Energy changes for Edexcel A-level Chemistry
This page covers the following topics:
1. Thermal energy
2. Enthalpy change
3. Reaction profile
Thermal energy released in a chemical reaction can be obtained by heating a known mass of water in an isolated system and measuring the change in temperature. The energy released is then found by multiplying the mass of water, specific heat capacity of water and the change in temperature.
Standard enthalpy change is the amount of energy gained by a reaction mixture per one mole of a reactant at standard room temperature and pressure. Enthalpy of combustion refers to the combustion of material, enthalpy of neutralisation to neutralisation reactions, and enthalpy of formation is used when considering elements bonding together to make a compound of consideration.
Negative enthalpy change suggests that a reaction mixture is losing energy and therefore the reaction is exothermic. Positive enthalpy change means that a reaction mixture is gaining energy and thus the reaction is endothermic.
Reaction profile shows the energies of reactants, intermediates and products in a reaction. The difference between the initial energy of the reactants and the highest point in a reaction profile is called activation energy which is the energy required for a reaction to happen. If reactant molecules have less energy than the activation energy, a reaction between them does not happen. Activation energy can be thought of as a hill that particles need to climb.
Reaction profile can be used to determine if a reaction is exothermic or endothermic. Exothermic reactions have the energy of reactants higher than the energy of products. Endothermic reactions have the energy of reactants lower than the energy of products.
Define standard enthalpy of formation.
Standard enthalpy of formation is the amount of energy gained by elements bonding to make one mole of a compound at standard room temperature and pressure.
amount of energy gained by elements making one mole of a compound at standard conditions
Name A and B shown in the reaction profile.
Reaction profile shows the energies of reactants, intermediates and products in a reaction.
A = reactants, B = products
George has drawn a reaction profile for a photosynthesis reaction provided below. What words should he include instead of X and Y in the diagram?
photosynthesis: water + carbon dioxide → glucose + oxygen
Reaction profile shows the energies of reactants, intermediates and products in a reaction. Reactants are on the left side of the photosynthesis equation provided, products are on the right.
X = water, carbon dioxide; Y = glucose, oxygen
Combustion of 4.00 g of carbon has increased the temperature of 500 g of water by 62.8 K. Find the enthalpy of combustion of carbon in kJ/mol. The specific heat capacity of water is 4180 J/(kgK).
500 g = 0.500 kg
E = mcΔT
E = 0.500 × 4180 × 62.8
E = 131 kJ (3 s. f.)
n = 4.00 ÷ 12 = 0.333 mol
ΔH = −E ÷ n
ΔH = −131 ÷ 0.333
ΔH = −394 kJ/mol
Define activation energy.
Activation energy is the energy required for a reaction to happen.
energy required for a reaction to happen
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