R1.1 - measuring enthalpy changes Flashcards
What is a chemical reaction?
A chemical reaction involves the transfer of energy between the system and the surroundings, with total energy being conserved.
What is the difference between heat and temperature?
Heat - process of energy transfer that occurs due to a temperature difference.
Temperature - measure of the average kinetic energy of particles.
What is energy?
Energy - measure of the ability to do work.
What is the system and the surroundings?
System - reaction mixture.
Surroundings - everything else
What is enthalpy?
Enthalpy (H) - is a measure of the chemical potential energy stored in a system. The enthalpy changes as chemical bonds and intermolecular forces change during a reaction, resulting in a heat transfer which can be observed.
What are the characteristics of endothermic reactions?
- ΔH is positive
- Surroundings –> System
- Bond breaking
- Reactants more stable than products
- E.g) Thermal decomposition
What are the characteristics of exothermic reactions?
- ΔH is negative
- System –> Surroundings
- Bond making
- Products more stable than reactants
- E.g) Combustion
What is an open system?
Open system - system where both energy and matter can be exchanged.
What is a closed system?
Closed system - a system where only energy can be exchanged, but not matter.
What is an isolated system?
Isolated system - system in which neither energy or matter can be exchanged.
What is activation energy?
Activation energy - minimum amount of energy required for particles to collide and for a reaction to occur.
What is standard enthalpy change?
Standard enthalpy change (ΔH) - heat transferred at constant pressure under standard conditions and states, determined by a change in temperature of a pure substance, measured in kJ mol-1
What are the standard conditions?
- Pressure of 100kPa
- Concentrations of 1 mol dm-3
- All substances should be in their standard states
What is the standard state of a substance?
Standard state - the pure form of the substance under standard conditions of 100kPa and 298K.
What is specific heat capacity?
Specific heat capacity (c) - heat needed to increase the temperature of a unit mass of a material by 1K.
How do you calculate specific heat capacity?
Specific heat capactity = (heat added (q)) / mass (m) x temperature change (ΔT)
What are 3 assumptions about enthalpy change?
1) Heat is lost as soon as zinc is added and the reaction starts.
2) All heat generated was absorbed by water.
3) Volume of CuSO4 = Volume of water (1g cm-3 density)
Why might enthalpy change be less than expected?
1) Not investigated under standard conditions
2) Heat lost to the surroundings
