09 - Thermodynamics Flashcards
State the equation for specific heat capacity.
ΔE = mc Δ θ
What is specific heat capacity?
It is the amount of energy needed to raise the temperature of 1kg of a substance by 1K (or 1°C).
What is specific latent heat?
Specific latent heat is the energy required to change the state of a substance without a change of temperature.
State the equation for specific latent heat.
Δ Е = Δ ml
What is internal energy?
The sum of the molecules’ randomly distributed potential and kinetic energies.
What can be said about the particles of a substance at absolute zero?
At absolute zero, the particles have zero kinetic energy.
Express absolute zero in degrees celsius.
OK =-273°C
What is temperature a measure of?
The average kinetic energy of the molecules in a given substance.
State the ideal gas equation.
pV = NkT
What is the relationship between pressure and temperature?
Pressure and temperature are directly proportional
What is the relationship between pressure and volume?
Pressure and volume are inversely proportional
What is a black body?
A black body is one that absorbs radiation of all wavelengths.
What determines the spectrum of radiation emitted by a black body radiator?
The temperature of the body.
What law can be used to link the power of radiation emitted from a black body to its temperature?
The Stefan-Boltzmann law.
State the Stefan-Boltzmann equation.
L = σ AT^4
How is the motion of gas molecules described?
Gas molecules move with Brownian motion, which is the random motion of molecules caused by collisions with larger particles.
How does a gas exert a force on its container?
• The molecules collide with the walls of their container.
• Collisions cause a change in momentum.
• A change in momentum produces a force equal to the rate of change of momentum.
What does Boyle’s Law state?
When a gas is at a constant temperature, pressure and volume are inversely proportional to each other.
Explain Boyle’s Law.
• When the volume of a gas increases, the space between molecules increases and so the time between collisions is larger.
• This causes the rate of collisions and so the rate of change of momentum to decrease.
• This means the force exerted is lower, causing a decrease in pressure.
What does Charles’ Law state?
When a gas is at a constant pressure, the volume is directly proportional to the absolute temperature.
Explain Charles’ Law.
• As temperature increases, the average kinetic energy of the molecules increases.
• Pressure is constant so the force and so also the rate of change of momentum, must remain constant.
• To achieve this, the volume increases so the faster speed of the molecules is compensated by there being larger gaps between them.
What does the Pressure Law state?
When a gas has a fixed volume, pressure is directly proportional to the absolute temperature.
Explain the Pressure Law.
• As temperature increases, the average kinetic energy, and so the speed of the molecules also increases.
• This increases the rate of collisions, and so produces a larger rate of change of momentum.
• This leads to a greater force exerted and so an increase in pressure.
In kinetic theory, what is assumed about the gases involved?
• The gas contains a large number of molecules.
• The molecules are identical to each other.
• All collisions between molecules and the walls of their container are perfectly elastic.
• The time taken for collisions is negligible compared to the time between collisions.
• There are no intermolecular forces between molecules.
• Molecules are in constant random motion.
• The gas particles obey Newton’s Laws of motion.
What follows all the assumptions made in kinetic theory?
Ideal gases.
State the relationship between volume and pressure for a given quantity of gas at a fixed temperature.
Pressure is inversely proportional to volume.
P = k / V
State the relationship between volume and temperature for a given quantity of gas at a fixed pressure.
Volume is directly proportional to temperature.
V = kT
State the relationship between pressure and temperature for a given quantity of gas with a fixed volume.
Pressure is directly proportional to temperature.
P= kT
Combine the relationships between pressure, volume and temperature into a single expression, with a constant.
pV
——— = constant
T
State the ideal gas equation.
pV = NkT
What does ‘k’ represent in the ideal gas equation?
The Boltzmann Constant
What does ‘N’ represent in the ideal gas equation?
The number of molecules
How do you convert between the number of moles and number of molecules in a sample?
Multiply the number of moles by the
Avogadro constant.
State 3 ideal gas assumptions about the motion of the particles in an ideal gas.
- The particles obey Newton’s Laws of motion
- They travel in straight lines
- They are in random motion and they experience perfectly elastic collisions
What can be said about the internal energy of an ideal gas?
There are no intermolecular forces and so there is no potential energy. This means the internal energy is entirely kinetic energy.
Describe the collisions between the molecules in an ideal gas.
The collisions between molecules and between molecules and the container are elastic.
What assumptions are made about the size of the molecules in an ideal gas?
- The volume of the molecules is negligible in comparison to the volume of the container
- All the molecules are identical
What’s Wien’s Law?
Peak wavelength is the wavelength at which the emitted radiation is most intense ie. there is a peak on the black body radiation curve
What is the peak wavelength?
The peak wavelength is the wavelength at which the emitted radiation is most intense ie. there is a peak on the black body radiation curve.
What is the Stefan-Boltzmann law equation?
L= σ AT^4
L is power output (W), σ is the
Stefan-Boltzmann constant, A is the surface area of the black body (m^2) and T is the temperature (K)
What equation can be used to calculate the kinetic energy of a molecule from its temperature?
KE=3/2kT
K is the boltzmann constant and T is the temperature (K)
Factors affecting gas pressures
Particle speed - where gas particles move faster due to increased temp, they collide with the container walls more frequently leading to increased pressure
Adding more particles- increasing the overall force they exert on the walls.
What is gay-lussac’s law
States that pressure of a given mass of gas is directly proportional to it’s absolute temperature, providing volume remains constant
What is boyle’s law and what does it assume
Pressure is inversely proportional to volume
It assumes thermal equilibrium (constant temp)
What is a black body radiator
An object that emits Em radiation in all parts of the spectrum perfectly in all directions
Explain why pressure increases with temperature
- As the temperature of the gas increases the (average) speed/Ek of the atoms increases
- Greater speed/Ek so the momentum of the atoms increases
- The rate/frequency of collision of atoms with the container walls increases Or the time between collisions with the walls decreases
- The rate of change of momentum at the walls increases
- Rate of change of momentum is equal to the force
- Pressure is force/area and the force (on the walls) is greater
3/4 must include reference to collisions with WALL
