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Lecture 2

Biochemistry 2EE3 Lecture 2.docx

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Karen Mc Garry

Biochemistry 2EE3 Lecture 2 • • • System: part of universe o Open o Closed o Cells have membranes which are like walls, but is an open system that allows things to pass in • Surroundings: rest of universe • • In most cells, volume does not change, therefore cells are highly resistant to pressure changes, and ∆Q is almost equal to zero and is practically equal to ∆H • • Spontaneous: occur without input of energy o Ex. Heat exchange, gas expansion o Ex. Diffusion of gas • • System wants to increase entropy (degree of randomness) o Boltzmann constant is number of energetically equivalent ways of arranging components of system o We estimate change in number of ways of energetically equivalent ways  Therefore we measure changes in entropy, not absolute values of entropy • Second law of thermodynamics: entropy wants to increase • • If ∆G negative, then spontaneous • If is equilibrium, ∆G=0 • Standard conditions are used because they are measurable • Kilocalories/mole usually used to give change in energy • Whether the value is large or small depends on what it is being used for • Chair conformation of cyclohexane is low energy, therefore more stable, more likely to be found in this conformation • Boat conformation of cyclohexane is high energy • When it undergoes conformation changes, cyclohexane uses energy to changes shape o If substituted, can discriminate between two chair conformations (axial/equatorial changes after chair flip) • Scientists are interested to know probability of cyclohexane in various conformations and how often it changes o You cannot answer this question by looking at energy changes o Can only compare probabilities • • Ratios of probability (i.e. which one is more likely to be present due to stability, low energy, etc.) is determined by Boltzmann equa
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