Physical Flashcards
Enthalpy change
The amount of heat released or absorbed by a chemical reaction, carried out at constant pressure
Exothermic reaction
Delta H=
A reaction where heat energy is released to the surroundings
Negative
Endothermic reaction
Delta H =
A reaction where heat energy is absorbed from The surroundings
Positive
Average bond enthalpy
The average enthalpy change for the breaking of one mole of bonds in gaseous molecules
Standard conditions
25 degrees C / 298k
1 atmosphere/ 100 kPa
Solution must have conc 1 moldm^-3
Standard enthalpy change of reaction
The enthalpy change that accompanies a reaction in the molar quantities expressed in a chemical equation under standard conditions, all reactants and products in their standard states
Standard enthalpy change of neutralisation
The enthalpy change that accompanies the formation of one mole of H2O from neutralisation, under standard conditions
Standards enthalpy change of formation
The enthalpy change when one mole of a compound is formed from its elements, in their standard states under standard conditions
Standard enthalpy change of combustion
The enthalpy change for the complete combustion of one mole of a substance under standard conditions, all reactants and products in their standard states
Hess’s law states that
The enthalpy change of a reaction depends only on the initial and final states an is independent of the route taken
First law of thermodynamics
Energy can be converted from one form to another and cannot be created or destroyed
Delta H is negative
4
System releases heat energy to the surroundings
Enthalpy of the system decreases
Temperature of surroundings increases
Enthalpy change is EXOTHERMIC
Delta H is positive
4
System absorbs heat energy from the surroundings
Enthalpy of the system increase
Temperature of the surroundings decreases
Enthalpy change is ENDOTHERMIC
When measuring temperature what are you recording the temperature of
Surroundings and NOT the system
Enthalpy change of a reaction calculation
Enthalpy of products - enthalpy of reactants
A molecular chemical reaction involves
Breaking covalent bonds in the reactant molecules and forming new covalent bonds in the product molecules
Breaking bonds is
Endothermic
Absorbs energy
Positive
Making/ forming bonds is
Exothermic
Releases energy
Negative
The enthalpy change of a reaction depends only on
the initial and final states and is independent of the route taken
The rate of a chemical reaction is
The change in concentration of a reactant or a product per unit time
Effect of concentration on rate
Collision theory
As the concentration of reactant molecules increases, the rate of rection increases
At a higher concentration there are more molecules in a given volume
More frequent successful collisions
Effect of pressure on rate
Collision theory
When the pressure of a gas is increased, the gas molecules are pushed closer together
The number of gas molecules in a given volume increases
More frequent successful collisions occur
The effect of temperature on rate
Collision theory
As the temperature of a reaction mixture is increased, the rate of reaction increases
At a higher temeperature, the average energy of the molecules increases
A greater proportion of the molecules have energy greater than or equal to the activation energy
More frequent successful collisions occur
Activation energy
The minimum energy required for a reaction to take place, by the breaking of bonds in the reactants
Catalyst
Increases the rate of reaction without being used up by the overall reaction
It allows the reaction to proceed via. Different route with lower activation energy
Heterogenous catalysis
Catalysis of a reaction in which the catalyst has a different physical state from the reactants
Homogenous catalysis
Catalysis of a reaction in which the catalyst and the reactants are in the same physical state
Key advantages of using a catalyst in an industrial reaction
Low temperature gives;
- reduced energy demand
- less CO2 emissions as less combustion of fossil fuels
- less cost
- Increased sustainability
Alternative reaction with higher atom economy and less waste can be used
Using an enzyme generates very specific product, no by products
Enzymes operate at close to room temperature and pressure
Effect of temperature
Boltzman
At a higher temperature, the average energy of the molecules increases
A greater proportion of the molecules have energy greater than or equal to the activation energy
More frequent successful collisions occur
Rate of reaction increases
Boltzman flattens and shifts to the right
Effect of a catalyst
Boltzman
A catalyst increases the rate of reaction without being consumed by the overall reaction
A catalyst lowers the activation energy for the reaction by providing an alternative pathway
A greater proportion of the molecules have energy greater than or equal. To the activation energy
More frequent successful collisions occur
Rate of reaction increases in the presence of a catalyst
Characteristics of a dynamic equilibrium
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- The rate of the forwards reaction is equal to the rate of the reverse reaction
- The system is closed (no materials added or taken away)
- The concentration of reactants and products do not change
- The macroscopic (big) properties (temperature, pressure, concentration) do not change
Le Chateliers principle
When a system in dynamic equilibrium is subjected to a change, the equilibrium position will shift to minimise the effects of the change
Effect of temperature on dynamic equilibrium
Exothermic
Increase= left
Decrease = right
Endothermic
Increase= right
Decrease=left
Significance of Kc
If Kc>1 the equilibrium position lies to the right
If Kc<1 the position lies to the left
If Kc=1 the equilibrium lies halfway between reactants and products
The effect of temperature on Kc/p
Exothermic forward reaction
If the temperature is increased:
-Thea equilibrium position shifts to the left
-to minimise the effect of an increase in temperature , by absorbing energy
-because the reverse reaction is endothermic
-the new mixture will contain more reactants and less products
The value. Of Kc/p decreases
The effect of temperature on Kc/p
Endothermic forward reaction
- the equilibrium position shits to the right
- to minimise the effect of an increase in temperature, by absorbing energy
- because the forward reaction is exothermic
- the new mixture will contain more products and less reactants
- the value of Kc/p increases
The effect of concentration Kc
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If the reactant concentration is increased
This means the expression for Kc no longer matches the original Kc value
The equilibrium position shifts to the right
The concentration of the product increases
The concentration of the reactant decreases
A new equilibrium is reached where Kc is restored to its original value
The effect of pressure KP
If total pressure is increased
All the partial pressures increase
There are more partial pressures of products than reactants so the numerator (top) of KP increases more than the denominator (bottom) of KP
The ratio in the KP expression increases
The equilibrium position shifts to the left so the partial pressure of the reactant increases, increasing the denominator (ratio decreases)
A new equilibrium is reached where KP is restored to its original value
Efffect of a catalyst on Kc and Kp
A catalyst increases the rat of both the forward and backward reaction by the same amount
The equilibrium position does not change
Value of Kc and Kp remains constant
The rate of a chemical reaction is
The change in concentration of a reactant or product per unit time
Rate equation
Rate = k [A]^m [B]^n
Rate at time ‘t’
Gradient of the tangent to the curve at time ‘t’
Initial rate of reaction
Is equal to the gradient of the tangent at time = 0
The half life of a reactant
The time taken for the concentration f the reactant to fall to half of its original concentration