Unit 7 Thermochemistry GenChem Study Guide

Draw and label the cooling curve of water (melting pt, boiling pt, s, l, g, phase changes, phases at equilibrium, increase/decrease in potential & kinetic energy, etc.).

* deposition g→l *↘ this arrow means decrease in kinetic energy meaning potential energy stays constant *→ this arrow means decrease in potential energy meaning kinetic energy stays constant

Sketch a potential energy diagram of a fast exothermic reaction.

*fast = lower bump meaning less activation energy *exothermic products have less energy than reactants * catalyst decreases activation energy which means there will be a lower bump

Draw and label the heating curve of water (melting pt, boiling pt, s, l, g, phase changes, phases at equilibrium, increase/decrease in potential & kinetic energy, etc.).

*sublimation s→g * ↗ this arrow means increase in kinetic energy meaning potential energy stays constant *→this arrow means increase in potential energy meaning kinetic energy stays constant

Using the diagram above, what is the value of ∆H for the reaction in kJ per mole of HCl? A) +35.0 B) -35.0 C) +92.0 D) -92.0

-14.0 kJ/0.400mol=-35.0 ANSWER: B) -35.0

True or False For a given substance, the kinetic energy always increases in the following order: gas⟶liquid⟶solid

-False For a given substance, the kinetic energy always increases in the following order: solid⟶liquid⟶gas

True or False In a calorimeter, the heat lost in a reaction is less than the heat gained by the water.

-False In a calorimeter, the heat lost in a reaction is equal to the heat gained by the water

True or False The container of an endothermic reaction will feel hot when the reaction is complete.

-False The container of an endothermic reaction will feel cold when the reaction is complete.

True or False If the potential energy of the products in a chemical reaction is 50 kilojoules, the reaction would be exothermic if the reactants had a potential energy of 35 kilojoules.

-False If the potential energy of the products in a chemical reaction is 50 kilojoules, the reaction would be exothermic if the reactants had a potential energy of any number greater than 50kJ for the reactants

True or False The enthalpy of the products is an exothermic reaction is greater than the enthalpy of the reactants. -False-endothermic

-False The enthalpy of the products is an endothermic reaction is greater than the enthalpy of the reactants

What is the value of q for the reaction (qrxn) in the diagram above? A) -14.0 kJ B) +14.0 kJ C) -18.3 kJ D) +18.3 kJ

-Just look at the answer for #33 and flip the sign ANSWER: A) -14.0 kJ

How many moles of HCl are produced in the reaction above? A) 0.100 mol B) 0.200 mol C) 0.400 mol D) 0.800 mol

14.6g HCl | 1 mol HCl/36.5g HCl=0.400mol ANSWER: C) 0.400 mol

Which of the following change of state operations is exothermic? A) Gas to liquid B) Solid to liquid C) Liquid to gas D) Solid to gas

A) Gas to liquid

The size of a temperature increase in a substance depends primarily upon the A) Specific heat and mass of the substance B) Original temperature of the substance C) Size of the calorimeter D) Temperature of the surroundings

A) Specific heat and mass of the substance

Which reactions in Figure 12-2 are exothermic? 1) W(s)-->W(g) change of heat = +60 2) W(g) + X(g)-->Y(g) change of heat = -40 3) 2Y(g)-->Z(g) change of heat = -90 4) Z(s)-->Z(g) change of heat = +30 A) 1 and 4 only B) 2 and 3 only C) All of the equations D) None of the equations

ANSWER: B) 2 and 3 only -Remember that exothermic is negative and endothermic is positive change of heat

The breakdown of glucose in your body (cellular respiration) is A) An endothermic reaction B) An exothermic reaction C) An unnecessary reaction D) The body's way of storing energy

B) An exothermic reaction

What does thermochemistry study? A) Rate of reaction B) Heat effects in chemical reactions C) Completeness of reaction D) Mole ratios

B) Heat effects in chemical reactions

Hess's law makes it possible to A) Balance tricky equations B) Indirectly determine the enthalpy change of a reaction C) Predict the products in exothermic reactions D) Determine the specific heat of a substance with a calorimeter

B) Indirectly determine the enthalpy change of a reaction

In a combustion reaction, the energy released as the products form is A) Less than the energy required to break apart the reactants B) More than the energy required to break apart the reactants C) Equal to the energy required to break apart the reactants D) Impossible to determine

B) More than the energy required to break apart the reactants

Why does the air around a combustion reaction feel warm?

Because a combustion reaction is exothermic it releases heat and makes its surroundinds feel warm

Why does the temperature of boiling water not change, even though the water is being heated?

Because the energy of the water is being used to change the phase from liquid to gas

Which change in temperature of a sample of water would result in the smallest decrease in the average kinetic energy of its molecules? A) 298K to 305K B) 298K to 302K C) 15℃ to 9℃ = decrease of 6 degrees D) 12℃ to 2℃ = decrease of 10 degrees

C) 15℃ to 9℃

What happens if Hif a chemical equation is reversed? A) It becomes zero B) It remains the same C) It changes in sign D) Depends on the reaction

C) It changes in sign

An exothermic reaction A) Is indicated by a positive enthalpy change B) Requires added energy to proceed C) Releases heat to its surroundings D) Does not involve energy transfer

C) Releases heat to its surroundings

A solid material X is placed in liquid Y. Heat will flow from Y to X when the temperature of A) Y is 20℃ and X is 30℃ B) Y is 10℃ and X is 20℃ C) Y is 15℃ and X is 10℃ D) Y is 30℃ and X is 40℃

C) Y is 15℃ and X is 10℃ -Remember heat always flows hot to cold

The heat of fusion of ice is 334 Joules per gram. Adding 334 Joules to one gram of ice at STP will cause the ice to A) Increase in temperature B) Decrease in temperature C) Change to water at a higher temperature D) Change to water at the same temperature

D) Change to water at the same temperature

Which serves as a unit of heat? A) Degree Celsius B) Pascal C) Kelvin D) Joule

D) Joule

Given the equation: H2O(g) + Cu(s) + 86.6 kJ → H2(g) + CuO(s), state whether the reaction is endothermic or exothermic and whether the reactants or the products contain more stored heat energy. Then find the quantity of heat transferred when 0.750 gram of copper reacts completely.

If you do dimensional analysis for this problem which should look like this: 0.750g Cu | 1mol Cu | +86.6kJ -------------------------------- |63.5g Cu | 1 mol Cu = +1.02 kJ ANSWER: +1.02kJ making this reaction endothermic since energy is absorbed so the products have more energy than the reactants

How much heat is given off when 24.0 g of methane (CH4) is burned in air? Is this reaction endothermic or exothermic? CH4(g)+2O2(g)CO2(g)+2H2O(l)+890kJ

If you do dimensional analysis for this problem which should look like this: 24.0g CH4 | 1 mol CH4 | -890kJ ----------------------------------- | 16g CH4. | 1 mol CH4 = -1335 ANSWER: -1335kJ making this reaction exothermic since it is negative

How much heat will be released if 30.0g of octane (C8H18) is burned in excess oxygen? Is this reaction endothermic or exothermic? C8H18+1212 O28 CO2+9 H2O H=-53483.4 kJ

If you do dimensional analysis for this problem which should look like this: 30.0g C8H18 | 1mol C8H18 | -53483.4kJ ------------------------------------------- | 114g C8H18 | 1 mol C8H18 = -14074.58 but you have to round it to 3 sig figs so -14100 ANSWER: -14100 kJ making this reaction exothermic since its negative

What is the activation energy for the reverse reaction in Figure 23-1? A) -100 kJ B) +100 kJ C) +300 kJ D) -300 kJ

If you look at the diagram the activation energy for the reverse reaction should be +100 kJ since it was -100kJ for the forward reaction ANSWER: B) +100 kJ

In Figure 23-1, are the forward and the reverse reactions exothermic or endothermic? A) The forward is endothermic, the reverse exothermic. B) The forward is exothermic, the reverse endothermic. C) Both are endothermic. D) Both are exothermic.

If you look at the diagram you see that the graph for the forward reaction is endothermic but once you reverse it, it becomes the opposite meaning the graph for the reverse reaction becomes exothermic ANSWER: A) The forward is endothermic, the reverse exothermic.

To answer the following question you need to check the diagram in the study guide for #'s 37-40 What is the value of Hr (heat of reactant) for the forward reaction shown in Figure 23-1? A) -200 kJ B) +100 kJ C) +200 kJ D) +300 kJ

If you look at the diagram you should see that the value of Hr for the forward reaction should be +100 kJ ANSWER:B) +100 kJ

What is the value of Hr for the reverse reaction in Figure 23-1? A) -200 kJ B) +100 kJ C) +200 kJ D) +300 kJ

If you look at the diagram you should see that the value of Hr for the reverse reaction should be -200 kJ as you are reversing the reaction ANSWER: A) -200 kJ

Explain when you use the formula Q= m∙∆H fus/vap for calculating the heat of a system. What is the difference between the ∆Hfus and the ∆Hvap?

Q= m∙∆H fus/vap we use this equation during a phase change meaning there is no temperature change -∆Hfus = melting/freezing and it is constant for these two phase changes (s→l) -∆Hvap = boiling/condensation (l→g)

What amount of heat energy is needed to change a 15.0 gram sample of water at 0oC to ice?

Q=mHfus Q=(15.0g)(334J/g) Q=5010J Remember to use Hfus when there is a phase change

What is the specific heat of carbon tetrachloride (CCl4) if it requires 77 J of heat to raise the temperature of a 6.5g sample from 21oC to 35oC?

Q=mc(change in temp) 77J=(6.5g)(c)(14℃) c=0.85J/g℃

If 6 grams of solid aluminum absorbs 8.6 joules of heat, the temperature of the aluminum will change by how many degrees?

Q=mc(change in temp) 8.6J=(6g)(0.897J/g℃)(change in temp) change in temp =1.598℃2℃

Liquid water in a container is heated from 35.5oC to 75.8oC. Calculate the amount of heat absorbed by the water, given 357g of the water.

Q=mc(change in temp) Q=(357g)(4.18J/g℃)(40.3℃) Q=60138.078J -->60100 J

To answer this question refer to the diagram in the study guide for #'s 33-36 What is the value of q for the surrounding (qsur)in the diagram above? A) -14.0 kJ B) +14.0 kJ C) -18.3 kJ D) +18.3 kJ

QH2O=mct=(112g)(4.18J/g℃)(30°) +14044.8J= +14.0 kJ ANSWER: B) +14.0 kJ

Your mom bakes an apple pie and you cut yourself a slice as soon as it gets out of the oven. Since your starving, you don't bother with getting a fork and pick up your piece of pie in your hand. You figure it is good to eat since the crust wasn't too hot but then you take a bite and burn your mouth. Explain why the crust isn't too hot to hold, but the apple pie filling was so hot it burned your mouth.

The crust has a higher specific heat capacity than the apple pie filling inside of it making it not hot to hold but extremely hot to eat right out of the oven

True or False Energy is required to break the bonds in chemical reactions.

True

True or False If the equation of an exothermic reaction is reversed, the sign of Hbecomes positive.

True

Temperature

a measure of the average kinetic energy of the particles in a sample of matter

Catalyst

a substance that speeds a chemical reaction by lowering the activation energy of the reaction

Enthalpy

a thermodynamic quantity equivalent to the total heat content of a system. It is equal to the internal energy of the system plus the product of pressure and volume

1st Law of Thermodynamics

energy can be transformed from one form to another but it cannot be created nor destroyed

Calorimetry

measurement of the amount of heat released or absorbed in a chemical reaction, change of state, or formation of a solution

Hess's Law

states that regardless of the multiple stages or steps of a reaction the toal enthalpy change for the reaction is the sum of all changes

Specific Heat Capacity

the amount of heat required to raise the temperature of 1 gram of a substance 1℃

Chemical Energy

the energy associated with chemical bonds and attractions between particles of a system

Heat of Reaction

the heat absorbed or released as a result of a chemical reaction

Activation Energy

the minimum energy needed to initiate a reaction

System

the part of the universe being studied; the chemical reaction or phase change

Equillibrium

the state in which the rates of opposing processes are equal