function rxn = calc_rxn_times(rxn,tnow)

global molecdata

for i = 1:length(rxn)
    
    permut = 1;
    
    for j = 1:length(rxn{i}.subs(:,1))
        
        molnum = rxn{i}.subs(j,2);
        
        permut = permut .* molecdata(molnum);
        
    end
    
    ai = rxn{i}.rate .* permut;
    
    if ai ~= 0
        rxn{i}.time = (1./ai) .* log(1./rand) + tnow;
    else
        rxn{i}.time = inf;
    end
end