Part 2 Flashcards
Energy Changes
Chemical reactions involve the breaking and formation of chemical bonds. The general rule is that energy is absorbed in the breaking of chemical bonds and energy is released when the bonds are formed.
Exothermic reactions
Exothermic reactions are reactions which release heat. The energy required to break the bonds in the reaclants is exceeded by the energy released when chemical bonds are formed in products.
Endothermic reactions
Endothermic reactions can be considered the opposite of exothermic reactions. They absorb energy from the environment in which they happen.
Enthalpy
Enthalpy is a term that describes energy within a chemical system. Enthalpy cannot be measured directly, so scientists talk about Enthalpy changes when discussing chemical reactions.
Energy diagrams.
Exothermic - we see reactants having more energy than the products. Enthalpy - change when energy his below reactant energy.
Endothermic - We see the products having more energy than the reactants. Enthalpy - change seen between the final product energy and reactant energy.
Breaking bonds
Breaking bonds is the exothermic process. Input energy required to seperate forces of attraction.
Forces include ionic, covalent, metallic and weak intermolecular attractions.
Changes of state
When a substance changes state it is because the substances chemical bonds have been either weakened or strengthened.
Energy is absorbed when this happens so is a endothermic process.
Energy is released when bonds strengthen so that is a exothermic process.
Bond Enthalpy
The strength of the covalent bond between atoms depends on the strength of the attractive forces between the nuclei and bonding electrons.
Moles
^ = mrm ^ = no of moles (mol) m = mass (g) M = molar mass (gmol -1)
Specific Heat Capacity
Q = mc AT Q = heat energy (J) m = mass (g) c = specific heat capacity (Jg -1 C-1) AT = temperature change .......
Equilibrium
Reversible Reactions
Physical changes are by definition reversible, eg melting ice blocks.
Reversible reaction =
A chemical reaction which changes the reactants physical state by can be changed back.
In a open system, reactions and physical changes are rarely reversible. Why?
This is due to the particles being able to enter and leave the system.
When the lid is on the system is closed. Nothing can escape.
See diagram.
Equilibrium state of balance
Equilibrium occurs when the forward reaction proceeds at the same rate as the reverse reaction. This means the rate of both reactions.
Equilibrium and physical change
Equilibrium may be established in physical processes. If the process can be represented by a reverse symbol equation there will be will be conditions where the process is in equilibrium.
The Equilbrium Constant - Kc
Any reverse reaction can be represented by:
aA + bB = cC + dD.
Where A, B, C and D represent the reactants and the products and a, b, c and d represent the mole ratio numbers.
The Equilibrium Constant Kc is a number value….
from the concentration of reactants and products for any reaction at equilibrium at any given temperature. it is calculated from the equilibrium expression for the equation.
The Equilibrium Expression
See diagram
Equilbrium Expression
- The concentration of the products (ie C and D) become the numerator.
- The concentration of the reactants (ie A and B) become the denominator.
- The squares brackets mean the concentration in moles per L.
- The co-efficients for the species become powers to which the concentration is raised.
- Don’t include solids or water as a reactant. (see chem equation).
Acids and Bases. What is an acid
Acids are compounds that have dissociated into a hydrogen ion (H+) and is one of several different types of anion. Hydrogen atoms are protons due to having lost a electron during ion formation. Acids can be defined as proton donors due to all acids containing one or more atoms of hydrogen in their formula.
Acid dissociation
- Can happen in solvents other than water.
- Most commonly dissociated in water.
- Dissociated protons/hydrogen ions will bind with water molecule to form hydronium ions (H3O+).
Bases:
- Are proton acceptos.
- In chemical reactions with acids, bases bond with the proton released from the acid.
Strong and weak acids
A strength of a acid is dictated by if the acid has fully dissociated or note.
What is a strong acid?
An acid that has fully disassociated in water.
What is a weak acid
An acid that has partially dissociated from water.
Define dissociation
The breaking up of a compound into similar consititments that are usually capable of recombing …….. other conditions.
Reactivity strong and weak acids:
The concentration of hydronium ions that determines the rate of reaction between a acid and a metal to produce metal salt and hydrogen gas. Because strong acids fully dissociate into ions when placed in water, they will have a higher concentration of hydronium ions in a solution compared to weak acids at the same concentration.
Conductivity between strong and weak acids
Conductivity of solutions is determined by the concentration of ions present in the solution. Strong acids are far more conductive that weak acids due to the higher concentration of ions in a solution due to them fully dissociating.
Strong acids create solutions with good electrical conductivity.
Weak acids create solutions with poor electrical conductivity.
Concentrated and Dilute Acids
The overall strenght of a acid does not change depending on concentration, unless the acid solution is extremely dilute. Concentration is the measurement of how many acid particles are in a volume of water.
In a lab, acids below 1 mol LY are considered dilute. The reactivity/conductivity of acids is determined by the concentration ions in a solution.
Monoprotic and Polyprotic Acids.
Define Monoprotic
The term monoprotic describes acids that donate only one proton to a water molecule.
Many other acids are able to donate more than one proton to water molecules. These acids are termed prolyprotic acids.
Nitric Acid
Monoprotic acid - only one proton is available to be donated to a water molecule.
Sulphuric Acid
Diprotic Acid - Two protons are available to be donated to water molecules
Phosphoric Acid
Triprotic Acid - Three protons are available to be donated to 3 water molecules.
Strong and Weak Bases
Bases are proton acceptors. Bases that dissociate fully into species that will accept protons are strong bases.
Weak bases do not fully dissociate in water. Weak bases are able to accept a proton from a water molecule in solution.
Weak acids will also react with acids to produce salts.
Conjugate acid / Base pairs
When a acid donates a proton to water the species that is left behind is called the conjugate base of the acid.
Acid + water = Conjugate base + hydronium ion.
Similarly, when a base such as ammonia accepts a proton from water, it becomes a conjugate acid.
Amphiprotic Substances
Some species are able to donate and accept protons such as water molecules when they accept a proton to become H3O or donate to become OH-. Such species are call “amphiprotic”.
Acidic Salts
Salts are ionic. Some salts form acidic solutions when one of their ions react with water and form hydroium ions. Acidic salts form weaker acidic solutions than strong acids.
The pH scale
The pH scale is used to measure concentration of acids and bases. pH ranges from O to 14 with O-6 being acidic, 7 being neutral and 8-14 being bases.
What is pH a measure of?
pH is a measure of the concentration of hydronium ions in a solution. The more hydronium ions present the more acidic the solution. The pH scale is logarithmic. Each unit on the pH scale represents a tenfold change in acidity. Eg, a solution with a pH of …… is 10 more times more acidic that a pH of 5. And a pH of 4 compared to 6 is 100 times more acidic.
Calculating pH.
pH = power of hydrogen pH = -log (H30+).
For example when the concentration of H30 is 0.1molL-1, pH =. (See diagram)
Kw - The ionic product of water
In a sample of pure water majority of particles = H20.
However, some water molecules have dissociated into H30+ and OH-.
We know that pure water is neutral. (see diagram).
Neutralisation - Strong acid / Strong base
When a acid reacts with a base, a neutralisation reaction occurs. Unless the same amount of acid and base are used, three will be a excess of one or the other left over.
