Gas laws, rate of reaction, equilibrium

Theoretical ideal gas model

All gases respond the same way to changes in volume, pressure, and temperature when mass of gas is fixed

1)Particles in a gas have negligible volume compared with the volume the gas occupies

2)There are no intermolecular forces between particles, except when the molecules collide

3)Gas particles have a range of speeds and move randomly. Average kinetic energy of particles is proportional to temperature

4) Collisions of particles with walls of the container and each other are elastic, kinetic energy is conserved

Elasticity

When a gas particle collides with the wall of a container, it bounces back with the same speed

Pressure of a gas is result of

A large number of collisions, and is the same on all walls (particles move randomly, no preferred direction)

Volume of a gas is

The total volume of its container as particle spread out to fill all available space

Effect of volume on pressure

volume increases, particles collide less frequently with walls as they take up more space, less pressure

volume decreases, take up less space and collide more frequently with walls, more pressure

Temperature effect on pressure

When temperature increases, particles have more kinetic energy, so collisions with the walls are more frequent and more energetic (particles moving faster), both increasing pressure

When temperature decreases, less kinetic energy, less frequent and less energetic collisions with the walls, less pressure

Deviations of real gases from ideal gas theory

Volume of gases not negligible, actually travel less distance between collisions with the wall, collisions more frequent, pressure is greater than predicted

Attractive forces between particles, when a particle approaches a wall attractive forces from other particles pull in the opposite direction, less energetic collisions and less pressure than predicted

This reduction of speed is most significant when average speed is low at low temperatures

Relation between pressure and volume

If temperature of a gas is held constant, it is found that increasing the pressure of a fixed mass of gas, decreases its volume

Pressure of a gas is inversely proportional to its volume

Product of pressure and volume is a constant

Unit for volume is pascal, Pa equivalent to one n/m² to get from m to cm, divide by 1000 twice, once for dm

Relationship between volume and temperature

If pressure is kept constant, and temperature is increased, volume will increase

If temperature is measured in degrees Celsius, a linear relationship

When the line is extended backwards, volume is only at zero at -273 degrees, minimum possible temp, volume can’t go below zero

If we use kelvin, relationship is directly proportional, absolute zero is when there is zero volume and that is the minimum temperature

Temperature increase then kinetic energy of particles increase so when particles collide more frequent and with more energy, to keep pressure constant, volume must increase to provide more space and less frequent collisions to keep pressure constant

Units of temperature

Used in Kelvin, absolute temperature, 0K is point when gas particles have no kinetic energy

To get from degrees to kelvin, add 273

Relationship between pressure and temperature

If volume is held constant, increasing the temperature will increase the pressure, if temperature measured in Kelvin, proportional, otherwise, linear

Increase in temperature increases the average kinetic energy of the particles, causing them to collide more frequently and with more energy with the walls, causing an increase in pressure

Molar volume

Volume occupied by one mole of any gas must be the same for all gases when under STP mv= 22.7 dm³/mol v/mv=n

3 laws for fixed mass of gas

Pressure inversely proportional to volume if temp constant

Temperature proportional to pressure if volume constant and measured in K

Temperature proportional to volume if measured in K and pressure constant

Combined gas law

P1 V1/ T1 = P2 V2/ T2

Cancel out anything it says that stays constant and do not include it

R

universal gas constant

8.31 Nm/K/mol

does not depend on the identity of the gas

Ideal gas equation

PV= NRT

Pressure in Pa (n/m²).

Volume in m³

Number of moles, mass/molar mass

Temperature in K, add 273.15 for if celsius is needed

Density can be found by

mass/volume

Gases deviate most at

Low temperature and high pressure

At low temperature gas particles are closer together have less kinetic energy, more attraction, more like a liquid

High pressure, less volume, closer together, more attraction between particles, more like a liquid

Real gases do not

obey the ideal gas law pv=nrt under all conditions

For one mole of a gas, pv/rt should be equal to one, so graph of pv/rt against p for an ideal gas is a horizontal line with intercept 1

(because when you increase P on one axis, you increase P on the other by the same, so it stays constant)

But, for real gases pv does not equal nrt so the value of pv/rt for one mole will vary

Gases behave most like ideal gas at low pressure and high temperature

Behave least at low temperature and high pressure

If the volume of the gas particles is not negligible, PV/nRT is greater than one, the collisions with the wall are more frequent and pressure is greater, so it will be greater than one

There are attractive forces between the particles, PV/RT > 1, reduces the speed of colliding particles, pressure lower than ideal gas laws, so PV/nRT <1

Rate of a chemical reaction

Speed at which reactants are converted into products during a chemical process

Rate of change in concentration over time

rate= 1/time, 1/s

Concentration rate and instantaneous rate

As the reaction proceeds, reactants converted to products so concentration of reactants decreases and concentration of products increases

ROR= Increase in product concentration over time OR decrease in reactant concentration over time

cannot be negative

Gradient here (rise/ run) is the rate of reaction

Gradient not linear, greatest at start (rate) as this is when the reactant concentration is highest, use as comparison point with different reactions, slows down next

Because of curved gradient, draw tangent to find instantaneous rate and find the gradient of that by making two points

Measuring rates of reaction using different techniques depending on the reaction

What is most convenient for that reaction

Concentration usually not measured directly, but by means of a signal that is relates to changing concentration

Change in gas volume produced

Collect gas and measure change in volume at regular time intervals, allows volume-time graph to be plotted

Gas syringe collects gas released and uses displacement to plot rate

Only if gas collected has low solubility in water

Change in mass

If the reaction is giving off a gas, the corresponding decrease in mass can be measured

Standing the reaction mixture directly on a balance. Method doesn’t work well if hydrogen gas is emitted as it is too light to show significant changes

Allows for continuous readings and a graph to be plotted

Calorimetry/spectrophotometry

Can be used if any reactants or products are coloured

Gives characteristic absorption in the visible region (320-800nm)

Indicator can be added to generate a coloured compound that can then be followed in the reaction

Pass light of specific wavelength through solution and measure intensity of light transmitted by reaction components

Concentration of coloured compound increases so absorbs more light, less light transmitted

Electric current generated according to amount of light transmitted which is connected to a graph of absorbance against time

Decrease in absorbance as coloured reactant is depleted

If it goes colour to colourless, the absorbance decreases until almost all light is transmitted through

Change in concentration measured using titration

Quenching is used, substance is introduced which stops the reaction in the sample at the moment when it is withdrawn

Freeze shot of reaction at an interval time

Repeat this many times at different intervals to see how concentration changes as reaction proceeds

Using conductivity

Total electron conductivity of solution depends on concentration of ions and their charges

If this changes when reactants go to products, Used to follow progress fo reactions

Use a conductivity meter

Machine can be calibrates using solutions of known concentrations so readings can be converted to concentrations of ions present

if 12 ions in reactants and 0 in products, show a decrease in electrical conductivity of the solution as reaction proceeds

Non continuous methods of detecting change during a reaction

Measure time taken to reach a chosen point fixed. Something observable used as an arbitrary end point that signifies to stop the clock

Different reactions take different times and this can be compared

Time is now dependent variable

Only gives an average rate over the time period

Ex time taken for a certain size of magnesium ribbon to react completely with HCl to where it is no longer visible

Magnesium ribbon is independent variable

If you see gas produced or ions in reaction

Use conductivity, gas produced, or mass decrease as rate of reaction measurements

Kinetic molecular theory

Particles in a substance move randomly as a result of their kinetic energy

Because of the random movements and collisions, not all particles in a substance at any one time have the same kinetic energy

Range of values, must take the average, directly related to its absolute temperature (K)

Solid to liquid to gas, increasing average kinetic energy and increasing temperature

Maxwell Boltzman energy distribution curve

Gas particles at a set temperature show a range in their values of kinetic energy

Area under the curve shows total number of particles in the sample

How reactants form products

Reactants placed together, Kinetic energy of particles causes collisions

Energy of collisions can cause bonds within reactants being broken, and new bonds forming

Resulting in products forming and reaction happening

rate of reaction depends on number of successful collisions, leading to formation of products

2 factors

Energy of collision

geometry of collision

Activation energy

The minimum amount of kinetic energy that particles need in order for a collision to lead to a reaction

Necessary for overcoming repulsion between molecules and for breaking some bonds in the reactants before they can react

Transition state

The state of energy that reactants must reach before the reaction can occur

Any particles that have KE greater than AE will have successful collisions

Particles with lower values of KE may still collide but these collisions won’t be successful in causing a reaction

ROR depends on proportion of particles that have a KE greater than AE

Magnitude of ROR varies from one reaction to the other, important deciding factor of ROR

Reactions with high AE proceed slower than those with lower AE, as less particles have required AE for a successful collision, will take longer for one to occur

Geometry of collision

Collisions between particles are random, they occur in many different orientations, in some reactions orientation can be crucial in deciding if the collision will be successful and so how many collisions will lead to a reaction

Collision theory

The ROR depends on the frequency of collisions that occur between particles possessing both values of KE greater than AE and appropriate collision geometry

Temperature on ROR

Increase in temp causes increase in KE of particles

Compare the same sample of particles at two different temperatures

Area under the curves the same, same number of particles

AT higher temperature, more particles have higher KE, peak of curve shifts to the right

Shift increases proportion of particles that have KE greater than AE

As temperature increases there is an increase in collision frequency due to increase in KE, but also an increase in collisions involving particles with AE needed to have successful collisions, so increase ROR

Concentration on ROR

As concentration increases the frequency of collisions between reactants particles increase therefore the frequency of successful collisions increases too, increasing ROR

More particles closer together, greater chance of reacting

As reactants are used up, their concentration falls and the ROR decreases which is why we see curve

Pressure on ROR

Reactions involving gases, increasing pressure increases ROR

higher pressure compresses gas, increasing its concentration, increasing frequency of collisions, increasing chance of successful collisions and ROR

Surface area on ROR

Subdividing a large particle into smaller parts increases the total surface area, allowing more contact and area to collide, higher probability of collisions and successful ones

Like stirring solid particles in liquids to break them up and speed up reaction

Catalyst on ROR

A substance that increases the rate of reaction without itself undergoing a chemical change

Provides an alternative route for the reaction that has a lesser AE

Larger number of particles will have the KE greater than the AE as catalyst will lower AE necessary and number of successful collisions will increase and ROR increase so product formation will too

Play an essential role in industrial processes, otherwise would proceed too slow and would have to use high temperature to increase ROR

Enzyme

Every biological reaction controlled by a biological catalyst called an enzyme which is specific for each reaction

Good for green chemistry- effective in small quantities, frequently reused, do not make waste, increase atom economy

Controls

When effecting temperature effect on ROR keep pressure, concentration, surface area constant

Graph errors

Systematic errors when all values higher than expected, precise not accurate errors in method

Random errors due to measurement equipment limitations, changes in surroundings, not enough repeats, accurate not precise

Bromine example physical system

Brome has a boiling point close to room temp so many particles have enough energy to form vapour in evaporation but as the container is sealed and vapour cannot escape, its concentration will increase and some of the vapour molecules collide with the surface of the liquid, lose energy, become liquid in condensation.

liquid to gas- evaporation

gas to liquid- condensation

Rate of condensation will eventually equal evaporation as an increase in concentration of vapour will cause more vapour to collide with the surface of the liquid (condensation)

At this point no net change in amount of liquid and has present

Equilibrium reached

Only occurs in closed system where bromine cannot escape as valour and must condense back

Characteristics equilibrium

Dynamic- Both forward and backward reactions are still occurring at same rate

Closed system- No exchange of matter with surroundings, reactants and products can react and recombine

Concentrations remain constant- Produced and destroyed at same rate

No change in observable properties- colour and density will not change as they depend on concentration

Either direction- Same equilibrium mixture result if under same conditions no matter if reaction starts with all reactants, all products, or mix

Chemical systems, 2Hi → H2 + I2

I2 is a purple gas

Start with 2HI in a sealed container

Increase in purple colour as I2 is produced

After awhile, increase in colour stops

Rate of disassociation of HI is fastest in start when concentration of HI is greatest and falls as reaction goes on

Reverse reaction (association of H2 and I2 to make 2HI) which initially is zero because no H2 or I2 produced and then speed up when concentrations of H2 and I2 increase

Rate of association = rate of disassociation so concentrations remain constant and colour in flask remains the same

Forwards vs backwards reaction

Reactants to products is forwards

Products to reactants is backwards

Equilibrium position

Proportion of reactant and product in mixture

Mixture contains more products (higher conc.) then it lies to the right

Mixture contains more reactants (higher conc.) then it lies to the left

K constant

Fixed value for a reaction at a set temp

Only use for equilibrium concentration

Products over reactants

High value of K means more products than reactants, lies to the right, almost goes to completion

Low value of K means more reactants than products, lies to the left, does not go to completion

equals one, significant amount of products and reactants at equilibrium

equilibrium constant

The equilibrium constant for one reaction is the reciprocal for its reverse reaction (products and reactants switched )

Weak vs strong acids

Weak acids go very little disassociation ( low K value )

Stronger acids more disassociation (high K value)

Normally lies left for acid disassociation

La chaterliers principle

A system at equilibrium that is subjected to change will respond in such a way to minimise the effect of that change

Whatever we do to a system, it will respond in the opposite way

After awhile a new equilibrium will be established with a different composition to previous mixture

Changes in concentration

Increase in concentration in one of the reactants

Cause rate of forward reactions to increase and backwards not effected, reactions rates now not equal

Equilibrium will shift in favour of products

New concentrations of reactants and products

Equilibrium re establishes itself

Value of K unchanged

Vice versa, decrease in concentration of product, shift equilibrium in favour of product, forwards reaction

New equilibrium position

In industrial processes

Products will be removed as soon as they form to ensure equilibrium is pulled to the right which will increase yield of product

Changes in pressure

Equilibria involving gases will be effected by a change in pressure if reaction involves change in number of molecules

If reaction is subject to an increase in pressure , system decreases this pressure by favouring side with smaller number of molecules, vice versa if decrease in pressure, favour side with larger number of molecules

temperature doesnt change so K value stays same

Count molecules by coefficient numbers of reactants vs products, which has more

Changes in temperature

K is temperature dependent, changing temp will change K

Exothermic releases energy, -H

endothermic absorbs energy, +H

Negative sign for enpalthy means forward reaction is exothermic and heat is a product. Therefore if you increase heat, reaction wants to make less, so endothermic reaction favoured ( K decreases as you have more reactants than products). decrease temp, wanna make more, exothermic reaction favoured (value of K increases as products more than reactants)

Positive sign for enpalthy means forward reaction is endothermic and heat is a reactant, so if temperature increases you wanna make less so forward reaction, endothermic is favoured, (K value gets higher as you make more products than reactants). if heat is decreased, then backward reaction, exothermic is favoured as you wanna make more, K value lowers (more reactants than products)

Addition of a catalyst

Because the forward and backward reactions pass through same transition state, catalyst lowers activation energy by the same amount for both

No effect on position of equilibrium or K value

Speeds up attainment of equilibrium and so cause quick formation of products

Used in processes to speed up Rate of product formation

Everything only in gaseous steam or heterogeneous mixture with reactants and products in liquid and gaseous states

Haber process

Based on N2+2H2 →← 2NH2 -93 kJ/mol.

Maximise conversion of reactant to product

Concentration- Increase concentration of reactants, or decrease concentration of products while it forms to switch equilibrium to the right

Pressure- High pressure to shift equilibrium right (less molecules on right)

Temperature- Low temperature, exothermic reaction favours, this causes low ROR so do not make temp too low

Catalyst- Used to speed up reaction and make up for low temperature

Haber process only achieves conversion of reactants to products of 10-20 percent per pass through reactor

Unconverted reactants recycled to reactor to obtain a 95 percent yield

Prevent and minimise waste for low equilibrium yield

concentration is constant but not

equal