react and the iron begins to oxidize and rust 4Fe + 3O2 → Fe2O3 + heat -other types use liquid sodium acetate encased in a plastic bag with a metal disc, pressing the disk makes a crystal of sodium acetate and the liquid becomes a solid- exothermic reaction, reaction can be reversed by boiling the bag which will liquify the sodium acetate

energy in dynamite

-chemical bonds in nitroglycerin constanly move, bend, stretch and rotate -bonds have potential energy and when they break it is released -4C3H5N3O9(l) →6N2 (g) + 12CO2 (g) + 10H2O (g) + O2 (g)

4.01 Conservation of Energy

-how thermal energy works

4.11 Hess's Law

-lets one calculate change in enthalpy of a reaction by adding enthalpies of each step

E85 cars

-reduce the amount of co2 emitted -can burn ehtanol and gas mixtures 85% ehtanol and 15% gas -E85 or FlexFuel cars

cold packs

-thin inner bag of water surrounded by crystals of ammonium nitrate, when you break the inner bag water dissolves the ammonium salts NH4NO3 + heat → NH4+ + NO3- NH4Cl + heat → NH4+ + Cl- -reactions are endothermic, bag absorves heat from surroudning

ice cools a liquid

-water gets colder and ice gets warmer and metls -scientists thought that a weightless invisible fluid called caloric made objects hot or cold= hot objects had caloric, cold ones dont

steps

1. This equation will first be used to calculate the heat absorbed by the water: q = mCΔT 2. Because the calorimeter is a closed system, the heat absorbed by the water equals the heat given off by the unknown substance. 3. Once you know the heat given off by the unknown substance, you can use the same equation to determine its specific heat capacity (C).1

A commercial hand warmer contains 100 g of iron and warms according to the thermochemical equation shown here: 4Fe (s) + 3O2 (g) → 2Fe2O3 (s), ΔH = -3,926 kJ how much heat (q) will the hand warmer produce when all of the iron is oxidized?

1. convert delta H in kj to kj/mol ΔH(kJ/mol)=ΔH(kJ)/molFe 2. calculate number of moles iron oxidized molFe=(m)molar massFe (100g)(55.8g/mol) 3. multiply moles of Fe by delta H

How much heat is required to evaporate 200 g of ethanol at its boiling point (ΔHv = 43.5 kJ/mol, FW = 46 g/mol)? Given: m = 200 g, ΔHv = 43.5 kJ/mol, formula weight (FW) = 46 g/mol

1. convert mass to moles 200g x 46 g/mol 2. calculate heat q = mΔHν= (4.35 mol)(43.5 kJ/mol) = 189 kJ

stepsssss

1. write out balanced thermochemical equations 2. write balanced equation for target reaction 3. reverse any steps so that products/reactants match target reaction 4. scale the step reaction so products/reactions that don't appear will cancel out 5. add the delta H values 6. scale the resulting reaction so it matches target reactionIf necessary, multiply or divide the summed ΔH by the scale factor.

Alfred Bobel

1866 Swedish chemists mixed nitroglycerin and diatomaceous earth or sawdust to make dynamite -rich for the invention, set aside money to establish Nobel Prizes in science, peace, and literature

heat

= transfer of thermal energy

specific heat equation

C = q/m∆T C= specific heat capacity q = heat m = mass in grams ∆T= change in temperature

ethanol combution

C2H5OH (l) + 3O2 (g) → 2CO2 (g) + 3H2O (l), ΔH = -1367 kJ -Ethanol combustion is exothermic (negative ΔH ). -Burning ethanol yields only 25 percent of the heat energy that burning gasoline does. Even though less heat energy is produced, E85 engines are more efficient, allowing them to produce the same amount of mechanical work. -Burning ethanol also yields significantly less carbon dioxide than burning gasoline. This reduces greenhouse gas emissions, so E85 cars contribute less to global warming.

Enthalpy changes when water changes phase

Converting ice to steam is an endothermic process. Heat is absorbed as temperature changes, and is used when ice is converted to water and when water is converted to steam. Therefore, all of the ΔH values are positive (endothermic), and enthalpy increases. Converting steam to ice is an exothermic process. Heat is released as temperature changes, and is released when steam is converted to water and when water is converted to ice. Therefore, all of the ΔH values are negative (exothermic), and enthalpy decreases.

overview

Enthalpy (H) is the total heat content of a system at constant pressure. It cannot be measured directly. Changes in enthalpy (ΔH) occur during chemical reactions [ΔH = H(of products) - H(of reactants)]. Because most chemical reactions occur at constant pressure, ΔH is equal to heat released or absorbed (ΔH = q = mCΔT). In endothermic reactions, ΔH is positive. In exothermic reactions, ΔH is negative. Enthalpy changes that occur during phase changes in the absence of a change in temperature are called latent heat.

Exothermic and endothermic reactions are opposite enthalpy changes.

Exothermic Opens in modal popup window reactions have a negative ΔH because the products have less enthalpy than the reactants. Endothermic Opens in modal popup window reactions have a positive ΔH because the products have more enthalpy than the reactants.

total heat required to change ice from -100°C to steam at 200°C?

Heating ice from -100°C to 0°C: 20.5 kJ Ice melting at 0°C: 33.4 kJ Heating liquid water from 0°C to 100°C: 41.8 kJ Liquid water boiling at 100°C: 226 kJ Heating steam from 100°C to 200°C: 20.8 kJ Total heat required: 342.5 kJ -You used q = mCΔT, q = mΔHf for melting, and q = mΔHv for vaporization.

gasoline combustion produces heat energy

Here is the combustion reaction of gasoline (C8H18): 2C8H18 (l) + 25O2 (g) → 16CO2 (g) + 18H2O (l), ΔH = -10,941 kJ

The sign of ΔH tells whether the reaction is exothermic or endothermic.

If ΔH is negative, the reaction is exothermic. If ΔH is positive, the reaction is endothermic.

cold packs absorb heat by dissoving salts

NH4NO3 (s) → NH4+ (aq) + NO3- (aq), ΔH = +27.7 kJ/mol NH4Cl (s) → NH4+ (aq) + Cl- (aq), ΔH = +14.8 kJ/mol -The ΔH is positive, indicating an endothermic reaction. -Because the reaction involves dissolution in water, the enthalpy of reaction is given a special name: the enthalpy of solution or heat of solution (ΔHs). -Enthalpies of solution occur when many salts, strong acids, and strong bases are dissolved in water. -Not all dissolutions are endothermic. For example, anhydrous calcium chloride has ΔHs = -81.2 kJ/mol (exothermic).

aerobic respiration

Respiration that requires oxygen, consists of many chemical reactions

latent heat

So there must be a change in enthalpy that is "hidden," or not reflected by a change in temperature. This hidden enthalpy change is called latent heat Opens in modal popup window .

differences

Thermal energy: the internal kinetic energy of all the particles in any substance - Temperature: the measure of the thermal energy within any substance; temperature measures the average kinetic energy of the particles in a substance -Heat: the change in thermal energy of a substance; heat is measured by a change in temperature temperature increase → heat gained temperature decrease → heat lost

What is the enthalpy of reaction for the initial combustion of liquid methane, given the delta HΔH for vaporization is +88 kJ and the net delta HΔH is -890 kJ?

Write the equation for the combustion of methane: CH4l+2O2g→CO2g+2H2OgCH4(l)+2O2(g)→CO2(g)+2H2O(g) The net enthalpy is equal to the enthalpy of the initial combustion plus the enthalpy of vaporization: ΔHinitial+ΔHvaporization=ΔHnet Rewrite the equation to solve for the initial enthalpy: ΔHinitial=(ΔHnet−ΔHvaporization) Substitute and solve: ΔHinitial=−890kJ−88kJ=−978kJ

thermochemical equation

a chemical equation that includes the enthalpy change

calorimeter

a measuring instrument that determines quantities of heat -insulated to prevent heat exchange with the outside -closed system

endothermic reaction

absorbs energy, -decrease in temp of surroudning examples: -Separating ion pairs -Changes of state: solid → liquid → gas

heating of ice

all heat energy goes into increasing kinetic energy of the ice molecules to make them vibrate faster

finding enthalpy of fusion

amount of heat absorbed/ mass of ice melted

specific heat capacity

amt of heat energy it takes to raise the temp of one gram of a specific substance by one degree Celsius -mass of substanace -metals have low specifc heat -water and ammonia have high specific heats

ice melting

as you continue adding heat the ice melts but the temo remains the same- enthalpy of fusion -increases kinetic energy so that water molecules are strogner than intermolecular forces ΔHf

glucose

body converts starches and sugars into glucose C6H12O6 + 6O2→ 6CO2 + 6H2O, ΔH = -2,803 kJ/mol. -many chemical reactions to capture the energy in pieces

methane

burning it yield much less heat energy and co2 than other fuels -burning propare yield less than gasoline but more than methane or ethanol

notes on thermochemical reactions

changes of sate contribute to the delta H -when a reaction is reversible the delta H of the reverse reaction has the same value as but the opposite sign of the forward reaction -mutiply the chem equation by some factor you must mutiply delta H by same factor -delta H unde standard condition= ΔHº

not all of the heat energy does work

conversion of heat energy to mechanical work is not 100% efficient -some is wasted heat that is vented by the exhaust system and removed by the coolant system

dynamite

early 19th century chemists recognized unstable nitroglycerin as an explosive -too dangerous to handle because nitroglycerin was so unstable -invented in 1867 and replaced gunpower

total energy remains the same

energy before and after is the same- law of conservation of energy

Hess Law

enthalpy change for an overall reaction equals sum of enthalpies for each step

energy flows from high to low

faster moving molecules- slower moving molecules -hotter- colder

problem with methane and propane

fules are gases at room temperature -must be stored in a low-pressure tank to keep liquefied -as liquid travels to engine it gets vaporized in a regulator, mixed with air, and burned in the engine

A 50 g copper rod absorbs 585 J of heat to raise its temperature by 30°C. What is the specific heat of copper?

given: q - 585, m = 50 g, triT- 30 degrees C unknown: C C = q/mΔT Sustitute values C = (585 J)/(50 g)(30°C) = 0.39 J/g-°C

enthalpy of fusion

heat fusion of water -during the melting process absorbed heat energy breaks the intermolecular forces that hold the ice crystals together, all of that energy converts ice to liquid -no temp change during this process -temp remains at 0degrees C until all ice is converted to water -amount of heat energy required to convert all ice to water is the enthalpy of fusion of water, depends on the mass of ice

enthalpy of fusionn

heat involved in changing a solid to a liquid

thermal energy

interal kinetic energy, combined motion of all particles within a substance -measured using temperature= represents the average kinetic energy of all the particles -high temperatures particles move fast -low temps particles move slow -particles= ions, atoms, molecules, ionic compounds, atomic-level particles

liquid natural gases

liquid methane= CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (l), ΔH = -890 kJ/mol liquid propane= C3H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2O (l), ΔH = -2,220 kJ/mol

How much heat is released when 50 g of water changes temperature by 15ºC?

m = 50 g C = 4.18 J/(g·°C) delta T ΔT = 15°C q= ? q = (50 g)(4.18 J/g·°C)(15°C) q = 3,135 J or 3.14 kJ

law of conservation of mass and energy

mass and energy of the products must equal the mass and energy of the reactants.

melting

melting ice- solid ice absorbs heat energy to become liquid water- endothermic reaction

equilibrium

molecules in each compartment move at the same rate with equal temps

space shuttle

must eliminate heat -heat comes from internal electronics and external sunlight -cooling system like cars -heat exchanges circulate water or liquid ammonia (high specific heat substances) -then transfers heat to a heat exchange system of aluminum tubes filled with Freon

The change in heat is equal to the heat absorbed.

q = mCΔT m = mass in gramsC = specific heat of the substanceΔT = change in temparature (°C)

calculate heat released

q= mCΔT m= mass of water in grams C= specific heat capacity, liquid water= 4.18 J/(g-°C) ΔT= temp change of water in degrees celsius

exothermic reaction

released energy in the form of heat -measure release of heat by increase temperature of surroundingds exampls: Forming ionic salts Combustion/oxidation Mixing some salts with water Mixing water with concentrated acids Changes of state: gas → liquid → solid

combustion

releases energy, burning match- releases energy in the form of heat and light as wood burns -exothermic reaction

total heat released when steam at 200 deg changes to ice at -100

steam molecules slow down and condense into liquid water -change in temp values are negative -heat of condensation, heat of solidification Steam cools from 200°C to 100°C: -20.8 kJ Steam condenses at 100°C: -226 kJ Liquid water cools from 100°C to 0°C: -41.8 kJ Liquid water freezes at 0°C: -33.4 kJ Ice cools from 0°C to -100°C: -20.5 kJ Total heat released: -342.5 kJ

transferring heat

substances with high specific heats are good for transferring heat

heat the water

temp increases to 100 degC -kinetic energy increases but not enough to overcome all intermolecular forces

steam heats

temp increases to 200DegC -kinetic energy of H2) increases and molecules move farther apart The specific heat of water vapor is 2.08 J/g-°C.

cars burn gasoline to produce heat

the more heat energy made, the more work can be done -fuel that produces the most heat energy is ideal

Hess's Law

total amt of heat used or released by a complex reaction can be determined by adding the changed in enthalpy of each individual reaction If you have reactants that get converted into products, there is a change in enthalpy. If the reactants get converted to the same products by a series of chemical reactions (the steps), then the enthalpy change is the same. For each reaction in the series (each step), there is an associated enthalpy change. So the enthalpy change of each reaction in the series (each step) adds up to the enthalpy change of the overall reaction. This is the idea behind Hess's law.

Enthalpy

total heat content of a system at constant pressure

collisions of molecules

transfers thermal energy -molecules with higher temp collide causing the other molecules to move faste -once they have collided they start to slow down -molecules in the cooler liquid move faster after each collision

water boiles

water vaporizes but temp remains the same liquid-gas change= enthalpy of vaporization -heat energy increases kinetic energy of water so that it is greater than intermolecular forces ΔHv - The enthalpy of vaporization may also be expressed in units of kJ/mol. To convert from kJ/mol to kJ/g, divide the change in the heat of vaporization by the molecular mass of the substance. So for water, (40.67 kJ/mol)/(18 g/mol) = 2.26 kJ/g

how to find to specific heat of a substance

with calorimeter 1. Fill the calorimeter with a known amount of water and measure its initial temperature. 2. measure mass of the substance of interest 3. heat the substance of interest to a known temp, boiling-water bath 4. place hot substance into calorimeter, temp increases 5. measure the final temp of the water

measuring enthalpy

∆H = (Hproducts - Hreactants)