What are the 4 ways to communicate enthalpy change?
Molar enthapy, enthalpy change, thermochemical equations, potential energy diagrams.
In most senarios, in terms of bonds, when is energy absorbed and when is it released
Breaking chemical bonds requires energy (endothermic/absorbed) and forming bonds releases energy (exothermic/released)
In more detail, whem more energy is absorbed to break bomds than is released when forming new bonds, the overall change is endothermic.
When more energy is released forming new bonds than is absorbed to break the existing bonds, the overall change is exothermic.
True or false: The individual enthalpy or the total energy of a system cannot be determined experimentally. However, enthalpy changes can be determined experimentally from the energy changes of the surroundings.
True
Standard molar enthalpy of a reaction:
The quantity of heat transferred in a reaton when products and reactants are at standard conditions. The circle symbol means standard.
Molar enthapy (Kj/Mol):
Is the amount of heat lost or gained by a substance undergoing a change (the enthalpy change that occurs for 1 mol of that substance). By convention, a subscript is used to indicate the change, for example, r=reaction, c=combustion, f=formation, d=decomposition, n=neutralization, s or sol= solution (dissolving)
Thermochemical equations (energy is a term in the equation)
Thermochemical equations are chemical reactions with the energy in them. The units are kJ NOT kJ/mol.
Delta H in the equation is NOT the same as experimental delta H
If a reaction is exothermic, it releases energy, so you can write the amount of energy released as part of your equation on the product side.
Ex. A+B ---> C+D+ 98kJ
Similarly, if a reaction is endothermic, it requires energy, so you can write the amount of energy it needs right into the equation on the reactnat side:
A + 300kJ ---> B+C
Enthalpy change (delta H)
Equal to the energy (kJ) absorbed or released by the system. This change is directly related to the change in the potential energy of the system. Enthalpy change depends on the number of moles of a reactant and product in a chemical reaction.
Enthalpy
The total internal energy of a system
How is energy transferred from a hotter substance to a cooler substance according to the particle theory of matter?
Moving particles have kinetic energy. Hotter substances contain particles that have a higher avg kinetic energy while colder substances contain particles that have a lower avg kinetic energy. When particles of a hot substance collide with particles of a cooler substance, they cause the particles in cooler substances to speed up and the particles in the hotter substance will slow down. Eventually both substances will have the same temp.
First law of thermodynamics:
energy cannot be created or destroyed, it can however be transformed from one type of energy into another.
Specific heat capacity:
The amount of heat required to raise the temperature of a unit of mass (usually one gram) by one degree Celsius.
Heat:
The amount of energy transferred between substances or objects. Is the amount of energy lost or gained.
Units: j, Kj, Mj
Second law of thermodynamics:
heat is spontaeneously transferred from an object at a higher temperature to an object with loweer temperature untill they both reach the same temperature
Kinetic molecular theory:
All matter is made of particles in motion.
Lots of motion= Lots of kinetic energy= High temperature
Temperature:
The average kinetic energy of the particles of a substance
Endothermic energy changes:
The temperature of the surroundings will decrease because the substance has absorbed energy. Denote with a positive sign.
Exothermic energy changes:
The temperature of the surroundings will increase because the substance has released energy. We denote exothermic energy changes with a negative sign
Explain what this potential energy diagram is trying to communicate.
At first you have the reactants, if they collide with the right orientations and enough energy (activation energy), the molecules will break the bonds between them. Now we are in the transition state, where the white atoms could re-bind to eachother or they could bind to a new black atom. In this graph, the white atoms bind to the black atoms. In order for these bonds to form, energy must be released, causing the potential energy of the system to go down. In this case, the energy released to form bonds was greater than the energy absorbed to break them, causing the reaction to be exothermic.
When will a collision between reactant particles actually form products?
1. if the reactant particles have the right orientation at the moment of impact
2. if the minimal amount of collision energy needed (called the activation energy) is present. (All reactant collisions below activation energy will not result in reactions.)
Increase temperature --> more collisions ---> more reactions
Collision theory:
Reactant particles must collide for a reaction to occur, but only a certain portion of the total collisions will result in a chemical reaction.
Which one is exothermic and which one is endothermic?
Left: Exothermic
Right: Endothermic
describe the law of conservation of energy and the second law of thermodynamics:
Law of conservation of energy: Energy is neither created nor destroyed, only transformed from one form to another. The energy released from a chemical reaction in a calorimeter changes the temp of the calorimeter (water) surrounding it. Heat lost by a system= heat gained by the surroundings (calorimeter/water)
Second law of thermodynamics: Thermal energy is spontaeneously transferred from an object at a higher temp to an object at a lower temp untill the two objects reach the same temp (equlibrium)
explain.
In a calorimeter, as a reaction occurs, the kinetic energy of the water will increase while the potential energy of the system goes down due to the exothermic nature of combustion. Then, if the reaction is reversible (often its combustion, so its very hard), the system will gain potential energy while the water will lose kinetic energy since it is a reverse reaction
Bomb calorimeter
a bomb calorimeter is a closed system (with changing pressure as the combustion reaction proceeds). Therefore the water product is in a liquid, rather than a gaseous state.
How do we calculate heat transfer in a calorimeter?
Energy is transferred netween the system and water. We can measure the temperature change of the water to calculate the heat lost/gained by the system but gained/lost by the water.
If the temperature of the surroundings (often water) increases, the chemical system has undergone an exothermic reaction (negative enthalpy change)
If the temperature of the surrounding (often water) decreases, the chemical system has undergone an endothermic reaction (positive enthalpy)
What is Calorimetry? WHat are the assumptions made for Calorimetry?
the process of measuring heat released or absorbed by a physical or chemical process, using an isolated system called a calorimeter.
Assumptions: The isolated system does not allow products to escape or thermal energy to be transferred to the air or other materials around it.
The calorimeter dosen't actuallt absorb any heat itself
The system being studied is surrounded by a small quantity of water with stable specific heat capacity and density. (Water has a high specific heat capacity, therefore a small valume is needed to see a temp change.)
Diffrentiate between an open system, a closed system, and an isolated system.
Open system: matter and energy can be exchanged (Ex. A cell)
Closed system: Matter cannot be exchanged but energy can (Ex. Earth)
Isolated system: neither matter nor energy can move in or out (Ex. Thermos)
Describe the common catalysts we need to know:
CFC- catalyzes the conversion of O3 (g) into O2 (g)
Sulfuric Acid- in the oil industry
Nitric acid- in making fertilizer
Catalytic converters in cars --> convert harmful NOx (g) and CO (g) into less harmful N2 (g) and CO2 (g)
Enzymes- in biological processes
What is a catalyst?
A catalyst is a substance that increases the rate of chemical reaction without being consumed in the reaction. It provides an alternate pathway (requiring less energy) for the reaction to occur.
Kinetic energy and potential energy
Kinetic energy: Energy of motion. Temperature is a measure of the motion of particles
Potential energy: Stored energy (in chemistry we deal with the energy stored in chemical bonds)
Explain what this potential energy diagram is trying to communicate.