Why do some photographs develop so slowly

The concentrations of each reactant and product will vary during the course of a chemical reaction. The so-called integrated rate equation relates the amounts of reactant remaining in solution during a reaction with the time elapsing since the reaction started. The integrated rate equation has a different form according to the order of reaction. 

Let us start by considering a first-order reaction. Because the reactant concentration depends on time t, we write such concentrations with a subscript, as [A]r. The initial reactant concentration (i.e. at time t = 0) is then written as [A]o. The constant of proportionality in these equations will be the now-familiar rate constant k (where the subscripted 1 indicates the order). 

Equation (8.24) is the integrated first-order rate equation. Being a logarithm, the left-hand side of Equation (8.24) is dimensionless, so the right-hand side must also be dimensionless. Accordingly, the rate constant k will have the units of s-1 when the time is expressed in terms of the SI unit of time, the second. 

The first-order rate constant k will have the units of s 1. 

Worked Example 8.7 Methyl ethanoate is hydrolysed when dissolved in excess hydrochloric acid at 298 K. The ester s concentration was 0.01 mol dm-3 at the start of the reaction, but 8.09 x 10 2 after 21 min. What is the value of the first-order rate constant k  

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