Energy and Rates

Energy and Rates

SCH 4U1Mr. Dvorsky

Chemistry = the science of matter, especially its chemical reactions. Thermochemistry = the study of energy changes that accompany these reactions

What is the universe?

What is energy?

• We all talk about energy: we eat energy snacks, we guzzle energy drinks, we pay energy bills, and try to use green energy…….but what is energy??

• -energy can manifest itself in two forms: kinetic and potential – these may take many forms – electrical, chemical, nuclear, etc.

• A very common form is heat. What often happens is that energy is transformed into heat and when you have heat at an elevated temp you can use it to do many useful things like driving an engine.

• Energy is often converted to heat when we burn fuel

What is heat?

• Thermal energy is the sum of all kinetic energies of all the particles of a sample of matter.

• You cannot measure the sum of the kinetic particles of a system

but....you can measure the temperature – a measure of the average kinetic energy of all the particles of a sample of matter.

…one more thing

Specific Heat Capacity, c-also needed for the calculation mentioned on the previous slide is the amount of energy needed to increase the temperature of one gram of a substance by one degree Celsius.

-value determined experimentally for every substance. See table 5.1 on page 280.

• The amount of heat that enters or leaves an object being heated or cooled:

Use the following equation

• Energy is often converted to heat when we burn fuel, but when we have energy available in electrical form you can generate light or propel an electric ventilator. This is all possible with different forms of energy.

• -energy comes in two major categories which subdivide into many different forms and can be converted from one form to another.

Examples: -The chemical energy in my muscles can be converted into mechanical energy to bend this metre stick.-solar panels transforming light energy into electrical

• Energy can be converted from one form to another this much we have shown - however not immediately intuitive is that energy cannot be created nor destroyed. Energy cannot be used up. This seems very odd. Especially when you pay your electric bill or fill your car with gas. Despite financial evidence to the contrary, none of the energy is used up but just converted to a different form.

We don’t use energy, that is not true.

• -a car burns fuel, chemical energy is transformed into heat. -part of this heat is transformed to do used to do work (propel a car), but the total amount of energy is entirely the same.

• The only difference is that you started with a certain amount of fuel (energy concentrated in a chemical form in the fuel) and once you started burning it you could displace yourself (which is good) but the fuel is transformed into different substances, and the concentrated form of energy has been dispersed over a large area in the environment. Total amount of energy before or after is the same but it is converted into a different form.

• So in daily language when we say we use energy, we are just actually transforming it from one form to another.

• You cannot make energy• You cannot destroy energy• You can only convert it from one form to another.

THIS IS THE 1ST LAW OF THERMODYNAMICS!

Euniverse = constantΔE = 0From before universe = system + surroundingsSoEuniverse = Esystem + Esurroundings

and…ΔEuniverse = ΔEsystem + ΔEsurroundings = 0

From this we get ΔEsystem = -ΔEsurroundings -warming up soup on stove, if the system gains energy it must come from the surroundings

Enthalpy, H• Also referred to as heat content• The total energy of a system plus the pressure x volume

(H = E+PV)• Impossible to measure but enthalpy change, ΔH, can be. ΔH = ΔE + ΔPV-enthalpy change = change in energy + change in pressure& volume-For reactions of solids and liquids in solution that take place in an open beaker, we can assume there is no change in volume of pressure. -So the heat exchanged by the system and its surroundings = enthalpy change or ΔH = ΔE = Q