Reactivity 1.1 Measuring Enthalpy Changes
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Heat
A form of energy, measured in Joules
Temperature
A measure of the average kinetic energy of the particles of a substance (its “hotness”). Measured in deg celsius or kelvin
Exothermic
When the energy given out when bonds are formed is greater than the energy required to break bonds. The excess energy is given to the surroundings as heat.
Endothermic
When the energy given out when bonds are formed is less than the energy required to break bonds. The required energy is drawn from the surroundings as heat.
Is energy released or required when bonds are formed?
Released
Is energy released or required when bonds are broken?
Required
Enthalpy (H)
The internal energy stored in chemical bonds
Activation energy
The minimum energy the reactant particles need to have to react and form the products.
What is the sign of the enthalpy change in an exothermic reaction?
negative
What is the sign of the enthalpy change in an endothermic reaction?
positive
Do the reactants or the products have a stronger bond in an exothermic reaction?
Products
Do the reactants or the products have a stronger bond in an endothermic reaction?
Reactants
What state changes are exothermic and why
Any in the direction of Gas–>Solid. The particles in the system have decreased Ek and so transfer this to the surroundings via heat.
What state changes are endothermic and why
Any in the direction of Solid–>Gas. The particles in the system have increased Ek and so draw this from the surroundings, making the surroundings colder.
Standard enthalpy change conditions
Pressure of 100kPa, conc of 1 mol dm-3 for all solutions, all substances in their standard states.
What is standard enthalpy change measured in?
kJ mol-1
what does Q stand for in Q=mcT
The heat added
what does c stand for in Q=mcT
The specific heat capacity of the substance
Specific heat capacity
The amount of heat energy required to raise the temperature of one gram of a substance by one degree Celsius (or one Kelvin)
Source of error in calorimetry experiments
Heat loss to the surroundings. This begins as soon as the temp of the system rises above the temp of the surroundings so the max recorded value is lower tham the true value obtained in a fully insulated system.