Enthalpy changes

Question 1

What is a reaction called where more heat energy is given out than taken in

Answer 1

Exothermic

Question 2

What is a reaction called when more heat energy is taken in than given out

Answer 2

Endothermic

Question 3

Example of Exothermic reaction

Answer 3

Mg + HCl –> MgCl2 + H2

Question 4

Example of Endothermic reaction

Answer 4

Ammonium nitrate —> ammonium ions + nitrate ions

Question 5

Is photosynthesis endo or exothermic?

Answer 5

Endothermic because the reactants are lower in energy than the product

Question 6

Specific heat capacity equation

Answer 6

Q= mxcxΔT

Question 7

What is C equal to in the Q = mxcxΔT

Answer 7

C= Specific heat capacity

Question 8

What is specific heat capacity measured in

Answer 8

J/kg°C

Question 9

How much heat energy is needed to raise the
temperature of 3 kg of copper by 6⁰C?
(Specific heat capacity of copper = 385 J/(kg ⁰C)

Answer 9

Heat energy = mass x specific heat capacity
x temperature change = 3 x 385x 6 = 6930J

Question 10

A heater of 800W is used to heat a 600g cast iron
cooker plate. How long will it take to raise the temperature of
the plate by 200 oC? Specific heat capacity of iron = 500
J/(kJ⁰C) (power = work done /time)

Answer 10

Heat energy needed = 0.6x500x200 = 60 000J
Time needed = 60 000/800 = 75 s = 1 minute 15 s

Question 11

Equation to calculate the moles

Answer 11

n=m/mr

Number of moles = mass/mr

Question 12

Calculate the number of moles in 15g of C2H6

Answer 12

n = m/mr = 15/30 = 0.5 mol

Question 13

What is calorimetry?

Answer 13

Measuring how much energy is released or taken in when a reaction takes place

Question 14

What is molar enthalpy change?

Answer 14

Energy change per mole of reactant used

Question 15

How to work out molar enthalpy change (ΔH)

Answer 15

energy released before/moles of reactants used

Question 16

How to measure molar enthalpy change

Answer 16

Use simple calorimeter

Question 17

Why can simple calorimeters be inaccurate?

Answer 17

Heat lost to surroundings, incomplete combustion

Question 18

A 0.5g sample of ethanol is burnt, raising the temperature of 250g of
water from 10˚C to 28˚C.
When 0.8g of butane is burnt, the temperature of 250g of water
increases from 10˚C to 40˚C.
c = 4.2 J/g/˚C
Which fuel produces the most heat energy per mole of fuel burnt ?

Answer 18

For ethanol: Q = (4.2 x 250 x 18) = 18,900 J
moles = mass/RFM = 0.5/46 = 0.01087
ΔH = -Q/moles = -18,900/0.01087
= -1,738,730 J/mol or -1,739 kJ/mol
For butane: Q = (4.2 x 250 x 30) = 31500 J
moles = mass/RFM = 0.8/58 = 0.01379
ΔH = -Q/moles = -31500/0.01379
= -2,284,264 J/mol or -2,284 kJ/mol