What is a system, and what is surroundings?
In a chemical reaction, what happens between them?
System: The reactants and products (usually)
Surroundings: Everything else outside the system
Energy is transferred between the system and the surroundings.
State the law of conservation of energy. If 10J of energy is lost by the system, how many J of energy is gained by the surroundings and why?
Law of conservation of energy states that energy can be changed from one form into another but cannot be created or destroyed. 10J, as the energy lost by the system is equal to that gained from the surroundings.
Internal energy definition
The internal energy of a substance is the sum of the total kinetic energy and potential energy of all particles in that substance.
Can enthalpy be measured directly? What is another name for it?
No. Heat content.
Enthalpy change equation
🔼H = Hp - Hr
or = Hf - Hi
Hp= enthalpy of the product
Hr= enthalpy of the reactant
Hf = final enthalpy
Hi = initial enthalpy
At constant volume, relationship between change in internal energy & heat change?
Heat change at constant volume = Change in internal energy
What does negative & positive sign of change in internal energy indicate?
Negative: That heat is released to the surroundings
Positive: That heat is taken in from the surroundings
Enthalpy change definition
Enthalpy change is the heat released or taken in during any change in a system, provided that the system is kept at constant pressure.
Heat change at constant pressure = ?
= enthalpy change of the reaction
= change in internal energy + work done on the surroundings
Define exothermic & endothermic reactions
Exothermic reaction is a reaction that the system releases heat to the surroundings.
Endothermic reaction is a reaction that the system takes in heat from the surroundings.
How do we measure whether the reaction is exothermic or endothermic?
Use a thermometer to measure the surroundings. (E.g. measure the temperature of water in the reaction of AgNo3 and NaCl)
Exothermic: Temperature of surroundings rises
Endothermic: Temperature of surroundings drops
Name 4 classic examples of exothermic reactions
Neutralization
Combustion
Precipitation
Calcium oxide (Quicklime) + water
Examples of endothermic reactions
Cracking of petroleum fractions (need heating!)
Thermal decomposition of CaCO3!
Deduce the sign for 🔼H in an exothermic & endothermic reaction
Exothermic: -ve.
- 🔼H= Hf-Hi
- heat is released to surroundings in exothermic reaction
- Hf<Hi
- it is -ve
Endothermic: +ve
- 🔼H= Hf-Hi
- heat is absorbed from the surroundings
- Hf> Hi
- +ve
standard enthalpy change conditions
- temperature of 298K
- pressure of 1 atm
- all substances in their standard states, the most stable physical state at 298K and 1 atm
- all solutions at a concentration of 1 M
standard state of carbon & how to write its state in equation?
graphite, not diamond!
C (graphite)
Standard enthalpy change of reaction definition
Standard enthalpy change of reaction is the enthalpy change when the number of moles of reactants specified in the thermochemical equation reacts completely under standard conditions.
Standard enthalpy of combustion definition
Standard enthalpy of combustion of a substance is the enthalpy change when one mole of the substance burns completely in oxygen under standard conditions (298K, 1atm)
function of polystyrene board when using simple calorimetric method for determining enthalpy changes of combustion
reduce heat loss to surroundings by conduction.
why should water be stirred when using simple calorimetric method for determining enthalpy changes of combustion
to ensure water is evenly heated
Equation to calculate heat released of a substance
mc🔼T (mass x specific heat capacity x temperature change)
- in water: maximum temperature rise of water, specific heat capacity of water
Specific heat capacity: symbol & definition
symbol c
Specific heat capacity is the amount of heat required to raise the temperature of 1g of the substance by 1 K (or 1 degree celsius)
Specific heat capacity of water is?
4.2 J/gK (4.2 J g-1 K-1)
