Unit 4 - Lesson 1: Calculating Enthalpy Change

Question 1

What is enthalpy?

Answer 1

Enthalpy is the heat energy absorbed or emitted from a reaction. It can be called the heat energy transferred (Q) or enthalpy change (ΔH).

Question 2

What formula calculates the amount of heat energy needed to heat a set mass of substance by a set temperature?

Answer 2

Q = m x c x ΔT

Heat energy transferred (J) = mass (g) x specific heat capacity (J/g/Celsius) x Change in temperature (Celsius)

Question 3

What is a specific heat capacity (c)?

Answer 3

The specific heat capacity (c) is the amount of energy needed to raise the temperature of 1 gram of a substance by 1 degree Celsius.

Question 4

What’s the specific heat capacity of water?

Answer 4

4.18 J/g/Celsius

(Sometimes rounded to 4.2)

Question 5

How do we arrange the formula using a pyramid to find change in temperature (ΔT)?

Answer 5

ΔT = Q / m x c

Change in temperature = Heat energy transferred / mass x specific heat capacity

Question 6

How do we rearrange the formula using a pyramid to find specific heat capacity?

Answer 6

c = Q / m x ΔT

Specific heat capacity = Heat energy transferred / mass x Change in temperature

Question 7

What is molar enthalpy change?

Answer 7

Molar enthalpy change is the amount of heat transferred per mole.

Question 8

What is the formula for calculating molar enthalpy change?

Answer 8

Molar enthalpy change = Enthalpy change / Number of moles of reactant

ΔH = Q / N

Question 9

How do you calculate the number of moles?

Answer 9

Moles = mass / relative particle mass

N = m / Mr

Question 10

What are the three steps for calculating molar enthalpy change?

Answer 10

1. Calculate Q using Q = m x c x ΔT. The m and c values will come from the substance that is changing in temperature.
2. Calculate the number of moles (N) of reactant (thing that is causing the change in temperature). Use moles = mass / relative particle mass.
3. Divide Q by N (divide the value from step 1 by step 2)

Question 11

812g of butane (C4H10) is burnt beneath 50g of water, heating it by 31 degrees Celsius. The amount of heat energy transferred from the butane to the water is…?

Answer 11

Q = m x c x ΔT
Q = 50 x 4.2 x 31
Q = 6510 J

Question 12

812g of butane (C4H10) is burnt beneath 50g of water, heating it by 31 degrees Celsius. The amount of heat energy transferred from 1 mole of butane is…?

Answer 12

Mr of butane = (4 x 12) + (1 x 10) = 58
N = m / Mr
N = 812 / 58 = 14

Question 13

812g of butane (C4H10) is burnt beneath 50g of water, heating it by 31 degrees Celsius. What is the molar enthalpy change?

Answer 13

Step 1. Q = 50 x 4.2 x 31 = 6510 J

Step 2. N = 812 / 58 = 14

Step 3. Q/N
6510 / 14 = 465 J/mol

465 joules of heat energy is transferred from 1 mole of butane to heat water by 31 degrees Celsius.

Question 14

How do we convert Kg into g? (Kilograms into grams)

Answer 14

Multiply Kg by 1000

Question 15

How do we convert g into Kg? (Grams into kilograms)

Answer 15

Divide g by 1000

Question 16

How do we convert J into kJ? (Joules into kilojoules)

Answer 16

Divide J by 1000

Question 17

How do we convert kJ into J? (Kilojoules into joules)

Answer 17

Multiply kJ by 1000