U1.1: Measuring Energy Changes Flashcards
Define energy
the ability to do work or cause a change
How can energy be transferred
- Conduction: Direct Contact
- Convection: Movement of fluids (air, water)
- Radiation: Emission of Electromagnetic waves
What is chemical change?
Total energy before = Total energy after as energy is never created nor destroyed
What is physical change
Change in physical states
What is the system
The chemical reaction itself
What is the surrounding
Everything not the system itself
Properties of exothermic reactions (5)
- More heat is released to surroundings than absorbed by the system
- Surrounding gets hotter
- Products are more stable than reactants
- delta H is negative
- Bonds form as removing heat brings atoms closer
- Bonds are more stable
- Reactant enthalpy > Product enthalpy
Properties of endothermic reactions (5)
- More heat is absorbed from surroundings than released by the system
- Surroundings get cooler
- Products are less stable than reactants
- delta H is positive
- Bonds break as adding heat separates atoms
- Bonds are less stable
- Product enthalpy > Reactant enthalpy
What are axis labelled in energy profile diagrams
- X axis: time (s)
- Y axis: Potential Energy (kJ)
How does the energy profile for exothermic reactions look?
- Reactants higher than products
- EA is the distance from the reactant base to the peak
- delta H is negative and it is the arrow below the EA (longer) from the base of Products
How does the energy profile for endothermic reactions look?
- Products higher than reactants
- EA is the distance from the reactant base to the top of the peak (longer)
- delta H is positive and it is the arrow from the reactant base to the base of the products (shorter)
Define heat
A measure of total kinetic energy of particles in a substance
Define temperature
A measure of the average kinetic energy of particles in a substance
Define enthalpy
Amount of heat energy contained in a substance
State the 2 formulas used in the calculation of the enthalpy change of a reaction (and their units)
- Q=mc(delta t): Q is energy (J), m is mass (g), c is SHC (J/G*C), t can be *C or K
- Delta H = - (Q/n): Delta H is change in enthalpy (kJ mol^-1), Q is energy (J), n is moles (mol)
c of water
4.18 J K^-1 g^-1
m of water
v in cm^3
What are the 2 types of calorimetry
Flame and Polystyrene Cup calorimetry
Outline what flame calorimetry is
- To measure enthalpy of combustion reactions
- Burning the material and see how much energy’s released to heat up the water
Outline what polystyrene cup calorimetry is
- To measure the enthalpy of reactions in solutions
- Combining different substances and measure the change in temperature of the solutions
In polystyrene cup enthalpy, what are the 3 types of enthalpy of solutions
- Enthalpy of hydrations (dissolving in water)
- Enthalpy of displacement reactions
- Enthalpy of neutralisation
in polystyrene cup enthalpy, what is the source of error and the assumption? State the 3 ways to minimise the error
- Heat energy can be wasted
1. Stirring the mixture to ensure even distribution of heat and so that the reaction proceeds faster
2. Use insulated containers
3. Use a lid
What is the enthalpy graph for the heat of neutralisation and where is the maximum temperature found
- Two lines drawn (one positive and one negative gradient) and where they intersect is the maximum temperature
- X axis is volume in mL
- Y axis is Temp (*C)
What is the enthalpy graph in general? Where is the maximum temperature found and what does the gradient represent
- Maximum temperature is where the line starts declining back down
- Line declining back down shows reaction is over and the mixture is cooling
- X axis is time in s
- Y axis is Temp (*C)
Define Specific Heat Capacity
The heat needed to increase the temperature of a unit mass of a substance by 1K or 1*C
Define Hess’s Law
- Enthalpy change for a reaction is independent of the pathway between the initial and final states
What does the Hess’s Law Energy Cycle look like? State the symbol too
- Arrow pointing from reactants to products
- Arrow pointing from Reactants to Elements (+)
- Arrow pointing from Elements to Products (-)
Hess’s Law formula?
- Delta H1 = Delta H2 + Delta H3
What happens when the arrow switch sides
The symbol switches too
When adding two reactions where one reaction had to swap its reactants with its products to make the overall reaction, what happens to the delta H value?
- Symbol switches
