In photosynthesis, plants form glucose (C6 H12 O6) and oxygen from carbon dioxide and water. Wri

In photosynthesis, plants form glucose (C6 H12 O6) and...

Key Concepts

Chemical Reaction Balancing

Chemical reaction balancing is the process of ensuring that the number of atoms for each element is equal on both sides of a chemical equation. This fundamental concept is crucial for obeying the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction. In the context of photosynthesis, balancing ensures that the carbon, hydrogen, and oxygen atoms from carbon dioxide and water correctly match those in the produced glucose and oxygen.

Enthalpy Change (?H)

Enthalpy change (?H) represents the total heat content change of a system during a reaction at constant pressure. It is a key thermodynamic quantity that indicates whether a reaction is exothermic (releases heat) or endothermic (absorbs heat). Calculating ?H for a reaction like photosynthesis helps in understanding the thermal energy requirements or outputs associated with the process.

Entropy Change (?S)

Entropy change (?S) measures the change in disorder or randomness in a system during a chemical reaction. An increase in entropy typically corresponds to a greater number of microstates or higher disorder in the products relative to the reactants, while a decrease indicates more order. In photosynthesis, evaluating ?S provides insight into the molecular rearrangements and the degree of organization or disorder created during the conversion of reactants to products.

Gibbs Free Energy (?G)

Gibbs free energy (?G) is a thermodynamic function that combines enthalpy and entropy to determine the overall spontaneity of a reaction. It is defined by the equation ?G = ?H - T?S, where T is the temperature in kelvin. A negative value of ?G indicates that a reaction can occur spontaneously under given conditions, whereas a positive value means the reaction is non-spontaneous, requiring an input of energy.

Reaction Spontaneity

Reaction spontaneity refers to whether a chemical reaction can proceed on its own without the need for an external input of energy. This is determined by calculating the Gibbs free energy (?G); if ?G is negative, the reaction is spontaneous, while a positive ?G signifies non-spontaneity. In the context of photosynthesis, understanding spontaneity is important because the process is endergonic, meaning it requires energy input to proceed despite occurring naturally within plants.

Standard Thermodynamic Conditions

Standard thermodynamic conditions refer to a set of conventional conditions (typically 1 atm pressure, 25°C temperature, and 1 M concentrations) under which thermodynamic data such as ?H°, ?S°, and ?G° are reported. Using these standard conditions allows for the consistent comparison and calculation of thermodynamic quantities across different reactions and systems, including photosynthesis.

Transcript

00:01 In this question we've been given a reaction of photosynthesis in which glucose is formed from a reaction of co2 and h2 of c6 h2 plus o2.

00:19 So we are going to be forming 6 moles of o2 from 6 moles of co2 and h2o.

00:27 So here we are to determine the standard enthalpy change.

00:30 The standard keeps energy change and the standard entropy change of this reaction.

00:36 And what we have to note about these is that these are state functions.

00:43 These are state functions.

00:45 So we can determine these by looking at the property of the products and subtract that of the reactants.

00:55 For example, if we are to look at the enthalpy change, the standard enthalpy change, this is going to be the sum of the individual enthalpy changes of formation of the products minus the sum of the individual enthalpy changes of formation of the reactants.

01:16 And we always have to make sure that we multiply these by the respective stoichiometric coefficients.

01:23 Because when we are looking at these expressions, the standard enthalpy changes from our booklets or our appendices, these are always given per mole of substance so if we look at our reaction we have at times we have certain amount of substances this stoichiometric coefficient that is not necessarily equal to one so we have to always multiply by this stoichiometric coefficient so that at the end of the day our property is given without this per more of substance so moving on to what we've been given applying this the end of the change the standard enthalpy change this is going to be equal to 1 multiplied by the standard enthalpy change of formation of glucose which is 1 273 .3 this is in kilojol plus 6 moles of 0 this is the standard enthalpy change of formation oxygen oxygen is a pure element and remember the standard endoppy changes of formation are always referenced at a standard condition and this is a pure element so this is going to be zero minus the standard end of the change of the reactants in this case co2 and h2o now applying this we're going to have six moles of negative 393 .5 minus six more of negative 285 .8 so the standard enthalpy change of that reaction of photosynthesis is going to be 2802 .5 and this is in kilojoules.

03:05 Some decide to write these in kilojoules per more of glucose that is going to be formed.

03:14 Then moving on, the standard gives energy change is going to be 1 multiplied by negative 910 .4 plus 6 multiplied by 0 minus 6 multiplied by negative 394 .4.

03:32 Minus 6 multiplied by negative 237 .1 and this is going to give us a standard gives energy change being equal to 287 .2 to 878 .6 and this is in kilojoules...

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