Measuring energy changes

Question 1

What are the forms of energy that are involved in chemical reactions?

Answer 1

• During chemical reactions, there is an energy change (enthalpy change) which is calculated from the changes in temperature of the surroundings
• The forms of energy are: heat, light, sound and electrical energy
• Most of the time there is a transfer in heat

Question 2

What is heat and when is thermal equilibrium reached?

Answer 2

• Definition: The transfer of energy between objects of different temperature
• Heat will spontaneously flow from an object of higher temperature to an object of lower temperature
• When two objects reach the same temperature, they are at thermal equilibrium
• The particles in an object of higher temperature have higher kinetic average energy than the one will lower

Question 3

What is enthalpy and how is it related to heat?

Answer 3

• The heat released or absorbed by a system at constant pressure
• The changes in enthalpy are represented by ΔH (H is enthalpy)
• If the enthalpy change is measured under standard conditions, it is known as standard enthalpy change of reaction (ΔHƟ)

Question 4

What are the standard conditions and what is the symbol used?

Answer 4

• Ɵ (plimsoll symbol)
• Pressure at 100kPa
• Temperature at 25°C (298K)
• These conditions differ to the standard conditions involving gases (STP)

Question 5

What is temperature and what is the kelvin scale?

Answer 5

• Definition: Average kinetic energy of the particles in a substance
• In science, absolute temperature is measured on the Kelvin scale
• Kelvin is directly proportional to the average kinetic energy of the particles in a sample (0K is -273°C), at which the particles have the least amount of kinetic energy

Question 6

What are systems and its surroundings?

Answer 6

• System: the chemical reaction itself

- Surroundings: the apparatus that contains the reaction and a thermometer, everything around the reaction

Question 7

What are the types of systems?

Answer 7

• Open system: in which matter and energy can move freely between the system and surroundings
• Closed system: only energy is able to move between the system and the surroundings. Any energy gained by the system is transferred from the surroundings to the system.
• Isolated system: does not exchange energy or matter with its surroundings e.g. universe

Question 8

What is exothermic and endothermic and what are the enthalpy changes?

Answer 8

• Exothermic: a reaction in which heat is released (transferred from the system of the surroundings, ΔH is negative
• Endothermic: a reaction in which heat is absorbed, when heat is transferred to a system from the surroundings, enthalpy of the system increases, ΔH is positive

Question 9

What is the law of conservation of energy?

Answer 9

• Energy cannot be created or destroyed, it is converted from one form to another. The total amount of energy in the universe is constant
• This means the total amount of energy is conserved in chemical reactions
• The total energy involved remains the same, when it is transferred from the system to the surroundings

Question 10

What are examples of exothermic reactions?

Answer 10

• Combustion and neutralization reaction
• In exothermic reactions in aqueous solutions there is an increase in the temperature of the reaction mixture (feel hotter)
• The mixture becomes warm, and then the heat is lost to the surroundings

Question 11

Check book for enthalpy level diagrams

Question 12

Explain the exothermic enthalpy level diagram.

Answer 12

• The enthalpy change is negative, the system loses heat
• The products of the reaction have a lower enthalpy than the reactants
• The less energetic substances (products) are more energetically stable
• To calculate the enthalpy change: products - reactants
• The energy that is used in calculations is inside the bonds of the reactants

Question 13

What are thermochemical equations?

Answer 13

• They show the enthalpy change (∆H) of a reaction

E.g. The equation ΔHƟc = −890 kJ mol−1

Question 14

Explain the endothermic enthalpy level diagram.

Answer 14

• Take in heat, decrease in temperature and therefore feels colder
• This is because the reaction mixture is part of the surroundings (not the system), the system gets warmer the reaction mixture gets colder
• E.g. thermal decomposition ΔHƟ = +180 kJ mol−1
• The products have a higher enthalpy than the reactants and therefore the products are less energetically stable

Question 15

What units are used for the enthalpy change?

Answer 15

• For ΔH Joules (J mol-1) used
• Sometimes kJ mol-1 are used which is:
  1kJ is 1000J

Question 16

What is a calorimeter and why is using a polystyrene cup a good calorimeter?

Answer 16

• A calorimeter is an object used for measuring the heat of chemical reactions or and the heat capacity
• Polystyrene cups are good heat insulators, it reduces heat loss to the surroundings
• The expanded polystyrene cup absorbs very little heat itself
• To further limit heat lost to the surroundings, the reaction can be carried out in a vacuum flask
• Once the min and max temperatures are known the enthalpy of the reaction can be calculated

Question 17

What is the formula for calculating the enthalpy changes?

Answer 17

• Used to calculate the heat change (how much is released or absorbed)
  q = m * c * ∆T
  q: heat absorbed or released in J (joules)
  c: the specific heat capacity of the solution in J g-1 °C -1 or J g-1 K-1
  m: mass of the solution in grams
  ∆T: change in temperature in °C or K

Question 18

What is the formula for enthalpy change?

Answer 18

ΔH = q/n (for exothermic it is -q)

• Given in kJ
• Divide J by 1000

Question 19

What is the specific heat capacity (c) and its units?

Answer 19

• The amount of heat required to raise the temperature of one gram of a substance by 1 °C or 1 kelvin.
• The units are either joules per gram per degree Celsius (J g−1 °C−1)
  OR joules per gram per kelvin (J g−1 K−1).

Question 20

What are the trends in specific heat capacity?

Answer 20

• Metals have a lower c, meaning that they heat up quickly and lose heat quickly
• Water has a high c, it takes a lot of energy to increase the temperature of water and holds on to the heat for longer
• A substance with higher specific heat capacities require more heat energy to increase their temperature and vice versa

Question 21

What values for q are there in exothermic and endothermic reactions?

Answer 21

• q is negative in exothermic reactions
• q is positive in endothermic reactions
  Given to 3 significant figures because the temperature is also measured to 3.s.f.

Question 22

How to calculate the enthalpy change (ΔH) for a reaction

Answer 22

• Calculate using the q=mcΔT formula the energy produced in J
• When the temperature increased, the answer will be negative (exothermic reaction)
• Next the limiting reactant needs to be determined. Calculate the number of moles using n=CV or n=m/Mr
• Divide each amount by the coefficient in the balanced equation
• The lower value is the limiting reactant, this value is used in the next formula
• Use the formula for the enthalpy change: ΔH = q/n (for exothermic it is -q)
• The enthalpy change is given in kJ (divide J by 1000)
• Don’t forget to convert cm3 to dm3

Question 23

How do you draw the graph of the reaction?

Answer 23

• Because heat my be lost to the surroundings, a graph can be made
• Change in temperature (y-axis) against time (x-axis) and extending the line to where the reactants were mixed
• This way the max temperature without any heat loss can be determined
  Check book

Question 24

What is the enthalpy change of neutralization (ΔHn)?

Answer 24

• The enthalpy change of neutralization (ΔHn): when an acid and base react together to form one mole of water
• Involves mixing known volume and concentrations of a strong acid and a strong base and measuring the temperature increase (equal volumes)
• On the graph temperature (y-axis) and volume of acid added in cm3 (x-axis)

Question 25

What is the molar enthalpy of combustion?

Answer 25

• Also called the standard enthalpy of combustion (ΔHƟc)
• The enthalpy change when one mole of a substance is burned completely in oxygen under standard conditions
• This is a thermochemical reaction
• The mass of the substances and temperature are involved
• The enthalpy change of combustion are always negative
• Generally the enthalpy changes calculated using the apparatus are lower than the accepted values in the data booklet
• Given in kJ mol-1

Question 26

What are four reasons why the experimental enthalpy is different to the ones in the data booklet? (Limitations)

Answer 26

• Heat loss to the surroundings
• Heat absorbed by the calorimeter
• Incomplete combustion of the substance due to the lack of oxygen
• Assumptions made about the specific heat capacity and density of aqueous solutions

Question 27

What is the formula for experimental error?

Answer 27

% error = (experimental value - theoretical value) / theoretical value
all x 100

Question 28

Assume that the density and specific heat capacities of aqueous solutions are equal to those of water. This might be a limitation.