IB Chemistry - SL Energetics
How to solve calorimeter questions (simple)
1. Determine if the reaction is endo or exo (temp decrease/increase respectfully) 2. Find the number of moles of the species which has been given a mass 3. Find 'q' with mass of water, 4.18 and temp change (which is always positive) 4. q/mol (write answer with correct sign)
What are the assumptions associated with a solution calorimeter?
1. Heat loss to the surroundings is minimal 2. The specific heat capacity of the solution in the calorimeter is the same of that of water 3. The mass used to calculate q is that of water (not the mass added) 4. The reaction goes to completion
How to calculate enthalpy change for a reaction when given two simultaneous equations
1. Put species on right side of the equation (if you flip, change symbol) 2. Multiply/divide to get correct coefficients (adjust enthalpy as you go) 3. Cross out reactants/products that overlap (or subtract is they don't cancel out) - you should be left with the equation that the question asks to find 4. Add the enthalpies together
What is the process of a solid dissolving in a solvent?
1. The species in the solid becomes separated from each other. The standard enthalpy of change for this endothermic process is the lattice enthalpy of the solid. 2. The separated species becomes surrounded by molecules of the solvent. This process is called solvation (with a solvent) or hydration (with water)
How to calculate enthalpy of reaction using bond enthalpies
1. Write out an equation 2. Formula is (reactants - products) 3. Use bond enthalpies, multiplying by the coefficient
How to calculate the heat of combustion using bond enthalpies
1. Write out an equation 2. Formula is (reactants - products) 3. Use bond enthalpies, multiplying by the coefficient
How to calculate standard enthalpy change of reaction
1. Write the equation (fractions must be used( 2. Work out the bonds between species (eg H₂ is H-H bond) 3. Add all the product bonds together and minus by all the reactant bonds together (use brackets) (remember 2HF is 2 x H-F)
Definition of a closed system
A closed system (lid, molecules cannot escape) can exchange energy with the surroundings but not matter.
Definition of an endothermic reaction
Absorbs energy from surroundings. Heat gain by the system. It involves bond breaking. Products are less stable and have more energy than reactants. Positive symbol (temperature decreases during reaction). eg. Ice packs
How to calculate bond enthalpy of methane
CH₄→C+4H, therefore go to data booklet at times energy to break C-H by four
When asked to calculate the energy released, what do you do?
Find the number of moles of the species which has information about its mass. Set up equation number before species given/mol found = enthalpy given/x Solve for x
Definition of heat
Heat is a form of energy that may be released during a chemical reaction. Heat is a mode of energy transfer which results in a temperature difference and which increases the average kinetic energy of the molecules present in a system (how fast the molecules are moving).
How to solve calorimeter questions (hard)
If you are given information about a concentration then the first step will be to work out which species is the limiting, this will be the mol used for q/mol
Definition of temperature
Is a measure of the average kinetic energy of the particles and is displayed by a Maxwell-Boltzman distribution diagram.
What is the convention for an element in its most stable state?
It's enthalpy of formation will be zero
Describing a Maxwell-Boltzman distribution diagram
Maximum point is average kinetic energy. A few slow-moving particles (low kinetic energy) near y axis. Most particles in the centre. A few fast-moving particles (high kinetic energy) far away from y axis.
Definition of specific heat capacity
Measure the amount of heat energy needed to increase the temperature of 1 gram of the substance by 1K.
Definition of an open system
Most chemical reactions take place in an open system (no lid, molecules can escape) which can exchange energy and matter (atoms, particles) with the surroundings.
Endothermic energy profile diagrams
Products greater than reactants. Activation energy is between reactants and maximum point. Enthalpy change is between reactants and products. If stronger bonds break than are formed, heat must be absorbed from the surroundings.
Exothermic energy profile diagrams
Reactants greater than products. Activation energy is between reactants and maximum point. Enthalpy change is between reactants and products. Stronger bonds are formed in the products than are broken in the reactants. Heat is released to the surroundings.
Definition of bond enthalpy
The amount of energy required to break one mole of particular covalent bond in the gaseous state into gaseous atoms.
What can be noticed when there is a 10 degree increase
The average kinetic energy shifts to the right indicating that more particles are on average moving faster.
Definition of the standard enthalpy change of reaction
The difference between the enthalpy of the products and the enthalpy of reactants under standard conditions (1M solution, 298K, 1 atm pressure). triangle H⁰ = H(products) - H(reactants)
Definition of standard enthalpy change of reaction
The enthalpy change of a reaction when carried out at 298 K and at a pressure of 100kPa
Definition of enthalpy of hydration
The enthalpy of hydration is the standard enthalpy change accompanying the production of a hydrated ion from an ion in the gas phase. Na⁺(g)+H₂O(l)→Na⁺(aq) EXO
Definition of enthalpy of solution
The enthalpy of solution is the standard enthalpy change accompanying dissolving a solid in a large excess of water. LiCl(s)+H₂O(l)→Li⁺(aq)+Cl⁻(aq) EXO
What does a calorimeter measure?
The heat energy released by the combustion of a fuel. This is done by measuring the mass burnt and the temperature change of the water.
What to do when asked to write the equation for the enthalpy of combustion
The species you are given is a reactant, must be one mol The other reactant is O₂(g) The products are water and carbon dioxide, and if necessary they have fractions as their coefficients Use data booklet for info
What to do when asked to write the equation for the enthalpy of formation
The species you are given is the ONLY product and must be one mol The reactants are what make up the species, and if necessary have fractions as their coefficients. Use data booklet for info
Which metal/species will experience the greatest temperature change
The species/metal with the lowest heat capacity
Definition of the chemical energy of a substance
The sum of its potential energy (stored energy) and kinetic energy (energy of movement). Called the species enthalpy (H). Each bond in a molecule contains a different amount of energy. The sum of all the bonds in a molecule gives the overall enthalpy of the substance.
Thermochemical equation answers
There are no fractions, the equation must be balanced. If the equation is flipped, the flip the symbol. If the equation is multiplied/divided, do the same with the enthalpy.
What occurs when there is less energy for species
There are stronger bonds. The more unstable a bond/the more energy it has, it is easier to break bonds.
Formula of specific heat capacity
q = mass(g) x specific heat capacity(from data booklet) x temp change
Axis's on the Maxwell-Boltzman distribution diagram
y axis = proportion of molecules with a particular kinetic energy x axis = kinetic energy
Rule with energy changes during reactions
Although a system may exchange energy with its surroundings the total energy cannot change during a chemical reaction. Any energy lost by the system is gained by the surroundings.
Definition of average bond enthalpy
Bond enthalpy averaged over compounds with a similar bond
How to calculate the enthalpy change using Hess' law
Combine enthalpy changes together. If you are following the direction of the arrow, you add, if you are going the opposite direction of the arrow, you minus.
Oxygen and enthalpy of formation
Does not have an enthalpy of formation
Definition of energetics
Energy is a measure of the ability to do work.
Definition of an exothermic reaction
Releases energy to its surroundings. Heat loss by the system. It involves bond forming. Products are more stable and have less energy than reactants. Negative symbol (temperature increases during reaction). eg. Combustion
What to do when asked to write the thermochemical equation of enthalpy of formation/combustion
Same process however no fractions as coefficients, must be whole numbers
Definition of Hess' Law
The heat evolved or absorbed in a chemical process is the same whether the process takes place in one or several steps
