3.2.1 Enthalpy changes

Question 1

what happens in an exothermic reaction

Answer 1

energy is released causing the temperature of the surroundings to increase

Question 2

describe the enthalpy change profile of an exothermic reaction

Answer 2

Energy is released
SO products are lower in energy than reactants
SO enthalpy change (𝚫H) is negative

Question 3

What happens in an endothermic reaction

Answer 3

Energy is absorbed causing the temperature of the surroundings to decrease

Question 4

Describe the enthalpy change profile of an endothermic reaction

Answer 4

Energy is absorbed
SO products are higher in energy than reactants
SO enthalpy change (𝚫H) is positive

Question 5

desribe the enthalpy change (𝚫H) arrow on an enthalpy profile

Answer 5

The arrow goes from the reactants to the products it will be up for endothermic (+ve) reactions and pointing down for exothermic reactions (-ve)

Question 6

what is activation energy (Ea)

Answer 6

Activation energy is the minimum amount of energy required to start a reaction (by breaking bonds)

Question 7

Describe the activation energy (Ea) arrow on an enthalpy profile

Answer 7

The arrow always goes up. it goes from the same level as the reactants to the top of the arch (therefore needing to be in the middle NOT on the reactant line)

Question 8

What 2 equations are used to calculate 𝚫H

and in what order

Answer 8

Q = mc𝚫T
then 𝚫H = Q/n

energy = mass of water x specific heat capacity x the change in temperature
enthalpy change = energy divided by number of moles

Question 9

what are the units for Q = mc𝚫T

Answer 9

Joules = grams x (J/g/℃) x ℃

Question 10

what

what are the units for 𝚫H = Q/n

Answer 10

(KJ/mol) = joules x number of moles

Question 11

what is the equation for 𝚫H using the enthalpy of the products and reactants

Answer 11

𝚫H = H(products) - H(reactants)

Question 12

what is the law of the conservation of energy

Answer 12

energy cannot be created or destroyed

Question 13

what is this symbol for 𝚫H⦵

the ⦵ should be smaller and in the top right (superscript)

Answer 13

standard enthalpy change

takes place under standard conditions

Question 14

what are standard conditions

Answer 14

• components in their standard states
• standard conditions:
  298K
  1atm (101kpa)
  1moldm⁻³

Question 15

what is 𝚫H⦵r for

the r would normally be in the bottom right (subscript)

Answer 15

standard enthaply change of reaction

Question 16

define standard enthalpy change of reaction

Answer 16

enthalpy change that accompanies a reaction in the molar quantities shown

Question 17

what is 𝚫H⦵c for

the c would normally be in the bottom right (subscript)

Answer 17

standard enthaply change of combustion

Question 18

define standard enthalpy change of combustion

Answer 18

enthalpy change when 1 mol of a substance completely reacts with O₂

Question 19

what is 𝚫H⦵f for

the f would normally be in the bottom right (subscript)

Answer 19

standard enthalpy change of formation

Question 20

define standard enthalpy change of formation

Answer 20

enthalpy change when 1 mol of a substance is formed from its elements in their standard states

Question 21

what is 𝚫H⦵neut

the neut would normally be in subscript

Answer 21

standard enthalpy change of neutralisation

Question 22

define standard enthalpy change of neutralisation

Answer 22

enthalpy change when an acid and a base react to form 1 mol of water

Question 23

state Hess’s Law

Answer 23

The enthalpy change of any 2 route is the same as long as the initial and final states are the same

Question 24

what does Hess’s Law do

Answer 24

Hess’s law provides a way of measuring 𝚫H indirectly

Question 25

why can some 𝚫H not be measured directly

Answer 25

• they have a very high Ea
• the reaction is very slow
• there are a mixture of side reactions or products

Question 26

How do you calculate 𝚫H using 𝚫Hf data

𝚫H of say reaction - not that important to the question

Answer 26

𝚫Hr = - 𝚫Hf(reactants) + 𝚫Hf(products)
OR
𝚫Hr = 𝚫Hf(products) - 𝚫Hf(reactants)

they are the same just different ways around

Question 27

How do you calculate 𝚫H using 𝚫Hc data

Answer 27

𝚫Hr = 𝚫Hc(reactants) - 𝚫Hc(products)

Question 28

how do you convert from kelvin to ℃

Answer 28

add 273 to the ℃

Question 29

using a spirit burner

describe an experiment that gives 𝚫Hc data

Answer 29

liquid fuels such as methanol can be burned easily using a small spirit burner.
1. Using a measuring cylinder, measure out 150cm³ of water. pour the water into the beaker and record the initial temperature of the water ( to the nearest 0.5℃)
2. Add methanol to the spirit burner. weigh this and record the mass.
3. place the spirit burner under the beaker, and light it whilst stiring the water with a thermometer.
4. after ~3mins extinguish the flame. Immediately record the maximum temperature reached by the water
5. reweigh the spirit burner with methanol in and assume the wick has not been burned

As the water has gained energy the methanol must have lost the same amount of energy so the 𝚫H is negative and the reaction exothermic - keep in mind for Q calculation and then the 𝚫H calculation use this symbol at the end to check

Question 30

why may experimental 𝚫Hc data be inaccurate

Answer 30

• there is heat loss to the surroundings other than water (this includes the beaker but maining the air surrounding the flame)
• incomplete combustion of methanol
• evapouration of methanol from the wick ( the burner must be weighed immediately after extinguishing the flame, otherwise some methanol my evapourate - spirit burners usually have a cover to minimise this)
• Non-standard conditions