Chemistry S2 Unit 4

−6.82kJ/mol

34.1 kJ of heat is released when 5 moles of a gaseous substance condenses. What is the heat of condensation?

+32.5J

A 10 g gold coin is heated from 25°C to 50°C (CAu is 0.13 J/g-°C).What is the ΔH?

Heat will flow from the ice cube to the dry ice.

A block of dry ice (−40°C) is placed in contact with an ice cube (−10°C). What will happen, assuming no chemical reactions occur?

chemical bonds of the gasoline

A car burns gasoline with air to make heat. Where does most of this energy come from?

100 kJ

A chemical reaction is carried out in a closed container. The energy absorbed by the chemical reaction is 100 kJ. What is the energy liberated from the air or the walls of the container?

o Molar mass of CH3OH = 32 grams o Moles = 16/32 = 0.5 moles o Q = 3.16 * 0.5 = 1.58 kJ

How much heat is given off when 16 g liquid methanol (CH3OH) at its freezing point changes to solid methanol? (methanol ΔHf = 3.16 kJ/mol)

80 J/g * 15.0 g = 1,200 Joules

If the heat of fusion of water is 80 J/g, calculate the amount of heat energy required to change 15.0 grams of ice at 0°C to 15.0 grams of water at 0°C.

are at the same temperature, the molecules will still collide, but there will be no heat transfer.

If two objects are at the same temperature

Q = 334 * 10 = 3,340 Joules or 3.340 kJ

In a calorimeter, 10 g of ice absorbs heat with an enthalpy of fusion of 334 J/g. What is the heat absorbed?

33.4 kJ

In a calorimeter, 100 g of ice melts at 0oC. The enthalpy of fusion of the ice is 334 J/g. How much heat was absorbed?

Q = 334 * 20 = 6,680 Joules or 6.680 kJ

In a calorimeter, 20 g of ice absorbs heat with an enthalpy of fusion of 334 J/g. What is the heat absorbed?

33.2 kJ

In the space station, liquid ammonia is used to transfer heat. If 100 g of liquid ammonia evaporates, then how much heat was removed? (ΔHv=5.65kJ/mol)

NH4Cl(s)→NH +4(aq)+Cl−(aq) , ΔH=+14.8 kJ/mol

In which thermochemical equation would the ΔH be considered a heat of solution? Responses

NH4NO3(s)→NH4+(aq)+NO3-(aq), ΔH=+25.7 kJ/mol

In which thermochemical equations would the ΔH be considered a heat of solution?

−6.32kJ

Methanol (CH3OH) has a heat of fusion of 3.16 kJ/mol.What is the heat of solidification that occurs when 64 grams of liquid methanol freezes?

Q = c * m * delta T ▪ Q = 1.01 * 20.0 * 20.0 ▪ Q = 404 Joules

Solid magnesium has a specific heat of 1.01 J/g°C. How much heat is given off by a 20.0 gram sample of magnesium when it cools from 70.0°C to 50.0°C?

the amount of heat in Joules required to raise 1 kilogram of a substance 1 degree Celsius.

Specific heat

substance A

Substances A-D have the following specific heats (J/g-°C): A = 0.90, B = 1.70, C = 2.70, D = 4.18. Which substance will cool the fastest when equal masses are heated to the same temperature?

endothermic process

any process which requires or absorbs energy from its surroundings, usually in the form of heat;

triple point

at this point, all three phases of matter are in equilibrium with each other

-melting -dissolving

example of an endothermic reaction

the letter Q in the equation

heat is symbolized by

Endothermic reaction

in this, the energy of the products is greater than the energy of the reactants.

exothermic reations

in this, the energy of the reactants is greater than the energy of the products.

Temperature

s a measure of the thermal energy in an object

endothermic reaction

the energy of the products is greater than the energy of the reactants

heat of fusion

the heat absorbed by amass of a given solid at its melting point that completely converts the solid to a liquid

Hess's Law

the net change in the enthalpy is equal to the sum of change in enthalpy for each reaction-bases on the conservation of energy

gas

this spreads out to fit whatever container it is in

Heat

a measure of the energy transferred from one object to another.

temperature

a measure of the thermal energy in an object.

The slower-moving molecules of air collide with the rapidly vibrating stone molecules. The air molecules gain energy raising the air temperature.

A stone is boiled in 100°C water for one hour. It is removed and placed in air that is 21°C.Which statement best explains what happens?

Steam delivers more energy because it condenses.

Consider the amount of heat energy delivered to an object by the same mass of liquid water at 100°C and steam at 100°C. Which statement is true?

−2.67×103kJ

Consider the reaction, CH4(g)+2O2(g)→CO2(g)+2H2O(l) , ΔH=−890kJ. What will be the change in enthalpy when 3 moles of methane react in excess oxygen? Responses

−198kJ

Consider these equations: 2S(s)+3O2(g)→2SO3(g)2S (s) + 3O2 (g)→2SO3 (g), ΔH=−792kJΔH = −792 kJ 2S(s)+2O2(g)→2SO2(g)2S (s) + 2O2 (g)→2SO2 (g), ΔH=−594kJΔH = −594 kJ 2SO2(g)+O2(g)→2SO3(g)2SO2 (g) + O2 (g)→2SO3 (g), ΔH=?ΔH =? What is the missing ΔHΔH?

−145kJ/mol

Consider these reactions. C(s)+O2(g)→CO2(g) , ΔH=−395kJ/mol H2(g)+12O2(g)→H2O(l) , ΔH=−285kJ/mol C5H12(g)+8O2→5CO2(g)+6H2O(l) , ΔH=−3.54×103kJ/mol What is the heat of formation of pentane (C5H12) gas from carbon and hydrogen gas?

melting or dissolving

Examples of an endothermic reaction

combustion or oxidation or condensation

Examples of an exothermic reaction

−304kJ

Given these reactions: NO(g)+O3(g)→NO2(g)+O2(g), ΔH=−199kJ O3(g)→32O2(g), ΔH=−142kJ O2(g)→2O(g), ΔH=+495kJ What is the ΔH for this reaction? NO(g)+O(g)→NO2(g)

from the object with the warmer temperature to the object with the colder temperature.

Heat always flows how?

-it's shape -it's volume -it's movement

How do we classify matter as a solid, liquid, or gas?

It can affect the ΔH value.

How does a phase change affect a thermochemical equation?

Heat always flows from the object at the higher temperature to the object with the lower temperature until they both reach the same temperature or achieve equilibrium.

How does heat flow?

from the object at the higher temperature to the object with the lower temperature until they both reach the same temperature or achieve equilibrium.

How does heat flow?

16.6

How many moles of liquid water must freeze to remove 100 kJ of heat? (ΔHf=−334J/g)

o Q = m * c * T o Q = 150 * 2.2 * 15 o Q = 4,995 Joules o Q = 4.995 or 5.000 kJ

How much heat do you need to raise the temperature of 150 grams of gasoline from -30 to -15 degrees Celsius?

1.58 kJ

How much heat is given off when 16 g liquid methanol (CH3OH) at its freezing point changes to solid methanol? (methanolΔHf=3.16kJ/mol)

−594kJ

Sulfur trioxide can be formed from sulfur dioxide and oxygen using the given set equations.What is the value of the missing ΔH in these equations? 2S(s)+3O2(g)→2SO3(g), ΔH=−792kJ 2S(s)+2O2(g)→2SO2(g), ΔH=? 2SO2(g)+O2(g)→2SO3(g), ΔH=−198kJ

0.78 J/g-°C

The amount of heat needed to raise 25 g of a substance by 15°C is 293 J.What is the specific heat of the substance? Use the equation C=q/mΔT .

A liquid is solidifying at the freezing point.

The latent heat in a substance has a negative value. Which situation is most likely to be true?

180.5 kJ

The reaction 2NO(g) → N2(g) + O2(g) has a ΔH=−180.5kJ . How much heat will be required to produce 44.8 L of NO by the reverse reaction?

10.5 kJ

The temperature of 100 g of liquid water in a calorimeter changes from 25°C to 50°C. How much heat was transferred? Use the equation C=q/mΔT. The specific heat of liquid water is 4.18 J/g-°C.

ΔH3=ΔH4-(ΔH1+ΔH2)

There is a series of three chemical reactions (1→2→3) that form the overall chemical reaction.How is the ΔH3 calculated if the others are known?

liquid water freezing

This diagram represents the enthalpy change in which example?

hot pack

This diagram would represent the enthalpy changes in which example?

Although the molecules collide, no heat transfers because there is no temperature difference between the objects.

Two objects, A and B, of the same temperature are in contact with each other. Which statement is true?

liquid, solid, gas, or plasma

What are the four states of matter?

the heat content of a system at constant pressure

What is enthalpy?

∆H>0

What is likely to be true about the ΔH for this reaction? C6H6 (s) → C6H6 (l)

565 kJ

What is the change in enthalpy when 250 g of water evaporates at 100oC? (ΔHv=40.67kJ/mol)

Only specific heat accounts for mass.

What is the difference between heat capacity and specific heat?

Liquid water has a relatively high specific heat capacity compared to most other substances.

What statement is true about the specific heat capacities of various substances?

1cal

Which amount of energy raises one gram of water by one degree Celsius?

ΔH<0

Which enthalpy of reaction would the reaction represented by the diagram have?

C3H8(g)+5O2(g)→3CO2(g)+4H2O(l), ΔH=−2,220kJ/mol

Which equation is a correctly written thermochemical equation?

a liquid changing to a gas

Which example is an endothermic reaction?

concentrated hydrochloric acid dissolving in water

Which example is an exothermic reaction?

condensation

Which example is an exothermic reaction?

temperature

Which parameter is measured directly in a coffee-cup calorimeter?

conservation of energy

Which principle is Hess's law based on?

an endothermic reaction

Which reaction does melting ice show?

4Fe(s)+3O2(g)→2Fe2O3(s), ΔH=−3,926kJ

Which statement is a correctly written thermochemical equation?

Energy is absorbed in an endothermic reaction.

Which statement is true in all cases?

In an exothermic reaction, the energy of the products is less than the energy of the reactants.

Which statement is true?

H

Which symbol represents enthalpy?

heat

Which term is the transfer of energy?

heat

Which term measures the energy transferred from one object to another?

a liquid with a high specific heat

Which type of substance would be best for use in a heating system?

alcohol

You have a block of dry ice (−40°C), an ice cube (−5°C), water (10°C), and alcohol (15°C).In which example do molecules move the fastest?

ΔH2=ΔH4−(ΔH1+ΔH3)

You have a series of four chemical reactions (1→2→3→4)(1→2→3→4). How do you calculate the ΔH2ΔH2 if you know the other ΔHΔH values?

exothermic process

a chemical reaction that releases energy, often through heat.