Change x x x x equilibrium initial x initial x

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Unformatted text preview: ADH][CH 3CHO] [NAD+ ][CH 3CH 2OH] In order to determine Keq for the reaction, we must know the concentrations of reactant and product at equilibrium. If you were doing this as a homework problem, your first step would be to set up an ICE table: € CH3CH2OH NAD+ CH3CHO NADH Initial [initial] [initial] 0 0 ! Change - x - x +x +x Equilibrium [initial]- x [initial]- x x x And then set up the equilibrium expression: K eq = ( x )CH CHO ( x ) NADH ([initial]EtOH − x)([initial]NAD 3 + − x) This equation (and the table) tells us that we need to know the initial concentrations of the reactants ([initial]) and the final concentration of the products (x) in order to determine the equilibrium constant. The € initial concentrations should be easy to determine experimentally: we simply record the initial concentrations of the ethanol and NAD+ at the start of the experiment. So, the tough experimental question becomes: “How can we measure the concentration of the products at equilibrium in a straightforward manner?” Luckily, the molecules in this lab have light absorbing properties. You’ll recall from the previous experiments that conjugated systems tend to absorb low energy (ultraviolet/visible) light. NAD+ contains adenine (a conjugated DNA base) and a pyridinium group that acts as the hydride acceptor. NADH contains the same adenine base as NAD+, but contains a reduced pyridinium (after hydride has been added). Since adenine is common to both molecules, both molecules absorb light where adenine does, which is at 260 nm. So...
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