3.2 Periodic Table And Energy 9.1-10.5 Flashcards
Enthalpy
H
A measure of the heat energy in a chemical system.
Thought of as the energy stored within bonds.
Enthalpy can’t be measured but enthalpy changes can
Enthalpy change
Change in enthalpy
(Triangle H)
Enthalpy charge= H(products) - H(reactants)
It can be positive or negative depending on whether the products contain more or less energy than the reactants
System
Refers to the atoms, molecules or ions making up the chemicals
Heat can be transferred in two directions
From system to surroundings- exothermic change
From surroundings to system- endothermic change
Exothermic
Heat out of the system
The chemical system releases heat to the surroundings
Any energy loss by the chemical system is balanced by the same energy gain by the surroundings
Enthalpy change is negative
The temperature of the surroundings increase as they gain energy
Endothermic
Heat into the system
The chemical system takes in heat energy from the surroundings
Any energy gain by the chemical system is balanced by the same energy loss by the surroundings
Enthalpy change is positive
The temperature of the surroundings decreases as they lose energy
Activation energy
Atoms and ions are held together by chemical bonds. During chemical reactions, the bonds in the reactants need to be broken by an input of energy. New bonds in the products can then form to complete the reaction. The energy input required to break bonds acts as an energy barrier to the reaction, known as the activation energy Ea. Activation energy is the minimum energy required for a reaction to take place
Standard enthalpy changes
The enthalpy change for a reaction can vary slightly depending on the conditions used. Chemists use standard conditions for physical measurements such as enthalpy changes, close to typical working conditions of temperature and pressure.
Standard pressure
100 kPa. This is very close to a pressure of one atmosphere, 101 kPa
Standard temperature
Is a stated temperature, usually 298 Kelvin (25°C)
Standard concentration
1 mol dm-3
Standard state
The physical state of the substance under standard conditions. Most data tables show the standard state at 100 kPa and 298 Kelvin
Enthalpy change of reaction
The standard enthalpy change of reaction is the enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states
Enthalpy change of reaction always refers to a stated equation, and its value depends on the balancing numbers.
Enthalpy change of formation
The enthalpy change that takes place when one mole of a compound is formed from its elements under standard conditions, with all reactants and products in their standard states
Enthalpy change of formation in elements
Enthalpy change of formation for an element refers to the formation of one mole of an element from its elements. This is clearly no change, so all elements have an enthalpy change of formation of 0kJ mol-1
Enthalpy change of combustion
Is the enthalpy change that takes place when one mole of a substance reacts completely with oxygen under standard conditions, with all reactants and products in their standard states
When a substance reacts completely with oxygen the products are the oxides of the elements in the substance
Enthalpy change of neutralisation
Is the energy change that accompanies the reaction of an acid by a base to form one mole of H2O(l) under standard conditions, with all reactants and products in their standard states
For enthalpy change of neutralisation, neutralisation involves the reaction of H+ with OH- to form one mole of H2O. The value of enthalpy change of neutralisation is the same for all neutralisation reactions
The Kelvin scale of temperature
It starts at absolute zero, zero Kelvin and is equivalent to -273°C. On the kelvin scale, ice melts at 273 kelvin (0°C) and water boils at 373 Kelvin(100°C)
So a 1 Kelvin rise in temperature is the same as the 1°C rise in temperature
Calculating an energy change
The energy change of the surroundings is calculated from three quantities – mass, specific heat capacity, and temperature change
Calculating an energy change – the mass of the surroundings
The mass is measured simply by weighing. You have to identify the materials that are changing temperature. Mass is usually measured in grams
Calculating an energy change – the specific heat capacity of the surroundings
Different materials require different quantities of energy to produce the same temperature change. The specific heat capacity is the energy required to raise the temperature of 1 g of a substance by one Kelvin
The specific heat capacity of water is 4.18J g-1K-1
Calculating an energy change – the temperature change of the surroundings
The temperature change is determined from the thermometer readings
Change in temperature = final temperature - initial temperature
Energy change equation
Heat energy = mass X specific heat capacity X temperature
q=mctriangleT
Causes of inaccuracy in experiments for finding the enthalpy change of combustion
- Heat loss to the surroundings other than the water. This includes the beaker but mainly the air surrounding the flame
- Incomplete combustion of methanol. There may be some incomplete combustion, with carbon monoxide and carbon been produced instead of carbon dioxide. You will see carbon as a black layer of soot on the beaker
- Evaporation of methanol from the wick. The burner must be weighed as soon as possible after extinguishing the flame. Otherwise some methanol may have evaporated from the wick. Spirit burners usually have a cover to reduce this error
- Non-standard conditions. The data book value is a standard value. The conditions for this experiment are unlikely to be identical to standard conditions
Determination of an enthalpy change of reaction
Using plastic cups made of polystyrene foam
These are cheap, waterproof and lightweight and offer some insulation against heat loss to surroundings
When carrying out reactions between aqueous solutions, the solution itself is the immediate surroundings. The chemical particles within the solution may react when they collide, and any energy transfer is between the chemical particles and water molecules in the solution. A thermometer in the solution will record any temperature change allowing the heat energy change to be calculated using mctriangleT
Average bond enthalpy
The energy required to break one mole of a specified type of bonds in a gaseous molecule
Energy is always required to break bonds
Bonds enthalpies are always endothermic
Bond enthalpies always have a positive enthalpy value
Limitation of average bond enthalpies
The actual bonds enthalpy can vary depending on the chemical environment of the bonds.
An average bond enthalpy is calculated from the actual bond enthalpies in different chemical environments
Bond breaking
Energy is required to break bonds
Bond breaking is endothermic
Enthalpy change is positive
Bondmaking
Energy is released when bonds form
Bondmaking is exothermic
And enthalpy change is negative
Calculating enthalpy changes from average bond enthalpies
The enthalpy change of reaction can be found by calculating the bonds enthalpies of the bonds in the reactants and the products
Hess’ law
If a reaction can take place by two routes, and the starting and finishing conditions are the same, the total enthalpy change is the same for each route
Rate of reaction
Measures how fast a reactant is being used up or how fast a product is being formed.
Change in conc/time
mol dm-3 s-1
Factors that alter the rate of a chemical reaction
Conc or pressure
Temp
Use of a catalyst
Surface area
Collision theory
Two reacting particles must collide for a reaction to occur.
Usually only a small proportion of collisions result in a chemical reaction. In most collisions, the molecules collide but then bounce off each other and remain chemically unchanged
Why are some collisions effective and others ineffective?
An effective collision is one that leads to a chemical reaction.
A collision will be effective if two conditions have been met:
– the particles collide with the correct orientation
– the particles have sufficient energy to overcome the activation energy barrier of the reaction
How does increasing conc affect the rate of reaction
When the conc of a reactant is increased, the rate of reaction generally increases. An increase in conc increases the number of particles in the same volume. The particles are closer together and collide more frequently. In a given time there will therefore be more effective collisions and an increased rate of reaction
How does increasing the pressure of a gas affect the rate of reaction
When a gas is compressed into a smaller volume the pressure of a gas is increased and the rate of reaction increases. The conc of the gas molecules increases as the same number of gas molecules occupy a smaller volume. The gas molecules are closer together and collide more frequently, leading to more effective collisions in the same time
Methods for following the progress of a reaction
Monitor the removal (decrease in concentration) of a reactant
Follow the formation (increase in concentration) of a product
Methods for following the progression of a Reaction that produces gases
Monitor the volume of gas produced at regular time intervals using gas collection
Monitor the loss of mass of reactants using a balance
Catalyst
A substance that increases the rate of a chemical reaction (by providing an alternative reaction pathway of lower activation energy) without undergoing any permanent change itself
– the catalyst is not used up in a chemical reaction
– the catalyst may react with a reactant to form an intermediate or may provide a surface on which the reaction can take place
– at the end of the reaction the catalyst is regenerated
Types of catalyst
Homogeneous
Heterogeneous
Homogeneous catalyst
Has the same physical state as the reactants. The catalyst reacts with the reactants to form an intermediate. The intermediate then breaks down to give the products and regenerates the catalyst
Heterogeneous catalysts
Has a different physical state from the reactants. Heterogeneous catalysts are usually solids in contact with gaseous reactants or reactants in solution
Reactant molecules are absorbed (weakly bonded) onto the surface of the catalyst, where the reaction takes place
After reaction, the product molecules leave the surface of the catalyst by desorption
Catalysis- sustainability and economic importance
It’s estimated that 90% of all chemical materials are produced using a catalyst. Catalysts increase the rate of many industrial chemical reactions by lowering the activation energy. This then reduces temp needed for the process and the energy requirements
If a chemical process requires less energy, then less electricity or fossil fuel is used. Making the product faster and using less energy can cut costs and increase profitability. The economic advantages of using a catalyst outweigh any costs associated with developing a catalytic process
The modern focus on sustainability requires industry to operate processes with high atom economies and fewer pollutants. Using less fossil fuel will cut emissions of carbon dioxide, a gas linked to global warming
Features of the Boltzmann distribution
No molecules have zero energy- the curve starts at the origin
The area under the curve is equal to the total number of molecules
There is no maximum energy for a molecule- the curve doesn’t meet the x axis at high energy . The curve would need to reach infinite energy to meet the x axis
The Boltzmann distribution and temperature
As the temp increases, the average energy of the molecules also increases. A small proportion of molecules will still have low energy, but more molecules have higher energy. The graph is now stretched over a greater range of energy values. The peak of the graph is lower on the y axis and further along the x axis- the peak is at a higher energy. The number of molecules is the same, so the area under the curve remains the same
Moving particles- at a higher temp
More molecules have an energy greater than or equal to the activation energy
Therefore a greater proportion of collisions will lead to a reaction, increasing the rate of reaction
Collisions will also be more frequent as the molecules are moving faster, but the increased energy of the molecules is much more important than the increased frequency of collisions
The Boltzmann distribution and catalysts
A catalyst provides an alternative reaction route with a lower activation energy. Compared to activation energy without a catalyst, a greater proportion of molecules now have an energy equal to or greater than the lower activation energy, activation energy with catalyst.
On collision, more molecules will react to form products. The result is an increase in the rate of reaction
Dynamic equilibrium
At equilibrium both the forward and reverse reactions are taking place. As fast as the reactants are becoming products, the products are reacting to become reactants
Therefore in an equilibrium system the concentrations of the reactants and products remain unchanged even though the forward and reverse reactions are still taking place.
For a reaction to remain in equilibrium, the system must be closed. A closed system is isolated from its surroundings, so the temperature, pressure, and concentrations of reactants and products are unaffected by outside influences
Le Chatlier’s principle
When a system in equilibrium is subjected to an external change the system readjusts itself to minimise the effect of that change
Effect of concentration changes on equilibrium
If there are more products formed, the position of the equilibrium has shifted to the right
If there are more reactants formed, the position of the equilibrium has shifted to the left
The effect of temperature changes on equilibrium
Forward and reverse directions have the same value for the enthalpy change – but the signs are opposite
An increase in temperature shifts the equilibrium position in the endothermic direction (enthalpy change is positive)
A decrease in temperature shifts the equilibrium position in the exothermic direction (enthalpy change is negative)
Effects of pressure changes on equilibrium
Decreasing the pressure shifts the position of equilibrium in the opposite direction, to the side with more gaseous moles.
Effects of a catalyst on equilibrium
A catalyst does not change the position of equilibrium; it merely speeds up the rate of the forward and reverse reactions equally. The catalyst will, however, increase the rate at which an equilibrium is established
The equilibrium law
Equilibrium constant= [C]^c [D]^d/[A]^a [B]^b
[ ] = concentration of
a,b,c,d = the balancing numbers in the overall equation
A,B,C,D= the equilibrium concentrations of the reactants and products of this equilibrium
Kc
Equilibrium constant
Kc=1
Indicates a position of equilibrium that is halfway between reactants and products
Kc >1
Indicates a position of equilibrium that is towards the products
Kc <1
Indicates a position of equilibrium that is towards the reactants
The larger the value of Kc…
The further the position of equilibrium lies to the right hand side and the greater the concentrations of the products compared to the reactants
