9 Enthalpy Flashcards
(35 cards)
What is enthalpy?
- Molar heat energy change
- The measure of the heat energy in a chemical system
- Energy stored within bonds
- Enthalpy cannot be measured but change can
What is a chemical system?
The atoms, molecules, or ions making up the chemicals
What is enthalpy change?
Difference in the enthalpies of the reactants and products
^H = H(products) - H(reactants)
What is the law of conservation of energy?
Energy cannot be created or destroyed
What happens during a chemical reactions involving an enthalpy change?
Heat energy is transferred between the system and the surroundings:
- System is the reactants and products
- Surroundings is what envelopes the system e.g. apparatus, lab
- Universe is everything
What is an exothermic change?
- System releases heat energy to the surroundings
- Energy loss by system = energy gain by the surrounding
- Enthalpy change is negative
- The temperature of the surroundings increases as they gain energy
What is an endothermic change?
- Energy transfer from the surroundings to the system
- Chemical system takes in heat energy from the surroundings
- Energy gain by system = energy loss by surroundings
- The temperature of the surroundings decreases as they lose energy
- Enthalpy change is positive
What is activation energy?
Definition: The minimum amount of energy for a successful collision to take place resulting in a reaction
- The energy input required to break bonds
What is the standard pressure?
100 kPa
What is the standard temperature?
298K (25C)
What is the standard concentration?
1 moldm-3
What is the standard state?
The physical state of a substance under standard conditions
What is the standard enthalpy change of reaction?
The enthalpy change that accompanies a reaction in the molar quantities shown in a chemical equation under standard conditions, with all reactants and products in their standard states
- Always refers to a stated equation
What happens if the balancing numbers are doubled in a reaction?
The standard enthalpy change is doubled
What is the standard enthalpy change of formation?
The enthalpy change when:
- one mole of a compound
- is formed from its elements
- under standard conditions
(with all reactants and products in their standard states)
What is the standard enthalpy change of combustion?
The enthalpy change when:
- one mole of a substance
- reacts completely with oxygen
- under standard conditions
(with all reactants and products in their standard states)
What are the products of combustion?
The oxides of the elements in the substance
- Carbon dioxide and water
What is the standard enthalpy change of neutralisation?
The enthalpy change when:
- one mole of water
- is formed when an acid and a base react
- under standard conditions
(with all reactants and products in their standard states)
How do you convert from Kelvin to celsius?
C = K - 273
What is the equation for an energy change?
q = mc^T
What is the ‘m’ in the energy change equation?
The mass of the material changing temperature
What is the ‘c’ in the energy change equation?
The specific heat capacity - the energy required to raise the temperature of 1g of a substance by 1K
4.18Jg-1K-1
Describe a method to find the enthalpy change of the combustion of methanol
- Measuring cylinder to measure 150cm3 of water into the beaker. Record the initial temperature
- Weight the spirit burner containing methanol with the lid on
- Place the spirit burner under the beaker. Light the burner and burn the methanol whilst stirring the water with the thermometer
- Heat until 30C rise and record the highest temperature reached
- Reweigh the spirit burner
What are the calculation to find the enthalpy change in a methanol combustion reaction?
- Energy change (q =mc^T) of water
- Mol of methanol burnt
- Enthalpy change
