Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Chemistry🧪🔫 > 9) ENTHALPY > Flashcards

9) ENTHALPY Flashcards

1
Q

What is enthalpy?

A

Measure of heat energy in a chemical system

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What does a chemical system refer to?

A

The:
Atoms
Molecules
Ions
making up a system

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What is enthalpy change?

A

The difference in enthalpies in a chem reaction

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is the law of conservation of energy?

A

Energy cannot be created or destroyed

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

In a chem reaction, what is the system, surroundings nd universe?

A

System- chemicals of reactants/products
Surroundings- apparatus (e.g thermometer)
Universe- includes both system + surroundings

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is an exothermic enthalpy △？

A

Energy transfer from system to surroundings

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What is an endothermic enthalpy △？

A

Energy change from surroundings to system

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Describe an Exothermic conservation of energy

A

-chem system releases heat energy to the surroundings
-energy lost by system is balanced by same energy gain by surroundings
-△H (-)
-temp of surroundings increases as it gains energy

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Describe an Exothermic conservation of energy?

A

-Energy transferred from surroundings to system
-chem system take in heat from the surrounding
-any enegy gained by chem system is balanced by same energy loss by the surroundings
-△H (+)
-temp of surroundings decrease as they lose energy

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is activation energy (Ea)?

A

Minimum energy required to for a reaction to take place

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

How do reactions with small Ea take place?

A

Energy needed to readily break bonds is available from the surroundings = reactions w small Ea are rapid

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What are the standard conditions?

A

-standard P. = 100kPa
-standard temp = 298k (25°c)
-standard conc. (Solutions ONLY) = 1moldm^-3
-standard state (the physical state of a substance under standard conditions)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Standard △H of a reaction (def.)?

A

△H of a reaction in molar quantities shown in a chem equation under standard conditions

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

△H of formation (def.)?

A

△H when 1MOL of a COMPOUND is formed from its constituent elements under standard conditions

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What is the △H of an element nd why?

A

formation of 1 ELEMENT from its own ELEMENT = no △H ＝ THEREFORE all elements △H is 0🙆

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

△H of combustion (def.)?

A

△H when 1 MOL of a substance REACTS COMPLETELY w oxygen under standard conditions

17
Q

△H of neutralisation (def)?

A

△H when the reaction of an acid nd base form 1 MOL of liquild H2O under standard conditions

18
Q

What is the value of △H for neutralisation nd why?

A

(-)57kJmol^-1 for all neutralisation reactions as the reaction involves H+(aq) w OH-(aq) to form 1 mol of H2O

19
Q

What is used to calculate energy changes?

A

-Mass
-SHC
-△temp

q=mc△T

20
Q

Describe the experimental method for determining enthalpy change of combustion of liquid fuels

A
  1. Measure vol & initial temp of water to nearest 0.5°c nd record
  2. Add liquid fuel to spirit burner nd weigh/record
  3. Place spirit burner under beaker of water nd burn the fuel while stirring water w thermometer
  4. Extinguish flame nd record max temp reached by water
  5. Re-weigh spirit burner nd record (assuming wick hasn’t been burnt)
21
Q

Determine the △H of combustion of methanol from these results:

mass os spirit burner + methanol before burning = 196.97g
mass of spirit burner + methanol after burning = 195.37g
mass of fuel burnt = 1.60g
initial temp of water = 21.5°c
final temp of water = 62.5°c
△T of water = 41.0°c
water density = 1.00gcm^-3 , c = 4.18Jg^-1K^-1
water volume = 150cm^3

A

Step 1) calculate energy change (q) of water in KJ

         q=mc△T
           =150 x 4.18 x 41.0 
           =25.707kJ

Step 2) calculate the amount, in mol, of CH3OH (methanol) burnt

      n(CH3OH) = m/Mr = 1.60/32.0 = 0.500 mol

Step 3) calculate △H in KJmol^-1

        △H of Combustion is for 1 mol of Ch3OH
        △H of Comb. = q/n
                                 = 25.707KJ/0.0500
                                 = 514.14KJ Answer: combustion △cH(CH3OH) = -514KJmol^-1
22
Q

Why might the actual △H of combustion be different to the calculated value?

&

How would it be diff?

A

-heat lost to surroundings
-incomplete combustion (black layer of soot on beaker)
-evaporation of spirit fuel from wick
-non-standard conditions

Less exothermic than the calculated number

23
Q

Describe how △H of reaction is calculated?

A

-chem particles within the solution may react when colliding
-energy is transferred between the chem & water particles
-thermometer records temp change
-heat energy calculated with q=mc△T
-△rH calculated using △H=q/n

24
Q

Find the △rH with these results:

Excess of Zn powder added to 50.0cm^3 of 1.00moldm^-3 Cu(II)SO4.
Mixture is stirred till max temp is reached.
Zn(s) + CUSO4(aq) → Cu(s) + ZnSO4(aq)
-initial temp =22.5°
-final temp = 60.5°
-temp△ = 38.0°
(Water density = 1.00gcm^-3)

A

Step 1) calculate q in solution in KJ:
50.0cm^3 of a solution has mass 50.0g
q=mc△T
=50.0 x 4.18 x 38.0
=7942J
=7.942KJ

Step 2) calulate mol of CuSO4 that reacted w excess Zn
n(CuSO4)= c x vol
= 1.00 x 50.0/1000
= 0.050mol

Step 3) calculate enthalpy change w △H=q/n
Ratio is 1:1
1 mol of CuSO4 lost = q/n
= 7.942/0.0500
= 159KJ

Ans: △rH = -159KJmol^-1

25
Q

Find the △neutH for the reaction below:

35.0cm^3 of 24.0moldm^-3 NaOH & 35.0cm^3 of 24.0moldm^-3 HCl mixed together.
Temp rises by 16.5°c.
Density of mixture is 1.00gcm^-3.

A

Step 1) calculate q (energy change) in solution:
Total vol. of solution that changes temp = 35.0 + 35.0 = 70.0cm^3
Density is 1.00gcm^-3, so Mass of solution = 70.0g
q=mc△T
=70.0 x 4.18 x 16.5
=48279J
=4.8279KJ

Step 2) calculate moles of NaOH nd HCl that reacted:
n(NaOH)=n(HCl) as ratio is 1:1
n=conc. x vol.
=2.40 x 35.0/1000
=0.0840mol

Step 3) calculate △neutH:
NaOH(aq) + HCl(l) → NaCl(aq) + H2O(l)
RATIO is 1:1 so 0.0840 mol of H2O transfers 4.8279KJ of Energy
△neutH=q/n
=4.8279/0.0840
=57.5KJ

Ans: △neutH = -57.5KJmol^-1

26
Q

What is average bond enthalpy?

A

Energy required to break one mol of a covalent bond in a gaseous molecule

27
Q

What happens in chem reactions?

A

Bonds break nd new bonds are formed

28
Q

What happens in an endothermic reaction nd what is △H?

A

△H (+)

energy required (taken in) to BREAK bonds 🔨👹

29
Q

What happens in an exothermic reaction nd what is △H?

A

△H (-)

Energy is released and bonds are FORMED 👨🏻‍🍳🥘

30
Q

what do calculations of average bond enthalpies need to be?

A

Calculations need to be gaseous molecules
If not gaseous, need to take into account △H of H2O(g) →→→condensing into H2O(l)

31
Q

What does Hess’s Law state?

A

The △H of a reaction is the same,whatever route it takes

32
Q

Calculation for finding △fH (formation)?

A

△H = Total△fH products — Total△fH reactants

Chemistry🧪🔫 (8 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions