Measuring energy changes Flashcards
What are the forms of energy that are involved in chemical reactions?
- 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
What is heat and when is thermal equilibrium reached?
- 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
What is enthalpy and how is it related to heat?
- 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)
What are the standard conditions and what is the symbol used?
- Ɵ (plimsoll symbol)
- Pressure at 100kPa
- Temperature at 25°C (298K)
- These conditions differ to the standard conditions involving gases (STP)
What is temperature and what is the kelvin scale?
- 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
What are systems and its surroundings?
- System: the chemical reaction itself
- Surroundings: the apparatus that contains the reaction and a thermometer, everything around the reaction
What are the types of systems?
- 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
What is exothermic and endothermic and what are the enthalpy changes?
- 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
What is the law of conservation of energy?
- 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
What are examples of exothermic reactions?
- 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
Check book for enthalpy level diagrams
Explain the exothermic enthalpy level diagram.
- 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
What are thermochemical equations?
- They show the enthalpy change (∆H) of a reaction
E.g. The equation ΔHƟc = −890 kJ mol−1
Explain the endothermic enthalpy level diagram.
- 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
What units are used for the enthalpy change?
- For ΔH Joules (J mol-1) used
- Sometimes kJ mol-1 are used which is:
1kJ is 1000J
What is a calorimeter and why is using a polystyrene cup a good calorimeter?
- 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
What is the formula for calculating the enthalpy changes?
- 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
What is the formula for enthalpy change?
ΔH = q/n (for exothermic it is -q)
- Given in kJ
- Divide J by 1000
What is the specific heat capacity (c) and its units?
- 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).
What are the trends in specific heat capacity?
- 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
What values for q are there in exothermic and endothermic reactions?
- 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.
How to calculate the enthalpy change (ΔH) for a reaction
- 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
How do you draw the graph of the reaction?
- 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
What is the enthalpy change of neutralization (ΔHn)?
- 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)
What is the molar enthalpy of combustion?
- 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
What are four reasons why the experimental enthalpy is different to the ones in the data booklet? (Limitations)
- 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
What is the formula for experimental error?
% error = (experimental value - theoretical value) / theoretical value
all x 100
Assume that the density and specific heat capacities of aqueous solutions are equal to those of water. This might be a limitation.
