Reactivity 1.1 Flashcards
Heat
A form of energy (joules), measure of total energy
Temperature
A measure for the average kinetic energy of the particles of a substance. Measured in degrees
Energy changing in chemical reactions
No overall energy changes
Exothermic reaction
Energy given out when new bonds are formed is greater than the energy required to break the bonds, excess energy is given out to the surroundings as heat
Endothermic reaction
Energy given out when new bonds are formed is less than the energy required to break the bonds in the reactants, then the excess energy required is taken in from the surroundings.
Enthalpy
Internal energy stored in chemical bonds (H)
Absolute value of the enthalpy
This cannot be measured, but change in H can be measured during a chemical reaction at constant pressure
H value exothermic reaction
Negative
H value endothermic reaction
Positive
Reaction profiles
Show the potential energy of the reactants and products changing during the reaction.
Activation Energy
Ea, the minimum energy required for a reaction to occur
What phase changes will be exothermic?
Gas to liquid, liquid to solid (where particles within the system have decreased kinetic energy and release it to the surroundings.
What phase changes will be endothermic?
Solid to liquid, liquid to gas (where the system has increased kinetic energy and must absorb this from the surroundings)
Standard Enthalpy Change Conditions
- Pressure of 100kPa
- Concentration of 1 mol dm-3
- All substances in their standard states
- Measured in kJ mol-1
What is absolute temperature a measure of?
Measure of average kinetic energy of particles
If the same amount of heat energy is added to two different objects made of the same material but different number of particles, what happens to the temperature?
The change isn’t equal due to the average kinetic energy of the particles increasing by a different amount
Increase in temperature when an object is heated depends on… (3 factors)
- Mass of the object
- Heat added (Q)
- Nature of the substance
Largest sources of error in reactions?
Heat loss to the surroundings
Where does heat travel?
From the hotter object to the cooler object until they are in thermal equilibrium
Exothermic reactions examples
Neutralisation and combustion
Endothermic reactions examples
Photosynthesis and thermal decomposition
How can you tell how stable a reactant or product is?
The enthalpy - the lower it is, the more stable the substance is
Change of temperature of surroundings depends on what? (2 factors)
Mass of the materials making up surroundings and the specific heat capacity
What assumption is made about dilute aqueous solutions?
That it has the same specific heat capacity and density as pure water.
Calorimeter
Used to measure the enthalpy change in reactions occurring in solution
Molar mass units
gmol^-1
Enthalpy of solution
The enthalpy change when one mole of a solute is dissolved in a solvent to infinite dilution (turning a solid to aqeous ions)
Enthalpy of formation
Formation of one mole of compound from its elements under standard conditions with reactants and products in their normal states
When calculating the enthalpy change of neutralisation, what do you calculate?
The amount of acid neutralised
